Swapped my boat's starter battery for a LiFePO4 — anyone else done this and had grief with the alternator?

Finally took the plunge last month and replaced the tired AGM starter battery on my 28ft sailing boat with a 100Ah LiFePO4 (Fogstar Drift). Boat's a 1998 Beneteau with a Yanmar 2GM20 diesel and the original 55A alternator. All seemed fine at first — engine starts beautifully and the battery holds charge brilliantly at the marina.

Problem is I've since read a fair bit about lithium batteries potentially wrecking older alternators because of the low internal resistance. The battery just hammers the alternator at full output for ages rather than tapering off like an AGM would. My alternator has no external regulator — just the built-in one — so I've got no easy way to limit charge current.

Has anyone actually had an alternator fail because of this, or is it more theoretical scaremongering? I'm wondering whether to stick a Victron Battery Protect or a Sterling Power BB1230 DC-DC charger in the circuit to protect things, or whether I'm overthinking it for a 55A alternator on a modest-sized bank. Running day sails mostly, rarely more than 2-3 hours motoring at a stretch.

@HazelPaddy my tiny house uses a Fogstar Drift too and the alternator drama is real — LiFePO4 accepts charge so fast it basically asks your alternator "is that all you've got?" until something melts to answer the question.

Grab a DC-DC charger (Victron Orion-Tr Smart is the go-to) between alternator and battery — it acts as a buffer so your Yanmar's charging system doesn't cook itself trying to satisfy a battery with no internal resistance worth mentioning.

Your 1998 alternator was designed for a lazy AGM, not this nonsense. 🔥

@HazelPaddy — yes, this is a well-known headache and you've stumbled into one of the genuinely problematic LiFePO4 applications.

The core issue: LiFePO4 has a very flat discharge curve, so your alternator regulator never sees the voltage drop it expects. It thinks the battery is perpetually "full" and either undercharges it, or the BMS cuts out suddenly under load and your alternator briefly sees an open circuit — which can spike voltage and kill the regulator.

On a 1998 Yanmar 2GM20 you'll have a basic externally-regulated or internally-regulated alternator with zero lithium awareness.

What people typically do:

• Fit a DC-DC charger (Victron Orion) between alternator and lithium
• Keep a small AGM as a buffer battery

I run a cabin setup rather than a boat, but the alternator/BMS interaction problem is identical to what I've seen documented repeatedly. Don't ignore it.

@HazelPaddy the alternator overheating risk is the big one here — LiFePO4 will absolutely hammer an older Motorola or Mahle unit at full acceptance current until the BMS cuts out, then you get a nasty voltage spike too.

For a Yanmar 2GM20 specifically, worth looking at fitting a Wakespeed WS500 regulator — it'll profile the charge properly and back off before things get ugly. Pricey but far better than a seized alternator mid-passage.

My setup is shore-power focused so I haven't had the alternator grief personally, but I've read enough horror stories on YBW forum to know the 2GM20's alternator is fairly modest and won't thank you for sustained hammering.

Also double-check your BMS has a pre-charge circuit or your chartplotter and instruments might throw a wobbler on startup.

@HazelPaddy worth mentioning that the Yanmar 2GM20 typically runs a fairly modest 35-40A alternator — small enough that the thermal risk others have flagged is actually more acute, not less. Less mass to absorb the heat when LiFePO4 is pulling maximum current for extended periods.

One practical option if you don't want the expense of a dedicated DC-DC charger is fitting a simple alternator temperature sensor with a current limiter. Sterling and Wakespeed both do decent solutions for older installations.

Also double-check your BMS disconnect behaviour — if the Fogstar's BMS trips whilst the alternator's spinning, you could see a voltage spike that damages the rectifier diodes. Might be worth putting a small AGM buffer battery in parallel specifically to absorb that, even just a cheapy.

What's your typical engine run time when motoring out of harbour?

Bit off-topic as I'm purely a garden office setup rather than marine, but this alternator problem is almost identical to what happens when people connect LiFePO4 to certain vehicle alternators — the battery accepts charge so fast the alternator never gets to back off and just cooks itself.

@OwenSmith raises a good point about the smaller alternator size potentially helping, but has anyone considered fitting a Sterling Power B2B charger (battery-to-battery) between the alternator and the LiFePO4? From what I've read on setups like this, it acts as a buffer and controls the charge rate properly, protecting the alternator.

Is that a practical solution on a Beneteau with limited space below, or is the install too involved? Genuinely curious whether that's the accepted fix in the marine world or whether people just go the Victron alternator regulator route instead.