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Help me understand the State of Health (SoH) report

9.8K views 7 replies 4 participants last post by  Qtown charger  
#1 ·
In my limited time here while considering a purchase i have come to appreciate the knowledge and discussion on the battery pack, it of course being a critical component to long term ownership happiness.

What I THOUGHT the SoH report indicated was the current battery pack capacity by cell pack. In other words, that if a battery pack when it is new offers 100% of its useable storage, that any number lower than that on the SoH readout meant that some of the capacity had indeed been lost, as is the case over time. Obviously there will be some, but you want it to be as little as possible.

The way it is being presented to me by the dealer and tech (which i should mention i have no reason to doubt, they’ve been very open with the info) is that it represents the value versus the car’s reported state of charge. In other words, if the car reports it is 100% charged, you want all 36 cells in that report to show 100% as well, However if the car is reporting it is 90% charged, the SoH report will show all cells at 90% as well and that’s perfectly fine. It would be an issue only if they car was showing 90% and one of more cells were showing less than 90%. But in that case it is not a concern that they do not show higher than 90%. Or if the car is reporting 100% charged and the SoH shows something in the low 90s.

so, in my thinking, regardless of the charge level of the car, my expectation is the SoH value is the lifetime capacity of the pack and you would want it to show 100%, Yes, its only charged to x% but it has 100% of its capacity available.

In the way it is being explained to me, it reflects the state of charge of the battery and you want it to match what the charge level is. The delta between the reported value and the charge level is the health degradation. So, if the charge level of the car is reported to be 90%, and the SoH reports shows all cells at 89%, then yes, you have lost 1%.

is one of those ways the correct understanding, if so which? Or is it something else.

Thanks so much.
 
#2 · (Edited)
Hello! Whatever the battery real capacity, once fully charged, it will display 100% charged. The available range will decrease when the battery gets older, but not the SOC. The very same happens with your laptop. For example, let's say your battery lost 50% of its original capacity, the SOC will display 100%, but the range will be half the expected value. All this being calculated by the computer only, the battery itself doesn't give any info about its real capacity nor health. Alcalin batteries for example have an increasing impedance allowing us to know very precisely the remaining capacity. Lithium batteries do not behave that way: the only sign appears at the very end, where the output voltage abruptely drops. But it's of no use in our case, we need to know the SOC at any time. How do we do that? Basically, the only way to do this is to count precisely the entering/outgoing energy, and depending of the age and numbers of charges/discharges, we can estimate the real battery health.
A friend has a very old Renault Twizzy. He rents the battery from Renault, that's their business model here. He complains that he gets only 60% of the original range, so Renault should replace the battery, as stated in the contract. Their answer? The soc is 100%, your battery is like new. No surprise from these assholes, but you get the idea.
In other words, the SoH, the real SoH I mean, is hard to guess, is based on statistics and experience, but it does NOT reflect the actual status of YOUR battery pack.
 
#5 ·
Would have to disagree with that blanket statement.
What is possibly less than definitive is the data you have been given. If I am not wrong, the official mechanism for JLR to determine a faulty battery under the warranty is to perform a SoH check under specific conditions which includes evaluating a fully charged (not 90%) battery. I would push for a full SoH report under the official conditions. As was stated above, the battery does not report anything, it is the software that manipulates the parameters of the battery and those parameters probably make no sense unless measured under standardized conditions.
 
#6 ·
So ask them to charge it to 100% and run it?

I got the pages just like the other posts here, cell 1….36 battery health (capacity fade)…

They are telling me that the readout for the health check is to determine variation from cell to cell, so if they all read approximately the same, that is what you want to see. I was under the impressions that is was the absolute value for each cell and that you wanted to see 100% or as close as possible, realistically.
 
#7 ·
Charging to 100% 1st. should be done...but
the problem is if you have one bad/weak battery cell, then the entire battery
will not charge to 100%. It will only charge to some value relative to the weakest cell.
This is not a problem with most of the cars.
I don't know of any EVs that can charge all but the bad cell to 100%.
We need smarter charging software. Just my 2 cents.
 
#8 ·
This may be semantics, but even with a bad battery, the car usually registers 100% at full charge. It is the kWh added to the battery that are reduced.
My understanding is that the individual battery cells are arranged in parallel in packs of 4 that accept charge independently. Thus, one bad cell will effect the pack of cells it is grouped with, but the other packs will still charge to full capacity.


The battery health report that a dealer can provide should document the voltage of each of the 108 packs of 4 cells. Each pack should register around 4v, and I assume the reported % is the percentage of this number that is measured. But, since battery voltage, and presumably each group of 4 cells, varies with state of charge, the health of the battery can only be evaluated at full (100%) charge. Since battery voltage and state of charge are not linearly related - a 50% charge does not equate to 200v, 90% should not equate to 90% of full voltage.
So, for the op I would definitely want a SoH report at 100% charge.