Is it safe to take apart a battery charger?

[Adrian_]

So I have this battery charger https://www.aliexpress.com/item/1005004128152511.html?spm=a2g0o.order_list.0.0.23001802LgOIId

I left my battery to charge overnight and in the morning, the battery voltage was at 79v not 84v. Tried replugging it but nothing. I thought maybe the battery was spanked like loose balancing wire, rode it just fine until BMS cut out due to under voltage protection around 60v which was normal. Tried charging again but nothing, the BMS doesn't even kick back on when I plug in the charger. I put a voltmeter to the charger output and I get 90mV which doesn't sound right. To me it sounds like faulty charger but it still turns on so I'm wondering if it's safe to take apart these 20S chargers to see if there's an internal fuse or some sort of protection that I tripped that needs to be reset? I don't want to get shocked by a high capacity cap. Never took a charger apart so I don't know how they are built.

 

[amberwolf]

What voltage does the battery read on it's charge port, and on it's discharge port?

If it is not enabling the charge port, the charger won't have a load, and if it's like many chargers it will not output the expected charging voltage if there isn't the expected load.


A battery that doesn't fully charge usually means it has cells (groups) that are less capable than others and so they reach full before the rest of them during charge, and reach empty before the rest of them during discharge. If there is enough of a problem with a group, it may keep dropping after reaching LVC (empty) and the battery may then be unable to recharge, as the BMS will turn off the charge port to prevent cell damage to an overdischarge cell (group) that can lead to a fire.


So if the charge port or discharge port has unexpectedly low voltage on it, I would recommend checking the actual cell (group) voltages in the battery to make sure they are all equal and none are below the BMS LVC (usually around 2.8-3v).

 

[Adrian_]

What voltage does the battery read on it's charge port, and on it's discharge port?

If it is not enabling the charge port, the charger won't have a load, and if it's like many chargers it will not output the expected charging voltage if there isn't the expected load.


A battery that doesn't fully charge usually means it has cells (groups) that are less capable than others and so they reach full before the rest of them during charge, and reach empty before the rest of them during discharge. If there is enough of a problem with a group, it may keep dropping after reaching LVC (empty) and the battery may then be unable to recharge, as the BMS will turn off the charge port to prevent cell damage to an overdischarge cell (group) that can lead to a fire.


So if the charge port or discharge port has unexpectedly low voltage on it, I would recommend checking the actual cell (group) voltages in the battery to make sure they are all equal and none are below the BMS LVC (usually around 2.8-3v).

Thanks for the response. I ordered a cheap 20s charger on Amazon prime and after modifying the charging port, I plugged it in to my battery and the BMS kicked back on and is charging fine, so it does seem that the charger just died while charging the battery which is why it only charged to 79v and why it refused to charge any further. From my experience, maybe cause I mostly deal with Chinese chargers and maybe you deal with smart chargers that don't behave this way, whenever I measure the output voltage of a charger without it being plugged into anything, it's always the full charge voltage, so a 20s charger will be 84v and a 13s charger will be 54v so when I only got 90mV on this faulty charger, my assumption was that it's broken which appears to be correct. I paid £125 ($140) for this charger so it's not particularly cheap and I'm shocked that it failed only after 4 charging cycles. Luckly it was withing 30 days since I ordered it so I was able to open a dispute on AliExpress. The reviews for it are generally good so it's possible I just got a bad unit. I'm glad that the charger atleast died peacefully and didn't cause a short or higher voltage to be sent to the battery, killing the BMS or worse; starting a fire.

 

[99t4]

Luckly it was withing 30 days since I ordered it so I was able to open a dispute on AliExpress.

Can you let us know how that resolves?