Battery imbalance

Had something similar crop up on my static caravan setup last winter — worth raising here because I don't see it discussed enough.

Running two 12V LiFePO4s in series for a 24V bank (Fogstar Drift cells). After a decent bulk charge, the Victron SmartShunt was showing the bank as "full" but one battery was sat noticeably lower than the other — about 0.4V difference at rest. BMS on that pack was throwing a fit.

A few things I've seen cause this:

• Unequal state of charge before connecting — did you top-balance the packs individually before wiring in series? This is the big one most people skip
• Internal resistance mismatch — even "matched" cells from the same batch aren't always equal
• Charger absorption settings — if your inverter/charger isn't holding absorption long enough, weaker packs never catch up

The AIMS units aren't something I've used personally — does it have proper LiFePO4 charge profiles or are you running it on a generic AGM setting? That'd be my first suspicion honestly.

Also — which BMS are you running across each individual battery? If it's cutting one pack off early during charge, the other just keeps taking current and they'll drift further apart every cycle.

Anyone else here managed this on a 24V series setup? Curious whether active balancers actually helped or just masked the problem. I've seen mixed opinions and haven't pulled the trigger on one yet.

RussThomas | 847 posts

@RelayDream good topic to raise. One thing worth checking that often gets overlooked — are your inter-battery cables absolutely identical in length and gauge? Even a small resistance difference between them can cause uneven current draw and accelerate imbalance over time, especially on a static setup where you're not regularly equalising things manually.

Also, with Fogstar Drift cells specifically, I'd have a look at whether your BMS is actually balancing passively during top-of-charge absorption or only during discharge. Some cheaper BMS units only balance one way, which is far too slow to keep pace with real-world use.

What charger are you running? Some 24V chargers marketed as "LiFePO4 compatible" have absorption voltages that aren't quite right and gradually pull the cells apart over a season.

PennineNomad | 1,203 posts

Something I've dealt with on the narrowboat — when you're running two 12V batteries in series, the individual BMSs often have slightly different cell-level balancing thresholds. What looks like inter-battery imbalance can actually be one BMS throttling charge acceptance earlier than the other.

Worth temporarily monitoring each battery's voltage independently (Victron SmartShunt on each unit if you can) during a full charge cycle. If one consistently hits absorption voltage while the other's still climbing, that's your culprit.

Also check your cell interconnects — a loose busbar on a series string creates resistance that skews voltage readings under load, making genuine balance look worse than it is. Seen it cause ghost imbalance warnings repeatedly.

Long-term fix on my setup was switching to a proper 24V battery rather than two 12V in series — halves the BMS complexity immediately.

ZI_Sparks | 412 posts

This caught me out badly in my tiny house build last year. Had two Fogstar Drift 100Ah packs in series and noticed my Victron BMV-712 was reading voltages that just didn't add up across the bank.

Turned out one cell group had drifted enough that under load, the weaker battery was hitting its low-voltage cutoff before the other had barely broken a sweat. The BMS on that unit was doing its job — but the system was working against itself.

What fixed it for me was a proper active balancer bridging the two batteries at the midpoint. Passive balancing inside individual BMS units simply isn't designed to handle inter-battery drift in a series configuration.

Worth checking your midpoint voltage regularly if you're not already — the BMV-712 has that feature built in and it's genuinely revealing.

PikeAndrea | 634 posts

Worth adding something nobody's mentioned yet — check whether your two batteries have matching internal resistance figures, not just matching state of charge. Even batteries from the same batch can drift apart over time, and the one with higher internal resistance will consistently lag behind during both charge and discharge cycles. This gradually worsens the imbalance.

A decent battery analyser will show you this. If they're significantly mismatched, no amount of top-balancing will fully sort it long-term.

Also @RelayDream — with a static caravan setup, are you leaving these unattended for extended periods over winter? LiFePO4 handles cold reasonably well but a prolonged low state of charge in freezing temperatures can accelerate cell drift considerably. Keeping them topped up before you leave helps enormously.