So is my charge controller determining when my batts are charged or is the battery BMS?

Both are in the loop, but doing different jobs — and it took me an embarrassing amount of time to properly get my head around this with my static caravan setup.

The charge controller is the one calling the shots on charging behaviour — it's deciding the voltage targets, the absorption time, the float level. That's where you programme in your battery profile. My Victron MPPT, for instance, is set to the correct absorption and float voltages for my Fogstar lithium bank.

The BMS is more like a last-resort bouncer. It's watching cell-level voltages, temperature, and current, and if something goes genuinely wrong — a cell running away, overcharge situation — it'll disconnect the battery entirely to protect it. That's its nuclear option.

So in normal operation? Your charge controller is determining when charging is "done." The BMS should barely be getting involved at all. If your BMS is regularly tripping, that's usually a sign your charge controller isn't configured correctly for your battery chemistry — lithium cutoffs set too high, that sort of thing.

Where it gets interesting is with cheaper controllers that don't let you customise voltage profiles properly. Renogy's budget stuff can be a bit limited here. You end up relying on the BMS more than you should, which causes nuisance disconnects.

Ideally these two should complement each other — controller handles the day-to-day charging cycle cleanly, BMS sits quietly in the background as your safety net.

Anyone else found their BMS tripping regularly? Worth sharing what controller you're running alongside it — might help narrow down whether it's a settings issue.

HalfAJob93 | 📍 North Yorkshire | ⚡ 400W panels / 200Ah LiFePO4

@Macca good explanation mate. Worth adding that the BMS is really your last line of defence rather than the primary charge management tool. Ideally your charge controller should be doing its job so well that the BMS never actually needs to intervene. If your BMS is regularly cutting out, that's usually a sign your charge controller parameters aren't quite dialled in correctly for your battery chemistry.

Spent a frustrating weekend last autumn assuming my BMS was faulty when actually I'd just got my absorption voltage set slightly too high on the controller. Once I dropped it by 0.2V, everything settled right down. Get the controller settings right first, then think of the BMS as your safety net rather than your charging brain.

RussScott | 📍 Array | ⚡ Shepherds hut / Static / Van

One thing that confused me for ages — if your charge controller's absorption voltage is set above what your BMS will tolerate, the BMS just cuts off charging entirely rather than gracefully tapering it down. Took me a while to realise my Renogy controller was set to a flooded lead acid profile and my Fogstar LiFePO4 cells were basically seeing a hard disconnect every cycle instead of a proper full charge.

Worth double-checking your controller's lithium profile actually matches your specific battery's recommended charge voltage — not all "lithium settings" are identical across manufacturers.

MarineGeoff | 📍 Array | ⚡ Motorhome / Shepherds Hut / Van / Backup

Think of it like this: the charge controller is the chef deciding how much food to serve, but the BMS is the bouncer who physically stops anyone entering if things go sideways — your Victron MPPT will dial back current based on absorption voltage targets, but if cells drift out of balance or temps go dodgy, the BMS just hard-cuts the charge circuit entirely, regardless of what the controller thinks it's doing. Two entirely separate mechanisms. The controller optimises, the BMS protects. Fogstar and similar quality LiFePO4 packs will have decent BMS protection thresholds baked in, but always make sure your controller's charge profile isn't fighting against them — mismatched settings are where the drama starts.

CliveBaker | 📍 Array | ⚡ Garden Office / Static Caravan / Battery Nerd

Worth flagging something that catches people out: the BMS doesn't just stop charging at the top — it can also throttle it mid-charge if individual cells are diverging. So your Victron MPPT might be merrily pushing bulk current, the BMS quietly bleeds that off internally balancing the pack, and from the outside it looks like your batteries are being awkward.

My Fogstar Drift cells used to do this constantly before the pack top-balanced properly. Looked like a controller fault. Wasn't.

Also — if your BMS cuts hard and the charge controller doesn't have a suitable absorption voltage programmed correctly for LiFePO4, you end up in a bizarre loop where the controller keeps attempting to restart charge. Set your voltages properly in the controller first, then let the BMS act as the last line of defence. Not the primary gatekeeper.