RELiON RB100-HP Group 31 12V 100Ah Lithium Dual-Purpose Battery

The RELiON RB100-HP takes the familiar Group 31 footprint and drops in lithium where you used to run lead-acid. It's a 12.8V, 100 amp-hour LiFePO4 battery built as a dual-purpose unit, so it'll crank a small outboard and still carry your house loads through a long day on the water. Same box size you already have room for. A whole lot less weight to haul around.

Dual purpose is the part worth slowing down on. A lot of lithium batteries are deep-cycle only and won't handle the current spike of an engine start. The HP in this one stands for the higher-performance cells RELiON uses to make it do both. That means one battery can start a small kicker or outboard and run your electronics, livewell pump, and lights between starts. On a center console or a bay boat where space and weight both matter, that flexibility is the whole point.

Fitment is straightforward. The case measures 13 inches long, 6.8 wide, and 8.8 tall, which is standard Group 31, so it'll sit in the same tray and box as the lead-acid battery you're replacing. The top terminals are M8 threaded studs, so you can bolt your existing lugs right on. We'd tell you to add a lock washer and snug them down properly. Vibration on an outboard transom will back a plain nut off over a season, and you don't want a loose lithium terminal.

The big win is weight. A Group 31 AGM or flooded battery is a back-breaker to wrestle into a locker. Swapping to LiFePO4 cuts that roughly in half, and on a smaller skiff that weight coming off the stern actually helps the boat get up on plane. The chemistry also gives you far more usable capacity. You can pull a lithium battery down deep without hurting it, where a lead-acid battery really only wants to give up about half its rated amp-hours before you start shortening its life.

Value shows up over time, too. LiFePO4 batteries hold up for thousands of charge cycles, far more than you'll ever squeeze out of a lead-acid Group 31 that gets cycled hard. So the higher sticker price spreads out over years of service instead of a battery you're swapping every couple of seasons. For a boat that gets real use, that math tends to land in lithium's favor.

There's a battery management system built into the case that watches cell voltage, temperature, and current, and it'll shut things down to protect itself if something goes sideways. One thing to get right at install is your charging voltage. Lithium wants a charger or alternator regulator set for a LiFePO4 profile. Leave an old regulator pinned at a high lead-acid absorption voltage and you'll stress the BMS over time. A DC-to-DC charger between the engine and the battery is the clean way to charge off an outboard, and it's what we'd point most rigs toward.

And it isn't only a boat battery. The same dual-purpose Group 31 lithium is a favorite in RV house banks, van builds, and small off-grid solar setups, anywhere you want a lot of usable power in a case you can actually lift on your own. Building a bigger house bank? These can be wired in parallel to add capacity. Just match your batteries and follow RELiON's guidance on how many you can string together safely.

If you've been on the fence about moving a Group 31 over to lithium, the RB100-HP is an easy place to start. You get the weight savings, the deeper usable capacity, and the dual-purpose flexibility to crank and cycle from one battery. We stock them, we ship same day on in-stock units, and we're glad to talk through your charging setup before you buy.