Re: What to do with LiFePO4 batteries when the boat is layed up?
Scott SV Tengah
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Avoiding charging the batteries to 100% during normal use is a completely different conversation than whether to leave batteries at high SOC or 50% SOC during wintering/long term storage. I know that Victron and likely Mastervolt do NOT charge to 100% even during normal use. It's obvious if you look at the absorption voltages preset by Victron lithium charging profiles and then compare it to what is considered 100% charged. I surmise that it's the same with Mastervolt, so the respective battery monitors are telling you it's 100% when it's really in the low-mid 90% SOC. They do this because the additional capacity going between 90%-100% SOC is more than offset by the faster deterioration of the batteries.
Regarding Mastervolt batteries "being different in design from other manufacturers batteries", I am a bit skeptical considering actual product teardowns indicate that they use Winston prismatic cells, just like Victron and just like the guy who spent a fraction of what we spent for our batteries. A not-so-hidden secret of these expensive batteries is that they use the same Chinese cells that others get at a 75% discount to build their own homebrew lifepo4 batteries. Of course, the internal connections, BMS system, warranty and hence safety and reliability are theoretically better and the system is more integrated, which is why I paid a premium for my Victrons. But I'd be very skeptical when the guy selling you the batteries at a huge profit tries to tell you that theirs is different somehow. I am under no illusion that the battery cells in my Victrons or your MV are made of unobtanium or handbuilt by an MIT engineer - they are Chinese Winston prismatic cells that are subject to the same limitations.
Trying to achieve a desired SOC by specifying voltage is difficult for reasons mentioned earlier. By setting a lower float voltage, I believe that all you're doing is setting the lower limit of voltage that the batteries will go to while connected to the charger. The absorption voltage/duration is really what determines the high SOC limit. Give it a try and set your float voltage to 24v and see if it still charges to 78%.
Let me try to dig up those papers for you. Here are the results of a simplified test that is much less dry.
To be fair, he's comparing 50% vs. 100% SOC but it's probably safe to assume that the damage associated with high SOC storage is not a step function.
At the end of the day, it's our individual boats, so we must do what we think is best. I find these engineering guys sometimes cross the border into marketing, so I always trust, but verify through objective means.
I have discussed the issue with a senior technician at MV support. His explanation for the recommendation to keep the batteries at 100% with a float voltage of 27.0V was that their batteries are different in design from other manufacturers' batteries. They are made for being kept at full charge over long periods. I further asked him if I wanted to keep them at a lower SOC, how to achieve that. His response was to change the float voltage to 26.5 from 27.0V rather than program an event that turns the charger off at 80% SOC. The event will result in a lot of switching which would be avoided with the adjustment of the float voltage. I've tried to do this and the SOC essentially goes to about 78% with the charger connected. So presumably you could set the float voltage to even less than 26.5V and end up with a SOC lower than 78%.
2007 A54 #69