Lithium polymer cells dropping down to 1.8V and not burning?

[FninjaP90]

I recently bought two Polyquest 3S 4300mah packs to power a Mamba Max on my project car. They were both well charged when I bought them, and one is still unused and at almost full voltage. I've been using the other one to set up the motor, servo, and testing the car out for a few minutes at low speeds outside.

Today I noticed that the car wasn't responding to transmitter input, so I measured the pack and saw that it was 5.5V. I was shocked and threw them on my TCM-equipped Triton charger, but the pack refused to take charge after a few seconds and topped out at 7.5V. I'm pretty sure I didn't use all 4300mah with the little use I've given it. In addition, my Mamba Max's auto lipo-cutoff at 3.0V/cell didn't kick in.

Wouldn't discharging the pack down to 1.8V/cell destroy the cells? Wouldn't it smoke or get hot? The pack appears perfectly fine from the outside. Jaime from Starluck said that I should try to individually charge the cells without the PCM and that it might accept charge.

Any idea what's going on?

[nicholcgn]

The voltage dropping down to 1.8 will not destroy the cell always. What it can do is damage them. IF they are damaged your chance of an issue when charging is greatly increased. IF you decide to keep using the cells charge them in some sort of safe container and store them in one also. You should do this with good cells anyway. Monitor their capacity . IF their capacity is 80% or less you should probably consider removing them from service and disposing of them. DO NOT CHARGE THEM UNATTENDED. This should be a rule for any types of batteries actually.


Most of my cells came about 1/2 charged. So you may not have had completely charged cells.

[tcolesen]

It must just be that one of the cells went bad, and the others are fine. Check the voltage of each cell, and that should tell you what's going on.

[FninjaP90]

I'm measuring the voltage diff between each of the green brackets:
http://img.photobucket.com/albums/v120/fninjap90/lipo.jpg
That's the PCM lead layout, and I am getting ~2.8V for each cell right now. I'm assuming that's how you charge each individual cell?

[kschauwe]

That's the PCM lead layout, and I am getting ~2.8V for each cell right now. I'm assuming that's how you charge each individual cell?
Yes. By charging each cell individually, it's also a cheap way to balance a pack.

[guver]

Your cells are still in balance then and maybe a slow charge (at first) will bring the pack above the lower limit. How did they get to 2.8 volts from 1.8?

Maybe there was a load on them?

[FninjaP90]

I'm charging them individually right now and they seem fine. The first two cells were charged 4100mah and 4300mah respectively, and the third cell is charging as I'm typing this.

When I first read them yesterday, they were at 1.8V. I put the pack on the PCM'd charger for a sec, and they shot up to 2.5V. Today, they somehow went up to 2.8V and are being charged from there.

I didn't overamp this pack, so I think it should be fine. Thanks for the help guys.

[nicholcgn]

4100 and 4300 mah? That would be an item of concern. These cells should be almost identical in mah. You might want to nicely charge and discharge each cell and get a baseling of capacity. This will let you know what is truly going on.

I would stilll closely watch this pack for quite a while. Make sure the cells are balanced before and after a run.

[FninjaP90]

After being charged, each cell was exactly 4.18V, so I think they should be fine.