Three ways to do it
| DC-DC charger | Isolator or relay | Second alternator + regulator | |
|---|---|---|---|
| What it does | Charges the house battery from the starter side at a set voltage and current | Joins the two batteries while the engine runs | A dedicated alternator, controlled by an external regulator |
| Lithium | Yes: right voltage, current capped | Not recommended | Yes, for big banks |
| Smart alternator | Yes | Often undercharges | Not affected |
| Current | set by the charger, up to about 50 A per unit | whatever the alternator gives | the most, set by the regulator |
| Who it suits | Most vans and RVs | Lead-acid on older vehicles | Big rigs and boats that need fast charging |
For almost everyone with lithium, it's a DC-DC charger. What size DC-DC charger picks the amps from your alternator and battery.
Wiring a DC-DC charger
Numbers below are from Victron's manual for the Orion XS 12/12-50A; follow your own charger's manual if it differs.
- Choose the cable. 6 AWG (16 mm²) for runs under 5 m (about 16 feet), 4 AWG for 5–10 m. Flexible, fine-stranded copper.
- Fuse both ends. A 60–70 A fuse close to the starter battery on the input cable, and another close to the house battery on the output cable.
- Switch it off first. Remove the remote on/off bridge so the charger stays off while you wire.
- Connect the input (starter battery or alternator side).
- Set the battery type in the app before connecting the house battery. For lithium, use the LiFePO4 profile or the settings in how to charge a LiFePO4 battery.
- Connect the house battery, then refit the remote on/off. It's live.
- Check engine detection. Start the engine and confirm it charges; switch off and confirm it stops. If it doesn't detect reliably, wire an engine-running signal to the remote input as the manual shows.
Then watch the cables and alternator on your first long drive. If anything runs hot, turn the current down in the app.
Lead-acid or AGM house battery
The same DC-DC charger works with its AGM profile, and it still helps with a smart alternator. A simple isolator is cheaper and was the standard way for decades; it just won't give a full charge if the alternator drops its voltage.
With solar
Driving and solar work together: the alternator covers driving days and cloudy stretches, the panels the rest. Some DC-DC chargers include a solar controller; a separate MPPT controller does the same job. What size solar panel for a van or RV sizes the roof.
Questions people ask
Can I connect a lithium house battery straight to the alternator?
Not safely. Battle Born warns that lithium batteries accept sustained high current, and an alternator trying to meet that demand for too long can overheat; its rule of thumb is that an alternator can handle about half its rated amps continuously. A DC-DC charger caps the current.
Does a DC-DC charger work with a smart alternator?
Yes, that's one of its jobs. Victron says the Orion XS's default engine-detection setting works with most conventional and smart alternators; if yours doesn't give it enough to work with, you wire in an engine-running signal instead.
What size cable and fuse for a 50 A DC-DC charger?
Victron's Orion XS 12/12-50A manual gives a 60–70 A fuse, with 6 AWG (16 mm²) cable for runs under 5 m and 4 AWG for 5–10 m. Use flexible, fine-stranded copper cable.
Can I use a battery isolator instead?
An isolator or voltage-sensitive relay simply connects the two batteries while the engine runs. It is the traditional cheap way for a lead-acid house battery, but it doesn't limit current or set a lithium charge voltage, so lithium makers point you to a DC-DC charger.
How long does it take to charge a house battery while driving?
Roughly the charger's amps per hour of driving: a 30 A charger puts back about 30 Ah an hour, so a 100Ah battery goes from half to full in under two hours.
Sources
- Orion XS 12/12-50A DC-DC battery charger manual (cable and fuse table, connection order, engine detection) Victron Energy
- Alternator charging your RV LiFePO4 lithium batteries (overheating, half the rated amps, DC-DC vs regulator) Battle Born Batteries
- Battle Born BB10012 datasheet (50 A max charge current) Battle Born Batteries
Every number on this page comes from these sources. Spotted something out of date? Tell me.
Size it for your place
A few taps and the guide works it out for your situation and your winter sun.