Fitting a 200Ah lithium bank on a 28ft bilge keeler — where to mount the batteries?

Finally taking the plunge and upgrading my old 110Ah AGM on my Westerly Centaur to a 200Ah lithium setup. Going with a Fogstar Drift 200Ah as the main bank, paired with a Victron SmartSolar 100/20 and a BMV-712 monitor. The plan is to run a 12V compressor fridge, nav lights, VHF and a bit of lighting — nothing mental, but the AGM was struggling by the end of a long weekend at anchor.

The awkward bit is placement. The standard battery box on the Centaur sits under the companionway steps, which is fine for a sealed lithium in terms of no gas venting worries, but it's pretty close to the engine and I'm not sure about heat exposure. The Fogstar Drift is rated to 45°C operating temp, but a warm summer day with the Yanmar ticking over could push things in a confined space I reckon.

Has anyone mounted their lithium bank in the aft cabin or quarter berth area instead? I've seen a few people go under the saloon berths too, though access for the wiring run gets messier. Weight distribution matters on a bilge keeler as well — she already sits slightly stern-heavy so shoving more weight aft feels wrong.

Would love to hear how others have tackled this on similar sized cruisers. What did you end up doing and did it cause any headaches down the line?

TU_Power | 📍 South Coast | ⚡ Solar & Marine

@Shaun1970 great shout on the Fogstar Drift, cracking value for money. On placement — the Centaur's quarterberth locker is a popular spot and keeps weight low and central-ish, but do check you can still get decent ventilation even though lithium is far more forgiving than AGM on that front.

One thing people overlook on bilge keelers specifically: those twin keels mean the boat sits on the hard at an angle depending on the surface, so make sure wherever you mount, the BMS isn't going to be stressed by repeated racking. Solid mounting board properly braced is worth the extra hour's faff.

Also worth confirming your Victron 100/20 is the correct model number — did you mean the 100/30? Just want to make sure it's properly matched to your panel wattage before you wire anything up. 👍

Chris1984 | 📍 East Anglia | ⚡ Off-grid & Narrowboat

@Shaun1970 nice project! On a Centaur, the most popular spot seems to be under the companionway steps or in the original battery box under the saloon floor — both keep the weight low and central which is exactly where you want it on a bilge keeler. Lithium is obviously much lighter than your old AGM so you've got a bit more flexibility than you might think.

One thing worth considering — make sure wherever you mount it has decent ventilation even though LiFePO4 is far safer than lead acid, and think about securing it properly for when you're heeled over. A good battery box with a proper strap-down arrangement is worth the effort. What's your planned charging setup beyond the SmartSolar — alternator via a B2B as well?

Callum1982 | 📍 West Scotland | ⚡ Off-grid & Narrowboat

@Shaun1970 On a Centaur the obvious spot is under the companionway steps or in the original battery box under the cockpit sole — just bear in mind lithium is far more tolerant of awkward positions than AGM, so lying flat isn't a problem with the Drift. Main thing I'd watch on a bilge keeler is moisture ingress; worth giving whatever space you use a good inspection first and maybe sitting the battery on a rubber mat to keep it off any damp surfaces. Also double-check the weight distribution — shifting from a heavy AGM to a lighter lithium can subtly affect trim, which matters more than people think on a Centaur. What's your charging setup beyond the SmartSolar? Alternator isolation will be important if you're running lithium without a DC-DC charger in the loop.

WayneClark | 📍 Yorkshire | ⚡ Off-Grid & Narrowboat

@Shaun1970 With a Centaur, the quarterberth locker is the obvious first choice — decent access, reasonably central, and keeps the weight low and near the keel. That said, a lot of folks shove batteries under the saloon sole if there's clearance, which is even better for stability.

Main things I'd watch for: lithiums don't love being in a sealed unventilated space if you've got any charging kit nearby, so make sure there's some airflow. Also worth checking the Fogstar Drift's IP rating before you put it anywhere prone to bilge splash — they're reasonably robust but no sense tempting fate on a boat that age.

Keep your cable runs as short as practically possible too, especially on the positive side. What BMS are you running alongside it?

Moorey79 | 📍 Lincolnshire | ⚡ Off-grid & Marine

@Shaun1970 One thing nobody's mentioned yet — with lithium you'll want to think carefully about ventilation, or rather the lack of requirement for it compared to your old AGM. That actually opens up options that were previously off-limits, like under-bunk spaces in the main cabin.

More importantly on a Centaur, keep an eye on the athwartships balance. A 200Ah lithium is considerably lighter than your old AGM so it's less critical, but if you're already carrying weight bias to one side, this is a good opportunity to correct it.

Also worth checking whether the Fogstar Drift's BMS handles the low-temp cutoff — Centaurs can get properly cold below the waterline in winter moorings. What charger are you pairing with the SmartSolar? 👍

RetiredEngineer61 | 📍 Array | ⚡ Motorhome & Emergency Backup

Not a Centaur owner, but I've wrestled with battery placement decisions on my motorhome and the principles carry across — weight distribution matters far more than people expect.

On a bilge keeler specifically, has anyone considered whether 200Ah of lithium (considerably lighter than equivalent AGM) might actually affect the ballast calculation? The Fogstar Drift is a decent chunk lighter than the AGM it's replacing. Worth checking with a naval architect if you're uncertain — probably overkill for 30-odd kilos difference, but worth a thought.

Also — are you planning a separate battery box, or relying on the compartment itself for containment? On my van build I learned the hard way that even "contained" spaces need ventilation clearance and securing straps rated for the load. What does the Centaur's original battery tray look like?

PylontechMaster | 📍 West Midlands | ⚡ Off-Grid & Marine

@Shaun1970 Great choice on the Fogstar Drift — cracking value for money. One thing worth considering specifically with a Centaur is that the bilge keel design gives you a surprisingly low centre of gravity amidships. Some owners fit a proper battery box directly over the keel bolts area, which keeps the weight central and low rather than aft in the quarterberth. Keeps your trim neutral too. Just make sure whatever location you choose has decent ventilation and you can realistically reach the BMS terminals without dislocating your shoulder! What Victron BMS or isolator setup are you running alongside that SmartSolar?

SmartSolarGuy | 📍 Array | ⚡ Motorhome, Solar & Cabin

One thing I'd add from my motorhome build — the Fogstar Drift runs absolutely fine on its side if you're tight for space in the bilge area. Confirmed with Fogstar directly when I was doing my own install. Just avoid fully inverted.

Also worth thinking about cable runs to your Victron SmartSolar. I lost significant efficiency on an early build by running undersized cable over a longer-than-expected route. Keep those runs as short as practically possible and size up generously — 16mm² minimum for a 200Ah bank in my experience.