Battery cycles vs SOH

So to my limited understanding the BMS is meant to show the battery SOH which translates to the number of cycles done where if I do 4000 - 5000 cycles it should read 80% (If battery cycles are based on it), am I correct on this?

My BSL battery 100Ah that’s about 1.5 years old now reports 100% SOH

If i have used 1200kWh That is 5.1kWh X 80% = 4.08kWh / cycle … 1200kWh / 4.08kWh/cycle = 295 cycles which is at least 6%

Am I completely wrong?

Also my BYD 2.54kWh also reads 100% SOH and its had a bad past life of every load shedding being taken down 10% from 2019 end to 2022 beginning

Good day,

I can only give you my theory of the SOH calculation as I believe different manufacturers use different methods to calculate SOH.

I see it as the actual dischargeable energy at the time of measurement VS the original capacity of the battery.

I did some google research regarding SOH parameters, as I am in the process of testing a 550 mah lipo cell that I salvaged from a disposable vape device ( the brand is Airpops)

So far after 33 100% DOD cycles at 1C discharge and 2C recharge the SOH of this tiny cell is 98.66% calculated from the actual capacity measured on the 1st test cycle 596 mah vs 588 mah.

If I calculate according to the data sheet of the battery the SOH will be more that 100%, 550 mah(claimed) vs 588 mah tested at cycle number 33.

That’s is my finding so far. And maybe this explains why your batteries still report 100% SOH

You most likely will have to have a cell or battery low voltage cut off event to update the SoH on the BSL. Basically you have to use 100% of it to know the total capacity. If you never go past 80% DoD, you will probably only update the SoH after it also drops below 80%.

Pylontech is different in that regard, they seem to interpolate SoH based on use, and their expectations of cell life.

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Made me think of 100ah cells installed in a battery bank with a cycle warranty.

The manufacturer says 100ah, the cells installed, if opened, warranty then shot obviously, are like 120ah.

Also, we imported 280ah cells, most of the cells, factory tested, were ±292ah.

My 2 cents.


May will be 4 years, an additional battery was added in 2021

It’s a typical trick used by battery makers. Add some extra capacity so that the battery lasts longer.

Technically computer hard drive makers (in the days of magnetic storage) did the same. Every drive came out of the factory with plenty of bad blocks… but the drive also came with extra “spares” and the firmware in the drive would swap them out on a write operation, so that you wouldn’t know the drive was going bad until years and years down the line when the spares were used up.

BMW does the same in their electric cars. The battery is a little larger than advertised, and the electronics keeps a little bit of it aside. Similar to the Pylontech batteries, they estimate the degradation, but from what I heard they overestimate it, so a reset followed by a full cycle may well improve the estimate.

So all in all the best guess is that it still in the period of “extra” capacity, I do wonder if it energy counts its self to determine its SOH i.e. used still reads it is getting 80Ah on a 80 DOD.

Begs the question how much you can rely on these numbers.