Fitted a 200W solar panel on my narrowboat last weekend — a few questions about charge controllers

I've just mounted a 200W mono panel on the roof of my narrowboat and wired it up to a Victron SmartSolar MPPT 75/15. Running it into a single 100Ah AGM leisure battery for now, with plans to add a second battery later in the season. The install seemed straightforward enough but I'm getting a few odd readings I wasn't expecting.

The main thing puzzling me is that on a fairly decent sunny afternoon (south of Birmingham, so decent light) I'm only seeing around 8–9A coming in rather than the ~11A I'd expect at peak. The panel is laid flat on the roof which I know isn't ideal, but I didn't think it'd knock that much off. Could the angle really account for that much of a loss, or should I be looking elsewhere — cable runs, connections, dodgy crimps?

Also wondering about the battery absorption voltage. The Victron app is currently set to 14.4V for the AGM, which I believe is standard, but the battery itself (a Varta LFD90) seems to have a recommended charge voltage of 14.7V on the label. Has anyone else had to tweak these settings for AGMs, and is it worth bumping it up or just leaving well alone?

Longest cable run from panel to controller is about 4 metres using 4mm² twin core. I used MC4 connectors straight off the panel leads. Wondering if the cable spec is fine for that distance or if I should have gone heavier.

YEL_Marine | 📍 Kennet & Avon | Posts: 847

@Liam1998 Nice setup to start with! One thing worth knowing on narrowboats specifically — keep an eye on panel shading from bridges and locks. Even partial shading can hammer your yield more than people expect, and if you're adding a second panel later, make sure you wire them in parallel rather than series to minimise shading losses. Your 75/15 will handle two 200W panels in parallel comfortably at 12V.

Also worth enabling the battery preset correctly in the VictorConnect app for AGM — the default absorption voltage is sometimes a touch high out of the box. Victron's recommended settings for AGM are typically 14.7V absorption and 13.8V float.

What's your primary usage on board? Knowing your loads will help work out whether that 100Ah will be sufficient before you commit to the second battery. 🚢

CaddyDream | 📍 Array | Posts: 312

Had a 75/15 on my narrowboat for a couple of years — solid bit of kit. One thing I'd flag: keep an eye on your battery voltage when you're moored up for a few days in summer. 200W into a single 100Ah AGM can push it pretty hard, especially if you're not drawing much. Worth setting the absorption/float voltages correctly in the VictronConnect app for AGM specifically — the defaults aren't always ideal.

Also when you do add that second battery, make sure they're identical spec and age. Mixing old and new AGMs in parallel causes headaches.

Planning to swap my AGMs for Fogstar Drift lithiums eventually — the MPPT handles the charge profile fine, just needs a quick reconfigure.

KateMason82 | 📍 Leeds-Liverpool Canal | Posts: 1,203

Great first setup @Liam1998! The 75/15 is a brilliant choice for your panel size — plenty of headroom there.

One thing I'd flag that nobody's mentioned yet: make sure you've got your battery temperature sensor connected if you're planning to moor anywhere without much ventilation. AGMs can be a bit fussy about charge voltage in winter, and the Victron will automatically compensate if it knows what temperature the battery is sitting at. Makes a real difference to battery longevity, especially on a narrowboat where temperatures in the engine bay can swing quite dramatically.

Also double-check your cable sizing between the controller and battery — undersizing that run is a really common mistake that costs you efficiency. What length of run are you working with?

BayLisa | 📍 Array | Posts: 2,156

200W into a 75/15 is fine for now but honestly that single 100Ah AGM is going to be your bottleneck way before the controller is. AGMs hate being pushed hard repeatedly — I learned that the hard way on my boat before switching to lithium (Fogstar Drift, no regrets).

Also check your absorption voltage settings in the Victron app. Default profile isn't always right for every AGM brand and I've seen people quietly cook batteries by just leaving it on factory settings. 🙄

The SmartSolar app is actually decent for this — you can see exactly what's happening with your charge cycles which is well worth monitoring when you're new to it.

Second panel when you're ready will make a massive difference on cloudy days on the cut.

BigAl31 | 📍 Trent & Mersey | Posts: 847

Nice one @Liam1998, good choice on the Victron — the Bluetooth monitoring alone is worth it on a narrowboat where you're constantly checking state of charge.

One thing nobody's mentioned yet: keep an eye on your battery temperature if you're cruising into autumn. AGMs can get unhappy being charged at full absorption voltage in cold conditions. The SmartSolar has a temperature compensation input if you pick up a Smart Battery Sense — well worth the tenner or so, particularly moored up overnight in winter.

Also double-check your cable sizing between panel and controller. Surprising how many folk undersize that run and lose efficiency before it's even reached the MPPT. What length is your roof-to-battery run roughly?