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paulz
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# Posted: 25 Apr 2021 02:16pm
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I'm keeping one of my three Valence 12v 140ah LFPs on the shelf, the other two being in use, in case of a system failure.
It's been sitting at 13.43v for the last month or two. As has been discussed, better for the battery so be at less than full charge for long term storage. Is there a voltage that would represent, say 80%, if that would be a good compromise and still give me some backup usage?
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gcrank1
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# Posted: 25 Apr 2021 06:53pm - Edited by: gcrank1
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From what Ive been reading about voltage readings and LFP, no. The range of use does not correlate to voltage drop/rise like we are used to with Lead Acid, it is 'too flat'.
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paulz
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# Posted: 25 Apr 2021 07:18pm
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Yep, you're right, I have seen the graphs. I guess the only way is to fully charge and go by ah?
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gcrank1
Member
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# Posted: 25 Apr 2021 07:46pm
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Maybe you could go to a top charge and based upon the Ah's of the battery do x ah's discharge to your target state of charge?
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ICC
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# Posted: 25 Apr 2021 08:12pm
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LFP's do have a voltage range that can be used to estimate charge level. Voltage is fortunately a very good guide to state of charge with lithium cells of all types.
A single LFP cell has a nominal voltage of 3.2 to 3.3 volts. A voltage of 3.6 volts is generally considered full and 2.5 to 2.8 is considered depleted. Long term storage is best at 3.2 to 3.3 volts for LFP. You will see slightly different suggestions but those numbers should see you safe.
If you also want the batteries to be ready for some use in a power out scenario then store them near the full voltage. You and I will probably never see any appreciable depreciation in life if we stored out LFP's at 3.45 to 3.5 volts per cell (or parallel string if we have a battery made of multiple cylindrical cells in series/parallel packs inside, like the Valence).
So for an assembled battery for a 12 volt system muliply those numbers by 4 and so on for 24 volt systems, etc.
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paulz
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# Posted: 26 Apr 2021 09:31am
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Thanks ICC, great info. I'm going to use it some today and run it down a bit.
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gcrank1
Member
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# Posted: 26 Apr 2021 09:43am
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Do I remember that those Valence bat's are made up of a bunch of small cylindrical cells? Not the same as 4 large cells for that 'volts per cell' calc.
On a practical matter Im inclined to agree that I may never see the diff in my lifetime as long as an LFP bat is something less than fully topped up, yet being a bit OC with an inquiring mind........
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ICC
Member
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# Posted: 26 Apr 2021 11:39am
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Quoting: gcrank1 Do I remember that those Valence bat's are made up of a bunch of small cylindrical cells? Not the same as 4 large cells for that 'volts per cell' calc.
Yes, there are a bunch of cells inside. 18mm diameter and 65mm long. They will be arranged in 4 groups that are all connected in parallel, so in voltage terms, those act like one big cell at 3.2-3.3 volts. Then those packs are series-connected to make it a "12-volt" battery. Tesla batteries are assembled similarly but on a larger and much higher voltage scale.
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paulz
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# Posted: 26 Apr 2021 06:06pm
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They are pretty heavy, that's all I know. Size of group 27 LA car battery and weigh probably 3/4 of one.
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