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BYD vs Polytech EOL capacity


Abnormal
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I have seen that these two batteries stipulate their end of life capacity using a different base of measurement. So I decided to do a comparison.

 

Pylontech USB2000D 2.4kWh usable 80% DOD

Usable capacity = 2.4Kwh * 80% = 1.92Kwh

Cycles at 80% DOD = 6000

End of life capacity after 6000 cycles at 80% DOD = 60%

End of life capacity = 2.4 * 60% = 1.44kWh

 

 

Pylontech USB3000D 3.5kWh usable 80% DOD

Usable capacity = 3.5Kwh * 80% = 2.8Kwh

Cycles at 80% DOD = 6000

End of life capacity after 6000 cycles at 80% DOD = 60%

End of life capacity = 3.5 * 60% = 2.1kWh

 

 

BDY 2.56kWh @100% DOD

Usable capacity = 2.56Kwh * 100% = 2.56Kwh

Cycles at 100% DOD = 6000

End of life capacity after 6000 cycles at 100% DOD = 80%

End of life capacity = 2.56 * 80% = 2.048kWh

 

Now BYD are publish the usable capacity which must mean that the total capacity of the battery must be more than their published numbers.

Assuming that their 100% DOD rating is actually only using 80% of the total battery capacity.

Then this mean that the total capacity is 2.56kWh / 80% = 3.2kWh

If after 6000 cycles the final capacity is 2.048kWh then

2.048kWh / 3.2kWh = 64%

So this means that their end of life capacity of the BYD is 64% of the total capacity

(or similarly the  Polytech end of life capacity is 75% of the usable capacity)

 

This makes them look very similar (in terms of cycles and remaining capacity)

Or of course I could be completely wrong, as i know just enough to make it dangerous 😊

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1 hour ago, Abnormal said:

End of life capacity after 6000 cycles at 80% DOD = 60%

Truth be told, this has never really made sense to me. Normally the battery is EOL when it can no longer do the job it has to do, so if you need 80% of the capacity to do the job, the battery is EOL when it has 79.9% of its original capacity left (it can no longer do the job).

A battery that has 60% of the original capacity left cannot be taken down to 80% DoD of the original  capacity... because it will be completely flat before it gets there. You cannot very well go to -20% now can you? :-)

So all I can think is that the testing is done at successively lower values (but still 80% of current capacity), so pretty much after 4500 cycles (or thereabouts) you are already EOL according to the old way of doing things, and if you used 1C discharge rates chance are it'd be about finished around the 2000 cycle mark... just like most others out there :-)

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One of the old forumites contacted Trojan on their cycles and all that.

They said that they are assured of the cycles, if used properly, for at the end of the cycles, the battery is at +-80% of it s life.

The last 20% is the buffer.

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@plonkster  I did wonder about how it would work.

All my old cellphones could still charge up to 100% although they may not last as long anymore.

I assumed that the BMS would always report 100% SOC and you could use this down to 20% SOC, but capacity of the battery would reduce with each cycle.

 

I see there may be an error depending how pylontech defines the remaining 60%. Will there be 60% of the total initial capacity available for use 2.4kWh * 60% = 1.44kWh or  will there only be 1.92kWh * 60% = 1.152kWh remaining.  If its only 1.152kWh then the 80% of the BYD is directly comparable to the 60% of the Pylontech.

 

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21 minutes ago, Abnormal said:

I assumed that the BMS would always report 100% SOC and you could use this down to 20% SOC, but capacity of the battery would reduce with each cycle.

I think so too. But that is a little bit of a departure from the traditional way of defining EOL, and one that allows boosting the cycles quite a bit. Of course one might point out that real life usage in a house will likely treat the battery well enough that it will last that many cycles, and that might well be true... my argument is merely that it makes comparisons hard.

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