Replacing a split charge relay with a DC-DC charger
How to swap a voltage sensitive relay (VSR) or split charge relay for a DC-DC charger, reusing the existing cables where they are up to the job.
Why replace the relay
A split charge relay or VSR simply joins the starter and leisure batteries once the starter battery voltage rises. A common example, the Durite 0-727-33, connects at 13.3V and disconnects at 12.65V. It has no control over how the leisure battery is charged: the leisure battery gets whatever the alternator happens to be producing.
A DC-DC charger sits in the same place but works differently. It takes power from the starter battery while the engine runs and delivers a proper charge profile to the leisure battery, at a fixed current, whatever the alternator is doing.
- Smart alternators. Many Euro 6 vehicles vary the alternator voltage and switch it off when the vehicle does not need it. Victron's words: "Controlled charging is indispensable." On these vehicles a relay can leave the leisure battery barely charged.
- A correct lithium charge. The charger runs a LiFePO4 profile and stops when it should, instead of passing on raw alternator voltage.
- A known current. The current is capped at the charger's rating, so the alternator and the wiring see a predictable load.
What we recommend
For most vans and motorhomes we recommend the Victron Orion-Tr Smart 12/12-30 non-isolated. It charges at 30A and Victron quotes an input draw of about 35A at full output, a load most split charge circuits were already built to carry. In most installations the existing relay cables can stay, but check them first using the table further down. Do not assume.
Non-isolated means the input and output share a common negative inside the charger. That keeps the wiring to three power connections:
- Positive in from the starter battery (the relay's starter-side cable).
- Positive out to the leisure battery (the relay's leisure-side cable).
- One negative, a new properly sized cable to the starter battery negative or the chassis.
Going up to a 50A or 70A charger is where the existing cables are more likely to need replacing. See Going bigger below.
Before you start
- Check what you have. This guide covers a standalone relay or VSR. If the split charging is built into a habitation power unit (Schaudt EBL, Sargent, CBE and similar), follow that unit maker's instructions for adding a DC-DC charger, because the wiring is different. VW with a factory second battery? Read VW factory relays below first.
- Identify the two relay cables before you isolate anything. On most VSRs the two main studs are interchangeable, so the relay will not tell you which cable is which. Trace each cable, or isolate the leisure battery only and, with the engine off, measure each stud to earth: the stud still showing around 12.5V is the starter battery side. If both studs show the same voltage, the relay is closed or stuck, so trace the cables instead. Label both cables.
- Isolate the circuit. Remove the fuses at both ends of the split charge cable and isolate the leisure battery. If you need to disconnect the starter battery, follow the vehicle maker's procedure.
Before: a typical voltage sensitive relay. Two main cables and a thin ground lead.
Where each relay wire goes on the DC-DC charger.
Check the existing cables
The positive cables from the old relay can usually be reused, but only once you have checked them. Check each cable separately, because the input run and the output run are often different lengths.
- Size. Read the cross-section printed on the insulation and measure the length of each run. Compare both against the table below for the charger you are fitting.
- Condition. Look for heat damage, hardened or cracked insulation, and corroded or loose crimps. Replace anything you are not sure of.
- Fuses. Fit the fuse ratings the charger maker specifies, one as close as practical to each battery. The fuse protects the cable, so the cable must be able to carry the fuse rating. If it cannot, upgrade the cable.
| Victron Orion-Tr Smart 12/12-30 | 0.5m | 1m | 2m | 5m | 10m | Fuse |
|---|---|---|---|---|---|---|
| Minimum cable, input or output | 6mm² | 10mm² | 10mm² | 16mm² | 16mm² | 60A, at each battery |
So a 10mm² cable can stay on runs up to 2m. Anything longer needs 16mm², which is also the largest the Orion-Tr terminals accept.
If in doubt, replace it. Cable is cheap. An undersized or tired cable on a 35A circuit gets hot, wastes charge and is a fire risk.
Add a proper negative
This is the one cable you will almost always need to add. A relay's ground lead only feeds its own coil and sensing electronics, so it is a thin wire. The Durite VSR, for example, comes with a 600mm lead and a small ring terminal for it.
A DC-DC charger's negative is part of the charging circuit, so it needs a proper current-rated cable. Match it to the positive cables. 16mm² is a sound default for a 30A charger.
- Run it from the charger's negative terminal to the starter battery negative. If that is impractical, use the vehicle's main chassis earth point close to the battery, with a crimped lug on bare metal and the paint removed.
- Many vehicles with stop-start or a smart alternator have a battery monitoring sensor on the starter battery negative terminal: a small module with a plug, clamped on the post. Connect the new negative on the vehicle side of that sensor, never to the battery post itself, or the vehicle cannot see the current the charger draws.
- The Orion-Tr is non-isolated, which means there is no isolation between its input and output. The negative is shared, so one negative cable serves both sides.
- Remove the relay's old ground lead, or cut it back and insulate it. It is not needed.
- Leave the leisure battery negative as it is, provided it already has a solid connection back to the vehicle negative. It will if the relay worked. If the leisure battery negative is not connected to the vehicle negative at all (some boats and custom builds), the output needs its own negative cable.
Sterling chargers are different. Sterling asks for all negatives to be common and recommends running the charger negative directly to the starter battery negative, not to the chassis. Where a battery monitoring sensor is fitted, that means the vehicle side of the sensor.
Fitting the Orion-Tr Smart 12/12-30
- Mount it. Fit it vertically on a non-flammable surface with the power terminals facing down, with at least 10cm clear above and below. Victron says to mount it close to the battery but never directly above one. It is rated IP43 (IP22 at the terminals), so choose a dry, protected spot such as under a seat base or in a locker. The old relay position often works, so the existing cables still reach.
- Remove the relay, keeping the two labelled cables.
- Starter battery cable to +IN.
- Leisure battery cable to +OUT.
- New negative to the negative terminal, run to the starter battery negative or a chassis point as above.
- Terminations. Victron specifies 1.6Nm for the screw terminals, held for at least 5 seconds so the screw settles.
- Leave the remote L-H wire link fitted. The charger then starts and stops itself by sensing the starter battery voltage, so a D+ or ignition wire is optional and does not change how it charges. If the old relay had an ignition or trigger wire, it is not needed. Insulate and secure it, or remove it back to its source.
- Check polarity before you power up. The Orion is not protected against reverse polarity, and Victron does not cover reverse polarity damage under warranty.
- Refit the fuses at both batteries, starter side first.
After: Victron Orion-Tr Smart 12/12-30 non-isolated in place of the relay.
Settings in VictronConnect
| Setting | What to set |
|---|---|
| Function | Charger |
| Battery | A lithium (LiFePO4) profile using TITAN's recommended values: absorption 14.4V, float between 13.3V and 13.8V |
| Engine shutdown detection | Leave enabled (default) |
| Start and shutdown voltages | Start with the smart alternator defaults: start 14.0V, delayed start 13.3V after 120 seconds, shut down at 13.1V. Adjust them to suit the vehicle if needed, see below. |
| Input voltage lock-out | Leave at the default, 12.5V (restarts at 12.8V). It is a backstop. The shutdown voltage is what normally stops charging when the engine is off. |
Tuning the voltages to the vehicle. The start and shutdown voltages can be changed in VictronConnect so the charger suits the vehicle. Some Euro 6 vehicles never lift the starter battery above about 13.2V at idle, so with the default 13.3V delayed start the charger will not run until the vehicle is driven. Measure the starter battery voltage with the engine idling and set the delayed start voltage just below it, for example 13.1V for a 13.2V idle. Keep a gap between the delayed start and shutdown voltages: if they are too close, the voltage drop under load reaches the shutdown level and the charger cuts back its current.
Never set the shutdown voltage below 12.8V. Below that, the charger keeps drawing from the starter battery after the engine has stopped and will start to discharge it.
Smart alternators that switch off while driving. If the alternator drops its voltage for long periods, the charger can read that as the engine being off. Victron's fix is an engine running signal on the remote L pin. That overrides the shutdown detection and keeps charging on while the signal is present. Victron does not recommend using the ignition switch for this, because it can flatten the starter battery when the ignition is on and the engine is not. On many Euro 6 vehicles a D+ signal is no longer provided.
VW factory relays
Many VW Transporters (T5, T6 and T6.1) left the factory with a second battery and a split charge relay under a front seat. The swap is the same as for any relay: the starter side cable goes to +IN, the leisure side cable to +OUT, and the new negative is added as above. The factory cables are fused at both ends. Check the cable size and both fuse ratings against the table for your charger, and change the fuses if they differ from the charger maker's figure.
- The thin control wire. Alongside the heavy cables and an earth, the VW relay has a thin control wire from the vehicle's body control module. On T5 and T6 models it is a switched +12V feed that closes the relay when the engine is running. It is a control feed, not a data line, and the Orion does not need it because it senses the voltage itself. Insulate it, tie it back and leave it disconnected. If the vehicle later logs a fault for the missing relay, a VW specialist can code the factory second battery out.
- Optional: use it as an engine running signal. If your smart alternator needs one, first check with a meter that the wire is live only with the engine running, not just with the ignition on. Reports differ on the T6.1. If it passes, it can feed the Orion's L pin as described in Settings in VictronConnect above, with the L-H wire link left in place.
- Battery monitoring sensors stay. Some VWs have a battery monitoring sensor clamped on the battery negative, with its own data cable to the vehicle. It is a separate part from the relay. Leave it in place and connect the new negative on the vehicle side of it.
- California, Ocean and Grand California. On these, the second battery relay is run by the camper control unit, which also reads the leisure battery through monitoring sensors and shows it on the control panel. Replacing it is more involved and may need the vehicle coding changing. Use a VW specialist.
Going bigger: 50A and 70A
Higher charge currents mean higher input currents, and this is where the existing split charge cables are often too small. Expect to replace the positive cables as well as adding the negative. Check that the vehicle's alternator can comfortably supply the extra load.
| Charger | Fuse, at each battery | Minimum cable | Notes |
|---|---|---|---|
| Victron Orion XS 12/12-50A | 60-70A | 16mm² under 5m; 4AWG (21.2mm²) from 5m to 10m | IP65. Victron's manual shows the GND going to the chassis with a lug on bare metal. The input current limit can be set from 1A to 50A in VictronConnect. |
| Victron Orion XS 12/12-70A | 80-100A | 25mm² under 10m | IP20, so only for a dry indoor location. |
Higher current: Victron Orion XS 12/12-50A, with 16mm² positives and the GND to a bare-metal chassis point.
Other brands
The same swap works with most non-isolated chargers, but each maker has its own rules on the negative, the engine signal and fusing. Isolated chargers need separate input and output negatives, so do not wire them with one shared negative. Most of these chargers switch on by sensing the starter battery voltage, so a D+ or ignition wire is optional. The table notes where a maker requires one or recommends one for smart alternators. Always fit the fuse ratings and cable sizes in the manual for your exact model. The figures below are from the manufacturers' manuals as of October 2026.
| Charger | Rating | Negative | Engine signal | Fuse (maker's figure) |
|---|---|---|---|---|
| Victron Orion-Tr Smart 12/12-30 non-isolated | 30A | One shared negative | Not needed. On smart alternators, use an engine running signal on the L pin, not the ignition switch. | 60A |
| Victron Orion XS 12/12-50A / 70A | 50A / 70A | GND to bare-metal chassis with a lug | As above | 60-70A / 80-100A |
| Sterling Pro Batt Ultra BB1230 / BB1260 | 30A / 60A input | Common negative, run directly to the starter battery negative, not the chassis | Voltage sensing by default. On Euro 6 and newer vehicles, Sterling says to use the ignition feed. | 50A / 100A, input and output |
| Lippert (Schaudt) WA 121525 / WA 121545 | 20-30A / up to 70A | 16mm² ground cables. Wiring depends on whether a Schaudt EBL is fitted. | Required. The booster switches on from the D+ signal. | See the manual wiring scenarios (40-50A on the WA 121525) |
| CTEK D250SE | 20A | Common earth to the chassis | On smart alternators, the red lead goes to a +12V source that is live only with the engine running. | 30A |
| REDARC BCDC1225D / 1240D / 1250D | 25A / 40A / 50A | Chassis earth | On variable-voltage alternators, the blue wire goes to the ignition. | 40A / 60A / 60A |
| Renogy DCC1212-20 / 40 / 60 | 20A / 40A / 60A | Common negative | D+ or ignition cable required. The charger will not run without it. | Input 30A / 60A / 90A, output 25A / 50A / 75A (runs up to 3m) |
| Votronic VCC 1212-30 | 30A | All negatives meet at the leisure battery negative | Optional. Without D+ it runs on voltage sensing, which Votronic rates as suitable for Euro 6. | See the manual cable and fuse table |
| Büttner MT-LB 30 | 25A / 30A | Isolated: run separate start and leisure negatives | D+ or ignition input, or voltage sensing with a wire link (see the manual) | See the manual cable and fuse table |
After installation
- Start the engine. VictronConnect should show the charger in bulk or absorption, and the TITAN app should show current going into the leisure battery.
- Stop the engine. The charger should stop on its own within about a minute, once the starter battery voltage falls below the shutdown voltage.
- After 20 to 30 minutes at full output, carefully feel the cables and terminals. Warm is normal. Hot means an undersized cable or a poor connection, so stop and fix it.
- If the charger cuts out under load, measure the voltage at the charger's input terminals against the starter battery terminals. A large difference points to cable or connection losses on the input side.
Not confident with vehicle electrics? Use a qualified auto electrician. Our stockists and installers can help.
Figures are taken from each manufacturer's published manual as of October 2026. Always follow the manual for your exact charger model.
