locked Re: Batteries starting to get weak - MMM - What to do -

Mark McGovern


No problem.  That article that I linked to by Compass Marine is by far the most comprehensive that I have found regarding using LiFePO4 batteries in a marine electrical system.  I have read it many, many times and every time I re-read it I learn something new.

If I was not intending to move aboard full time and do extensive off-grid cruising, I would not have made the switch to LiFePO4.  The Firefly batteries you mentioned would be my choice for "part time" cruising.  They appear to have many of the same benefits of LiFePO4 regarding charge acceptance rate and Partial State of Charge use without the relative complexity and cost of installing a LiFePO system.

When thinking about SOC keep in mind that part of the beauty of LiFePO4 batteries is that, unlike Lead Acid batteries, they really don't care what the SOC is within a really wide range. They seem to be as happy at ~30% SOC as they are at ~90%.  They really just dislike the extremes, so the main job of the BMS (at least how Victron designs their BMS) is to keep the batteries from getting over-charged (over voltage) and getting over-discharged (under voltage) by cutting off charging source(s) or load(s).  Otherwise, the BMS doesn't really intervene in normal daily use (outside of intra-battery cell balancing which is a separate topic to itself and disallowing charging at temperatures below 5 deg C).

To give you some idea of the "new thinking" required with these batteries compared to Lead Acid when I took delivery of my batteries it was going to be several weeks before I could install them.  New batteries are delivered at about a 50% SOC.  When I asked my dealer how long I could leave them "as is" and what I needed to do to maintain them he said I could leave them that way for many months and probably years, and that I didn't need to do anything other than store them at a reasonable temperature.  Can you imagine what would happen to your Lead Acid batteries if they were left at 50% SOC for months?

Mark McGovern
SM #440 Cara
Deale, MD USA

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