Anyone else running Victron MPPT in parallel on a boat — how are you handling the wiring?

I've got two Victron SmartSolar 100/30 controllers on my narrowboat, both feeding into the same 12V lithium bank (three 100Ah Fogstar Drift cells in parallel). Each controller has its own panel string — 2× 175W Renogy monocrystalline on the cabin roof, and a foldable 200W on the stern when I'm moored up. Works brilliantly most of the time, but I've been second-guessing my busbar setup lately.

Currently both MPPTs share a positive busbar before the battery, with individual 30A ANL fuses on each controller's output. My concern is whether the cable runs being slightly different lengths (about 0.3m difference) is causing any measurable imbalance. I know with lithium the voltage is fairly flat anyway, but I've noticed on the Victron Connect app that one controller consistently hits absorption a few minutes before the other, and I'm wondering if that's just normal behaviour or a sign of something worth investigating.

Has anyone used the VE.Smart Networking feature to sync two SmartSolars and seen a noticeable improvement in charge efficiency? I've enabled it but honestly I'm not 100% sure both units are properly paired — the app seems a bit vague on confirming the sync is actually active. Would be good to hear from anyone who's got this dialled in properly.

DalesSolar | 847 posts

@ForestBoater good setup! One thing worth flagging with parallel Victrons into lithium — make sure you've got them networked via VE.Smart Networking (Bluetooth mesh) so they're sharing voltage sense and temperature data. Without that, they're essentially making independent charging decisions and you can get one controller "leading" whilst the other lags behind, which isn't ideal for properly balancing your absorption phase.

Also, are you running individual fuses/breakers per controller back to the busbar? Definitely worth it for fault isolation. I'd keep cable runs as equal in length as possible too, otherwise you'll see slightly different voltage readings at each unit.

The Fogstar Drifts are solid cells — are you feeding the battery temperature sensor data into the Victrons? Makes a noticeable difference over winter on the cut.

Ran almost the same rig in my van before I upgraded — the thing that actually bit me wasn't the controllers arguing, it was the busbar rating not keeping up when both MPPTs decided to bulk simultaneously at full chat... 30A + 30A into undersized copper = warm surprises 🔥

Victron's VE.Smart Networking sorts the communication side beautifully (both units sync their charge states over Bluetooth), but double-check your busbar and fuse ratings are sized for the combined worst-case current, not just one controller's output — rookie mistake I made so you don't have to.

GemmaFisher82 | 312 posts

@ForestBoater I've got a similar setup on my widebeam — two 100/30s into a Fogstar bank, so very relevant! One thing I'd add that hasn't been mentioned yet: make sure your cable runs from each controller to the busbar are as equal in length as possible. Unequal resistance can cause one controller to shoulder more of the load than the other, which skews your charge data and can cause odd behaviour with the Victron networking. I learnt this the awkward way after wondering why one unit always seemed to be working harder than the other. Also worth double-checking your busbar rating — people often underspec these on boats and forget the cumulative current from two controllers. Are you using a proper tinned copper busbar or just terminal blocks?

ChloeScott76 | 203 posts

@ForestBoater one thing nobody's mentioned yet — on a narrowboat specifically, keep an eye on your cable runs between each controller and the battery. With the damp environment you get on the cut, even a tiny difference in resistance between the two positive runs can cause one controller to "see" a slightly higher voltage than the other, which upsets their charge coordination. Try to keep both runs the same length and cable gauge, even if it means running a bit of extra cable on the shorter one. Sounds fussy but it genuinely matters with lithium. I'd also suggest labelling both controllers clearly in VictronConnect so when you're troubleshooting remotely you know which is which! 🙂

PartnerBuild | 847 posts

Worth adding something nobody's touched on — cable length matching between your two controllers and the battery.

I learned this the hard way wiring my shepherd's hut setup. Both MPPTs were Victron SmartSolar 100/20s feeding the same Fogstar bank, but one run was noticeably longer than the other. The voltage drop difference was enough that one controller consistently thought the bank was fuller than it was, so they were essentially working at crossed purposes half the day.

Matched my cable lengths (or compensated with slightly heavier gauge on the longer run), problem vanished.

On VictronConnect, if both controllers are in the same VE.Smart Network, they'll share battery voltage sensing directly — that genuinely sorts the communication issue. @GemmaFisher82's point about shared sensing is the key bit here. Without it you're really just hoping for the best.