Did I kill my battery?

kiltedcelt

100 W
Joined
Aug 19, 2015
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158
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Chicago, IL USA
I have a 36v battery (Dolphin case from em3ev), that is about three years old. I haven't used it in over a year and a half but I thought I'd been recharging it regularly enough to keep some life in it. I thought I'd just give it a recharge but the charger light is showing green, and while the LED power light on the side of the battery (that push button on/off switch), lights up, the push-button battery gauge doesn't light up and when I throw a multimeter on it, it only shows 3.7 volts. Have I killed this battery? Is there any way, or some specific charger I could use to bring it back? I wasn't planning on using it in an e-bike but was thinking it could power a large power bank kind of thing I was planning making for use for disaster/power outage situations or powering a small sound system, etc. Is this thing just a giant toxic paperweight now?
 
That means the BMS has shutdown to protect the pack. To find out why, you'd have to open it up and start measuring the cell groups at the BMS, starting at the main battery negative and going up to the main battery positive, moving both probes at the same time so you're only measuring one cell at a time. List the voltages here, starting from main negative, and it'll show which cell group(s) are high or low and preventing charge and discharge.


Most of the time cheap BMSs seem to be powered by just a few cells toward the negative end of the pack, so it drains those cells, making htem unbalanced compared to the others.

If it's only ever quick-recharged, then the BMS never gets to top those cells off, because the other cells are high enough voltage already that it stops the charger when they're full. The difference between the full and not-so-full cells gets greater and greater and eventually teh not-so-full cells are mostly empty, and the BMS shuts off output to keep you from overdischarging them, and then at some point the BMS shuts off input to keep you from recharging the low cells after they drop below the "safety cutoff" point.

When instead the charger is left on the battery until it stops cycling on and off, the BMS will rebalance the battery. It can take anywhere from several hours to several days or even weeks, depending on the level of imbalance.
 
Thanks for the concise answer Amberwolf. Since this is an em3ev battery I'd be inclined to think that the BMS should be decent quality, because they build pretty solid batteries. I'm just betting I wasn't charging it regularly enough and it just went dead too far. The charger that came with the battery was a slow charger that would typically take about six hours or so, maybe more to bring the battery up to full charge. Any kind of "fast" charger was never an option because apparently the tiny charger connector and the BMS wouldn't allow anything other than something like a 5amp input. Would that charger that came with the battery even work to recharge it if the cells needed balancing somehow? When I plug it in, the charger just has a green light on which in theory indicates the pack is charged, but the multimeter says otherwise, and a simple USB rechargeable light plugged into the USB accessory port on the pack also doesn't charge which indicates again that the battery is in a severely depleted state. Originally this battery didn't have enough range which is why I upgraded to a 52v 28 Ah battery. I thought about trying to sell the 36v battery, but at the time 36v systems were already pretty much passed on by so I couldn't imagine anyone would want to buy the battery. I'd hoped to be able to still use it, as I mentioned above as a kind of really big powerbank for powering accessories and such when I was on camping trips or biking fun rides and and such. If the cells can still be balanced, what charger would be able to do it, since it seems like the stock charger isn't capable? (fingers crossed) Please don't say Cycle-Satiator - I don't have that kind of money. :(
 
If you can get to the cell connections and measure the voltages of the individual cell groups we can figure out happened to cause the BMS to shut things down.

If it's just a cell group or groups that it considers too low to charge, depending how low it is you may be able to bring it back to life by bringing them back up individually. It's not a matter of a strong enough charger, it's just a matter of figuring out the current state of the battery pack.

As amberwolf said it's more than likely because the BMS was pulling power for itself from a subset of the cells, instead of the whole pack.

Lots of them do this, probably the most common cause of "battery I haven't used in a while doesn't work now"
 
kiltedcelt said:
When I plug it in, the charger just has a green light on which in theory indicates the pack is charged, but the multimeter says otherwise, and a simple USB rechargeable light plugged into the USB accessory port on the pack also doesn't charge which indicates again that the battery is in a severely depleted state.
I'm just going to quote myself from above, rather than retyping, then add more below that:

https://endless-sphere.com/forums/viewtopic.php?p=1409122#p1409068
amberwolf said:
That means the BMS has shutdown to protect the pack. To find out why, you'd have to open it up and start measuring the cell groups at the BMS, starting at the main battery negative and going up to the main battery positive, moving both probes at the same time so you're only measuring one cell at a time. List the voltages here, starting from main negative, and it'll show which cell group(s) are high or low and preventing charge and discharge.



kiltedcelt said:
If the cells can still be balanced, what charger would be able to do it, since it seems like the stock charger isn't capable? (fingers crossed) Please don't say Cycle-Satiator - I don't have that kind of money. :(

No full-battery charger is capable of fixing the problem, because your BMS has probably shutdown to protect you from overdischarged cells. So nothing can charge thru it. After fixing those low cells, so the BMS reactivates the input, then any full-battery charger meant for that voltage pack (about 42v) including your original could fix the rest of the balance, along with the BMS.


After you do the quoted part from my post above, we can see if it's even safe to rebalance it. If cells drop too far, you don't want to reuse them, because chemistry changes occur that can make them unsafe. That's why the BMS shuts off even the charge input.

If they're still in the safe range, then a low-current (100mA to maybe 200mA) power supply that puts out no more than 4.2v can be used to recharge the low cell(s) until they get to about the same as the rest of the cells that are higher, and then you can just plug in the regular charger to do the normal charge, and leave it on for a few days or weeks to let the BMS do the balancing it's designed to do. (depends on how close you can get the low cells to the high cells). They usually call them single-cell chargers, but you may well have a wallwart (AC power supply adapter) for some battery powered device laying around that already does what you need. Or maybe find one at Goodwill, etc.
 
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