I have a 36V (3) 12V battery 36V trolling motor application, I have (3) IP65 12V chargers

Been thinking about this setup a fair bit lately — running three 12V batteries in series for a 36V trolling motor application is something I've seen crop up more on the canals and reservoirs here in the UK.

The Blue Smart IP65 chargers are a solid choice (got two of them myself for a different setup), but the multi-shunt arrangement is where it gets interesting. If you've got three Smart Shunts across three batteries in a series string, you need to be careful about what you're actually measuring vs what the Victron ecosystem is reporting back to you via the app.

My concern would be:

• Are the IP65 chargers charging each battery individually in parallel, then you reconnect in series for the motor?
• Or are you trying to charge in-series, which most standard 12V chargers really aren't built for?

The safest wiring I've seen recommended is to break the series string for charging — charge each 12V unit independently, then reconnect. Otherwise you risk imbalance across the pack, which kills longevity fast.

Also worth thinking about whether LiFePO4 (Fogstar Drift cells are popular for this sort of thing now) would simplify the whole arrangement — integrated BMS handles a lot of the balancing headache.

Anyone here actually running a similar 36V series setup on the water? Curious what wiring approach you've landed on and whether the Victron Connect app plays nicely with multiple shunts like this.

Reply by MarineCallum:

@MidlandsNomad good setup in principle, but one thing worth flagging — when you're charging three 12V batteries in series with individual IP65 chargers, you absolutely need to ensure your charger negatives are properly isolated from each other, otherwise you'll create ground loops and potentially damage the chargers or get very uneven charging.

The cleanest approach is to charge them individually whilst disconnected from the series string, or use chargers specifically designed to handle series-connected batteries with isolated outputs.

Also worth checking whether your IP65 chargers have the same charge profile — mismatched profiles across the three batteries will cause imbalance over time, which is a real headache with trolling motor applications where you want consistent runtime.

What chemistry are your batteries — flooded, AGM, or lithium? That'll affect how critical the balancing issue actually is in practice.

@MidlandsNomad the elephant in the room nobody's mentioned yet — cell balancing.

Three separate IP65 chargers in series sounds tidy but each one is fighting the others. If one battery drifts even slightly in internal resistance, you'll get uneven charge termination across the bank. Over time that compounds badly.

What you actually want is either:

• A proper 36V charger designed for series banks
• Or individual chargers wired to each battery independently (not series-connected chargers) — meaning your negative return paths need careful thought to avoid ground loops

Done this myself on a shepherd's hut build using a similar bodged series arrangement. Ended up with one battery consistently undercharged after six months. Killed it.

If budget allows, a Victron MultiPlus with proper battery bank configuration is the cleaner long-term solution. Costs more upfront, saves you replacing batteries prematurely.

@FormerMechanic14 nails it on balancing, but the practical fix nobody's mentioned is wiring your three chargers to charge each 12V battery individually (not across the series string) — so positive to that battery's positive, negative to that same battery's negative, which means you'll need proper isolated chargers (which IP65 units typically are, happy days).

Run them simultaneously and each battery gets its full CC/CV cycle independently — basically the poor man's Victron MultiPlus solution for a fraction of the cost.

Worth checking your charger outputs aren't accidentally sharing a common neutral though, or you'll create a ground loop that'll make your BMS weep — learned that the hard way in my van conversion before I knew better. 🚐⚡

@BoatPaddy so what's the actual wiring diagram for that then? You can't just say "wire them to charge each battery individually" and leave it there — that's obvious, the question is how without creating a ground loop nightmare when the batteries are in series.

Running a similar lash-up on my narrowboat for the electric outboard and this is exactly where it falls apart in practice. Three separate chargers, three separate AC feeds — fine. But the DC negative commons get messy fast when your batteries are series-connected.

Has anyone actually done this cleanly, or is the real answer just to bin the IP65 chargers and get a proper 36V charger with built-in balancing? Because that's where I'm leaning after burning a week on this.

@BurnWalker it's simpler than it sounds. Each charger's positive clips to the positive terminal of its battery, negative to that same battery's negative. The series chain stays intact — you haven't broken anything. Charger 1 sits across battery 1, charger 2 across battery 2, charger 3 across battery 3. Done.

I ran exactly this arrangement on a static van with three 12V AGMs before I moved to a proper 36V lithium setup. The chargers work in isolation against their individual battery voltage, so balancing sorts itself out naturally. Just make sure your charger negatives aren't somehow bridged to each other externally or you'll create a short across cells — that's the one gotcha people trip over.

Draw it out on paper before you touch anything. Seriously.