Trickle charging vehicle main battery

Been thinking about this lately after fiddling with my own motorhome setup, and wondered if anyone else has gone down this rabbit hole.

I've got a Victron Orion-Tr Smart DC-DC charger isolating my leisure bank from the starter battery, and it works brilliantly for keeping the leisure side happy. But what about the other direction — keeping the vehicle's main battery topped up when you're parked up for extended periods?

My scenario: I'm often sitting on a site for two or three weeks at a stretch, engine not running, solar doing all the heavy lifting via an MPPT. The leisure bank stays fat and happy, but the starter battery just slowly self-discharges doing nothing.

I've seen some people wire in a small trickle feed from the leisure bank back to the chassis battery — essentially reversing the usual flow — but I'm wary of doing anything daft without proper isolation. The last thing I want is to flatten both banks trying to start a cold engine on a January morning in Wales.

Has anyone set up a proper bidirectional arrangement, or is the cleaner solution just a dedicated small trickle charger run off the inverter/shore power when available?

Curious whether anyone's using the Orion XS for this — I know it supports bidirectional charging in theory, but configuring it sensibly seems like it needs a bit of careful thought around the BMS settings if you're running lithium on the leisure side.

What's your lot doing to keep the starter battery honest during long static periods?

Good shout @VickyFisher - the Orion-Tr Smart is a solid choice for that. One thing worth considering is whether you've got the engine detection set up properly. If you're relying purely on voltage sensing rather than a dedicated ignition wire input, you can sometimes get false triggers when your leisure batteries push voltage back and the Orion thinks the engine's running. Worth checking your alternator's actual output voltage too - some modern smart alternators with variable voltage regulation can confuse DC-DC chargers or cause them to throttle back unexpectedly. What alternator are you working with? Euro 6 engines especially can be awkward in this regard. The Victron app gives you decent diagnostics so you can see exactly what's happening in real time, which helps enormously for troubleshooting this sort of thing.

Slightly different angle on this — I've been looking at the same question for my static caravan setup where the tow vehicle sits for weeks at a time.

One thing worth flagging: if your alternator has smart charging (variable voltage), the Orion-Tr Smart handles this well in standalone mode, but make sure you're triggering it via the engine running detection input rather than just voltage sense. A smart alternator can sit at 12.8V at idle and fool a voltage-triggered charger into thinking the engine is off.

Also worth checking your vehicle's parasitic draw — if the Orion is pulling even 50mA in standby and the vehicle sits unused, you can flatten the starter battery over a few weeks regardless of what the leisure side is doing.

@VickyFisher what voltage threshold have you configured for the low-voltage disconnect on yours?

@BlownFuse for the static caravan scenario just wire in a basic VSR (voltage sensing relay) — dead simple, no programming faff, kicks in when the tow vehicle hits ~13.7V and disconnects before it drags the starter battery down. Sterling and Ring both do decent units for under £30.

That said, if your leisure bank is lithium (looking at you, Fogstar Drift owners), don't use a VSR — the higher charge acceptance of LiFePO4 will hammer your alternator. Victron Orion non-isolated is the smarter play there, proper current limiting keeps everything happy.

Main thing people get wrong: assuming the vehicle's smart alternator actually charges properly — most modern ones see a DC-DC charger as a load and throttle back. Check your alternator type before spending a penny on any of this.

@Rob1963 VSR worked a treat on my static until the tow car sat there for three weeks in January and the leisure bank decided to have the starter battery for breakfast 🪦