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

Mark McGovern


Voltage may not be as bad an indicator of SOC of LiFePO4 batteries as you might think.  Here's a quote from the owner of MV Tanglewood who has had his LiFePO4 bank in operation for about ~18 months now:

First, counter to what everyone says, I think battery bank voltage is a sufficiently close indicator of SOG.  People say it's a much worse indicator for LFP vs Lead, but I disagree.  My lead bank was 50.4 volts when fully charged, and 48.0V when is was 50% empty and the generator started.  That's a spread of 2.4 volts.  In contrast, my LPF bank is full at 53.6V, and ready for recharge at 50.4V.  That's a 3.2V spread and is 30% more voltage swing than lead.  The difference, though, is that in the mid area of charge, the LFP voltage doesn't vary much, so there isn't as much differentiation between 60% and 50%.

Granted, this 3.2v spread is for his nominal 48v system but even with our nominal 24v system it would be a 1.6v spread which is a significant enough spread to monitor and act on.  I am literally in the middle of installing my LiFePO4 system and I am not yet living full time on the boat so I don't have a ton of personal experience with it just yet.  However, the experience that I do have so far lines up with Tanglewood's experience.  I think that if you operated an LiFePO4 system simplistically such that it is assumed to be "full" at 26.8V (stop charging) and then "empty" at 25.2v (start charging), the battery bank would be quite happy and would give you many years of service.

Here is the full blog post:  http://www.mvtanglewood.com/search/label/Lithium%20Batteries  

Be sure to read his other LiFePO4 related posts.  They are full of great information.
Mark McGovern
SM #440 Cara
Deale, MD USA

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