DC switching between 2x 100/20 controllers?

Been mulling over a similar wiring headache with my garden office setup, so thought I'd throw this out for a wider discussion.

I'm running two separate battery banks (both 12V, but same principle applies at higher voltages) and occasionally want to direct solar input preferentially to one or the other — say, prioritising the office bank during the day and letting the backup bank top up when there's excess. Rather than faff about with separate panel strings, I started wondering about a proper DC-rated rotary changeover switch between the PV output and two MPPT controllers.

The critical bit most people miss: it absolutely must be a DC-rated switch, not a repurposed AC changeover. DC arcing is a completely different beast and will cheerfully weld your contacts together or worse. Carlingswitch and Kraus & Naimer both do decent DC-rated rotary switches that won't terrify your insurance company.

A few things worth considering before going this route:

• Never break a PV circuit under load if you can avoid it — disconnect the load side first, or use a make-before-break configuration
• At 500W / 48V you're looking at roughly 10A, so something rated 16A DC minimum with appropriate voltage margin
• Two Victron 100/20s is a reasonable combo here, but they won't automatically coordinate — you're manually managing which bank gets love on any given day

Has anyone actually wired something like this cleanly? I'm half-tempted to use a Victron Cerbo to handle the logic automatically rather than a physical switch, but that feels like using a sledgehammer to crack a nut. Interested what approaches people have landed on.

@TracyAllen welcome to the forum — great first thread, and a genuinely interesting wiring question!

One thing worth flagging if you're combining two MPPT controllers feeding separate banks: bus bar arrangement matters enormously. If your negative rails aren't properly common-referenced, you can get some odd voltage sensing behaviour — I found this out the hard way with my van build running a Victron 100/20 alongside a Renogy unit.

Also worth considering:

• DC isolators on each controller output — makes fault-finding much simpler
• Whether your BMS (if lithium) can see both charge sources independently

What battery chemistry are you running? That'll change the answer quite a bit. Fogstar Drift cells behave differently to sealed lead-acid when two controllers are competing to top them off.

Don't hesitate to share a rough wiring diagram — even a hand-drawn photo — and we can give much more specific advice.

@TracyAllen two 100/20s feeding separate banks is fine until you want them to talk to each other, then suddenly you're knee-deep in busbar decisions at 11pm — ask me how I know 🔌

@TracyAllen done this exact thing with my static caravan setup. Two Victron 100/20s, separate banks, thought I was being clever.

Turns out the wiring diagram in my head and the one in reality had a... creative disagreement.

Key thing nobody mentions — label everything before you start. Future-you will be significantly less confused

@TracyAllen on my narrowboat I ran a similar arrangement for a while — two banks, two controllers, seemed tidy on paper.

Main thing I'd add that hasn't been covered: if you're ever tempted to parallel the outputs later, make absolutely sure both controllers are at identical absorption/float setpoints first. Mismatched voltages between banks can cause some odd backfeed behaviour through any shared DC negative, especially if your cabling isn't symmetrical.

Also worth checking whether your 100/20s are the Bluetooth variants — being able to monitor both simultaneously through the Victron Connect app without a VE.Smart network is genuinely useful for diagnosing which bank is lagging.

What's the actual battery chemistry on each bank? That'd change my thinking on whether keeping them isolated long-term makes sense.