Anyone else had issues with Fogstar Drift cells losing balance after deep cycling?

Running a 280Ah 24V LiFePO4 bank in my motorhome built from four Fogstar Drift 280Ah prismatic cells with a Daly 8S BMS (100A). Been using it since last spring and generally very happy — charges cleanly from a 400W roof array through a Victron SmartSolar 100/30 and a Victron Orion-Tr Smart for alternator charging.

The problem started showing up after a few full discharge cycles, mainly when I'm running the inverter hard (2kW Victron Multiplus-II, kettle and induction hob use). Cell 3 keeps drifting high during top balancing — sitting at 3.42V when the others are 3.38–3.39V. The Daly trips on cell overvoltage before the pack hits full capacity. I've done a few manual top-balances with a bench PSU direct to the cell terminals and it reins it back in, but it creeps out again within 10–15 cycles.

I'm wondering whether the Daly is just not cutting it as an active balancer — it's only passive and the balance current is laughably small (~30mA). Considering swapping to a Seplos BMS or adding a standalone Daly Active Balancer on top of the existing passive BMS. Has anyone gone the hybrid passive BMS + active balancer route with prismatic cells, and is it actually worth the faff, or is a better BMS the cleaner solution?

VanCarl | 847 posts

@Smudge the Daly 8S is likely your culprit here rather than the cells themselves. Daly's passive balancing current is notoriously weak — typically only 35-50mA — which simply can't keep pace when you're regularly deep cycling. The imbalance creeps in gradually and you won't notice until it's quite pronounced.

Worth checking whether your balancing is set to trigger at the right voltage threshold. Some Daly units default to starting balance too low in the charge cycle, meaning cells barely get any balancing time before the BMS hits full charge and cuts off.

If the cells themselves were faulty, you'd likely see the same cell consistently drifting rather than random variation. What pattern are you seeing — is it always the same cell leading or lagging?

Interesting thread — I'm in a similar situation planning a 24V build for my tiny house and was looking at Fogstar Drift cells paired with a Daly BMS.

@VanCarl makes a good point about the passive balancing. Quick question for @Smudge — are you seeing the imbalance appear during discharge, charge, or both? And roughly what depth of discharge are you regularly hitting?

I've been reading that some people switch to a JK BMS for the active balancing and it supposedly makes a noticeable difference with prismatic cells that have drifted slightly in capacity. Has anyone here actually compared the two back-to-back on Fogstar cells specifically?

Also wondering whether a top-balance before installation was done properly — apparently that's crucial with prismatics and if it wasn't done thoroughly it can cause persistent drift issues down the line.

@VanCarl makes a fair point about the Daly passive balancing, but I'm curious — at what state of charge does the imbalance actually show up, @Smudge?

I've got a similar Fogstar Drift build in my garden office (16S, 48V) and noticed the cells drift most noticeably below about 20% SoC. Switched to a Victron Smart BMS and the active monitoring made a huge difference in spotting when the divergence was happening.

@PaulMurray — for a tiny house build I'd seriously look at pairing whatever BMS you choose with a Victron SmartShunt just so you've got proper visibility of what's actually going on at cell level before committing to a config.

What's your typical depth of discharge looking like, @Smudge? That might narrow down whether it's genuinely a cell issue or just balancing struggling to keep up.

@CliffWill good question on the SoC point — in my experience with prismatic cells the imbalance tends to surface most visibly at the top end, when cells hit absorption and the weaker ones start hitting the high-voltage cutoff early. That's when passive balancing really struggles because there's limited time to bleed off the delta before the BMS trips.

@Smudge worth logging individual cell voltages at the point the BMS disconnects. If you're seeing one or two cells consistently high whilst others are sat at 3.40V-ish, that confirms a balancing issue rather than cell degradation. I ran a Daly on my narrowboat build briefly — replaced it with a JK BMS specifically for the active balancing. Night and day difference on cell spread, especially after deep discharge cycles.

What's your actual depth of discharge typically? Going below 20% regularly can accelerate divergence.

@Smudge had very similar with my 280Ah Daly-based build last summer. The passive balancing on those units really does struggle to keep up once cells start drifting — it only kicks in above about 3.4V and the balancing current is tiny, something like 35mA if I recall correctly. What helped me was doing a proper top balance on the cells individually before reassembling — got a bench power supply and brought each cell up to 3.65V individually until current dropped right off. Made a noticeable difference straightaway.

Worth checking your charge parameters too. If your absorption voltage is set a touch low the BMS balancer never gets enough time to do its work. I'd also have a look at whether one particular cell is consistently the outlier — usually tells you if you've got a weak cell rather than just a balancing issue.

Great thread, this. @Spud79 is onto something about where the imbalance shows up — I'd add that with LiFePO4 the flat voltage curve in the mid-range makes it almost impossible for passive balancing to do meaningful work until you're right up at the top.

One thing worth trying @Smudge: deliberately hold the bank at absorb voltage (around 28.4V for 24V LiFePO4) for a longer period — a couple of hours if your charger allows it. Gives the Daly's balancing circuit more time to bleed down the higher cells. It's a workaround rather than a fix, but it helped my own Daly build considerably between spring and when I eventually upgraded to active balancing.

Long term though, a decent active balancer wired across the pack is probably the cleaner solution if the drift keeps worsening.

Really resonates with me this. Had almost identical drama with my motorhome build — four EVE 280Ah cells, Daly BMS, same passive balancing headache.

What sorted it for me was a deliberate top-balance session before reassembling the bank. Charged each cell individually to 3.65V with a bench PSU, held it there until current dropped to almost nothing. Took the better part of a weekend but the bank's been rock solid since.

@SilverCaptain makes a fair point about that flat LiFePO4 curve hiding sins until the very top. That's exactly where Daly's passive balancing gets overwhelmed — it's fighting too little voltage differential with too little current.