Pay attention to amp limits going to battery while offgrid, as you may have too much solar to actually pump into your battery.

Learned this one the hard way last summer when I expanded my array from 800W to 2.4kW.

My Fogstar Drift 200Ah has a max charge rate of 100A. Sounds plenty, right? Except once I'd wired in the extra panels and a second MPPT controller, I was theoretically pushing well over 150A into a battery that simply wouldn't accept it. The Victron BMV was telling me the panels were clipping themselves just sitting there in decent July sun.

The maths that trips people up:

• Array output ÷ battery voltage = charge amps
• 2400W ÷ 24V = 100A (and that's at peak, which doesn't last)
• But on a crisp autumn morning with cold panels running at peak efficiency? You're regularly nudging past that figure

The BMS will protect the cells, obviously, but you're essentially wasting generating capacity you paid good money for. Your MPPT will throttle back, your panels sit in partial shade of their own making, and you wonder why the battery isn't full by noon despite a gorgeous clear sky.

Solutions I've seen work well:

• Size your battery bank to match the array, not just your consumption needs
• Add a dump load controller (I divert excess to a water heater)
• Parallel batteries if you're regularly hitting charge current limits

Anyone else running oversized arrays relative to their bank? Curious whether others have found creative ways around it rather than just throwing more batteries at the problem.

Classic case of "more panels, same bottleneck" 😄

Had the exact same realisation when I upgraded my static caravan setup. Stood there feeling smug with my shiny new panels, wondering why the battery wasn't charging any faster. Turns out physics doesn't care how much you spent on solar.

Worth everyone checking their BMS spec sheet before buying more

Good point but worth being precise here — the actual limit to watch is the C-rate, not just the raw amp figure.

Your 200Ah battery at 100A is 0.5C, which is actually fairly decent for LiFePO4. The issue is your MPPT controller's output rating is often the real bottleneck before you even hit the battery limit.

On my narrowboat I've got a Victron SmartSolar 100/50 — that "50" is 50A output max, full stop, regardless of panel wattage coming in. Lots of people wire up 1.5kW+ of panels into a controller that physically cannot push more than 50A out.

Check:

1. MPPT max output amps
2. Battery max charge amps
3. Any inline fusing limits

Whichever is lowest wins. The excess solar capacity isn't wasted necessarily — helps in low light — but you won't see it at peak.

Good shout @SolarJo — this caught me out on the narrowboat too.

Worth noting that your MPPT controller is often the first bottleneck people hit before they even reach the battery's charge limit. My Victron SmartSolar 100/50 is right there in the name — 50A output max. So even if your battery could theoretically accept more, your controller is capping it anyway.

The calculation I use:

Max usable solar input (A) = min(MPPT output limit, battery max charge current)

Anything above that figure in panel capacity is essentially wasted on a sunny day with a partially charged bank. Doesn't mean the extra panels are useless — cloud cover, morning/evening angles, shading losses all mean you'll still benefit — but you won't be pulling full rated wattage into the battery simultaneously.

Sizing your MPPT and battery together before buying panels would save a lot of people some expensive lessons.

Good thread. What nobody's mentioned yet is that your MPPT controller rating is often the first bottleneck you'll hit, before you even stress the battery's charge limit.

My Victron 100/50 will only push 50A regardless of how many panels I throw at it — so in my case the controller saves the battery from itself, almost accidentally. Not exactly elegant system design on my part, but it works out.

If you're running multiple MPPTs to handle a bigger array, suddenly that battery amp limit becomes very real very fast. Add up your controllers' max output currents and compare that against your battery spec before you buy anything.

@OakSpirit is right about C-rate being the proper way to think about it — especially if you ever swap battery capacity later. Doing the maths once saves a lot of grief.

So basically I spent £££ wiring up a load more panels on the motorhome roof, feeling dead clever, only to find my Victron SmartSolar was just... sat there throttling everything back like a bouncer turning away a queue.

Solar panels: ready to party 🎉
Battery: "100A max, mate, read the sign