Anyone else running MPPT and PWM controllers in parallel on the same battery bank?

I've been experimenting with a bit of an odd setup on my narrowboat over the past few weeks. I've got a Victron SmartSolar 100/30 MPPT handling two 200W panels on the roof, and I recently bolted on a cheap 20A PWM unit connected to an older 80W panel I had sitting in the shed. Both controllers are charging the same 200Ah AGM bank. The system seems to be working fine so far — voltages look stable and nothing has exploded — but I'm not entirely sure what's happening under the bonnet.

My main concern is whether the two controllers are fighting each other in any meaningful way. The Victron does its absorption and float transitions based on its own voltage sensing, but obviously the PWM unit has no idea what the Victron is doing. I've set both to the same absorption (14.4V) and float (13.8V) voltages, which I thought would at least keep things broadly aligned. But I've noticed the PWM occasionally seems to drag the voltage down slightly when the Victron is trying to push into absorption — or maybe I'm imagining it.

Has anyone actually done this properly, or know if there's a smarter way to wire it so the PWM defers to the MPPT? Or is it just not worth the bother and I should run a second dedicated wire from that 80W panel into the Victron instead? The 100/30 probably has enough headroom to handle it.

@WillStevens curious timing on this thread — I've been running almost exactly this configuration on my static caravan for the better part of two years.

The key thing most people miss is charge stage synchronisation. Your MPPT will hit absorption voltage and throttle back, but if your PWM is still pushing hard it'll essentially override that — the battery never properly transitions between stages.

What I did was set the Victron SmartSolar absorption voltage slightly lower than the PWM's fixed setpoint, so the MPPT gracefully steps aside and lets the PWM handle finishing. Not textbook, but it works.

Worth monitoring your battery temps closely during the first few cycles. I use a Victron BMV-712 to catch any anomalies.

What battery chemistry are you running? That changes the tolerance for this kind of parallel oddity considerably — AGM is far more forgiving than lithium here.

Really interested in this thread as I ran a similar hybrid setup on my shepherd's hut for about eight months. The main thing I'd flag is making sure your PWM controller's float voltage is set slightly lower than the MPPT's — otherwise they end up fighting each other and your MPPT keeps getting knocked out of absorption prematurely. Cost me a fair bit of efficiency before I worked it out.

Also worth checking whether your PWM controller has any load output terminals you're using, as some of them get confused about battery state when another controller is simultaneously pushing current in.

@EdHamilton — would be curious whether you've noticed any temperature-related issues with yours? Mine got a bit odd during last summer's warmer spells with both controllers active simultaneously.

Had this exact combo running in my shepherd's hut last summer — MPPT doing the heavy lifting, older PWM bolted on as a secondary. Main thing I'd watch is the PWM dragging panel voltage down to battery voltage, which can mess with what your MPPT "sees" if they're sharing the same array. Keep them on separate panel strings and it's much cleaner.

Also worth checking your Victron's absorption/float settings match whatever the PWM is set to — otherwise they're fighting each other over charge termination. Had some weird state-of-charge readings on my BMV-712 until I sorted that.

Not a perfect setup but it works. Running Fogstar lithium here so the tighter voltage tolerances made it more obvious when something was off.

Really interesting thread — been running a similar hybrid arrangement on my van conversion for about two years now, so happy to share what I've observed.

The key thing nobody's mentioned yet: charge controller priority. If your PWM is terminating at bulk/absorption before the MPPT finishes its cycle, you can end up with both controllers fighting over the absorption voltage setpoint. I saw some genuinely weird battery behaviour on my LiFePO4 Fogstar cells until I staggered the absorption voltages slightly — MPPT set 0.2V higher so it always "leads" the charge profile.

Also worth checking your cabling — both controllers feeding the same busbar should have appropriately rated fusing on each run independently, not just at the battery terminal. Easy to overlook when retrofitting a second controller.

@WillStevens what battery chemistry are you running on the narrowboat? That changes the answer considerably.

Great thread this, been running something similar on my static caravan setup for about 18 months now.

One thing nobody's mentioned yet — watch your absorption voltage settings carefully. If your MPPT and PWM aren't programmed to identical charge profiles, you can end up with them essentially fighting each other. The MPPT might hit absorption and throttle back while the PWM is still hammering away at a slightly different voltage target. Not catastrophic, but it's inefficient and can confuse both controllers.

@WelshCamper curious whether you've noticed any difference in battery health over your two years running it? That's my main lingering concern with the setup longer term.

I'd also suggest anyone trying this keeps the PWM on a separate, physically closer panel string to minimise cable losses — PWM suffers far more from resistance than MPPT does. Small thing but it adds up.

Running this exact combo on my garden office — MPPT doing the proper job, old PWM controller basically just showing up for moral support. 🤣

One thing worth flagging: if your PWM is set to a slightly different absorption voltage than your MPPT, they'll essentially be fighting each other like two people trying to parallel park the same car. Check your voltage set