[RE-wrenches] Lithium battery in non cycling use

[Daniel Young]

The main thing you can do when non-cycling is to set the charge voltage a little lower than you normally would. 

Check the manufacturer for the recommended full charge voltage (usually 3.6V/cell) that's 57.6V for a 48V pack. I set my absorb voltage to 57V even to begin with. And float is closer to 3.5V/cell or 56V. That constitutes roughly 90% capacity (check your battery manufacturer spec, it can vary by brand).

Ageing of LFP tends to accelerate at both higher temperatures and higher charge voltages. There are some research papers out there testing this in various ways for various applications, but the conclusions are usually the same. Keep the lithium from getting too hot (I say stay below 30-35C when possible) and do not charge them to 100% unless you really need the capacity.

So my common sense tells me to set my absorb and float voltages such that the LFP batteries are not regularly going above 90% SOC in backup applications.

Another factor to account for is the BMS's cell balancing feature, you do want the float voltage set high enough that the BMS does get triggered to balance cells (many BMS's do not balance until the cells are near full, as this is where imbalances really start to show up, and it saves energy by not bleeding current off the cells constantly). For example, if the battery BMS starts balancing when cells are above 3.52V (56.32V for 48V packs), then the float voltage needs to be just above this (56.4-56.5) that way the BMS has the ability to balance cells regularly. The manufacturer would be the source for the cell balancing voltage threshold. If float was set to 55V for example, the cells could get relatively out of balance and the BMS would do little to stop it, only bleeding off current when a cell gets particularly out of balance with a higher voltage than its peers.

Always ask the manufacturer for advice on this first of course.

--

Danny Young
Engineering Team Lead
Solar Energy Solutions
Lexington | Louisville | Bloomington | Cincinnati | Evansville | Indianapolis
513-448-5176 (mobile)
877-312-7456 (Main Office)
danny at sesre.com

-----Original Message-----
From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> On Behalf Of Jay
Sent: Wednesday, February 23, 2022 10:02 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: [RE-wrenches] Lithium battery in non cycling use

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
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