How to monitor the permanent and the removable battery on caravan 12V solar system

Running a similar dual-battery setup in my motorhome — permanent lithium under the bed, removable AGM in the locker for longer hauls — and the monitoring situation is genuinely a head-scratcher when they're chemistries apart.

The Victron SmartSolar sees them as one lump on the bus, which is fine until you want to know which battery is actually struggling. My workaround has been a Victron BMV-712 on the permanent bank and a cheapo Renogy inline meter on the removable one — bodged, but it tells the tale.

Where it gets interesting is when you disconnect the removable battery mid-trip. The BMV recalibrates its "full system" assumptions and starts throwing numbers that are technically wrong until you do a sync. Bit annoying on a proper excursion.

Has anyone gone the Cerbo GX route to track two separate banks independently? Feels like overkill for a caravan but I'm tempted. Or is there a cleaner way to wire two BMV-712s into the same SmartSolar setup without them arguing with each other?

Also curious whether anyone's running Fogstar Drift lithium as their removable bank — wondering if the self-discharge difference between it and a permanent AGM causes the MPPT to prioritise one over the other in practice, or if it just doesn't care.

Would love to hear how others have tackled this — especially if you've got a tidy solution that doesn't involve seventeen apps on your phone.

@Smithy on the boat I run a Victron BMV-712 on the main bank and honestly it's the best thing I did. Smart shunt on each battery, then the BMV app shows both via Bluetooth — no faff.

The removable one's trickier mind. I just put a cheap Daly BMS with its own Bluetooth on that battery so I can pull it out and still see state of charge independently on my phone.

Two separate apps isn't ideal but it works a treat. If you want everything in one place, Victron's Cerbo GX ties it all together but that's proper money.

@LDVWanderer already nailed the smart shunt approach, so I'll add — the key thing people miss is which battery you set as the "reference" for SOC.

On my static caravan I made the mistake of shunting the wrong side of the split and the readings were garbage for months. Took me ages to figure out.

For a removable battery specifically, worth looking at whether you actually need continuous monitoring on it, or just a standalone voltage display on the locker. Something like a cheap Victron Battery Sense or even a basic panel voltmeter might be sufficient for the AGM side — AGMs are reasonably predictable from voltage alone, unlike lithium.

Save the proper shunt/BMV setup for the permanent lithium. That's where accurate SOC actually matters.

Really useful thread this. To add to what @LDVWanderer and @ShedGenius have touched on — don't overlook the Victron GX portal side of things if you go the smart shunt route. Once both shunts are talking to a Cerbo GX or even just the VRM app via a cheap Raspberry Pi running Venus OS, you get a single dashboard showing both batteries simultaneously. Massive quality of life improvement over squinting at two separate monitors.

For your removable AGM specifically @Smithy, worth checking the shunt stays with the battery rather than the locker wiring — otherwise your SOC readings go daft every time you swap it out and you're back to square one recalibrating. Learned that one the hard way on my last trip to Pembrokeshire! 😄

Good shout from @Taffy55 on the GX, but if that's overkill for a straightforward dual-bank setup, worth knowing the BMV-712 actually handles two batteries natively — the main shunt monitors your primary bank, and there's a second voltage input for the auxiliary. No extra kit needed.

The catch is it's voltage-only on that aux channel, so you won't get accurate SOC on your removable AGM — just a voltage reading. Fine for a quick sanity check but don't rely on it for accurate capacity monitoring.

For my lithium bank I've got a Victron SmartShunt doing the heavy lifting. The AGM I treat as secondary and just watch voltage. Honestly that's usually enough — AGMs are more forgiving to read from voltage alone than lithium ever would be.