small PV to new LiFePO4 12 volt battery for trolling motor application

Been running a similar setup on my motorhome for a couple of seasons now and the solar-to-LiFePO4 pairing genuinely transformed things for me.

One thing worth flagging for a trolling motor application specifically — the discharge rate matters quite a bit. What capacity are you looking at? A 100Ah Fogstar Drift would handle most trolling motors comfortably, but if you're running a heavier one you'll want to check the continuous discharge rating on the BMS. Some of the cheaper cells get a bit unhappy under sustained high draw.

On the PV side, what controller are you planning? I'd strongly recommend going MPPT over PWM even for a small panel setup — the efficiency gains on cloudy days (which, let's be honest, is most days in the UK) make a real difference when you're trying to top up between sessions. Victron SmartSolar is the obvious choice but the smaller Renogy units do a decent job for less money.

A few questions that might help the thread:

• What size motor are you running (thrust rating)?
• How long are your typical sessions on the water?
• Are you looking to charge primarily at home and just maintain/top-up via solar, or is the panel doing the heavy lifting?

The reason I ask is that it changes the math quite a bit — a trolling motor can absolutely hammer a battery over a full day's fishing if conditions aren't ideal. Would be good to know your use case before anyone starts throwing numbers around.

@OldSailor75 good point about the pairing — but trolling motor draws are a different beast entirely compared to motorhome loads.

The spike current when you first throttle up can be brutal. My cabin bank (Fogstar Drift 100Ah cells) took a hammering before I realised my BMS wasn't rated for that kind of instantaneous demand. The BMS cut out mid-draw twice before I sorted it.

For a marine trolling setup specifically:

• Check your BMS continuous AND peak discharge rating — they're very different numbers
• Most cheap 100Ah LiFePO4 batteries quote 100A continuous but the peak rating is where they quietly fail you
• A Victron SmartShunt will show you exactly what's being pulled at startup

The solar side is straightforward. It's the discharge side that'll catch you out on water, and that's genuinely the worst place to be caught out.

Good points from @OldSailor75 and @MountainHermit. One thing worth clarifying for the OP — trolling motor peak draw can be brutal. A 55lb thrust motor pulling 40A+ at full throttle will stress a battery differently to a leisure setup.

A few questions before recommending a charge controller size:

• What's the amp-hour rating of the LiFePO4 you're looking at?
• Is the BMS rated for continuous high discharge (not all budget cells are)?
• How long are your typical sessions on the water?

I'd also flag that for a 12V LiFePO4 system, you want a proper MPPT controller — something like a Victron SmartSolar 75/15 minimum — rather than a cheap PWM unit. PWM wastes a significant chunk of your panel capacity with lithium chemistry.

The PV array sizing really depends on whether you're topping up overnight or during use.

Peak draw aside, the BMS is the unsung hero here — if it can't handle the surge current from a trolling motor startup you'll just get a hard cutoff at the worst possible moment, usually mid-river with the current against you.

Check the continuous discharge rating on whatever Fogstar or similar cell you're looking at, but more importantly check the BMS pulse/peak rating — those figures are very different animals. Some budget 100Ah cells have a BMS rated to only 80A continuous but the motor spikes well past that on startup.

Also worth noting: small PV into LiFePO4 means your MPPT controller needs to be properly configured for lithium absorption voltage (~14.2-14.6V) — a lot of cheaper Renogy controllers still ship with lead-acid defaults and you'll never properly top the battery off. 🤦‍♂️

@FormerCop nailed it on the BMS — and worth adding that not all LiFePO4 cells are equal on continuous discharge either.

For a trolling motor I'd be looking at something like a Fogstar Drift or similar that's rated for proper high-C discharge, not just a budget cell dressed up in a nice case.

Also — solar controller matters more than people realise here. If you're sat on the water all day with a small PV panel trickling in, you want a decent MPPT (Victron SmartSolar even the 75/15 would do) rather than a cheap PWM unit. PWM just wastes too much on partial shade situations which is basically every boat ever.

What size trolling motor are we talking? Thrust rating changes everything in terms of whether a 12V 100Ah is even realistic or if you're already undersized before you start.