Controller for 24v panel with 12v battery?

So I'm in a similar boat — got a 24v panel sitting on the roof of my static and a 12v Fogstar lithium under the bed, and I'm trying to work out the best controller setup without spending a fortune.

From what I've read, MPPT is the obvious answer here since PWM basically can't handle the voltage step-down properly. But there's a massive price range out there and I genuinely don't know where to draw the line.

Few things I'm unclear on:

• Does the MPPT controller just handle the voltage conversion automatically, or do I need to configure it?
• Is there a minimum controller spec I should be looking at for a single 400w panel?
• Anyone running a Victron SmartSolar on this kind of setup? The 75/15 looks like it might do the job but I want to make sure before I drop £80-odd on it

Also wondering whether the cable run length matters much here — my panel is probably 4-5 metres from the battery. Presumably thicker cable the longer the run?

Had a bad experience before where I just grabbed the cheapest thing on Amazon and ended up with a controller that cooked itself within a month, so trying to be a bit more careful this time round.

Anyone done exactly this kind of 24v-to-12v setup? What controller did you land on?

@SmartSolarNerd this is exactly where an MPPT controller earns its keep over a PWM. A PWM simply can't step voltage down properly — you'd be throwing away a big chunk of your panel's potential.

For a 24v panel into a 12v Fogstar lithium, something like the Victron SmartSolar 75/15 handles that voltage differential no problem, and the Bluetooth app makes dialling in the lithium charge profile dead easy. Worth the extra tenner over a no-name unit.

Few things worth checking first though:

• What's your panel's Voc? Needs to sit comfortably within the controller's input limit
• What's the panel's wattage? Make sure your controller's amperage rating covers it
• Does your Fogstar BMS require a specific charge voltage? Victron's user-defined profiles sort this easily

Running a similar setup in my motorhome for emergency backup — genuinely solid combination.

@ExTrucker73 is spot on, and I've run almost exactly this configuration on my boat for three seasons now. A Victron SmartSolar MPPT handles the voltage conversion beautifully — the 75/15 handles most modest panel setups and sits around £60-70. What people often overlook is that you're actually gaining here: the MPPT algorithm recovers that "lost" voltage as additional current into your 12v battery, so your Fogstar charges faster than you'd expect. Just double-check your panel's open-circuit voltage against the controller's input rating before buying — a 24v nominal panel typically has a Voc around 36-38v, which catches people out on cheaper units.

Picking up on what @ExTrucker73 and @FormerMariner36 have already covered — one thing worth double-checking before you buy anything is your panel's Voc (open circuit voltage) at cold temperatures, not just the rated figure. UK winter mornings can push that noticeably higher than the spec sheet suggests, and if you're sizing an MPPT controller you want to make sure the input voltage rating has enough headroom.

I ran into exactly this when planning the electrics for my shepherd's hut — ended up going slightly oversized on the controller just to be safe.

Also, what's the panel's wattage? If it's modest, a Victron SmartSolar 75/15 might be all you need and the Bluetooth monitoring is genuinely useful for keeping an eye on a Fogstar LiFePO4 charge profile.

@AshDweller raises something important but nobody's mentioned the actual sizing maths yet.

Take your panel's Voc, multiply by 1.25 for temperature derating, and make sure your chosen MPPT can handle that input voltage. A cheap Renogy 20A unit might look fine on paper until you realise its max PV input is 50V and your panel Voc is 45V — that's uncomfortably close on a cold morning in January when Voc climbs.

Also check the controller's maximum charge current against your Fogstar's BMS limits. Fogstar Drift cells are decent but they're not indestructible. Exceeding the BMS charge current rating will trip it out at best, damage cells at worst.

I run a Victron SmartSolar 100/20 on my shepherd's hut for exactly this reason — proper voltage headroom and Bluetooth monitoring so I can actually see what's happening rather than guessing.

@SolarJunkie has the sizing nailed, so I'll add the practical bit nobody's mentioned — lithium absorption voltage matters here. Fogstar cells typically want 14.2–14.6V, and some cheaper MPPT controllers misreport this when stepping down from 24V input. Worth pulling up the charge curve in whatever app the controller uses before trusting it blindly.

On my static I'm running a Victron SmartSolar 100/20 doing exactly this 24V-panel-to-12V-battery conversion — the Bluetooth monitoring lets you verify the actual charge voltage hitting the battery rather than guessing. The Renogy Wanderer series is cheaper but lacks that visibility, which I'd consider a real disadvantage with lithium specifically.

Check your panel's Voc against the controller's maximum input voltage with some headroom for cold-morning voltage spikes — panels can read significantly higher than rated Voc in cold UK conditions.