Anyone else running Victron MPPT in parallel? Wiring questions and lessons learnt

I've got two Victron SmartSolar 100/30 controllers running in parallel on my narrowboat setup — one feeding a 200W panel on the roof and the other handling a 175W panel mounted on the stern. Both charging the same 200Ah LiFePO4 bank. Been working reasonably well over the summer but I'm getting some odd behaviour now the days are shorter and the sun angle is lower.

The problem I'm seeing is that the two controllers seem to be fighting each other a bit in the absorption phase. One will hit absorption voltage (around 14.2V on lithium profile) and throttle back, while the other is still pushing hard in bulk. End up with some weird voltage spikes and the BMS on my battery occasionally throwing a high voltage warning — nothing catastrophic, but it's not right. Both controllers are networked via VE.Direct to a Cerbo GX so they should be talking to each other through VE.Smart Networking.

Has anyone actually confirmed whether VE.Smart Networking properly synchronises the charge phase between two separate MPPTs, or is it just sharing voltage and temperature sense data? I've read the Victron docs three times and it's still not entirely clear to me whether they'll genuinely coordinate absorption timing or just use the shared voltage reading to make individual decisions.

Also curious whether anyone's had better results keeping the two controllers on completely separate battery banks and just using a DC-DC charger to balance them — feels like overkill for my setup but wondering if it's cleaner in practice.

Doing something similar in my shepherd's hut build — two Victron SmartSolar 75/15s running independently into the same 100Ah Fogstar lithium.

Couple of things I learnt the hard way:

• VE.Smart Networking between the two controllers is absolutely worth setting up — they share voltage/temperature data and charge in sync rather than fighting each other
• Make sure your battery cable lengths are matched between each controller and the battery, otherwise one will see slightly different voltage and behave differently
• Both controllers need identical absorption/float settings (or let VE.Smart handle it)

@YorkshireExplorer are you using the Victron app to monitor both simultaneously? You can see them on the same dashboard which makes spotting any disagreement between them much easier.

What battery type are you running? That changes how fussy the parallel setup needs to be — lithium is less forgiving than AGM if they drift out of sync.

Great thread, @YorkshireExplorer — narrowboat setups always throw up interesting challenges with awkward panel positioning.

One thing worth mentioning that catches people out: make sure your battery positive connections from both MPPTs are fused independently at the battery end, not just at the controller. I learnt this the hard way when I had a short develop in the cabling between controller and battery — the upstream fusing wasn't doing what I assumed.

Also, are you using VE.Direct cables to pull both controllers into the Victron Connect app? You can monitor them side by side which is really handy for spotting if one panel is underperforming due to shading or a dodgy connection. On a narrowboat with trees and bridges, you'll likely see quite different harvest between your roof and stern panels depending on orientation.

@MuddyRanger — what battery bank size are you running with those 75/15s?

Good thread. Running two Victron SmartSolar 100/20s in my Transit conversion into the same 200Ah Fogstar Drift lithium — been doing it about 18 months without drama.

One thing worth flagging that catches people out: make sure both MPPTs are networked via VE.Direct or VE.Smart Networking (Bluetooth). Without it, each controller is making independent charge decisions and you can end up with one sitting in absorption whilst the other is still bulk charging. They're not fighting exactly, but they're not coordinating either.

With VE.Smart Networking enabled, they sync their charge states properly and share voltage/temperature sense data across both units. Makes a noticeable difference to charge efficiency, particularly in the absorption-to-float transition.

@MuddyRanger — same applies to your 75/15s, both support it natively. Just create a network in the Victron Connect app and add both devices.

Great thread! I've got a similar setup on my static caravan — two SmartSolar 100/30s feeding into the same 200Ah Battle Born bank. One thing worth mentioning that I haven't seen covered yet: make sure you've got your battery voltage sense wired correctly on at least one controller. I had voltage offset issues early on where both controllers were slightly disagreeing on battery state, which caused some odd charging behaviour. Connecting the VS+ and VS- directly to the battery terminals on the "master" unit sorted it immediately. Also worth enabling the shared voltage sense via VE.Smart Networking in the Victron Connect app — once both controllers are talking to each other properly, the coordination is noticeably better. @Tommo the Fogstar Drift should respond really well once they're properly synchronised. Happy to share my VE.Smart config if useful.

Really useful thread, this one. I ran a similar parallel setup in my static caravan before I moved to full off-grid living, and the one thing nobody warned me about was keeping the cable runs to the battery as equal in length as possible between both controllers. Unequal resistance caused one controller to do the bulk of the work while the other barely contributed. Took me an embarrassingly long time to diagnose!

Also worth mentioning for @Tommo and @BreezyCaptain — make sure your battery's BMS communication is properly configured if you're using VE.Smart Networking. With lithium banks especially, having both controllers receiving accurate state-of-charge data made a noticeable difference to how evenly mine charged. The Victron Community forum has a good guide on setting absorption voltage offsets when running multiple units. Hope that helps someone avoid the head-scratching I went through!