MPPT charge controller

Bit of a rabbit hole I fell down last winter when my garden office panels were buried under three inches of snow for a fortnight — started wondering if I could just feed the Victron SmartSolar directly from my van's alternator to keep the LiFePO4 ticking over instead of rigging up a proper B2B charger.

Short answer after much Googling and one slightly singed wire: don't do it, at least not without understanding why it's a bad idea first.

The MPPT algorithm is hunting for a peak power point on a curve that solar panels produce naturally — feed it a flat DC source like a battery or alternator and it'll either do nothing useful, behave unpredictably, or in some cases just pull maximum current with zero regulation. The controller essentially expects a source that droops under load, which is exactly what PV does.

That said — there are dedicated DC-DC B2B chargers (Sterling and Victron both do decent ones) specifically designed for alternator-to-battery scenarios, and they're genuinely worth every penny rather than trying to bodge an MPPT into a job it wasn't designed for.

Anyone here actually tried running a secondary battery bank into an MPPT input experimentally? Curious whether the Renogy units behave any differently to Victron in this regard — some of the cheaper controllers seem to have very relaxed opinions about input source requirements based on what I've seen on threads elsewhere.

@RockyMender yeah been there with the snow situation, nightmare isn't it

Worth knowing the SmartSolar absolutely can handle panels in series — actually prefer it tbh. Higher voltage, thinner cable runs, less resistive loss. My garden office runs two 200W panels in series into a 100/30.

Few things to check though:

• Voc (open circuit voltage) combined must stay under the controller's max input — 100V on mine
• Factor in cold temperature Voc because panels spike voltage in the cold, could push you over the limit
• Make sure your battery bank voltage makes sense for the MPPT range

The Victron app calculates all this if you plug in your panel specs, really handy. Don't skip the cold correction — that's where people get caught out in UK winters especially.

Slightly different angle worth considering here — have you looked at the adaptive absorption behaviour on the SmartSolar when it's only seen partial charge cycles? After a snow period like that, the battery history algorithm can get a bit confused and extend absorption longer than needed once panels are clear again.

Worth connecting via VictronConnect and checking the battery charge state history after the snow clears. If your batteries have been sitting partially discharged for a fortnight, you may want to manually trigger an equalisation cycle (assuming flooded lead-acid) or at minimum a full absorption run before going back to normal float.

What battery chemistry are you running? That changes the recovery approach quite a bit. My static caravan setup with Fogstar lithium handles the gap periods differently to the AGMs I had previously.

Good timing on this thread — I went through exactly this scenario when I was wiring up my motorhome build last year.

The key thing nobody's mentioned yet: if you're running multiple panels with different orientations or shading profiles, separate MPPT inputs genuinely earn their keep. My setup has roof panels facing slightly different angles, and combining them into one MPPT was leaving real watts on the table.

For a garden office though, @RockyMender, if your panels are matched strings, same orientation, a single SmartSolar input is perfectly fine. Just watch your Voc at minimum winter temperature — UK winters can push open-circuit voltage higher than you'd expect, and you absolutely must stay within the controller's input limits.

Fogstar do decent panel specs that make this calculation straightforward. Run the numbers before committing any wiring.

Really useful thread this — @RockyMender your snow situation is exactly why I got interested in panel configuration flexibility too.

One thing nobody's mentioned yet: if you do end up wiring panels in series to boost your Voc, just double-check your combined open-circuit voltage against the SmartSolar's maximum input. Even on a cold morning (which obviously you'll be seeing plenty of!) panel Voc climbs noticeably — manufacturers quote it at 25°C but on a crisp January day you can easily add another 10-15% on top. Last thing you want is to fry the controller's input stage.

The formula is straightforward enough — check the temperature coefficient on your panel datasheet and work backwards from your lowest expected temperature. Victron's MPPT calculator tool online is actually quite decent for sanity-checking this before you commit to any rewiring.