Jay, I believe that the secret to long LiFePO4 life is low charging voltage. (Of course, the temperature, and depth of discharge do make a big difference as well, but you probably can't control the temperature, and you already said it's not cycling, so DOD isn't an issue either.) There are two problems with just going and reducing charging voltages...
1. As has been mentioned, cell balancing might not work (depending on BMS) 2. BMS calibration errors and/or battery monitor calibration errors may occur I have had this issue once, I tried to reduce the absorb & float voltage to somewhere around 53.2 volts for a non-cycling LiFePO4 battery. I don't remember the exact setting I used. This should keep the battery well over 90% full (probably around 95%) and the client figured that even storing the battery as low as 90% full was a good trade-off in order to extend the shelf life. The only issue is that this specific internal BMS calibrates when the batteries reach a voltage of at least 56 volts, so the BMS slowly kept reporting a lower and lower battery percentage. I was unaware of this, and the client didn't notice, but then the BMS shut the system shut down due to "low SOC". So to sum it up, as far as the chemistry, if you could keep it around 90% SOC for non-cycling use, that would be awesome for shelf life, but because of BMS-related problems, that's probably not going to work well. So here's my own unconventional answer that I've never heard anywhere else, that I have decided is probably best for non-cycling use... 1. Set your float and absorb fairly low. 2. Set an automatic EQ cycle (perhaps once a month) to charge at a voltage that's high enough to calibrate the BMS and balance the cells (depending on the brand, probably around 56V?) 3. Don't let anyone else look at your settings, because nobody will understand why you're "equalizing" a lithium battery, and they'll think you're ignorant and you don't understand the lithium chemistry. Disclaimer: consult battery manufacturer because I don't want any liability 😉. But honestly, it would be good to know what the BMS calibration voltage is, as well as what voltage it starts balancing the cells. Caveat: the only issue with this that I can see is if the BMS needs a significant time to balance the cells. In this scenario, it would be better to get the battery up to the calibration voltage for just a couple of minutes to calibrate the SOC, and then go down to the balancing voltage for a while, then go back to the storage voltage. Unfortunately, I don't know any way to have this kind of cycle happen once a month... Thanks, Kienan Maxfield Solar maxfieldso...@hotmail.com<mailto:maxfieldso...@hotmail.com> (801) 631-5584 (Cell) ________________________________ From: RE-wrenches <re-wrenches-boun...@lists.re-wrenches.org> on behalf of Darryl Thayer <darylsol...@gmail.com> Sent: Wednesday, February 23, 2022 10:10 PM To: RE-wrenches <re-wrenches@lists.re-wrenches.org> Subject: Re: [RE-wrenches] Lithium battery in non cycling use Hi Jay I am not sure what non-cycling use is, I have mentioned before in my experience where a cell in the string can become lower in charge and the string loses capacity in discharge because the BMS shuts off the complete battery when the troubled cell reaches low voltage cut out. To prevent this, a string should go tho top balance. This is of course true for both cycling and little cycling. On Wed, Feb 23, 2022 at 9:36 AM Jay <jay.pe...@gmail.com<mailto:jay.pe...@gmail.com>> wrote: Hi all the lithium experts. Are there any issues with using LFP lithium in non cycling battery backup mode. Or specific settings, brands etc that should be considered when designing for GTBB, non cycling. Thanks Jay _______________________________________________ List sponsored by Redwood Alliance Pay optional member dues here: http://re-wrenches.org List Address: RE-wrenches@lists.re-wrenches.org<mailto:RE-wrenches@lists.re-wrenches.org> Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org There are two list archives for searching. When one doesn't work, try the other: https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: http://www.re-wrenches.org/etiquette.htm Check out or update participant bios: http://www.members.re-wrenches.org
_______________________________________________ List sponsored by Redwood Alliance Pay optional member dues here: http://re-wrenches.org List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org There are two list archives for searching. When one doesn't work, try the other: https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/ http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: http://www.re-wrenches.org/etiquette.htm Check out or update participant bios: http://www.members.re-wrenches.org
