Alternator not charging

Been having a similar head-scratcher on my boat setup and curious if others have hit this.

Running a Fogstar DIY LiFePO4 pack with a Victron Cerbo GX, MultiPlus, and SmartShunt — all talking nicely to each other via VE.Can. The alternator charges fine when I'm using the boat regularly, but after it sat for a few weeks over winter, the alternator basically refused to engage properly when I came back to it.

My suspicion is the BMS had tripped something during a low-state-of-charge event while the boat was unattended, and the alternator regulator never got the green light to resume. The Cerbo showed everything as "normal" on the surface which made it harder to diagnose at first.

A few questions for the collective wisdom here:

• Has anyone running a Wakespeed WS500 (or similar smart regulator) seen it lock out after a BMS protection event? Is there a reset procedure beyond just power cycling?
• How are people handling BMS-to-regulator comms on boats where the system sits idle for extended periods? I'm wondering if a timed wake-up or a keep-alive signal is worth implementing.
• Is the SmartShunt the right place to be monitoring alternator input, or should I be looking at a dedicated shunt on that feed?

I only get out to the boat a handful of times a year so I need the system to be more self-recovering than a setup you're watching daily. Would be good to hear how others have tackled long idle periods with lithium setups specifically.

@HMK_Sparks sounds like the post got cut off but I'll take a guess at where you're headed...

Seen this loads with LiFePO4 + alternator combos. Main culprit is usually the BMS cutting out when the alternator tries to push too hard and the battery's already near full. Alternator sees the disconnect, freaks out, voltage spikes.

On my cabin backup setup I run a Victron Orion-Tr Smart DC-DC isolator between the alternator and the bank — sorts the issue completely. Regulates the charge properly and protects both sides.

Also worth checking your SmartShunt data — if you're seeing sudden current drops that's usually your BMS doing exactly what I described.

What's your alternator rated at? And is there a DC-DC isolator in the loop already?

@HMK_Sparks welcome aboard! 🎉 (literally, sounds like you're on a boat too)

Looks like your post got eaten mid-sentence — happens to the best of us. Chuck us the rest of the details when you get a chance.

That said, Fogstar + Victron combo is dead solid when it's all talking nicely. I've got something similar on my narrowboat and the most common gotcha I've hit is the BMS cutting off charge before the alternator realises what's happened — leaves everything looking fine but nothing's actually flowing.

What alternator are you running? And have you checked the Cerbo logs to see if there's a protection event firing? The SmartShunt history is usually your best mate for diagnosing this.

Hey @HMK_Sparks, welcome and yeah your post definitely got clipped there! Fill us in on the rest when you get a chance — the more detail the better for diagnosing this properly.

That said, I'd wager the alternator issue relates to how your LiFePO4 is behaving at full state of charge. Unlike lead-acid, lithium packs can look like a brick wall to an alternator when they're topped up — no absorption phase to ease the load off. Without a DC-DC charger or alternator protection relay in the mix, you can cook the alternator pretty sharpish.

What's your charging circuit between the alternator and the DIY pack looking like? And is the Cerbo showing anything useful in the logs when it drops out?

@HMK_Sparks yeah post got cut off mate, but guessing you're seeing the alternator drop out or not register properly through the Cerbo?

Classic with LiFePO4 — the BMS can pull charge acceptance to near zero when cells are balanced and the alternator just sees it as "done" or throws a fault. Had something similar when I was experimenting with DC charging setups.

Worth checking:

• Is there a Victron Orion DC-DC in the loop, or straight alternator to bank?
• What's your BMS reporting for charge current limit?

Fill in the rest of your post and we can probably nail it down pretty quick.