EcoFlow 12V 100Ah Trolling Motor Battery charging drops to 12.9V after charge sometimes until big load

Had something similar on the boat a while back with a different LiFePO4 — charged to what looked like full, resting voltage settled around 12.9V and I thought something was off.

Turned out the cells just hadn't fully balanced. The BMS was cutting off charge early because one cell hit the top voltage threshold before the others caught up. So the overall pack looks full but it isn't really — hence the slightly lower resting voltage.

Few things worth checking:

• Leave it on absorption for longer if your charger supports it — some cheaper units just blip to "full" too fast
• Check what your charger's charge profile is — LiFePO4 needs a proper 14.4–14.6V absorption stage, not a generic AGM profile
• Does the voltage drop happen under load or just at rest? Big difference diagnostically

The "big load fixes it" thing is interesting — could be the load forcing cell balancing activity or just burning off surface charge weirdness. Bit unusual tbh.

What charger are you running with it? If it's a basic unit it might not be holding absorption long enough for the BMS balancer to do its thing. Victron stuff lets you tune this properly, which made a big difference for me.

Anyone else seen this pattern with EcoFlow packs specifically? Curious whether it's a BMS tuning thing from the factory.

@BenJohnson95 that's a classic surface charge situation — sounds like yours had the same thing mine did on the narrowboat last summer.

I had a Fogstar Drift 100Ah that would read 13.2V straight off the charger, then settle back to around 12.9–13.0V after an hour or so. Completely normal resting voltage for a LiFePO4 that's genuinely full, actually — the flat discharge curve means you can't read much into voltage alone.

The penny dropped for me when I put a proper load on it and watched it hold steady rather than immediately sag. That's your real indicator — voltage under load, not resting voltage.

Worth checking your BMS isn't cutting charge early due to a cell imbalance though. If one cell is racing ahead, you might not actually be hitting full capacity even when it looks like you are.

Worth adding a bit more technical context here — 12.9V at rest on a LiFePO4 pack is actually perfectly normal and doesn't indicate a problem at all.

The voltage plateau on LFP chemistry is famously flat between roughly 20–80% SoC. Once surface charge dissipates (can take 30–60 minutes after charge terminates), resting voltage of 12.9–13.1V is representative of a genuinely full pack.

What matters far more than resting voltage is:

• Coulomb counting if your BMS supports it
• Cell-level balance readings if you can access them
• How the pack performs under actual load — does voltage hold steady or sag immediately?

On my narrowboat setup with Fogstar Drift cells and a Victron BMV-712, I learned to stop trusting voltage readouts almost entirely for SoC and rely on the shunt-based Ah counting instead. Far more reliable picture of what's actually in the bank.

@FenlandSolar is right that 12.9V is normal, but worth clarifying why it drops there specifically — LiFePO4 has an extraordinarily flat discharge curve between roughly 20–90% SoC, so the resting voltage genuinely tells you very little about actual state of charge. A coulomb counter (Victron BMV-712 is what I run) is far more useful than voltage monitoring alone for these packs.

The "big load reveals the truth" behaviour you're describing is essentially the pack's internal resistance unmasking where it actually sits on the curve. If the voltage holds stable under trolling motor load rather than diving, the cells are almost certainly fine — you're just misreading a healthy chemistry as a fault.

@24V_Queen's point about the flat voltage curve is the key bit here. I learned this the hard way on my old Ducato — spent a week convinced my Fogstar cells were dying because the voltage looked "low" after charging. Nearly sent them back.

The trick that finally gave me peace of mind was running a proper load test rather than staring at resting voltage. Chuck a decent draw on it for 20 minutes, then measure. Voltage under load tells you far more than that resting figure ever will.

My Victron BMV shunt was what eventually convinced me everything was fine — watching the actual state of charge percentage rather than trying to interpret voltage on a chemistry that laughs at your multimeter.