Smartsolar 75/15 prematurely entering absorption and float

Had this exact head-scratcher with my static caravan setup last summer — Victron SmartSolar 100/20 paired with a Fogstar 100Ah LiFePO4, and the thing was smugly declaring absorption after pulling in barely 400Wh on a decent sunny morning, which in Scotland counts as a miracle so you'd think it'd crack on a bit longer.

Turned out my battery voltage was rebounding so quickly (LiFePO4 flat discharge curve being the culprit) that the MPPT thought it had cracked the job early. Classic case of the charger being too clever for its own good.

Few things worth checking if you're seeing the same:

• Tail current setting — if it's too high relative to your actual battery capacity, it'll trigger float prematurely. Drop it to around 1-2% of battery capacity in amps
• Absorption voltage — make sure it's set correctly for LiFePO4 (typically 3.45-3.55V per cell, so ~27.6-28.4V on a 24V system)
• Re-bulk voltage — worth nudging this up slightly so it doesn't just sit in float all afternoon doing nowt

Also worth connecting via Bluetooth and watching the actual charge current in VictronConnect when it makes the switch — if it's jumping to float whilst still pulling meaningful current, something's misconfigured rather than the battery giving up early.

Anyone else found the factory LiFePO4 presets a bit optimistic on Victron kit, or is it just mine being awkward? Genuinely curious whether this is a firmware thing or just user error (almost certainly the latter, if I'm honest).

@DailyWanderer had the same on my static caravan — turned out the default absorption voltage was set too low for LiFePO4. The SmartSolar was seeing the voltage climb quickly (as LiFePO4 does under light load) and misreading it as a full battery.

Check your charge profile in VictronConnect:

• Absorption voltage: 14.2V
• Float voltage: 13.6V
• Absorption time: reduce to 1hr max, or use the adaptive setting

Also worth checking your battery wire gauge and connections — any resistance will cause voltage drop at the controller end that throws the readings off.

If your Fogstar has a BMS with comms, see if you can get actual cell-level data. The MPPT is only seeing terminal voltage, which can be misleading.

@DailyWanderer also worth checking your battery sense — if the MPPT is only reading terminal voltage rather than actual battery voltage, voltage drop across your cables will fool it into thinking it's fuller than it is, and you'll be in float before you've had a proper brew.

Interesting one — are you certain the battery profile is set to lithium rather than the default AGM/gel in VictronConnect? I made this exact mistake on my motorhome setup initially; the MPPT was using absorption voltage of ~14.4V which LiFePO4 hits almost immediately under partial load, so it looked like it had charged when it really hadn't.

Also worth checking: what's your tail current configured to? If it's not set (or set too high), the controller may be declaring absorption complete far too early based purely on voltage rather than waiting for current to taper properly.

@LochLover's point about battery sense is solid too — have you got the Victron Smart Battery Sense dongle, or are you relying on the MPPT's own voltage reading at the terminals?

Right so I had almost the exact same saga with my static caravan last spring — Fogstar 100Ah, SmartSolar 75/15. Turned out my tail current setting was the culprit.

The MPPT was hitting the absorption voltage threshold and seeing low current draw (battery was actually fairly full already from the night before on shore power), so it was legging it straight through to float.

Worth checking what your tail current is set to in VictronConnect — mine was left on the default which is frankly useless for LiFePO4. Drop it down to around 1-2% of battery capacity (so roughly 1-2A on a 100Ah pack) and suddenly the charge stages actually make sense again.

@WattLiz makes a good point on the lithium profile too — that should absolutely be step one before poking anything else.

@WattLiz makes a good point but assuming that's already sorted — the one that bit me with my garden office setup was the absorption voltage threshold being set too low.

On LiFePO4 the MPPT triggers absorption when voltage hits your set point, not when the battery's actually full. If your absorption voltage is set around 14.2V and you've got any voltage drop on the cables, it'll trip early constantly.

Check your cable run length and gauge — I was using undersized wire between my Fogstar and the 75/15 and it was causing exactly this. Bumped the absorption to 14.4V and sorted the wiring, problem gone.

Also worth looking at your tail current setting in VictronConnect if you're on a recent firmware — that's probably the most reliable way to end absorption properly rather than relying on voltage alone.