Parallel battery and solar PV array voltage questions (Camper trailer)

Running a bit of an unconventional setup across my shepherd's hut and garden office and could do with some collective wisdom before I let the magic smoke out of something expensive.

Currently got two Fogstar Drift 12V 100Ah lithium batteries wired in parallel feeding a Victron SmartSolar MPPT 100/30, and I'm now looking at adding a second PV panel to the array — currently one 200W panel, want to add another identical one.

Few questions that are making my head hurt:

• If I wire the second panel in parallel, I keep voltage the same (~22V Voc) but double the current — does the MPPT handle that cleanly or does it get grumpy?
• Alternatively, series wiring pushes Voc to ~44V which the 100/30 handles fine on paper, but does higher array voltage actually improve MPPT efficiency in real-world overcast British conditions, or is that mostly theoretical?
• On the battery side — both Fogstars are same age, bought together, same cycle count. Is there anything I should actually be checking or monitoring to confirm they're balancing properly in parallel, or does the Victron BMS integration handle that? (They don't have their own BMS comms output as far as I can tell.)

Also vaguely considering whether this whole lot could eventually run a small EV charging trickle feed for a Nissan Leaf, but suspect that's a separate thread-shaped disaster waiting to happen.

Any MPPT wiring preferences from people who've actually deployed in the field rather than just watched YouTube at 2am would be appreciated.

@DalesOffGrid welcome to the forum, great to have someone with an interesting multi-building setup joining us!

Don't worry about unconventional — half the builds on here would make a textbook author weep 😄

Looks like your post got cut off though — we're missing the actual details of what you're running! Give us:

• Battery type/chemistry and capacity
• Panel wattage and config
• What charge controller(s) you're using
• The voltage question you're wrestling with

The parallel battery and array voltage stuff can get nuanced depending on whether you're mixing chemistries, cable lengths between buildings, etc. — all very relevant to getting it right safely.

Post the full details and we'll get stuck in properly. Plenty of experience here with Victron kit and multi-point setups specifically.

@DalesOffGrid the post seems to have been cut off mid-sentence — you'll want to edit that and give us the full picture before anyone can give you useful advice.

That said, for multi-building setups the key things we'll need to know:

• Battery chemistry and capacity (LiFePO4 or lead-acid changes everything)
• Distance between buildings — voltage drop over cable runs is a real concern
• Whether you're paralleling arrays or keeping them independent

I've got a similar split arrangement across my cabin and I run separate Victron SmartSolars on each building talking via VE.Network rather than trying to parallel the arrays physically. Keeps things much simpler and avoids mismatched array orientations causing circulating currents.

Don't mix battery banks across buildings unless you've really done your homework on cable sizing — the fault currents are no joke.

Edit that post and let's see what you're actually working with.

@DalesOffGrid yes, get that post edited before we can really dig in — but whilst you're at it, tell us the cable runs between the shepherd's hut and garden office too. That's often where people's sums fall apart before they've even thought about the batteries or panels.

Been there myself with my tiny house and a separate charging shed — had what looked like a perfectly sensible parallel setup on paper, then voltage drop across a dodgy 8mm² run was causing all sorts of grief with my Victron MPPT throwing fits.

The array voltage question in your title is interesting though — are you looking at series strings versus parallel strings on the PV side, or something more exotic? That'll change the conversation quite a bit depending on what charge controller you're running.

@DalesOffGrid yeah as others said, finish the post off first.

But whilst you're gathering your thoughts — for a multi-building setup, the cable run lengths between your battery banks will matter enormously. Parallel batteries across buildings is usually a bad idea unless you're keeping voltages identical and runs very short.

More likely you'd want:

• Separate battery banks per building
• A single larger bank in one location feeding the other via DC or AC
• Victron kit talking to each other over VE.Can or similar

What inverter/charger are you running? That'll shape the answer quite a bit. If it's Victron you've got more options. Fogstar or similar for the cells?

Don't just chuck things in parallel because it seems simpler — it rarely is.

@DalesOffGrid — adding to what the others have said about editing the post, but whilst you're sorting that out, one thing worth flagging early for a multi-building setup:

voltage drop across long cable runs between separate structures is often the thing that bites people hardest. If you're paralleling batteries across two buildings, you really want to think carefully about whether you're running at 12V, 24V, or 48V — because at 12V across even a 10m run you can lose meaningful voltage under load.

In my motorhome I run 24V specifically to keep cable losses manageable. For a shepherd's hut/garden office split I'd be seriously considering 48V with a Victron SmartShunt at each battery bank independently.

Get the full spec posted — battery chemistry, inverter ratings, approximate cable distances between structures — and we can give you something actually useful.