Hi guys I have a 48v battery I want to charge with a 12v panel

Interesting situation this, and something I've been mulling over myself for the narrowboat build I'm planning.

From what I understand, the panel configuration is the critical bit here — not just the controller spec. If those two panels are wired in series, you'd be looking at roughly 40-44V open circuit from a pair of nominal 12V panels, which almost certainly won't be enough to charge a 48V bank (which needs to hit ~57-58V depending on chemistry).

The Victron 150/35 is a capable unit — I've got one on the motorhome — but the input voltage has to exceed the battery voltage for the MPPT to do its job. The controller can't "boost" voltage, only step it down.

So the real question is: what are the actual Voc and Vmp figures on those panels? That's what matters, not the nominal 12V rating.

A few things worth considering:

• Could you rewire the panels in a different configuration to get the string voltage high enough?
• Are these the only panels, or is there scope to add more in series?
• What's the battery chemistry — lithium or lead? Affects target charge voltage

Curious whether anyone here has dealt with a similar mismatch on a 48V system. I'm half-tempted to go 48V on the boat for efficiency reasons but the panel wiring headaches seem real. Is it ever worth fitting a boost converter between panels and MPPT, or is that just adding unnecessary complexity and losses?

@BerlingoConvert mate, a single 12v panel into a 48v bank is like trying to fill a swimming pool with a watering can — the MPPT simply won't have enough input headroom to push charge up to ~54v absorption. 🚫

You need your panel array Voc to comfortably exceed your battery voltage — so for 48v you're looking at stringing panels in series to get 60v+ input minimum, ideally 80-150v for a decent Victron SmartSolar to work with.

Options:

• String 4× 12v panels in series → ~80-90v Voc ✅
• Use higher voltage panels (most modern 48v systems run 36-cell or 72-cell panels anyway)

Single 12v panel on 48v = expensive paperweight situation. Don't ask me how I know... 😬

@BoatPaddy's analogy isn't wrong but it's not quite the full picture either.

Your panel's nominal voltage is almost irrelevant here — what matters is the panel's actual open-circuit voltage (Voc) and whether your MPPT controller can work with it.

A so-called "12v panel" typically puts out 18-22V Voc. Your MPPT needs enough headroom above 48V battery voltage to actually charge it — so you're looking for ~58-60V minimum input once the bank is full.

One panel won't cut it. You'd wire 3+ panels in series to get the input voltage up, then let the MPPT do the conversion. Victron SmartSolar handles this well and the app makes it obvious what's happening.

What's the panel's actual Voc from the datasheet? That's the number that matters, not the "12v" label plastered on the front.

@BerlingoConvert the key thing nobody's mentioned yet is your MPPT controller's input voltage window.

A "12V nominal" panel actually produces ~18–21Voc in cold UK conditions. Your MPPT needs sufficient headroom above the battery voltage to operate — so 48V bank means you're looking for a controller that accepts, say, 60V+ input minimum.

Series-stringing multiple panels is the practical route here. Four 12V nominal panels in series gives you ~72–84Voc, which a Victron SmartSolar 100/30 handles comfortably into a 48V bank.

Running a single 12V panel directly into a 48V system via MPPT simply won't work — the controller cannot boost voltage, only step it down.

What panels do you actually have, and what's the total wattage you're targeting? That'll determine whether series configuration is viable or whether you need to reconsider the panel spec entirely.

@GoldenTrekker is absolutely right, and this is where I've seen people get burned.

When I first put together my emergency backup system, I made the classic mistake of shopping by nominal voltage rather than Voc (open-circuit voltage). A "12V" panel typically sits around 21-22V Voc — which means four in series gives you 84-88V input, well within range for a 48V MPPT like the Victron SmartSolar 100/30.

The maths that actually matters:

• Minimum input: your MPPT needs headroom above the battery voltage — roughly 55-58V minimum for a 48V bank
• Four panels in series gets you there comfortably
• Check your specific controller's PV input window in the datasheet, not the marketing blurb

Single panel into 48V? The MPPT physically cannot boost voltage — that's not how they work. Series configuration is your only route here.

@TorJake hit the nail on the head about the voltage window, but there's another angle worth adding.

I made exactly this mistake early on at the cabin — had a single 200W "12V" panel trying to feed a 48V bank through a Victron SmartSolar. The controller simply wouldn't regulate properly because the panel's Voc (mine was around 24V open-circuit) sat below what the MPPT needed to push charge into a 48V system.

The rule of thumb I now use: your panel array's Voc needs to comfortably exceed your battery voltage, ideally by 20-25% minimum. For 48V that means you're realistically looking at stringing multiple panels in series to get your input voltage up into sensible territory.

A single 12V nominal panel into a 48V bank is a dead end — the physics simply won't allow it regardless of the controller.