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[brose]

I have an infamous slowly dying stock Calex charger on my S I am replacing soon. I was wondering if anyone could explain like I'm 5 (ELI5) how the slow decrease in maximum SOC happens. Well, maybe not 5, but assuming basic understanding of electrical components and circuits (resistor, capacitor, diode, transformer, etc.). As a first guess, it seems strange that it only charges up to, say 90%. Why does it not just stop charging?

[DonTom]

I have an infamous slowly dying stock Calex charger on my S I am replacing soon. I was wondering if anyone could explain like I'm 5 (ELI5) how the slow decrease in maximum SOC happens. Well, maybe not 5, but assuming basic understanding of electrical components and circuits (resistor, capacitor, diode, transformer, etc.). As a first guess, it seems strange that it only charges up to, say 90%. Why does it not just stop charging?

There could be several possible reasons. Perhaps the value of the resistors are increasing with the age and heat. Perhaps the electrolytic capacitors dry out and can no longer charge up to peak. Perhaps a diode shorts out so half the waveform is gone in a bridge rectifier. The outcome of any of these could mean that the voltage under load can only go to 112 VDC which will be 90% SOC. So from then on, that will be as high as it can go and from there, it is somewhat likely to drop further depending on the exact cause. As the voltage /current capacity  drops under load, the SOC max possible will also be less. It can eventually get below 90.2 volts where there will be no charge at all.

-Don-  Auburn,  CA

[Richard230]

My daughter's charger eventually dropped to 87% maximum SOC and it was starting to look like there was no end in sight.  And I have no answer to your question, other to say that a new $800 charger will bring you back to 100%. 

[DonTom]

My daughter's charger eventually dropped to 87% maximum SOC and it was starting to look like there was no end in sight.  And I have no answer to your question, other to say that a new $800 charger will bring you back to 100%.

Did it slowly drop down,  IOW,  used to charge to 95% and then 90%, etc?

-Don-  Auburn, CA

[Richard230]

My daughter's charger eventually dropped to 87% maximum SOC and it was starting to look like there was no end in sight.  And I have no answer to your question, other to say that a new $800 charger will bring you back to 100%.

Did it slowly drop down,  IOW,  used to charge to 95% and then 90%, etc?

-Don-  Auburn, CA

Yes, it slowly dropped down over a period of about 2 years.  I first noticed the SOC drop to 98% and then it slowly went down from there.   

I should add that for the first year I could leave the charger plugged in and it would turn on every day and charge another 250 Wh and eventually reach 100% SOC. After 6 months, it wouldn't go past 98% after being charged for a week. Then I gave the bike to my daughter and she didn't care all that much about fully charging the battery pack, so I don't know if keeping the charger plugged in forever would bring the SOC back up to 100%.