Charging a small Lithium Battery from the main LiFePo

Got a similar head-scratcher going on with my shepherd's hut setup actually.

Running a Fogstar 280Ah as the main bank, and I want to keep a separate small 20Ah lithium topped up for the EV charging controller and some ancillary bits — basically a dedicated "don't drain the big bank" buffer.

Been looking at the Victron Orion-Tr Smart 12/12-18A as the obvious candidate to bridge them. Seems almost overkill for a 20Ah battery but the isolation and smart charging profiles are the appeal really.

Few things I'm mulling over:

• Does the Orion play nicely when both batteries have a BMS? Wondering if they'll argue with each other when the small one hits full and its BMS starts throttling
• Worth enabling the engine detection / input voltage lockout so it only charges when the main bank is genuinely healthy — say above 13.0V?
• Anyone running this kind of LiFePo → LiFePo setup rather than the more typical alternator-to-leisure-bank scenario?

I think the Orion-Tr Smart handles it fine because it's essentially just a DC-DC converter with proper absorption/float stages, but I'd rather not fry a perfectly good small battery learning the hard way.

Anyone done something similar — charging a small lithium from a big lithium? Curious whether the Victron Connect app gives you decent enough visibility over what's actually happening between the two banks.

@GazAllen a B2B charger is the obvious answer here. Victron Orion-Tr Smart is the go-to but probably overkill for a 20Ah battery tbh.

Renogy do a cheaper DC-DC that'd do the job fine for that size load. Just make sure whatever you use is lithium-to-lithium compatible — some of the budget ones still assume lead acid charging profiles and you'll end up with an unhappy little battery.

Also worth checking if your 280Ah has a BMS that plays nicely when it's at low SOC — last thing you want is the main bank cutting out because the B2B is still pulling from it. Set a low voltage disconnect if you can.

Ran a similar bodge on my motorhome for years, never had grief once I sorted the disconnect thresholds. 🤷

@KevClark overkill is Victron's entire brand identity though, isn't it — meanwhile a cheap Renogy DC-DC charger will do the job without requiring a marine electrician and a remortgage.

@NickBennett not wrong, though I'd push back slightly on "cheap" — the Renogy DC-DC chargers are properly specced for LiFePO4 profiles, not just slapping a bulk voltage at it and hoping for the best.

For a 20Ah battery, the Renogy 20A DC-DC is genuinely the sweet spot. Running exactly this on my shepherd's hut — main Fogstar 200Ah feeding a small 20Ah auxiliary for LED lighting circuits. Works flawlessly.

Key thing @GazAllen needs to check: make sure the charger has a dedicated LiFePO4 charge profile, not just a "lithium" setting that's actually tuned for NMC chemistry. The absorption voltage difference matters more than people realise on small batteries. Gets them either perpetually undercharged or cycling unnecessarily hard.

Victron's lovely if you enjoy remortgaging your hut, mind.

@Renogy_Pro what current output are you actually getting through to the 20Ah though? With such a small bank I'd worry about over-speccing the charge rate and stressing the cells — most B2B chargers seem to start at 20A+ which feels aggressive for a 20Ah lithium.

Relevant to me as I'm planning something similar for my garden office build — main Fogstar bank in the shed, small auxiliary battery inside the office itself for a few USB points and low-draw stuff. Been wondering whether a proper B2B is even necessary at that scale or whether there's a simpler solution I'm missing?