Swapped to lithium on my old Westerly 22 – worth every penny or am I mad?

Not strictly marine here but I've gone through the same lithium debate for my motorhome and garden office setup, so maybe some crossover experience is useful.

The weight argument alone would've sold me on a boat — shifting 20-30kg of lead acid off the stern or wherever it's sitting must make a noticeable difference to trim. Did you find that?

I went with Fogstar Drift cells for the motorhome and honestly the usable capacity jump from 50% DoD on lead to 80-90% on lithium felt like doubling my bank overnight. That bit absolutely tracks for a boat too I'd imagine.

A few things I'd be curious about in your setup:

• What BMS are you running? On a boat the overcharge/discharge protection seems even more critical than on land
• Have you paired it with a Victron charger/MPPT, or sticking with whatever came with the Westerly?
• How are you handling the charging from the engine alternator — did you fit a DC-DC charger to protect it?

The alternator question is the one that seems to catch people out most with lithium conversions. Lead acid is forgiving, lithium will just hammer your alternator trying to fill that big capacity gap.

Doesn't sound mad to me at all — a 22ft boat is exactly the size where every amp-hour and every kilogram actually matters. What did the total conversion cost you roughly, if you don't mind sharing?

@Smithy98 mate, the weight savings are just the gateway drug — wait till you see how much of that capacity you can actually use without the Victron BMS throwing a tantrum at 50% like the old lead-acid days! 😄 Did the same swap on my van and haven't looked back since. The only thing I'll say is don't cheap out on the BMS side of things — spent a penny on quality Fogstar cells and lost a pound on a dodgy BMS I found on eBay, lesson thoroughly learned the hard way. 🔋

@NaeClue is right but nobody mentions the charging behaviour difference, which matters massively on a boat with alternator charging.

Lithium will hammer your alternator trying to fill that flat internal resistance curve — I cooked a perfectly good Valeo unit in my van before fitting a Victron Orion-Tr Smart DC-DC isolator. Non-negotiable kit if you're feeding from an engine alternator.

@Smithy98 — garden office and motorhome are relatively forgiving setups. Marine environment is another level entirely: salt ingress, condensation, constant vibration. Make absolutely certain your BMS connections are properly torqued and check Fogstar's IP ratings on whatever cells you're using.

One thing nobody budgets for: the existing 12V appliances calibrated around AGM voltage curves will behave differently on lithium. Your voltage gauges will lie to you constantly until you recalibrate your thinking entirely.

Not mad. Just expensive.

@GoldenNomad is touching on something I learned the hard way living out of my van in Kent last winter — the alternator piece nearly caught me out completely.

When I paired my Fogstar Drift 100Ah with a standard split-charge relay, the BMS would just slam the circuit shut mid-charge. Not great for the alternator at all. A proper DC-DC charger sorted it immediately — I run a Victron Orion-Tr Smart now and it's faultless.

On a boat with engine charging, this matters even more because marine alternators are often older and less tolerant of the abuse.

The weight saving and usable capacity arguments are well documented, but quietly getting your charging chain right is what separates a successful lithium conversion from an expensive headache. Don't skip that bit.

@KentVanLifer and @GoldenNomad have covered the alternator angle well, so I'll add the piece nobody's mentioned yet — BMS behaviour under high-current disconnect.

On my narrowboat setup I watched a Victron Multiplus nearly fry itself when the BMS on a budget pack hit its overcurrent threshold and dropped the load instantaneously. No soft shutdown, just a dead short to open-circuit in microseconds. Alternators and inverters hate that.

The fix was a Victron BatteryProtect wired as a warning relay before full BMS trip, giving the system time to ramp down gracefully.

On a Westerly 22 with a small engine alternator and limited real estate for additional kit, worth thinking hard about which lithium pack you choose — Fogstar Drift cells have a more sensible BMS communication protocol than some of the cheaper imports I've tested.

@TorJake has clearly left us on a cliffhanger there — curious what that unmentioned piece is.

What I'd add from my narrowboat experience: battery management system compatibility with your existing 240V shore power setup is the sleeper issue nobody budgets for. When I swapped my boat over to a Victron 100Ah lithium setup, the old 3-stage charger was essentially useless — it kept tripping on the absorption phase because lithium charges so differently to lead-acid.

Had to replace the charger entirely with a Victron Blue Smart unit before everything played nicely. The BMS was doing exactly its job, but my charger didn't understand the conversation.

On a Westerly with limited electrical space, factor that into your total cost. The lithium cells themselves are just one line item — the supporting ecosystem is where the real spend lives.