Playing with fire

[(*steve*)]

A week or so (it seems, it may have been a month) I checked one of my el cheapo electric screwdrivers and it was telling me the battery was VERY low.

Having misplaced the charger, I've been promising myself to either find it or if something up.

Today it wasn't reading a charge at all. Worse, it wasn't accepting a charge.

Opening it up I find the cell was at 1.05V. not good. Clearly the charger prevents charging below some lower limit.

Well, what's the worst that can happen? I started charging it at 20mA and the voltage started rising. At 2V I increased the charger to 40mA. Eventually I had it being charged at 100mA with a voltage of just over 3V when the low battery warning started working again.

At this point I could again charge it via the charging port, and at 5.5V it was drawing a bit more than 350mA.

Also at this point, I extended the leads from my power supply so the drill can sit outside on paving bricks. If it catches fire, I don't want it setting fire to anything other than itself!

My research indicates that copper dendrites (the cause of catastrophic over discharge failure) don't actually form unless a cell gets a reverse voltage on it. The damage at deep discharge is limited to changes in the cathode which can significantly reduce capacity. Since it's a single cell, reverse charge won't happen.

I just checked it, placing it in a plastic bag (because there's a very small chance of light rain tonight) and the drill shows good torque, so the cell can handle moderately high discharge currents.

I'll probably replace the cell, because I'm not going to trust it now. At least a replacement cell is slightly cheaper than a new screwdriver :-D

 

[(*steve*)]

No explosions during the night.

Now I'm carrying around this screwdriver like it's a pipebomb :-o

Let's see how it goes...

 

[ChosunOne]

Copper dendrites? Voltage up to 5.5V? What's the composition of this cell?

 

[(*steve*)]

Lithium cells when overcharged produce lithium dendrites which short the cell. When heavily discharged (I've read 12% reverse charged) you get copper dendrites which short the cell.

The 5.5V was the input to the inbuilt charger (it is rated for 6V). There is a linear regulator in there as part of the charger. Since the voltage on the cell was initially low, a slightly lower input voltage would reduce dissipation a little (it also reduced the charge current).

Yeah, it's a single unprotected 18650 lithium cell.

 

[kellys_eye]

I've yet to see any real battery 'event' of any sort...... and I'm not the best at respecting their needs either. Whether it's because I've been lucky (so far!) or haven't appreciated just how much abuse they really can take I'll never know.....

Same goes for the 'exploding battery charger' scares you read about. Nothing. Nada. Zilch. Not that I'm 'hoping' to be the victim of one but I wonder just how many such events are as a result of social media reporting rather than being common as they are claimed????

I've even seen Youtubers trying to deliberately explode/damage batteries by over-charging, hammering nails through them etc. Not many spectacular results.

But.... who's going to chance it now?

 

[(*steve*)]

But.... who's going to chance it now?

When I consider the cost of that drill compared to even one piece of test equipment in my shed...

 

[shrtrnd]

I've seen car batteries explode, cell phone batteries explode, and laptop computer batteries start on fire.
I give-up trying to figure-out why. Most of the time I figure it's a short somewhere, but I don't discount the construction of the battery, considering the lack of quality control in some of the places making batteries these days.
*steve*'s got more guts than me with these things anymore. If I question the battery, I toss it. My kid had a laptop
battery start a fire when he had the computer plugged-in, and nobody home. Just paranoia, I'm sure, ... but that's just me.