State of Health determination

[mvonw]

I've been curious how State of Health (SOH) is calculated in LiFePO4 batteries by BMS's. Is this just a fancy count of charge cycles, or is measuring some other electrical/physical characteristic of the battery?

Have these SOH values been empirically validated by any independent group?

Sounds like a good Will video: Server rack torture test...

 

[curiouscarbon]

a commonly used SOH indicator is capacity relative to initial capacity.

i don't know if this is used in any specific BMS but it is commonly regarded as a decent way to estimate SOH.

if a cell labeled 100Ah tests as 98Ah out of the box, then that would be the initial capacity. once it reaches 80% of that capacity, some seek replacement, that is to say zero SOH.

once the pack reaches a point where it tests 78Ah or less, then one might say it has 0% SOH

by this measure,
SOH = 1 - (1-(current capacity / initial capacity)/0.8)
where 1 = 100% and 0 = 0%

or something. basically cell starts at 100 SOH and reaches 0 SOH when capacity is 80% of initial.

another measure is internal resistance at a given temperature and state of charge. i dunno how exactly that works but generally SOH would decrease as internal resistance increases at a given temperature and SOC.

hope this helps somewhat ??️

 

[curiouscarbon]

a basic SOH indicator is cycle count, where 0 cycles is 100% SOH and X cycles is 0% which would be 2000 for most LiFePO4 cells.

this is less of an intrinsic measurement, since cycles can vary in depth of discharge and temperature and time at a given voltage, which affects the degradation of SOH.

with LFP cells, cycles that involve high temperature, extended times above 3.5 V per cell, very high ampere charge or discharge, are prone to reducing SOH more per cycle

 

[curiouscarbon]

voltage droop at a given ampere load is another way. i think this is related to internal resistance. voltage droop per ampere will increase as cell ages.

@RCinFLA has posted a few diagrams about cell terminal voltage slump that helped me learn

 

[curiouscarbon]

I've been curious how State of Health (SOH) is calculated in LiFePO4 batteries by BMS's.

may i ask, are there any specific BMS you are interested in?

the BMS i use only reports cycle count, but no SOH data field.

 

[mvonw]

thank-you. I have Trophy batteries that use Seplos BMS's. I think I generally understand how the SOH is calculated, but real question is how reliable is this in terms of actionable information?? For example, in 5 years, will my batteries report a 50% SOH, and then decrease proportionately over the expected 10 year life, or will they one day just go from 100% to 0%?