Ask PYS: Power

Firefly Oasis vs. AGM vs. Flooded - Increasing Effective Bank Capacity – David, on May 26, 2015 asked:

I have a 35’ cutter boat that needs to have its house batteries replaced next year. I spoke with Nigel Calder and he recommended the Firefly Oasis batters. My existing house batteries have a capacity of about 200 Ahr. They are conventional lead acid, 6V golf cart batteries which are around 10 years old. What should I budget to replace the effective bank capacity if I want to increase capacity? On the last cruise our daily Ah draw was around 50Ah, no refrigeration etc. Ocean passages of 5 to 28 days, at anchor weeks at a time. For the most part, with our 400W solar panels, I would run the engine or generator once every 5 or 6 days at most. Most of the loads are lights (now LED), navigation equipment, auto helm, and radio equipment (HF, VHF, Radar, AIS). The HF is probably the biggest amp draw, and the time used can be significant.

Thanks for all the background on your boat and electrical needs.

Flooded Deep Cycle:       ~$150 @ 30% of useable capacity
Standard AGM:               ~$300 @ 55% of useable capacity
Firefly AGM:                    ~$750 @ 80% of useable capacity

In terms of value of the standard deep-cycle flooded lead-acid provides best cost to capacity ratio. You’d select either AGM (standard or firefly) based on space or weight constraints, where desired capacity can only be achieved with higher-density batteries.