Hi! Someone is selling me a used 12s4p battery made of Samsung 30q cells. Can you tell me what to look for when buying to make sure it is safe and still has great performance (seller who is also a battery builder says the battery has 30 cycles).
Should I check for power output with multimeter? What should I expect?
Should I ask him if he has put a fuse for every P group?
Please share your thoughts… Thank you!
You should post up a bunch of pics of the build process. Otherwise, tough to say. If they aren’t a builder on here, I’d be very skeptical until proven different with pics.
Yes what @abusfullofnuns said take lots of pics and post it on here before you buy 
and the crew will have a field day but they will give you the honest truth if it’s worth it 
Unfortunately there’s no way for us to know if the pack is free of hidden defects but if the pack is unwrapped we can check its construction. That’s important for both performance and safety.
You can make sure none of the p-groups are below 2.5V (which speeds up aging of the cells and can damage them) and then balance charge the pack. Wait a couple of hours and then measure the voltage of each p-group. Then wait at least a couple of days and remeasure the voltages. If any are more than a few millivolts lower than the others then one or more cells in those p-groups might be bad.
There is no easy way to check how well it performs at home. If we don’t see any issues with its construction and the voltages are okay then you could ride it a couple of times. That’s about the only way to check without expensive equipment.
Price, if its too cheap, there is usually a good reason for it…
As mentioned above, you must get photographs of the construction of THAT battery, not simply one like it.
Also if it’s suspicously cheap, then that’s a massive red flag as mentioned above.
30 cycles at 4A is a lot different than 30 cycles at 125A. You should ask for screenshots of their VESC current settings if they use VESC, or find out how much load was on the battery.
Crash — was it ever in a crash?
Does it charge all the way to 50.4V? What voltage has it been stored at?
These are all relevant discussions, hardware-wise.
As far as humans go, does the person have a history of shady deals? What do you know about them?
Also keep in mind your personal safety and the safety of anyone who lives in your home could be affected. Also the safety of anyone in your building if you’re in a highrise or dormatory. It’s not something to take lightly.
So many things to look for and others you just don’t have the equipment to test
Only real way is to take it apart to check construction the rest is chemical
It ends up more exspencibe to test and check a used pack that you may as well buy a new one from a reputable builder.
It’s a Roll a dice buying a used pack all you can do is stack the Dice in you faver by geting as mutch info from them as possible.
If you got any pic of the construction many egel eyed keyboard Warriors on this forum will eye it up and rip any thing slightly off to shreads for you