Victron IP65 12/15 Charger & Victron 100/50 Solar

Had a similar head-scratcher with my motorhome setup last year when I added a Victron IP65 12/15 to work alongside my existing MPPT 100/50.

The big question is always where to connect the mains charger — before or after the MPPT output, or straight to the battery terminals? I went direct to the battery bus bar in the end, which keeps things clean and lets each Victron device do its own thing independently.

The key thing with lithium (I've got two 200Ah Fogstar cells) is making sure both chargers can see the BMS cutoff signal properly. If your BMS trips on high voltage and only one charger gets the memo, you've got a problem. Victron's VE.Direct network helps massively here — you can set the MPPT to "external control" and let the BMS call the shots.

Few things worth checking:

• Are both chargers set to the same charge profile? Sounds obvious but easy to miss
• Is your wiring rated for the combined output? 15A mains + whatever the solar's pushing can add up quick
• What BMS are you running? Some play nicer with Victron kit than others

Honestly the Victron ecosystem makes this less painful than most — if you're on VictronConnect you can at least monitor everything from one app rather than guessing what's happening.

Anyone else running a combined MPPT + mains setup on a static or motorhome with lithium? Curious whether people bother with a proper battery protect relay between the two sources or just trust the BMS to sort it out. Feels like there should be a neater solution than what I bodged together. 😄

@KevClark the connection point matters less than people think — what actually causes issues is having the IP65 and MPPT fighting each other because their absorption voltages are slightly mismatched.

If your battery has a proper BMS with charge termination, both chargers will just back off naturally. If you're on a dumb AGM or gel, I'd strongly recommend setting identical absorption and float voltages across both units via VictronConnect.

On my narrowboat I run an IP65 12/15 alongside a 75/15 MPPT — both connected directly to the busbars, not daisy-chained. Never had a conflict in 18 months.

The only genuine gotcha: if one charger sees the bus voltage rising from the other and interprets it as a full battery, you can get premature float. Check your bulk termination current settings carefully.

@KevClark this is something I've been working through on my static caravan setup too. One thing worth flagging that I don't think has been mentioned yet — if you're running a lithium bank with a BMS that has a hard disconnect, both the IP65 and the MPPT must be on the same side of that disconnect. Had a scenario where the BMS tripped under load and the IP65 was effectively charging into an open circuit, which isn't ideal.

Also worth checking your MPPT 100/50 absorption/float voltages match the IP65 profile exactly — even a 0.1V discrepancy can cause them to "fight" each other and neither reaches its target properly. What battery chemistry are you running? That would help narrow down which charger should effectively be taking the lead role.

@KevClark this is almost identical to what I'm dealing with on my narrowboat — Victron IP65 12/15 arriving next week to sit alongside my existing MPPT 100/50.

My specific worry is whether the two will "fight" over charge states when the battery is sitting at around 80-90% SOC. Does the MPPT 100/50 effectively back off when the IP65 is pushing mains charge in, or do you end up with both hammering the battery simultaneously?

Running Fogstar Drift lithium so the BMS should handle overcurrent, but I'd rather not rely on that as my primary safeguard.

Did you end up using any kind of load balancing, or did it just sort itself out naturally through the Victron ecosystem?

Been through exactly this with my garden office build — ran a Victron IP65 12/15 alongside a 100/30 MPPT for about eight months before I finally got the settings dialled in properly.

The thing that sorted it for me was making sure both devices were seeing the same battery profile — sounds obvious but my IP65 was still on a generic AGM profile whilst the MPPT had been tuned properly for my Fogstar Drift lithium. They were essentially arguing with each other over absorption voltage.

@Emma1996 on a narrowboat I'd pay particular attention to your battery temperature compensation settings — bilge temperatures can swing wildly and both chargers need to respond consistently or you'll get one undercutting the other constantly.

If you've got VE.Direct on both, getting them talking through a Cerbo GX is honestly the cleanest solution long-term.