Relion RB36V40 36V 40Ah Group 31 LiFePO4 Deep Cycle Battery

The Relion RB36V40 is a 36 volt, 40 amp-hour lithium iron phosphate battery in a standard Group 31 case. If you run a 36 volt trolling motor, this is the kind of battery that changes how you rig the bow. Instead of stacking three 12 volt lead-acid batteries in series and eating all that weight up front, you drop in one LiFePO4 pack and wire it straight to the motor.

Let's talk about what that means on the water. A 36 volt trolling motor pulls hard, and lead-acid banks sag under that load and give back only about half their rated capacity before the voltage drops too low to be useful. LiFePO4 is a different animal. This pack holds a nearly flat voltage deep into the discharge, so your motor keeps full thrust right up until the battery is close to empty. You get the usable capacity you paid for, not just the top half. Relion rates it at 99 percent efficiency, and 40 amp-hours at 36 volts is real, deliverable energy for a day of fishing. And it's a fraction of the weight of the three-battery lead-acid bank it replaces, which is weight you feel every time you load the boat and every time you're up on plane.

The RB36V40 puts out 175 amps of peak discharge current. That headroom matters when a high-thrust motor spikes on start-up, or when you punch it to reposition against wind and current. The internal battery management system watches cell voltage, temperature, and current, so the pack protects itself without you thinking about it. LiFePO4 packs like this one also cycle far more times than lead-acid before they wear out, which is why the up-front cost pencils out over a few seasons.

Install is straightforward if you've wired a battery before. The case measures 13 inches long by 6.8 inches wide by 8.4 inches tall, so it drops into a standard Group 31 tray. Terminals are M8 threaded studs on top, which take a proper ring lug torqued down tight. That stud connection is far more secure than a post clamp under the constant vibration of a boat pounding across chop. Use marine-grade lugs, get them torqued to spec, and seal them against corrosion. If you're mounting in a tight console, measure your hold-down and hatch clearance first, since the 8.4 inch height is a little taller than some lead-acid Group 31 batteries.

The case is molded ABS and carries an IP67 ingress rating, so it shrugs off bilge spray, rain, and a washdown at the ramp. The battery itself is certified to UN38.3, CE, and UKCA, and the cells inside are UL1642 and IEC62133 certified. That certification stack matters if you ever ship the battery, and it tells you the cells have been through real safety testing, not just a spec sheet.

One thing to get right is charging. A LiFePO4 pack wants a lithium-profile charger. Use a 36 volt charger set for lithium, or a lithium-compatible on-board charger, and it'll top off fast and hold a long service life. A charger locked to an old lead-acid-only profile can confuse the BMS and cut out mid-charge, so match the charger to the chemistry. If you're charging off the outboard's alternator, plan on a DC-to-DC converter rather than a direct connection.

Who's this for? Anglers running 36 volt bow-mount motors on bass boats, bay boats, and center consoles who are tired of hauling and replacing lead-acid banks every couple of seasons. It's not the battery for a 12 volt or 24 volt motor, and it's not a starting battery for your outboard. It's a deep-cycle battery for a 36 volt trolling system, and on the right boat it saves weight, saves space, and just plain lasts.