# Li-ion battery damage

Joined Mar 28, 2014
18
I connected a 3.7 Li-ion battery to a 100uF capacitor with 30 volts initial voltage. The capacitor is discharged and the battery voltage became -1.4 volts (negative!).
Is the battery damaged?

#### MrSoftware

Joined Oct 29, 2013
1,866
I'm not a battery expert, but I would be surprised if 100uF of charge would hurt anything larger than a tiny battery. However if it has protection circuitry built into the battery, maybe the tiny voltage spike tripped something? What else do you know about the battery?

Joined Mar 28, 2014
18
It's 600mAH and It did not have any charge when I connected it.

#### gerty

Joined Aug 30, 2007
1,291
I connected a 3.7 Li-ion battery to a 100uF capacitor with 30 volts initial voltage. The capacitor is discharged and the battery voltage became -1.4 volts (negative!).
Is the battery damaged?
Did you cross the polarities (positive to negative) ?

Joined Mar 28, 2014
18
Did you cross the polarities (positive to negative) ?
The polarities were correct.

#### gerty

Joined Aug 30, 2007
1,291
It's 600mAH and It did not have any charge when I connected it.
Are you saying you connected a charged capacitor to a dead battery?

#### BobaMosfet

Joined Jul 1, 2009
1,185
The Farad charge of the capacitor is irrelevant in a DC circuit (in this case). Electrons flow into the capacitor from the negative terminal. A capacitor is nothing more than an open circuit-- no current ever flows through the capacitor). As negative charge builds up on the negative capacitor plate, the field that generates pushes back against the incoming electrons until voltage potential is equalized-- as such, no more electrons can flow onto that negative plate. The positive plate is empty-- all loosely-bonded (covalent) electrons have been pushed away by the same field and drawn towards the positive supply terminal.

Essentially, what you're seeing is a capacitor acting as a battery connected to the battery in parallel. You will see a negative value-- and coincidentally it's interesting that it's about half of what the overall voltage is. The difference between the two is likely due to losses from the internal resistance of the battery.

Joined Mar 28, 2014
18
Are you saying you connected a charged capacitor to a dead battery?
The battery was healthy; but it's not charged now.

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#### BobaMosfet

Joined Jul 1, 2009
1,185
Makes me want to try this myself, but using an oscilloscope to see if there is an oscillation going on (a bit like a tank circuit).

#### wayneh

Joined Sep 9, 2010
16,399
However if it has protection circuitry built into the battery, maybe the tiny voltage spike tripped something?
That's my bet, although I can't explain a negative voltage reading. The battery itself is not likely effected, only the circuitry. The capacitor's charge would be a drop in the ocean compared to the battery's capacity.

#### MrAl

Joined Jun 17, 2014
7,763
Hi,

I FIRST have to ask why the heck anyone would do such a thing?

Li-ion cells have to be charged carefully and with a special charging routine.

A 100uf capacitor at 30 volts can put out 300 amps for a short time, about 10us.
The current depends on the ESR of the cap and the lead resistance mostly.
But most important is that if 30 volts appears across the battery for even a short time, it can jump the small gaps inside the material and short out the cell, and if there is enough energy stored or left from that it can cause the cell to explode and/or catch fire.

Li-ion cells are not the things to be played around with or some serious damage may occur.

Be careful what you try.

If there was a protection circuit in there it probably had activated to try to stop such an explosion and so may have permanently incapacitated the cell. Best bet is to get a new cells and discard the damaged one as it is very hard to determine if any damage is a safety hazard in an Li-ion cell.

Joined Mar 28, 2014
18
Hi,

I FIRST have to ask why the heck anyone would do such a thing?

Li-ion cells have to be charged carefully and with a special charging routine.

A 100uf capacitor at 30 volts can put out 300 amps for a short time, about 10us.
The current depends on the ESR of the cap and the lead resistance mostly.
But most important is that if 30 volts appears across the battery for even a short time, it can jump the small gaps inside the material and short out the cell, and if there is enough energy stored or left from that it can cause the cell to explode and/or catch fire.

Li-ion cells are not the things to be played around with or some serious damage may occur.

Be careful what you try.

If there was a protection circuit in there it probably had activated to try to stop such an explosion and so may have permanently incapacitated the cell. Best bet is to get a new cells and discard the damaged one as it is very hard to determine if any damage is a safety hazard in an Li-ion cell.
thanks, I learned alot.

#### Lestraveled

Joined May 19, 2014
1,946
Hold on. Was this a battery pack or a discrete cell? Many battery packs have the charge controller built in them. You may have fried the controller not the battery.

Joined Nov 5, 2010
215
Have you even tried charging the battery? How do you know there's any problem with it? If the battery was discharged, the protection circuit may have disconnected it from the terminals until it gets a reasonable charge current. The -1.4V may be an artifact of this disconnection. Or as Les says it may be fried. But trying to charge it should shed some light on it.

Unless it's a bare cell with no protection circuit. In which case if it's at or below zero volts, it's dead.

Joined Mar 28, 2014
18
Have you even tried charging the battery? How do you know there's any problem with it? If the battery was discharged, the protection circuit may have disconnected it from the terminals until it gets a reasonable charge current. The -1.4V may be an artifact of this disconnection. Or as Les says it may be fried. But trying to charge it should shed some light on it.

Unless it's a bare cell with no protection circuit. In which case if it's at or below zero volts, it's dead.
After some charging, now it has zero voltage.

#### Lestraveled

Joined May 19, 2014
1,946
Is it a battery pack or a discrete battery??

Joined Mar 28, 2014
18
Is it a battery pack or a discrete battery??
It's like the battery in the attached figure. Dimensions 3x4x.5 cm with two wires.
I can't read the nameplate it's in chines.

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#### Lestraveled

Joined May 19, 2014
1,946
OK, as it is right now the pack is dead. I suggest that you open/unwrap it. I suspect that something other than the battery failed. If you open it you may have the opportunity to fix it.

Joined Mar 28, 2014
18
OK, as it is right now the pack is dead. I suggest that you open/unwrap it. I suspect that something other than the battery failed. If you open it you may have the opportunity to fix it.

I opened the battery as u see in below picture:

It has a small PCB with 2 ICs. I fear to work the battery without its PCB. Maybe it gets fire or explodes.