Power surge issue when battery reaches full power (12VDC system)

Had this exact headache on my narrowboat last summer. Took me three weekends of cursing at the bilge to figure it out.

Nine times out of ten when you're seeing surges at full charge on a 12V system, it's the BMS cutting out momentarily rather than anything exotic happening. The battery hits its overvoltage protection threshold, BMS trips, load collapses, voltage drops slightly, BMS reconnects — and you get that nasty spike cycling through your system. Lovely.

Things worth checking first:

• What's your charger's absorption voltage set to? If it's pushing 14.6V+ into a LiFePO4 that's got a 14.4V BMS ceiling, you're going to have a bad time
• Are you running Victron kit? The MPPT charge controllers let you fine-tune absorption voltage properly — worth the investment if you're on cheap Renogy gear currently
• Check whether the surge correlates exactly with that top-of-charge moment or slightly after — timing matters for diagnosis

On my boat I run a Victron SmartSolar paired with Fogstar Drift cells and had to drop my absorption setpoint down to 14.2V to keep the BMS happy. Zero issues since.

Also — what's actually experiencing the surge? Sensitive electronics will show this far more dramatically than something like a pump or lighting circuit. If it's an inverter throwing an alarm that could narrow things down considerably.

What BMS and charger combination are you running? Makes a massive difference to where the fault actually sits. Would be useful for anyone else reading this thread too — these setups vary enormously.

@Boycie25 nine times out of ten it's the BMS doing its job too well — cutting absorption and confusing your inverter/charger into a brief unloaded voltage spike before it rebalances.

Check your Victron MPPT or charger absorption/float transition settings; a poorly tuned tail current threshold is the usual culprit, and dropping it from the default 2A to something more sensible for your battery bank size sorts it sharpish.

If you're running Fogstar or similar LiFePO4 cells, also confirm the BMS communication is actually talking to your charger properly — a dumb charger on a smart battery is a recipe for exactly this drama at the top of charge.

@Boycie25 @RetiredChef both onto something here. Had this exact drama in my static caravan setup two winters ago — Victron SmartSolar was seeing the Fogstar battery's BMS kick in and interpreted it as a dead short. Inverter went absolutely mental.

What sorted it for me was lowering the absorption voltage by 0.2V and extending the float period. The BMS stopped doing its frantic on/off dance and everything settled down beautifully.

Also worth checking your cable gauge — undersized cables cause voltage drop that confuses the whole system when current suddenly changes at full charge. I was running 6mm² where I needed 10mm² minimum. Embarrassingly obvious in hindsight.

Pull your Victron app logs (assuming you're running something similar) and look for the voltage spikes — they'll tell you exactly when the BMS is interrupting.

@Boycie25 @RetiredChef @FellKev been through this on the narrowboat — nine times out of ten it's also worth checking your Victron MPPT charge profile doesn't have absorption voltage set above your BMS high-voltage cutoff, because that's basically telling your BMS to panic on a schedule.

@Boycie25 mine turned out to be the absorption voltage set 0.3V too high on the Victron MPPT — battery hit float, BMS freaked out, inverter saw a 2-second dropout and my garden office NAS drive had a little existential crisis every sunny afternoon for three weeks before I twigged.