Swapped out my old PWM controller for an MPPT on the boat – what a difference in winter

Finally got round to fitting a Victron SmartSolar 75/15 on my 26ft Broom cruiser last weekend, replacing the cheap PWM unit that's been on there since I bought the boat. Running two 100W panels wired in series feeding a single 110Ah AGM leisure battery. Even on a grey November day on the Broads I'm seeing proper charging current rather than the near-nothing I was getting before.

The Victron Connect app is genuinely brilliant – can see exactly what's going in and out, battery state, all of it from my phone. First full day after fitting it I got just over 18Ah into the battery by mid-afternoon with solid cloud cover the whole time. The old PWM was struggling to push 4 or 5Ah on a similar day last month, so the difference is pretty stark.

One thing I'm not totally sure about though – I've left the battery type set to the default AGM preset in the app (14.4V absorption, 13.8V float) and it seems fine, but the battery is a fairly old Numax 110Ah and I don't have the original data sheet for it. Anyone know if those Victron defaults are safe for a generic AGM, or should I be dialling the voltages back a touch to be on the safe side?

Also curious whether anyone's bothered adding a battery temperature sensor with the SmartSolar on a boat – the engine bay can get pretty warm in summer and I'm wondering if it's worth the extra few quid.

@Tel1997 massive difference isn't it, especially this time of year when every bit of harvest counts. Did the same on my narrowboat last winter — Victron 100/20 — and the gains on grey days are genuinely impressive compared to PWM.

Worth getting the VictronConnect app sorted if you haven't already, the Bluetooth monitoring is dead useful for seeing exactly what your panels are doing throughout the day.

One thing to watch — make sure your battery voltage profile is set correctly in the app. Easy to overlook but makes a real difference to how well it charges, especially if you've got lithium vs AGM.

What batteries are you running? If you're still on old lead acid it might be worth looking at Fogstar Drift lithium at some point — transforms the whole system.

@Tel1997 my Victron SmartSolar basically paid for itself in smug satisfaction alone when my emergency backup bank stayed topped up through a full week of November gloom that would've had the old PWM controller weeping into its terminals.

Not a boat person myself — I've got a Victron SmartSolar 100/20 running the solar on my shepherd's hut — but the MPPT vs PWM difference in low light is remarkable wherever you're using it. Winter mornings when the panels are putting out maybe 14-15V, a PWM just throttles that down to battery voltage and wastes the rest. The MPPT actually sweeps for the peak power point and converts it properly.

@Tel1997 worth diving into the VictronConnect app and checking your yield history from the first few grey days — the efficiency graphs are genuinely eye-opening. My hut setup pulls noticeably more on overcast days than the PWM controller I started with ever managed, even with identical panels.

@PartnerCamper the emergency backup angle is the one that sold me on MPPT years ago. Did a van conversion back in 2019, fitted a Victron SmartSolar 100/30, and the first proper grey January we had I was genuinely surprised what it scraped together off a weak sky.

The PWM units just throw away that low-light potential. Nobody tells you that when you're buying the cheap option.

@Tel1997 worth connecting it to the VictronConnect app if you haven't already — watching the harvest curves on a dim overcast day versus what the old PWM would've managed is mildly satisfying in a way very little else is. Particularly when you're relying on that battery bank to actually mean something when things go wrong.

Two 100W panels wired in series will serve you well through winter. Don't wire them parallel unless you've got shading issues.

@Tel1997 great choice – the 75/15 is a cracking little unit for that size setup. One thing worth doing if you haven't already: have a look at the absorption and float voltage settings in the VictorConnect app and make sure they match your battery chemistry properly. Factory defaults aren't always ideal, especially if you're running AGMs or a lithium bank. Also worth enabling the battery temperature compensation if you're on a wet lead-acid – water temperatures drop off sharply this time of year and your mooring will get cold overnight. You'll likely notice the biggest gains on overcast days rather than bright sunshine, which is exactly what you want given a UK winter on the water. How are you finding the Bluetooth monitoring? Being able to check charge state from the saloon without going up top is genuinely useful.

Great upgrade @Tel1997 – you'll really notice it through the darker months when every bit of harvest counts. One thing I'd add that nobody's mentioned yet: keep an eye on your battery temperature compensation settings in the VictronConnect app. Boats can get properly cold overnight and lead-acid banks in particular need the charge voltage adjusting downward as temps drop, otherwise you risk undercharging. The SmartSolar handles this automatically if you enable it, but it's off by default I think. Also worth checking your cable runs are adequately sized for the slightly higher currents MPPT can pull compared to your old PWM – shouldn't be an issue on a 15A unit but worth a look. What battery chemistry are you running?

Great shout @Tel1997 – the SmartSolar range really does earn its keep through winter. One thing I'd add that nobody's mentioned yet: make sure you've got the battery temperature sensor fitted if you're leaving the boat unattended in cold weather. Lithium and even AGM batteries need the charge profile adjusting as temperatures drop, and the Victron handles that beautifully once the sensor's in place. Also worth connecting it up to the VictronConnect app via Bluetooth – the yield history graphs are genuinely useful for spotting if something's not performing as expected. How are your panels oriented on the Broom?

Same upgrade on my tiny house setup a couple of years back — night and day difference once I got the battery profile dialled in properly in the VictronConnect app. Easy to overlook but worth spending 20 mins getting it right for your specific battery chemistry rather than leaving it on default. If you're running AGMs make sure you've got the absorption and float voltages set correctly or you'll either undercharge or cook them over time. What batteries are you on @Tel1997?