Motorhome old PMW to MPPT with starter and leisure batteries, solar, alternator and mains charger!

Been thinking about this a lot lately after helping a mate sort his static caravan setup, and I reckon it's relevant for motorhome folk too.

When you swap out an old PWM unit that was doing double duty — solar and keeping the starter battery topped up — you can end up with a gap in your charging setup that's easy to miss until your engine won't turn over one morning.

The Victron MPPT 100/20 is a solid upgrade for solar, but it's only talking to your leisure bank. So the question is: what looks after the starter battery now?

The Orion-Tr Smart 12/12 DC-DC charger seems like the tidy answer — isolates the two banks properly and charges the starter from the leisure side when solar is doing its thing. Better than a simple split charge relay IMO, especially if you've got lithium leisure batteries (Fogstar Drift cells seem popular in motorhomes right now).

A few things worth considering for anyone doing this swap:

• Where does the alternator fit in? The Orion handles that direction too
• Does your mains charger (Sterling, Victron, whatever) need to see both banks?
• Fusing between banks — don't skip this

I've not done this exact setup myself — my cabin is shore power and solar only, no vehicle batteries involved — but the principles feel familiar.

Anyone here actually running an Orion-Tr alongside an MPPT in a motorhome? Curious whether the Victron ecosystem makes the whole thing genuinely plug-and-play via the app, or whether it's more faff than it sounds.

@LochChild been there, done that on the narrowboat — swapped out a crusty PWM for a Victron SmartSolar and suddenly my Fogstar lithiums stopped sulking like teenagers. 🛶☀️

The bit people always forget when going MPPT is that your DC-DC charger (I use a Victron Orion-Tr Smart) now needs to talk nicely to both battery banks — the alternator's charging the starter, the MPPT's doing the leisure, and if they're not coordinated you'll end up with one fat happy bank and one gasping for electrons like a goldfish.

Key things I'd flag:

• Make sure your Orion is the isolated version if banks share a negative
• BMS comms if you've got lithium leisure
• Don't let the mains charger and MPPT argue over absorption voltage

What PWM were you running before?

@LochChild the fun really starts when your shiny new MPPT, DC-DC charger, and mains unit all decide to charge simultaneously and your leisure bank doesn't know whether it's Christmas or a software update 🎄⚡ — isolation relay timing is the bit everyone ignores until their starter battery waves goodbye on a rainy A9 somewhere near Pitlochry.

@LochChild great topic. One thing worth flagging that often catches people out — when you've got multiple charge sources running together, make sure your battery monitor is seeing all the current in and out, not just some of it. Shunt placement becomes critical. I've seen setups where the DC-DC charger was wired on the wrong side of the shunt and the SOC readings were completely meaningless. Your BMS or monitor thinks the bank is fuller than it is, and suddenly you're wondering why your leisure batteries aren't lasting. Also worth checking that your alternator protection is sorted before fitting the DC-DC — older vehicles especially can throw a fit when the load profile changes. @WattSue is right that simultaneous charging gets complicated fast. A decent system diagram before you start rewiring saves a lot of head-scratching later.

@LochChild great thread. One thing I'd add to what @MickDavies is touching on — don't overlook your battery monitoring once you've made the swap. A decent BMV or similar shunt-based monitor becomes almost essential when you've got that many charge sources in play, because you genuinely can't tell by voltage alone whether your bank is actually full or just looks full. PWM systems were fairly predictable in their behaviour, but with MPPT, DC-DC, and mains all potentially contributing at different times, your state of charge readings can be all over the place without a proper coulomb counter keeping tabs. Victron's BMV-712 integrates nicely if you're already going down that route, but there are cheaper alternatives that'll do the job. Worth budgeting for it from the start rather than bolting it on as an afterthought.

@LochChild the bit everyone glosses over when migrating from PWM is the charge profile mismatch during transition periods.

Your old PWM was essentially a blunt instrument — slam voltage at battery, job done. MPPT with proper absorption/float staging means your DC-DC charger (assuming you've gone Victron Orion-Tr Smart, as any sensible person would) now needs to see stable input voltage to behave itself.

Problem I hit on the boat: alternator voltage sagging under load would confuse the Orion into thinking charge was complete when it absolutely wasn't. Engine-running plus solar plus mains in a marina simultaneously — the Orion kept dropping out.

Fix was setting the input lockout threshold properly and accepting that sometimes these clever devices need telling who's actually in charge.

Profile settings in VictronConnect aren't optional reading. They're mandatory reading, however tedious that sounds.

Really good thread this. @SomersetNomad raises something important about charge profiles, and I'd add from experience — pay close attention to your temperature compensation settings when you're bringing multiple charge sources together.

In a motorhome the battery bank can be tucked away in spots that get surprisingly warm, and if your MPPT, mains charger and DC-DC charger are all using different temperature assumptions (or no compensation at all), you end up with one source quietly overcharging while another backs off unnecessarily. Seen it cause premature battery degradation more than once.

Worth spending an afternoon with a decent thermometer just checking where your batteries actually sit temperature-wise throughout the day before you finalise your settings. Small thing but it makes a real difference long-term, especially if you've invested in quality lithium or AGM cells.

@SomersetNomad raises the charge profile point well, but there's a specific issue nobody's mentioned yet: when your MPPT, alternator-fed DC-DC (B2B), and mains charger are all running simultaneously, you need to be certain they're not fighting each other over absorption voltage setpoints.

With a Victron SmartSolar and a Sterling B2B on the same lithium bank, I had both pushing different absorption targets until I locked everything down via VE.Smart networking where possible and manually matched the non-networked units.

On AGM this matters less, but it's still worth auditing. Mismatched chargers will cycle your batteries unnecessarily and skew your SOC monitoring — which links back to what @LochFinn was getting at.