question on multiple battery banks of different capaciities - each with their own BMS - in a Victron system

Been down this rabbit hole myself with the static van setup, so thought I'd chip in.

The short answer is it can work, but the devil's in the detail. When I first expanded my bank, I had a mismatched pair of Fogstar Drift 100Ah and an older 200Ah unit, each with their own BMS. On paper, fine. In practice, the smaller bank's BMS kept tripping under heavy load because the Victron MPPT was essentially treating the whole lot as one big happy family — which it very much wasn't.

The core issue is that each BMS will protect its own cells independently, so if one trips, you get a sudden voltage shift and the Multiplus can throw a wobbler. Seen a few folk on here report the same.

A few things worth considering:

• Parallel only if voltages are closely matched before connecting — don't just slam them together
• Victron's DVCC settings become really important here; worth reading the Cerbo GX manual properly rather than guessing (as I did initially)
• If the BMSs support comms (Pylontech-style or similar), that changes the conversation entirely

My current setup runs smoother since I went matched-capacity, but I appreciate that's not always practical when you're expanding incrementally.

Has anyone actually run genuinely different capacities long-term without drama? Curious whether the BMS brand matters as much as the capacity mismatch itself.

@MoreTeaVicar the devil AND his entire extended family are in those details 😅

Right, one thing nobody mentions — when you've got banks of different capacities, your Victron MPPT/MultiPlus will just gleefully charge the whole lot as if it's one happy family, but the individual BMS units will have very different opinions about

@MoreTeaVicar been through exactly this with my setup. The bit that catches people out is the BMSs fighting each other at end-of-charge — one trips offline first, dumps the load onto the other bank, which then also trips. Cascade failure.

Victron handles it better than most IF you let the DVCC system take control and disable individual BMS charge control where possible. Run everything through a single Cerbo GX and use the BMS CANbus integration properly.

Practically though — mismatched capacities are less of a problem than mismatched chemistry or age. Throwing a new Fogstar cell next to a 3-year-old bank is asking for trouble regardless of how clever your BMS is.

What are the actual specs of your two banks? That'd help narrow down whether you're looking at a wiring solution or a fundamentally bad idea.

The sneaky gotcha nobody warns you about: parallel banks with different capacities will charge unevenly because the smaller bank hits its BMS cutoff voltage first, potentially leaving the larger bank starved — your Victron MPPT or MultiPlus won't know which BMS threw its toys out the pram.

Worth considering:

• Run each bank on a separate charge source if possible (Victron's load-sharing plays nicely here)
• Or use a battery balancer between banks to keep voltages honest
• Same chemistry is non-negotiable — mixing LFP with AGM is a recipe for a very expensive paperweight

Had a similar headache on my narrowboat before I separated the bow and stern banks onto dedicated Victron SmartSolar controllers — night and day difference. Fogstar Drift cells in both now, sleeping soundly.

@MarineGaz is right that the BMS conflict is the real villain here, not the capacity mismatch itself.

@RetiredChef raises the uneven charging point which is crucial, but there's another layer to this — different internal resistances between banks will compound that imbalance significantly.

In my van build I ran two Fogstar Drift 100Ah units alongside an older 200Ah AGM temporarily, and the lithium bank would hit absorption voltage whilst the AGM was still genuinely half-empty. The Victron SmartShunt helped me actually see this happening in real time rather than guessing.

The practical fix I landed on: treat each bank as a completely isolated system with its own MPPT or DC-DC charger, rather than paralleling them directly. More hardware cost upfront, but you're not relying on the BMSs to negotiate charging priority — because they won't, not reliably.

Victron's VE.Smart Networking helps coordinate charging sources but it won't solve fundamentally mismatched banks at the battery terminal level.