Split charge relay or DC-DC charger: which one your van needs

The internet answers this with a model year. The published determinants are technical: what the battery is, what the alternator does, what voltage the battery needs, how far the cable runs and how fast you want to charge. Every cell below is a quoted manufacturer statement with the date it was checked, or the line “No published statement found”. This page gives no wiring instructions; each product links the maker’s own diagram.

The decision table

Alternator	Lead-acid battery (flooded, EFB, AGM, gel)	LiFePO4 battery
Conventional: roughly fixed voltage, about 14 V	Relay defensible Rules D7, D4 A DC-DC charger remains the more controlled route, and the only one if you later change to lithium.	DC-DC charger Rules D1 Chemistry settles it before the alternator does.
Smart / ECU-controlled: variable, 12.5 to 15 V (Victron)	DC-DC charger Rules D2, D3	DC-DC charger Rules D1, D2 Chemistry settles it before the alternator does.
Not known: measure it first	Measure the alternator first Rules D8-measure	DC-DC charger Rules D1, D8-measure Chemistry settles it before the alternator does.

Relay kits indexed here: 6. DC-DC chargers: 6, of which 5 publish a lithium charge profile.

Read first.

Loading the published dataset…

Test, don’t guess: how to tell what your alternator does

“Every UK vehicle after 2015 has a smart alternator” is a convention with no published source found, and it is wrong for enough vehicles to matter. The checker records that claim as rejected (rule D9-year) and answers from a measurement instead.

Engine running and at normal running temperature, headlights and blower off.
Voltmeter across the starter battery terminals.
A conventional alternator holds a roughly fixed voltage; Victron cites about 14 V as typical.
A smart alternator varies. Victron publishes the range as 12.5 V to 15 V for a 12 V system and notes that vehicles with regenerative braking often show large voltage variations.
Victron’s own Orion XS charger detects a smart alternator with a default delayed-start threshold of 13.3 V and shuts down at 13.1 V. Those are the manufacturer’s detection thresholds, not a diagnostic standard.
If the reading wanders, or sits low once warm, a voltage-sensing relay will not charge the leisure battery properly.

Rule D8-measure Manufacturer document

Test, don't guess: with the engine at normal running temperature, measure the voltage at the starter battery terminals. Victron cites a fixed ~14V for a conventional alternator and 12.5V to 15V for a smart alternator. Its Orion XS default delayed-start voltage of 13.3V and shutdown voltage of 13.1V (smart alternator) are the manufacturer's own engine-detection thresholds and are cited as such, not as a diagnostic standard.

Conventional alternators will generate a fixed voltage (e.g. 14V), whereas the voltage of smart alternators can vary between 12.5V to 15V (for a 12V system). Smart alternators in a regenerative braking system often show large voltage variations.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.1. Engine shutdown detection, Manufacturer document, checked 16 September 2026

Delayed start voltage (Vstart(delay)): Smart alternators can generate a lower voltage when the engine is running, therefore a lower start level is necessary for these systems. ... Default: 13.3V/26.6V (smart alternator) and 13.8/27.4V (regular alternator).
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.5.6. Engine shutdown detection & input voltage lock-out, Manufacturer document, checked 16 September 2026

Shutdown voltage (Vshutdown): This level corresponds with the engine being off. ... Default: 13.1V/26.2V (smart alternator) and 13.5V/27V (regular alternator).
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.5.6. Engine shutdown detection & input voltage lock-out, Manufacturer document, checked 16 September 2026

Start voltage (Vstart): At this level, charging starts immediately. Default: 14V/28V.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.5.6. Engine shutdown detection & input voltage lock-out, Manufacturer document, checked 16 September 2026

The determinants, each with its published source

Rule D1 Manufacturer document

Controlled charging also protects the alternator in lithium systems from overloading, as the low impedance of lithium batteries results in a high alternator current.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 2.1. Introduction, Manufacturer document, checked 16 September 2026

NOT RECOMMENDED FOR USE WITH LITHIUM BATTERIES WITHOUT AN EXTERNAL CURRENT LIMITER - The AB system is not a current limiting system and will allow your lithium to draw as much current as it sees available.
Sterling Power Products, Alternator to Battery Chargers up to 130A (product page), Product description, Manufacturer document, checked 16 September 2026

Rule D2 Manufacturer document

It is common for vehicle manufacturers to install "smart" ECU (Engine Control Unit) controlled alternators to increase fuel efficiency and reduce emissions. Smart alternators provide variable output voltage and shut down when not needed.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 2.2. Features / Smart alternator compatibility, Manufacturer document, checked 16 September 2026

Conventional alternators will generate a fixed voltage (e.g. 14V), whereas the voltage of smart alternators can vary between 12.5V to 15V (for a 12V system). Smart alternators in a regenerative braking system often show large voltage variations.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.1. Engine shutdown detection, Manufacturer document, checked 16 September 2026

Especially in the case of vehicles with a Euro 5 or 6 smart alternator, which often supplies charging voltage that is too low even with the engine running or in the event of a voltage drop over long cables, controlled charging is crucial.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 2.1. Introduction, Manufacturer document, checked 16 September 2026

The alternator to battery charger is an easy way to add advanced charging to many vehicle and engines, it is still very much suitable for most American Vehicles and most marine and other engines, but it is not suitable for modern vehicles / vans sold in Europe (Ford, VW, Merc, Renault, Citroen etc) and Japanese vehicles which have the new Euro Re Generative braking control systems fitted to the vehicle, for these vehicles you must use the Battery to Battery charger range off products, look for the Regenerative Friendly Braking logo.
Sterling Power Products, Alternator to Battery Chargers up to 130A (product page), Product description, 'Please note:' paragraph, Manufacturer document, checked 16 September 2026

Rule D3 Manufacturer document

A relay delivers whatever voltage the alternator delivers. If the leisure battery's published charge voltage is above the alternator's working voltage a relay cannot fully charge it. Victron LFP Smart: charge voltage between 14 V and 14,4 V (14,2 V recommended); Victron AGM Deep Cycle absorption 14,2 - 14,6 V (normal cycle service); smart alternator working range 12.5V to 15V.

Charge voltage: Between 14 V/28 V and 14,4 V/28,8 V (14,2 V/28,4 V recommended)
Victron Energy datasheet: 12,8 & 25,6 Volt Lithium-Iron-Phosphate Batteries Smart, Battery specification table, Charge voltage row, Manufacturer document, checked 16 September 2026

Victron AGM 'Deep Cycle': Absorption, Cycle service Normal (V) 14,2 - 14,6; Fastest recharge (V) 14,6 - 14,9; Float 13,5 - 13,8; Storage 13,2 - 13,5
Victron Energy datasheet: Gel and AGM Batteries, Table 3: Recommended charge voltage, Manufacturer document, checked 16 September 2026

Conventional alternators will generate a fixed voltage (e.g. 14V), whereas the voltage of smart alternators can vary between 12.5V to 15V (for a 12V system).
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.1. Engine shutdown detection, Manufacturer document, checked 16 September 2026

Rule D4 Manufacturer document

Cable run length penalises a relay specifically: a relay passes the full cable voltage drop through to the leisure battery, whereas a DC-DC charger regulates its output regardless of input. Victron's worked example: 5 m run, 16 mm2, 50 A gives 550 mV drop across the round trip.

Especially in the case of vehicles with a Euro 5 or 6 smart alternator, which often supplies charging voltage that is too low even with the engine running or in the event of a voltage drop over long cables, controlled charging is crucial.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 2.1. Introduction, Manufacturer document, checked 16 September 2026

Example: Calculate the input cable voltage drop: Distance between starter battery and charger: 5m. Vshutdown= 13,1V. Recommended wire gauge: 16mm2. Cable resistance: ~1,1 mΩ/m @20 °C, thus Rcable = 1,1mΩ x 10m (2 x 5m) = 11mΩ. At a maximum input current of 50A, this results in: Vcable = 11mΩ x 50A = 550mV.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.5.6. Engine shutdown detection & input voltage lock-out (Input voltage lock-out), Manufacturer document, checked 16 September 2026

If there is a voltage drop over the battery cables, the Orion XS will output the correct voltage, but the batteries will receive a lower voltage.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 5.2.4. Battery cable voltage drop, Manufacturer document, checked 16 September 2026

Rule D5 Manufacturer document

A DC-DC charger lets the installer cap the current drawn from the alternator; a relay (or an uncontrolled alternator-to-battery charger) does not. Victron's Orion XS input current limit is configurable from 1 A to 50 A 'to avoid overloading an alternator'; Sterling states its AB system 'is not a current limiting system'.

Input current: Limits the maximum current drawn from the input source. This is particularly important to avoid overloading an alternator or other weak input sources. The value can be adjusted between 1 A and 50 A in 0.1 A increments.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.5.2. Charger mode settings, Manufacturer document, checked 16 September 2026

If VIN ≤ Vshutdown, the charge current will be reduced to prevent VIN to drop below Vshutdown.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.2. Engine shutdown detection sequence, step 2 -> 3, Manufacturer document, checked 16 September 2026

The AB system is not a current limiting system and will allow your lithium to draw as much current as it sees available.
Sterling Power Products, Alternator to Battery Chargers up to 130A (product page), Product description, Manufacturer document, checked 16 September 2026

Rule D6 Unverified not implemented

Alternator output headroom: the DC-DC charger's input current must be within what the alternator can spare. No published rule was found for what share of an alternator's rated output may be drawn continuously. The checker states the constraint and tells the reader to check the vehicle manufacturer's alternator figure; it outputs no percentage.

Why not: No manufacturer document read on 2026-09-16 publishes what share of an alternator's rated output may be drawn continuously by a DC-DC charger. Spec section 2.4.1 D6.

This is particularly important to avoid overloading an alternator or other weak input sources.
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.5.2. Charger mode settings (Input current), Manufacturer document, checked 16 September 2026

Rule D7 Manufacturer document

A voltage-sensing relay remains defensible where all three hold: a conventional fixed-voltage alternator, a lead-acid leisure battery, and a short cable run. Sterling states its uncontrolled alternator-to-battery charger is 'still very much suitable for most American Vehicles and most marine and other engines'; Victron gives ~14V as the conventional alternator's fixed voltage.

The alternator to battery charger is an easy way to add advanced charging to many vehicle and engines, it is still very much suitable for most American Vehicles and most marine and other engines, but it is not suitable for modern vehicles / vans sold in Europe (Ford, VW, Merc, Renault, Citroen etc) and Japanese vehicles which have the new Euro Re Generative braking control systems fitted to the vehicle, for these vehicles you must use the Battery to Battery charger range off products, look for the Regenerative Friendly Braking logo.
Sterling Power Products, Alternator to Battery Chargers up to 130A (product page), Product description, 'Please note:' paragraph, Manufacturer document, checked 16 September 2026

Conventional alternators will generate a fixed voltage (e.g. 14V), whereas the voltage of smart alternators can vary between 12.5V to 15V (for a 12V system).
Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, 4.1. Engine shutdown detection, Manufacturer document, checked 16 September 2026

Common UK base vehicles: what the vehicle maker has published

The question people actually ask is “does my T6 charge the leisure battery?”. This table records what each vehicle manufacturer’s own documentation says about variable-voltage, regenerative or “smart” charging, or the absence line with the document that was checked. It never infers from the model year and never gives wiring instructions.

Base vehicle	Manufacturer statement on alternator charging behaviour	Statement on charging a second battery	Document checked
Citroen Relay (Jumper) Relay / Jumper third generation (2006 on; handbook edition ed01-21 read) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	Citroen Relay / Jumper HANDBOOK (eGuide ed01-21, en-GB, document 9999_9999_369), Manufacturer document, checked 16 September 2026 Citroen's Relay/Jumper handbook (eGuide edition ed01-21, en-GB) describes Stop & Start for diesel versions but contains no wording about variable-voltage or regenerative control of the alternator. The handbook's 'Regenerative braking' and 'energy recovery' passages refer to the electric e-Relay's traction battery ('the energy recovered is then used to recharge the traction battery'), not to the diesel 12 V system.
Fiat Ducato X290 (2014 on, including Series 8 and 9) Ducato X290 (2014 on) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	FIAT DUCATO OWNER HANDBOOK (MY19, print 603.85.089, EN, 12.2019), Manufacturer document, checked 16 September 2026 Fiat's Ducato MY19 Owner Handbook (print code 603.85.089, 12.2019) describes the Start&Stop system and battery recharging but contains no wording about variable-voltage, regenerative or smart alternator control; the word 'alternator' appears only as an instrument-panel item ('Alternator Status'). The handbook's battery note reads: 'The replacement battery should be of the same type (L6 105Ah/850A) and should have the same characteristics.'
Ford Transit (2014 on) Transit fourth generation, Ford code TTS (2014 on) · verified 16 September 2026	regenerative charging (48 volt mild hybrid system); Ford's fuse table also names a 'Smart regenerative charge inhibit relay': The system uses regenerative charging during engine braking. It switches the engine off when your vehicle is coasting at a speed less than the engine stop speed that you have set. Applies to: HYBRID ELECTRIC VEHICLE (MHEV) section - 'Your vehicle has a 48 volt mild hybrid system.' - of the owner's manual for Transit (TTS) Vehicles Built From: 17-01-2022	No published statement on charging a second battery found (checked 16 September 2026).	FORD TRANSIT Owner's Manual (CG3796en, Transit (TTS) Vehicles Built From: 17-01-2022), HYBRID VEHICLE OPERATION - HYBRID ELECTRIC VEHICLE (MHEV) - HOW DOES A HYBRID ELECTRIC VEHICLE WORK, Manufacturer document, checked 16 September 2026 Ford's charging-behaviour wording in this manual is in the mild-hybrid (MHEV) section. The same manual's fuse specification table lists a 20 A 'Smart regenerative charge inhibit relay', which is Ford's own name for the function, but the manual does not otherwise describe how the alternator is controlled on non-hybrid variants. Manuals for builds from 2014 to January 2022 were not read.
Ford Transit Custom (2013-2023) Transit Custom first generation, Ford code TTF (2013-2023) · verified 16 September 2026	regenerative charging (48 volt mild hybrid system): Your vehicle has a 48 volt mild hybrid system. The system is designed to help reduce fuel consumption and CO2 emissions by using regenerative charging and stopping the engine when your vehicle is coasting at a speed of 12 km/h (7 mph). Applies to: HYBRID VEHICLE OPERATION - MILD HYBRID ELECTRIC VEHICLE (MHEV) section of the owner's manual for Transit Custom (TTF) Kocaeli, Vehicles Built From: 27-01-2020	No published statement on charging a second battery found (checked 16 September 2026).	FORD TRANSIT CUSTOM Owner's Manual (CG3763en, Transit Custom (TTF) Kocaeli, Vehicles Built From: 27-01-2020), Unique Driving Characteristics - HYBRID VEHICLE OPERATION - MILD HYBRID ELECTRIC VEHICLE (MHEV), Manufacturer document, checked 16 September 2026 Ford's published charging-behaviour wording in this manual is limited to the mild-hybrid (MHEV) variant. For non-hybrid diesel variants the same manual describes Auto-Start-Stop but states nothing about alternator voltage behaviour, and does not use the phrase 'Smart Regenerative Charging'. Ford manuals for builds before 27-01-2020 were not read.
Ford Transit Custom (2023 on) Tourneo Custom/Transit Custom second generation, Ford code TTU (Vehicles Built From: 12-06-2023) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	FORD TOURNEO CUSTOM / TRANSIT CUSTOM Owner's manual (CG3964en, Tourneo Custom/Transit Custom (TTU) Vehicles Built From: 12-06-2023 Vehicles Built Up To: 22-10-2023), Manufacturer document, checked 16 September 2026 Ford's owner's manual CG3964en (Vehicles Built From: 12-06-2023 Vehicles Built Up To: 22-10-2023) describes Auto-Start-Stop ('The system is designed to help reduce fuel consumption and CO2 emissions by stopping the engine when it is idling, for example at traffic lights.') but contains no wording about the alternator, regenerative charging or smart charging.
Mercedes-Benz Sprinter (W907/V907, 2018 on) Sprinter 907 (2018 on) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	Auxiliary battery for retrofit consumer's vehicle interior. Includes: cut-off relay for auxiliary battery (E36)	The Mercedes-Benz Sprinter. Model information & price list. Prices effective from August 2018 production., Manufacturer document, checked 16 September 2026 Mercedes-Benz UK's Sprinter model information and price list (prices effective from August 2018 production) lists alternator options (14 V / 180 A to 14 V / 280 A) and the 'ECO start/stop function' but says nothing about how the alternator's voltage is controlled. It does list a factory auxiliary battery option (code E2I) with a cut-off relay (E36). The UK online operator's manual pages timed out.
Mercedes-Benz Vito (W447, 2014 on) Vito 447 (2014 on) · verified 16 September 2026	Manufacturer document could not be read on 16 September 2026; not recorded as absent.	Could not be read.	Manufacturer page attempted, Unverified, checked 16 September 2026 No Mercedes-Benz document for the Vito could be read on 2026-09-16: the UK online operator's manual pages timed out and the UK Vito brochure URL returned an XML error document. This record is a could-not-read placeholder, not a checked absence.
Nissan NV200 NV200 (M20, 2009 on; UK van brochure read) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	NISSAN NV200 (UK van brochure: Introduction \| Performance indicators \| ... \| Technical specifications \| Price list), Manufacturer document, checked 16 September 2026 Nissan's UK NV200 van brochure gives alternator and 12 V battery capacities in its technical specifications table and explains diesel particulate filter 'REGENERATION', but says nothing about how the alternator's output voltage is controlled. Nissan UK no longer lists the NV200 and does not host its owner's manual publicly.
Peugeot Boxer Boxer third generation (2006 on, facelifts 2014 and 2024) · verified 16 September 2026	Manufacturer document could not be read on 16 September 2026; not recorded as absent.	Could not be read.	Manufacturer page attempted, Unverified, checked 16 September 2026 No Peugeot document for the Boxer could be read on 2026-09-16: peugeot.co.uk returned HTTP 403 and the public Service Box handbook portal exposes Boxer PDFs only in other languages at the URLs found (a guessed en_gb URL returned an HTML error page). This record is a could-not-read placeholder, not a checked absence.
Renault Trafic III (X82) Trafic III, X82 (2014 on) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	Stop and Start function - Renault Trafic - user manual (user-manual.renault.com), Manufacturer document, checked 16 September 2026 Renault's online Trafic user manual (user-manual.renault.com) describes the Stop and Start function and the battery. The Stop and Start page lists 'the battery is not sufficiently charged;' as a condition and a '"Battery charge optimisation" to prioritise battery charging;' message; the Battery page says 'The charge status of the battery can decrease, especially if you use your vehicle: for short trips; in an urban setting; when the temperature is low; with the vehicle stationary and the engine off.' Neither page describes variable-voltage or regenerative alternator control.
Toyota Proace Proace second generation (2016 on) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	Toyota Proace Medium Van \| Electric & Diesel \| Toyota UK, Manufacturer document, checked 16 September 2026 Toyota UK's Proace model page and its 2023 Proace price and specification sheet carry no wording about the diesel alternator, regenerative charging or Stop & Start battery management (the page's charging references concern the electric Proace traction battery). Toyota's owner's manual is served only through a VIN-gated finder and was not read.
Vauxhall Vivaro (2019 on) Vivaro C / New Vivaro (2019 on) · verified 16 September 2026	Manufacturer document could not be read on 16 September 2026; not recorded as absent.	Could not be read.	Manufacturer page attempted, Unverified, checked 16 September 2026 Vauxhall's servers returned HTTP 403 to every request on 2026-09-16: the New Vivaro owner's manual PDFs (April 2019 and June 2020 editions), the owners' manuals index page and the Vivaro model page. This record is a could-not-read placeholder, not a checked absence.
Volkswagen Crafter (2017 on) Crafter SY/SZ (2017 on) · verified 16 September 2026	regenerative braking: All our Euro 6 TDI and BiTDI engines include efficiency technologies consisting of Start/Stop, low rolling resistance tyres and regenerative braking as standard. Applies to: All our Euro 6 TDI and BiTDI engines (Volkswagen Commercial Vehicles UK TDI technology page; the page names no individual van model)	No published statement on charging a second battery found (checked 16 September 2026).	Volkswagen Van Turbocharged Direct Injection (TDI) \| VW Vans, Manufacturer document, checked 16 September 2026 Volkswagen's statement is engine-based and names no model. The Crafter owner's manual is VIN-gated and was not read.
Volkswagen Transporter T5 T5 (2003-2015) · verified 16 September 2026	No published statement on the alternator’s charging behaviour found (checked 16 September 2026).	No published statement on charging a second battery found (checked 16 September 2026).	Volkswagen Van Turbocharged Direct Injection (TDI) \| VW Vans, Manufacturer document, checked 16 September 2026 No Volkswagen publication about the T5's alternator charging behaviour was found. The only Volkswagen Commercial Vehicles UK page describing diesel charging technology refers to Euro 6 TDI and BiTDI engines and does not name the T5. The T5 owner's manual is not publicly hosted by Volkswagen UK.
Volkswagen Transporter T6 T6 (2015-2019) · verified 16 September 2026	regenerative braking: All our Euro 6 TDI and BiTDI engines include efficiency technologies consisting of Start/Stop, low rolling resistance tyres and regenerative braking as standard. Applies to: All our Euro 6 TDI and BiTDI engines (Volkswagen Commercial Vehicles UK TDI technology page; the page names no individual van model or model year)	No published statement on charging a second battery found (checked 16 September 2026).	Volkswagen Van Turbocharged Direct Injection (TDI) \| VW Vans, Manufacturer document, checked 16 September 2026 Volkswagen's statement is engine-based (Euro 6 TDI and BiTDI), not model-based. Whether a given T6 carries a Euro 5 or Euro 6 engine is a matter for the vehicle's own documents; this record asserts nothing about model years. The T6 owner's manual is VIN-gated and was not read.
Volkswagen Transporter T6.1 T6.1 (2019 on) · verified 16 September 2026	regenerative braking: All our Euro 6 TDI and BiTDI engines include efficiency technologies consisting of Start/Stop, low rolling resistance tyres and regenerative braking as standard. Applies to: All our Euro 6 TDI and BiTDI engines (Volkswagen Commercial Vehicles UK TDI technology page; the page names no individual van model)	No published statement on charging a second battery found (checked 16 September 2026).	Volkswagen Van Turbocharged Direct Injection (TDI) \| VW Vans, Manufacturer document, checked 16 September 2026 The only Volkswagen Commercial Vehicles UK publication found that describes the diesel charging system is the TDI technology page; it does not use the words alternator, recuperation or smart alternator. The T6.1 owner's manual is served by Volkswagen only through the VIN-gated online manual, so it was not read.

DC-DC (battery-to-battery) chargers, by published output and profile

Output current, input voltage window, ignition or engine-detection requirement, lithium profile and the maker’s own smart-alternator statement. The maker’s cable and fuse table is linked, never reproduced: this site publishes no ampacity table. Price bands are what UK retailer pages showed on the date recorded and are not live.

Charger	Output	Input window	Trigger	Lithium profile	Smart alternator statement	Maker’s cable/fuse table and diagram	Price band	Where sold
CTEK D250SE dual input 20A DC-DC charger with MPPT verified 16 September 2026 ctek.com	20 A with solar (MPPT) input	11.5 to 23 V	Either D+ or engine detection	Published: 14.2 V, 20 A, LiFePO4.	The D250SE can maintain a stable output up to 20A to vehicles fitted with smart ECU controlled alternators and also has a temperature sensor for optimised charging, regardless of weather conditions. D250SE Dual input 20A charger with selectable charge voltages (product sheet), Manufacturer document, checked 16 September 2026	Maker’s cable and fuse recommendations, CABLE AND FUSE REQUIREMENTS Maker’s wiring diagram	£257 to £257 seen 16 September 2026	12 Volt Planet £257 checked 16 September 2026 Programme exists, rate not published
REDARC BCDC1225D dual input 25A in-vehicle DC-DC battery charger verified 16 September 2026 redarcelectronics.com	25 A with solar (MPPT) input	9 to 32 V	Either D+ or engine detection Input Mode Standard, Blue Wire Connection Not Connected, or Connected to Ground: 12 V Mode ON above 12.9 V OFF below 12.7 V. Input Mode Low Voltage, Blue Wire Connection D+ for Idle-Stop, 'Ignition' for others: 12 V Mode ON above 12.0 V OFF below 11.9 V. *1 Tested every 100 seconds	Published: Charging Profile Li: Maximum Voltage 14.5 V (LiFePO4); Float stage maintains 13.3 V (13.6 V for LiFePO4) on the output battery	NOTE: The REDARC BCDC Series In-Vehicle chargers are suitable for use on vehicles with conventional alternator charging systems, including standard, variable voltage and idle stop type alternators. BCDC Dual Input Multi-Stage In-Vehicle Battery Charger instruction manual, 2 Installation, Manufacturer document, checked 16 September 2026	Maker’s cable and fuse recommendations, 2.4 Cable Sizing, Table 2.4.1; Specifications: Vehicle Input Fuse Rating / Output Fuse Rating Maker’s wiring diagram	not recorded	No UK retailer link recorded.
Ring Automotive RSCDC30 DC-DC Smart Battery Charger 30A verified 16 September 2026 ringautomotive.com	30 A with solar (MPPT) input	12.6 to 50 V	Not stated	Published, voltages not stated	It also has a programmable input voltage range to allow compatibility with smart alternators. Ring Automotive Vehicle Conversion Catalogue (cat739), DC-DC Smart Battery Charger page, Manufacturer document, checked 16 September 2026	No published cable and fuse table found (checked 16 September 2026). Maker’s wiring diagram	not recorded	No UK retailer link recorded.
Sterling Power Pro Batt Ultra BB1230 12V-12V 30A battery to battery charger verified 16 September 2026 sterling-power.com	30 A configurable	11 to 20 V	Either D+ or engine detection The unit, by default, shall start charging when the input voltage exceeds 13.2V and switches off when the voltage drops below 13.0V for more than 240s (adjustable). Going above 13.2V for 5 seconds shall reset this timer.	Published: LiFePO 4	This is required to complement the regenerative braking aspect of modern Euro 5/6+ engines - where the alternator's voltage can drop below 13V for a short period of time. When the alternator's voltage rises above 13.2V the 240 second timer ends (+ reactivates the timer) and the charger remains charging. Sterling Power Pro Batt Ultra English Manual, software V71+, Quick Installation, Manufacturer document, checked 16 September 2026	Maker’s cable and fuse recommendations, Recommended cable size for current required Maker’s wiring diagram	£110.28 to £118.80 seen 16 September 2026	Sterling Power (own shop) £118.80 checked 16 September 2026 Could not be established 12 Volt Planet £110.28 checked 16 September 2026 Programme exists, rate not published
Victron Energy Orion-Tr Smart 12/12-30A Isolated DC-DC charger verified 16 September 2026 victronenergy.com	30 A	8 to 17 V	Either D+ or engine detection Delayed start voltage (Vstart(delay)) ... Default: 13.3V (Smart Alternator) and 13.8 (Regular Alternator). Shutdown voltage (Vshutdown) ... Default: 13.1V (Smart Alternator) and 13.5V (Regular Alternator). Start voltage (Vstart): At this level, charging starts immediately. Default: 14V. Delayed start voltage delay (tstart delay) ... Default: 2 minutes.	Published: lithium batteries, absorption time is fixed, default 2 hours.	Vehicle manufacturers are now introducing smart ECU (Engine Control Unit) controlled alternators to increase fuel efficiency and reduce emissions. Smart alternators deliver a variable output voltage and are shut down when they're not needed. The converter has an engine-running detection mechanism. This prevents the converter from discharging the starter battery when the alternator does not supply power. Orion-Tr Smart DC-DC Charger Isolated (manual), 3. Features - Smart alternator compatibility, Manufacturer document, checked 16 September 2026	Maker’s cable and fuse recommendations, 4.2. Cable and fuse recommendations, Figure 6 Maker’s wiring diagram	£211.46 to £211.46 seen 16 September 2026	12 Volt Planet £211.46 checked 16 September 2026 Programme exists, rate not published
Victron Energy Orion XS 12/12-50A DC-DC battery charger verified 16 September 2026 victronenergy.com	50 A configurable	9 to 17 V	Either D+ or engine detection Delayed start voltage (Vstart(delay)) ... Default: 13.3V/26.6V (smart alternator) and 13.8/27.4V (regular alternator). Shutdown voltage (Vshutdown) ... Default: 13.1V/26.2V (smart alternator) and 13.5V/27V (regular alternator). Start voltage (Vstart): At this level, charging starts immediately. Default: 14V/28V. Delayed start voltage delay (tstart delay) ... Default: 120 seconds.	Published: For lithium batteries, the absorption time is fixed, 2 hours by default.	It is common for vehicle manufacturers to install "smart" ECU (Engine Control Unit) controlled alternators to increase fuel efficiency and reduce emissions. Smart alternators provide variable output voltage and shut down when not needed. The Orion XS has an integrated mechanism that detects whether the engine is running (engine shutdown detection), so the charger is only activated when the alternator is supplying power. Manual - Orion XS 12/12-50A DC-DC battery charger, 2.2. Features - Smart alternator compatibility, Manufacturer document, checked 16 September 2026	Maker’s cable and fuse recommendations, 3.3. Cable and fuse recommendations Maker’s wiring diagram	£231.43 to £231.43 seen 16 September 2026	12 Volt Planet £231.43 checked 16 September 2026 Programme exists, rate not published

Split charge relays, by published thresholds

Voltage-sensing relays engage and disengage at published voltages. The columns that matter here are the maker’s own statements on smart alternators and lithium, which are usually absent, and the link to the maker’s own wiring diagram, which this site never redraws.

Relay	Type	Rating	Engage / disengage	Smart alternator statement	Lithium statement	Maker’s diagram	Price band	Where sold
Durite 0-727-10 12V 100A extra heavy duty make/break relay verified 16 September 2026	relays	100 A	No published engage voltage found (checked 16 September 2026). / No published disengage voltage found (checked 16 September 2026).	No published statement on smart or variable-voltage alternators found (checked 16 September 2026).	No published statement on lithium batteries found (checked 16 September 2026).	Maker’s wiring diagram	not recorded	No UK retailer link recorded.
Durite 0-727-23 12V 70/20A double contact split charge relay verified 16 September 2026	relays	70 A	No published engage voltage found (checked 16 September 2026). / No published disengage voltage found (checked 16 September 2026).	No published statement on smart or variable-voltage alternators found (checked 16 September 2026).	No published statement on lithium batteries found (checked 16 September 2026).	Maker’s wiring diagram	not recorded	No UK retailer link recorded.
Durite 0-727-33 12V 140A voltage sensitive relay verified 16 September 2026	relays	140 A	13.3 V / 12.65 V after 5 s	No published statement on smart or variable-voltage alternators found (checked 16 September 2026).	No published statement on lithium batteries found (checked 16 September 2026).	Maker’s wiring diagram	£39.99 to £39.99 seen 16 September 2026	Simply Split Charge £39.99 checked 16 September 2026 Could not be established
Durite 0-727-43 12V 200A Smart Programmable Voltage Sensitive Relay verified 16 September 2026	relays	200 A	No published engage voltage found (checked 16 September 2026). / No published disengage voltage found (checked 16 September 2026).	The Durite Smart Relay has been designed to be used with the latest range of vehicles fitted with smart alternators and regenerative braking systems. In addition, there are 9 available programmes allowing the user to set the charging/protection voltages to suit their requirements.	No published statement on lithium batteries found (checked 16 September 2026).	Maker’s wiring diagram	not recorded	No UK retailer link recorded.
Victron Energy Cyrix-ct 12/24V 120A intelligent battery combiner verified 16 September 2026	relays	120 A	13 V / 12.8 V	No published statement on smart or variable-voltage alternators found (checked 16 September 2026).	No published statement on lithium batteries found (checked 16 September 2026).	Maker’s wiring diagram	£42.30 to £42.30 seen 16 September 2026	12 Volt Planet £42.30 checked 16 September 2026 Programme exists, rate not published
Victron Energy Cyrix-Li-ct 12/24V 120A lithium battery combiner verified 16 September 2026	relays	120 A	13.4 V after 120 s / 13.2 V after 0 s	No published statement on smart or variable-voltage alternators found (checked 16 September 2026).	The functionality of the Cyrix-Li-ct is analogous to the Cyrix-ct. The Cyrix-Li-ct will parallel connect a lead acid starter battery and a LiFePO4 battery: - if the Charge Disconnect output of the VE.Bus BMS is high, and - if it senses 13,4V (resp. 26,8V) or more on one of its power terminals. The Cyrix will disengage immediately: - when its control output becomes free floating, signalling cell over voltage or cell over temperature, and/or - when battery voltage drops below 13,2V.	Maker’s wiring diagram	not recorded	No UK retailer link recorded.

Why there is no wiring diagram on this page

A relay or a DC-DC charger connects to the vehicle’s starter battery and, on many installations, to an ignition feed. Which feed, which fuse-box position and which cable is a design decision on a vehicle nobody on this site has seen, and a wrong answer can start a fire. This site therefore never publishes wiring instructions, fuse-box tap points or its own cable and fuse tables. Each product row links the manufacturer’s own published diagram and its cable and fuse recommendations; the manual’s figure wins over anything here. Connection to the vehicle’s own wiring, alternator or ECU is a job for a competent auto electrician.

Lithium note, sourced: fitting a lithium battery to a charging system designed for lead-acid can require a DC-DC charger: Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger, Controlled charging also protects the alternator in lithium systems from overloading, as the low impedance of lithium batteries results in a high alternator current. (checked 16 September 2026). Whether it affects a vehicle or battery warranty is a question for the warranty terms, which this site records per battery where published.

Common questions

Do all vans made after 2015 have a smart alternator?

No published source found; the checker never answers from model year. This site never answers from the model year. Of the 16 base vehicles indexed, 5 have a manufacturer statement about variable-voltage charging on record; the rest carry the line “No published statement found” with the document that was checked.

How do I tell whether my alternator is a smart alternator?

Measure it rather than guess. Test, don't guess: with the engine at normal running temperature, measure the voltage at the starter battery terminals. Victron cites a fixed ~14V for a conventional alternator and 12.5V to 15V for a smart alternator. Its Orion XS default delayed-start voltage of 13.3V and shutdown voltage of 13.1V (smart alternator) are the manufacturer's own engine-detection thresholds and are cited as such, not as a diagnostic standard. Victron’s published figures: “Conventional alternators will generate a fixed voltage (e.g. 14V), whereas the voltage of smart alternators can vary between 12.5V to 15V (for a 12V system). Smart alternators in a regenerative braking system often show large voltage variations.” (checked 16 September 2026).

Can I use a split charge relay with a lithium (LiFePO4) leisure battery?

Battery chemistry LiFePO4 points to a DC-DC charger. A relay connects a low-impedance lithium bank directly across the alternator. Victron: controlled charging protects the alternator in lithium systems because low impedance results in a high alternator current. Sterling: its alternator-to-battery system is not a current limiting system and is not recommended for lithium without an external current limiter. Victron Energy, Manual - Orion XS 12/12-50A DC-DC battery charger: “Controlled charging also protects the alternator in lithium systems from overloading, as the low impedance of lithium batteries results in a high alternator current.” Sterling Power Products, Alternator to Battery Chargers up to 130A (product page): “NOT RECOMMENDED FOR USE WITH LITHIUM BATTERIES WITHOUT AN EXTERNAL CURRENT LIMITER - The AB system is not a current limiting system and will allow your lithium to draw as much current as it sees available.” (checked 16 September 2026). 5 of the 6 DC-DC chargers indexed publish a lithium charge profile.

Why does a smart alternator stop a voltage-sensing relay working?

A smart / ECU-controlled (Euro 5-6) alternator points to a DC-DC charger. A voltage-sensing relay needs a sustained high alternator voltage that a smart alternator does not provide: Victron publishes the smart-alternator range as 12.5V to 15V and says such alternators often supply charging voltage that is too low even with the engine running; Sterling says its uncontrolled alternator-to-battery charger is not suitable for European and Japanese vehicles with regenerative braking systems. Victron: “It is common for vehicle manufacturers to install "smart" ECU (Engine Control Unit) controlled alternators to increase fuel efficiency and reduce emissions. Smart alternators provide variable output voltage and shut down when not needed.” Sterling Power: “Conventional alternators will generate a fixed voltage (e.g. 14V), whereas the voltage of smart alternators can vary between 12.5V to 15V (for a 12V system). Smart alternators in a regenerative braking system often show large voltage variations.” (checked 16 September 2026).

How big a DC-DC charger can my alternator supply?

No manufacturer document read on 2026-09-16 publishes what share of an alternator's rated output may be drawn continuously by a DC-DC charger. Spec section 2.4.1 D6. The checker states the constraint and will not put a percentage on it.

Does this page tell me how to wire a relay or a DC-DC charger?

No. This site gives no wiring instructions for any vehicle. Each relay and charger record links the manufacturer’s own published diagram and manual, and the manual’s figures win over anything here. Connection to the vehicle’s own wiring, alternator or ECU is a job for a competent auto electrician.

What do a relay kit and a DC-DC charger cost?

Relays indexed here were listed at £39.99 to £42.3; DC-DC chargers at £110.28 to £257, on UK retailer pages read on 16 September 2026. Prices are recorded on the date shown and are not live.

Where next

Battery size checker: how much capacity the loads need before you decide how to charge it.
Mains chargers by published charge profile: the hook-up side of the same question.
Methodology and every source.

This site publishes what manufacturers and standards bodies have published. It does not design electrical installations and is not a substitute for a competent auto electrician or a habitation check.