Volt spikes 24v-39v

Been having a weird issue on the boat lately and can't quite nail down the cause.

Running a 24v system — Victron MultiPlus, 200Ah of Fogstar Drift lithium, and a mix of solar + alternator charging. Every now and then the BMS is logging voltage spikes up to 38-39v, which obviously shouldn't be happening on a 24v setup.

It seems to coincide with the alternator cutting in when the engine starts, but I'm not 100% sure. Could also be a grounding issue — narrowboats can be a nightmare for that with all the damp and metal hull.

Few questions:

• Could a dodgy B2B charger cause this, or would it be more likely the alternator regulator?
• Anyone had similar spikes logged in VictronConnect? Are they real or could it be a sensor glitch?
• Is there a way to confirm whether it's a genuine transient vs a measurement artefact?

The reason I'm asking here rather than just calling my installer is... well, I've been chasing him for weeks and getting nowhere. System's only been in since spring and it's already been playing up on and off. Getting a bit fed up tbh.

Don't want to just start swapping parts blindly. Hoping someone's seen this before and can point me in the right direction before I either damage something or lose my rag completely.

HollyGraham83 | 47 posts

@BreezyDrifter that voltage spike to 39v is seriously concerning for a 24v system — that's well beyond what your BMS should be tolerating. First thing I'd check is whether you've got a proper isolation switch between your alternator and the battery bank. Alternators absolutely hate being disconnected from a battery under load, and on boats it can happen more easily than you'd think — dodgy connections, corroded terminals, even a BMS cutting out under high charge current. If the BMS trips and the alternator is still spinning, that spike will go straight through your system. Do you have a Sterling alternator protection device fitted? Pretty much essential on lithium setups with alternator charging. What's your alternator running through — directly to the bank or via a DC-DC charger?

LutonAdventure | 134 posts

@BreezyDrifter quick question — when exactly do the spikes occur? Specifically, are they happening when the alternator kicks in and the solar is already pushing decent amps? Because on my static caravan setup I had something similar when two charge sources weren't properly coordinated and briefly fought each other.

Worth checking whether you've got a Victron Battery Protect or any isolator in the line that might be disconnecting under load — that'll cause a classic inductive spike. Also, is your alternator going through a proper DC-DC charger (like a Victron Orion-Tr Smart) or wired direct? Direct-wiring an alternator to lithium is basically asking for drama.

What does the Victron app show in the history logs around the spike timestamps? That'd narrow it down considerably.

DownsCruiser | 203 posts

@BreezyDrifter alternator's my first suspect. When you disconnect a lithium bank under load, the alternator can't dump its energy anywhere and spikes hard — seen it take out kit before. Lithium doesn't absorb like AGM does.

Had a similar headache in my motorhome before I fitted a Victron Orion DC-DC. Fixed it completely.

Check if the spikes coincide with the BMS tripping. If it's cutting out under charge and leaving the alternator spinning with no load, that's your culprit.

• Is there a battery-to-battery charger or alternator isolator in the setup?
• What's the BMS set to for overvoltage cutoff?

39v on a 24v system will fry things fast. Worth checking your Victron hasn't already flagged anything in VictronConnect history.

Robbo51 | 312 posts

@BreezyDrifter worth checking whether you've got a battery-to-battery charger (B2B) between your alternator and the lithium bank, or if the alternator's connected directly. Lithiums accept charge so aggressively that a standard alternator can cook itself, and without proper isolation you can absolutely see those kinds of spikes when the BMS cuts out under load.

Also — has your Victron thrown any alarm codes? The MultiPlus should log high voltage events which might help pinpoint the exact trigger. Check VictronConnect if you haven't already.

A 39v spike on a 24v system will damage electronics fast, so I'd be cautious running things until you've traced it properly. What alternator are you running and is there anything between it and the batteries?