Anyone else finding their MPPT settings make a massive difference in winter vs summer?

Just had a bit of an eye-opener this week. I've been running a Victron SmartSolar 100/30 with a 400W panel array on my narrowboat, and I'd left the absorption voltage at the default 14.4V all year round. Noticed my 100Ah lithium (a Fogstar Drift) was only hitting about 87% SoC on sunny days last month, which seemed odd given we had some decent November sun.

Tweaked the absorption voltage up slightly to 14.6V and bumped the absorption time from the fixed 2 hours down to a tail-current-based cutoff at 2A, and suddenly I'm consistently landing at 97-98% SoC on the same kind of day. Felt like I'd basically found a free extra 10Ah from nowhere. Dead simple change in the VictronConnect app, took about three minutes.

Wondering if others are doing seasonal adjustments on their MPPTs, or have you found a single set of settings that works well enough year-round? I'm also curious whether anyone on AGM or gel batteries notices the same kind of drift — I imagine temperature compensation makes it even more complicated for lead-acid users.

CrispyRigger | Posts: 847

@RustyCaptain absolutely this. Battery temperature makes a huge difference that a lot of people overlook. Cold batteries need a higher charge voltage to actually reach full charge - your 14.4V in July is doing a completely different job in January when the batteries might be sitting at 5°C.

If you're not already using a Victron temperature sensor (the Smart Battery Sense is brilliant for this), it's worth the £25 odd. The MPPT will automatically compensate the absorption and float voltages based on actual battery temp rather than just guessing.

Also worth checking your absorption time settings going into winter - panels are producing less, so sometimes the bulk stage barely finishes before the sun drops and you're cutting absorption short. I bumped mine up from 2 hours to 4 last October and noticed the difference immediately.

SmartSolarFan | Posts: 1,204

@RustyCaptain great timing on this thread actually. Worth mentioning that if you've got a Victron BMV or Smart Battery Sense, you can have the MPPT automatically compensate based on actual battery temperature rather than adjusting manually with the seasons. The temperature coefficient for most lead-acid batteries is around -4mV per cell per degree Celsius, so on a cold January morning your absorption target could legitimately be pushing 14.8-15V rather than the default 14.4V. On a narrowboat particularly, those batteries in the bilge can get properly cold overnight. Lithium owners largely sidestep this issue, but for anyone on AGM or flooded lead-acid it's genuinely worth the investment in a temperature sensor. Makes a noticeable difference to how fully the batteries actually top up during those short winter charge windows.

RustySpanner | Posts: 2,341

@RustyCaptain this is one of those things that genuinely catches people out. What @CrispyRigger is alluding to is that flooded lead-acid and AGM both want roughly +0.03V per degree Celsius drop below 25°C — so on a cold canal morning at 5°C, you're looking at absorbing closer to 14.8–15.0V to actually fill the battery properly.

On my boat I run the Victron temperature sensor cable directly to the battery bank. The SmartSolar handles the compensation automatically once you configure the coefficient in VictronConnect — dead easy.

Without compensation, you're essentially undercharging all winter, then wondering why your capacity feels rubbish by February.

If you're on lithium, different story entirely — Fogstar Drift cells want tighter voltage windows and generally don't want temperature compensation applied the same way.

PW_Sparks | Posts: 634

Good thread this. One thing nobody's mentioned yet — if you're on a narrowboat and the batteries are sitting in a cold bilge space, they could easily be 5-10°C colder than ambient air temperature. That gap matters quite a bit for your voltage compensation calculations.

Worth grabbing a cheap temperature probe and actually measuring at the battery terminals rather than guessing. The Victron SmartSolar supports a proper battery temperature sensor (the VE.Smart dongle route if your BMV supports it), which takes the guesswork out entirely and adjusts automatically. Been running that setup myself through two winters now and the difference in absorption behaviour is noticeable compared to just manually tweaking settings seasonally.

@RustyCaptain what chemistry are you running? AGM, lithium, or flooded? Makes a fair difference to how aggressively you'd want to compensate.

JackHunt | Posts: 487

Great thread. To add something slightly different — have you looked at the temperature compensation setting in VictronConnect? If you haven't got a Smart Battery Sense or temp sensor fitted, your MPPT is essentially guessing, especially on a narrowboat where your battery bank temperature can swing quite a bit between a cold bilge overnight and after the stove's been going all day. The compensation is usually set at -16.2mV/°C for lead acid, but without actual temp data it's just running on assumptions. Picked up a Smart Battery Sense for about £30 and it made a noticeable difference to how the charge profile behaved through autumn. Works wirelessly over Bluetooth so no awkward cable runs either. Might be worth considering before tweaking absorption voltages manually. @PW_Sparks curious what you were going to say about battery temps on boats!