Epoch Batteries has been making a BIG splash the past year or so, and now have what I consider the best value LiFePO4 (LFP) batteries for house battery use by far. In addition, they've recently introduced the battery in the attached picture that can be used for ALL outboard engines up to 250HP. Today I rigorously questioned the applicability of this battery in an outboard powered boat, and got impressive answers back from their US rep. Bottom line: I can recommend this as a start battery in place of AGM, and at 30 lb, 120Amp-hour, a CCA of 1100 amps, in a group 27 size, the specs are simply amazing and precedent-setting.
This is 25-35 lb lighter than an equivalent AGM, can be installed INSIDE the cabin without ventilation (AGM requires a ventilated cabinet), or outside depending on your weight balance, and even under high cranking loads will deliver a solid 12.8 volts, whereas AGM voltage will sag to mid or high 11-ish volts on high winch/windlass loads. Thus this will lead to better windlass/winch performance and a longer life for the motors. Other thoughtful features are an external on/off switch, state of charge monitor, very useful bluetooth phone app, and internal heating for cold temperatures. Altogether this battery blows my mind and represents the very first lithium battery that meets all my specs for safety and reliability as an outboard start battery. You pay for it at 2x the cost of AGM, but I think it's worth it and I'll finally be changing my own boat to all lithium this year.
I would set the boat up with this as the start battery and also connect the anchor winch/windlass and pot puller to it. All other loads would go to house. I'd set up a Victron battery to battery charger between this battery and the house to limit charge current from the outboard, otherwise if this start battery is used for deep discharge loads and frequently has to be charged up by the alternator for 2+ hours or more from 20% state of charge, you risk burning up the alternators, which are NOT meant to be running anywhere near their nameplate capacity for long time periods.
