How to safely rebalance banks?

Been meaning to post about this for a while. Running two separate 12V LiFePO4 banks in the shepherd's hut — a pair of Fogstar Drift 100Ah cells I built up myself and an older Renogy 200Ah drop-in I had kicking around from the motorhome days. Both sat under the same Victron MPPT and share the load bus.

Problem is they've drifted apart over winter. The Fogstar bank is sitting noticeably higher than the Renogy when I check individual cell groups, and the BMS on the Renogy keeps throwing a high-voltage warning when the Fogstar's already comfortably full. Bit of a mess, frankly.

I know the textbook answer is "balance at rest, top-balance individually before connecting," but that ship has sailed. What I'm actually after is practical advice on rebalancing two already-installed banks that are misbehaving mid-season without pulling everything apart.

A few things I'm considering:

• Isolating one bank temporarily and letting it self-discharge slightly via a small load
• Using a proper cell-level active balancer on the Renogy pack
• Just accepting some inefficiency and letting the Victron absorption cycle sort it over time

None of these feel especially clean. Anyone dealt with a similar situation? Particularly interested if you've used an active balancer after the fact on a drop-in pack — I'm sceptical you can even access the cells properly on the Renogy without voiding the warranty and losing a finger in the process.

Happy to be told I've bodged this from the start. Wouldn't be the first time.

WayneClark | 847 posts

@BatteryTim good timing on the post — this catches a few people out. The key thing with mixing a DIY bank and a drop-in like that Renogy is that they'll almost certainly have different internal BMS characteristics, so forcing them to balance against each other can end up fighting the individual BMS units rather than actually balancing anything useful.

What's your current setup for connecting them — parallel, or are you switching between them? That changes the approach quite a bit.

Generally I'd recommend getting each bank to the same resting voltage (around 13.3V for LiFePO4 at storage) independently before you ever tie them together. A decent programmable charger on each separately first saves a lot of headaches. What charger are you working with?

DaleSpirit | 312 posts

Had almost the exact same situation with my static caravan setup last summer — two banks that had drifted apart in resting voltage after I'd been running them separately for a few weeks.

What sorted it for me: bring both banks to the same resting voltage independently before ever connecting them together. Charged each one up, left them 24 hours with loads disconnected, then measured. Mine were still about 0.3V apart which sounds tiny but caused a proper rush of equalising current when I bridged them — learned that the hard way with some very warm cables.

Got a cheap battery capacity tester off Amazon, ran both banks through a discharge cycle first. Knowing their actual state rather than guessing from voltage alone made the whole thing much safer.

@BatteryTim what's your Victron showing for each bank's resting voltage right now?

DownsCruiser | 1,203 posts

Done this in my motorhome. Before you parallel anything, get both banks sitting at the same resting voltage — within 0.05V ideally. Charge them separately first, let them rest a few hours, then connect.

The danger is if there's a big voltage gap between banks when you parallel them — you'll get a huge inrush current and potentially trip your BMS or worse.

Also worth checking your Renogy drop-in's BMS will actually play nice with the Fogstar setup. Some drop-ins get funny when you try to balance through them — the internal BMS can block current in ways that confuse the picture.

Slow and boring wins here. No shortcuts.

CornishBoater | 234 posts

Something I'd add from doing this on the boat — even once both banks read the same resting voltage, I'd leave them sitting disconnected for a good few hours first. Surface charge can be deceptive and make you think they're closer matched than they actually are. I learned that the hard way when I rushed it and got a decent equalising current spike through my busbar connections.

Also worth asking — @BatteryTim, do you know what BMS your Fogstar cells are running? I've found some of the cheaper ones don't respond well to sudden current inrush when you parallel banks, even briefly. Might be worth current-limiting the connection somehow, even just a resistor temporarily across the link. Anyone here done it that way with LiFePO4 specifically?