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How To Charge Starter Battery From House Battery And Solar Panel System for RV, Van and Camper


Ok, so we're successfully keeping the house batteries charged up using the solar panels. Now, how can the starting battery be kept charged when the RV or van is sitting unused for some time, or to prevent accidentally discharging the battery?

The solution is a 12V to 12V DC-DC charger that goes from the house batteries to the starter battery. If there's already a 12V-12V DC-DC charger (like the VICTRON Orion XS 50A) going from the engine alternator to the house batteries, you need to make sure that both of these chargers are not running at the same time to prevent a charging loop that can run down the batteries. The easiest way is to use the ignition signal to only turn on the DC-DC house battery charger and turn off the DC-DC starter battery charger when the engine is running. The Victron Orion XS and Tr Smart DC-DC chargers used for charging the house battery have an auto-sense mode for when the engine and alternator are running. This needs to be deactivated, and the house battery charger needs to only turn on from the ignition on signal.

The VICTRON Orion-Tr Smart 12/12-Volt 18 amp 220-Watt DC-DC Charger Isolated (Bluetooth) is a bit overkill for keeping the starter battery charged, but it's the smallest DC-DC charger that Victron offers and it will do the job very well.

The wiring diagram below is for charging the house batteries. Reverse the wire connections for charging the starter battery. Connect S1 input to the ignition signal in a way that opens the circuit to prevent charging when the engine is running.


For charging the starter battery, the house batteries are the INPUT and the starter battery is the OUTPUT.

We want to sense the ignition on signal, which is +12V from the starter battery to tell the charger to shut off.

The remote on/off connector references the INPUT side of the charger, which is the house battery.

If the ground is not isolated between the input and output batteries (as is the case in my setup, where the house batteries and the starter battery use the frame as a common ground), then triggering the charger on/off is easier. The +12V output from the ignition signal and a 10 pull-down resistor can be connected into the L (low) side of the REMOTE input.

  • When the +12V ignition signal is off, the L pin is grounded through the 10KΩ pull-down resistor and turns on the starter battery charger.

  • When the +12V ignition signal is on, the L pin is raised to +12V and turns off the starter battery charger.

If the ground is isolated between the INPUT (house batteries) and OUTPUT (starter battery), then a NC (Normally Closed) isolated relay (mechanical or solid-state) is needed. A solid-state relay is ideal. but NC solid-state relays are not common. A pre-wired mechanical relay like this is an easy solution:


12/12-Volt 18 amp 220-Watt DC-DC Charger

I wanted to install the charger as close to the starter battery as possible, so that the voltage drop is minimal, and therefore the charger can accurately read the battery voltage. I don't care as much about input voltage drop.

At first I thought I would install it above the battery by fabricating a bracket to mount it from the battery tray or battery. I decided that wasn't such a great idea, because it would make the battery more difficult to remove or service.

There's a large empty space under the driver's side headlamp that the charger can fit in and still be accessed from above. The installation is easiest by removing the headlamp. I considered mounting it flat at the bottom. Since the design is intended to be upright for heat dissipation and water resistance I decided it would be better to make a bracket to hold it an angle. The angle makes it still serviceable from above without removing the headlamp.

The bracket was glued to the sheet metal with RTV (RTV takes a long time to cure because of the large encapsulated volume. A flexible epoxy like G-Flex might be a better choice). I chose the RTV because it also acts as a vibration damper. Easier than drilling holes and screwing it on.

Connection To The Starter Battery

A fuse should be used between the charger and starter battery, close to the battery. The Sprinter van already has a fuse box attached to the battery for large current draw devices, and alternator input. There was an unused fuse slot. These are MIDI style fuses. I added a 30A fuse to connect the charger.


The starting battery will be a standard lead acid battery or an AGM lead acid battery. Choose the correct settings inside the Victron Orion charger by using the VICTRON CONNECT app for the type of starting battery.

I have an AGM starter battery. The settings are

The first time you connect to the device using the VICTRONCONNECT app, the default VICTRON ORION-TR is setup as a power supply.

Click on FUNCTION Power Supply and select CHARGER.

Now it should show the FUNCTION as CHARGER.

Select ENGINE SHUTDOWN DETECTION and disable it. The engine shutdown detection won't work for this purpose, because in this case we're going from the house batteries to the starter battery, and want the charger to turn on when the engine is off.

This setting is useful if you want to charge the battery only when the solar power kicks in (still need to use the inputs to turn it off when the engine is on). In that case you can set the start voltage below the peak charging voltage (14.2V LiFePO4), and the delayed start voltage just below the solar power charger float voltage (13.5V for LiFeP04), and the shutdown voltage set just above the nominal battery voltage (13.2V for LiFePO4).

I just turned it off, because I want the battery to stay charged no matter what the sun is doing.

The ORION-TR already has a default setting for AGM [Spiral Cell]. I used basically the same settings but dropped the absorption and float voltage by 0.1V. I also set the Re-Bulk to 1V.

There are conflicting opinions on float charging for long periods of time. It would be nice if there was a time limit for the float charge. An alternator does not usually float charge.

Affiliate Disclaimer

The products shown here were purchased by me with the intent to use them. I did not receive any free items, and I am not being paid or compensated for this review. The video, description, and comments may contain affiliate links. If you click on a link, I may receive a commission. Money earned helps to support my channel and bring you more informative videos about engineering, crafting, and DIY


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