Anyone else struggled to balance two 100Ah lithium batteries from different brands?

I've got a bit of an odd situation and wondering if others have hit the same wall. I picked up a Fogstar Drift 100Ah a while back and recently added a second 100Ah lithium — a cheaper Epoch unit I found secondhand — thinking I'd just wire them in parallel and double my capacity. Simple enough in theory, right?

The problem is they don't seem to want to play nicely together. The Fogstar's BMS cuts out earlier than the Epoch's, so I end up with one battery doing most of the heavy lifting while the other sits there barely touched. Running a 40A load through a Victron SmartShunt, I can see the current draw is noticeably uneven. I've checked the cable lengths and they're matched to within a few centimetres, terminals are clean and tight.

Has anyone managed to get mismatched lithiums to balance properly in parallel, or is it basically a fool's errand? I'm wondering if it's purely down to the different BMS cutoff voltages, or whether there's something else going on — internal resistance mismatch maybe? Would genuinely appreciate any thoughts before I start pulling things apart again.

@EwanScott mixing brands is always a gamble with lithium. The issue is almost certainly different BMS cutoff voltages and internal resistance between the two units. Even if both are nominally 100Ah, they'll have slightly different state-of-charge curves and the BMSs will fight each other to some degree.

Couple of things worth trying:

• Fully charge both independently before connecting in parallel
• Make sure your cable runs to each battery are identical length — unequal resistance makes balancing worse
• Keep an eye on whether one BMS is tripping before the other under load

I ran a Fogstar alongside a Renogy in my motorhome for a few months. Worked okay but the Fogstar always seemed to carry more of the load. Ended up replacing the Renogy with another Fogstar and the difference was noticeable. Same brand, same batch ideally, saves headaches long-term.

Hey @EwanScott, had almost exactly this with a Fogstar and a generic Chinese cell last year. What helped me was doing a full top-balance on both separately before connecting in parallel — charge each one individually to 14.6v, let them rest, then join them. The Fogstar BMS is generally pretty well-behaved but if the Epoch unit has slightly different cell chemistry it'll resist equalising under load. Also worth checking whether the Epoch has a low-temp cutoff that might be triggering earlier than the Fogstar's.

One thing @ExChippie is right about — the BMS tolerances matter a lot here. If you've got a way to monitor individual battery voltages separately rather than just the combined bank voltage, that'll tell you quickly which one is pulling its weight and which isn't. What charger are you using?

Hey @EwanScott, worth checking whether both batteries are actually reaching the same resting voltage before you connect them in parallel. If they're more than about 0.1V apart, the BMS on whichever is lower will see a sudden inrush current when connected and potentially trip out.

What I'd suggest is fully charging each one independently first, letting them sit for an hour or so to settle, then measure both with a decent multimeter before paralleling them up. If the Epoch is consistently sitting lower at "full," its BMS threshold may just be calibrated differently to the Fogstar.

Some people use a small resistor or a brief load to bring the higher one down slightly before connecting — a bit fiddly but effective. Have you noticed whether it's always the same battery tripping, or does it vary?

@EwanScott ran into something similar on the narrowboat — two different 100Ah units (both "lithium" but very different internal BMS logic) and one was constantly throttling the other's charge acceptance.

What finally sorted it for me was isolating each battery, doing a full charge cycle on each individually first, then letting them rest for a couple of hours before paralleling. Even a 0.1V difference at rest causes the BMS units to fight each other initially.

Also worth checking if the Epoch has adjustable BMS parameters — some do via Bluetooth app. If you can nudge the absorption voltage to match the Fogstar's profile you'll have a much better time long-term.

Honestly though, mixing brands in parallel is always a compromise. If the headache continues, a Victron Battery Protect on each bank separately might give you more control over how they're managed.

Hey @EwanScott, one thing nobody's mentioned yet — check whether the two batteries have different internal BMS charge/discharge cutoff thresholds. Even if both are nominally 12.8V LiFePO4, the Fogstar Drift's BMS is reasonably well-documented, but some of the budget Epoch units I've seen have slightly tighter low-voltage cutoffs that cause them to drop out first under load, leaving the other battery doing all the heavy lifting. You end up thinking it's a balancing issue when it's actually one BMS being more protective than the other. Worth pulling the specs sheets side by side if you can find them. If the cutoffs are mismatched, there's not a huge amount you can do short of running them through a dedicated battery-to-battery balancer. What symptoms are you actually seeing — is one getting noticeably warmer, or is it more a capacity issue?

@EwanScott had almost this exact situation — Fogstar Drift paired with a budget unit I grabbed off eBay. The resting voltages were fine but under load they'd diverge noticeably, with the Fogstar doing the heavy lifting almost every time.

What actually helped me was running them in parallel through a Victron Smart BatterySense so I could at least monitor what was happening in real time. Turned out my cables weren't quite equal length between the batteries and the busbar, which was making the imbalance worse than it needed to be.

Worth equalising cable lengths as a first step — it sounds trivial but it made a real difference here. Are both your batteries accessible enough to rewire easily, or are they boxed in?

@EwanScott this rings very true from my narrowboat days. Had two "matched" 100Ah cells from different batches and spent weeks scratching my head wondering why my Victron BMV-712 was reading nonsense.

The thing that actually sorted it for me was running them through several full charge/discharge cycles individually before ever wiring them in parallel. Lithium cells from different manufacturers often ship with quite different state-of-charge, and the BMS on each will have subtly different voltage thresholds.

Once I'd done three or four solo cycles on each, they settled remarkably close together and have played nicely ever since. Worth the faff before assuming there's a deeper compatibility issue.