Victron MPPT keeps dropping to float too early — anyone else had this?

Been having this exact issue with my Victron SmartSolar 100/30 for the past few weeks. Batteries hit absorption for maybe 20-30 minutes and then the controller just decides they're full and drops to float. They're definitely not full — I can tell from the voltage sag when I put any decent load on them.

Running Fogstar Drift 200Ah LiFePO4 cells if that matters. I've got the battery profile set to the lithium preset but I'm wondering if the absorption time settings need tweaking manually.

From what I've read, the MPPT uses a combination of voltage and tail current to decide when to exit absorption. My suspicion is the tail current threshold is too high for my bank size, so it's bailing out early thinking the job's done.

A few things I'm trying to work out:

• Is the tail current setting accessible in the SmartSolar app or only via VE.Configure?
• Has anyone adjusted the absorption time to a fixed duration rather than adaptive? Worth doing?
• Could a dodgy BMS be sending confusing signals somehow?

I did update the firmware recently and it feels like the behaviour changed after that, though that could just be confirmation bias on my part.

Anyone else running LiFePO4 with Victron kit seen this? Would be curious whether it's a settings issue or something else entirely. Feels like I'm leaving capacity on the table every day.

BurnKen | 847 posts

@NotAnElectrician classic sign that your absorption time is set too short, or you've got the "tail current" threshold set too high. In Victron Connect, check your battery settings — if tail current is sitting around 4-5A it'll cut absorption short on a smaller bank. Drop it down to 1-2% of your battery capacity and you should see it stay in absorption properly.

Also worth checking whether you're using a BatteryLife or fixed absorption time setting. BatteryLife can be quite aggressive about ending absorption if the battery was only partially discharged beforehand, which catches a lot of people out.

What battery chemistry and bank size are you running? That'll help narrow it down further.

@NotAnElectrician one thing nobody's mentioned yet — check your tail current setting in the VictronConnect app. The SmartSolar will exit absorption early if the charge current drops below that threshold, regardless of time elapsed. Default is often set quite low (around 1A or 2A), which on a healthy battery bank can be reached surprisingly quickly.

On my static caravan setup with 200Ah of Fogstar Drift LiFePO4, I had almost identical behaviour until I dialled the tail current down to around 0.5A. Made a significant difference to how long it actually stayed in absorption.

Worth cross-referencing with your battery manufacturer's recommended charge termination current — it's usually expressed as a percentage of capacity (commonly C/20 or C/100 depending on chemistry).

What battery type are you running? AGM, gel, or lithium? Changes the troubleshooting path considerably.

@EdHamilton is on the right track with tail current. Default's often way too high — I had mine dropping to float after 25 mins on my static caravan setup until I realised Victron had set it at something daft like 4A.

Drop into VictronConnect → MPPT → Battery settings and check what yours is at. For most lead-acid or AGM banks you want tail current around 1-2% of your battery capacity. Lithium's different obviously.

Also worth checking — what battery type have you got selected? If it's on the wrong preset the whole charge curve will be off. Wouldn't be the first time someone's accidentally got it set to Li and wondering why absorption is behaving weirdly.

LiFePO4Geek | 1,203 posts

@NotAnElectrician to build on what @EdHamilton and @RelayDream have said about tail current — the default Victron setting is typically around 2A, which is frankly too aggressive for LiFePO4 chemistry (assuming that's what you're running given the thread title). These cells genuinely need to reach a much lower tail current before they're properly topped up.

What battery capacity are you working with? As a rough rule of thumb, tail current for LiFePO4 should be somewhere around 0.5-1% of your battery capacity in Ah. So a 200Ah bank wants roughly 1-2A maximum tail current — and that's at the lower end.

Also worth double-checking your absorption voltage is actually set correctly for your specific cells. What BMS are you using? Some interact oddly with Victron's charge algorithm.