Maintaining charge in 2 12v batterys

Been looking into this myself for my garden office setup. Got two 12V lithiums running in parallel and was initially worried about keeping them balanced, especially since my solar input's a bit inconsistent with the UK weather.

The Victron balancer's solid kit, but I reckon the real issue is whether your charge controller's pushing equal voltage to both batteries. If they're genuinely in parallel, a decent MPPT should handle it, but older PWM controllers can sometimes favour one battery over the other depending on cable routing and resistance.

What I did was upgrade to a Victron SmartSolar — the monitoring side alone is worth it. You can actually see what each battery's pulling. Caught a dodgy connection on one of my terminals that was causing a 0.3V differential. Sorted once I cleaned it up properly.

The balancer works best as a safety net rather than a primary solution though. It'll top up the weaker one, but if you're running them hard (high discharge), they'll still drift unless your charging's bang on.

Few questions for you:

• Are they sealed, AGM, or lithium?
• What's the solar controller model?
• How far apart are the batteries physically?

Cable runs and resistance matter more than people think. Even 2-3 metres of thin cable can cause issues. I've seen folk recommend keeping them as close as possible, which isn't always practical with a caravan layout.

Would be curious what others are doing with dual battery setups. Seems like there's quite a few of us running this config.

Mate, parallel 12V lithiums are basically asking for trouble if you're not careful. The inconsistent solar input is your real problem here — you need a proper MPPT controller that actually knows what it's doing, not some dodgy PWM unit.

Have you got battery management systems (BMS) on both cells? If they're communicating with each other that's

The parallel setup isn't inherently problematic if you've got decent kit, but @EcoFlowMaster's got a point about inconsistency being the real headache. I ran two 12V lithiums on my caravan for years—worked fine, but I needed a proper MPPT controller (Victron SmartSolar in my case) that could handle the varying input without causing charge imbalances.

Key thing is cell-level management. If your batteries are BMS-equipped, they'll naturally balance themselves, but only if they're seeing consistent voltage. With dodgy solar input, you're asking the BMS to do overtime.

For a garden office with presumably more stable conditions than my setup, I'd suggest:

• Decent charge controller (not a cheap PWM)
• Matching battery specs exactly
• Monitor actual voltages with a shunt

What's your current controller situation? That's usually where people trip up.

I've got a similar setup on my narrowboat actually, and the key thing I found was getting a proper multi-channel charger rather than relying on a single controller feeding both batteries. Are you using individual MPPT controllers for each battery, or trying to run them off one?

The real issue isn't the parallel connection itself—it's voltage droop when one battery's slightly lower than the other. They'll try to balance through the connecting cables, which creates heat and uneven charging cycles.

What's your current controller setup? If it's a single Victron or Fogstar feeding both in parallel, that's where your inconsistent input becomes problematic. Consider either:

• Separate controllers per battery (bit pricey but bulletproof)
• A quality combiner box to manage input more intelligently

What's your battery capacity and typical daily draw? That might affect whether this matters practically for your setup.

Parallel 12V lithiums need a decent controller to keep them happy, tbh. I'd lean toward a multi-channel charger like what @LindaClark90's mentioned — Victron's MPPT controllers handle this pretty well with their balancing firmware.

Inconsistent solar input is the real killer here. You'll get voltage drift between cells if one's charging faster than the other. A Fogstar or similar with proper load balancing helps, but honestly? If your solar's genuinely patchy, you might want to consider a small DC-DC converter between them as an interim measure.

What capacity are we talking, and how variable's your input? That'd help narrow down whether you need something serious or if a decent 60A controller sorts it. BMS should be communicating too — can's both talk CAN protocol?

Parallel lithiums are fine if you're not a muppet about it — matching specs and decent cabling is half the battle. The real trick is your charger: a Victron SmartCharger or similar multi-channel kit keeps them from drifting apart when solar's doing the on-off dance that UK weather loves.

Inconsistent solar input's actually your mate here because it forces you to size properly. I run two 12V cells on my narrowboat and they'll happily sit for weeks if the charger's dialled in right. BMS comms help too if your cells support it.

What controller are you currently using? That's usually where people go wrong — cheap PWM units treat parallel strings like a toddler treats spaghetti.