high internal impedance

[viorel00]

Hi, I have two NIMH cells that came with my cordless phone. Rated 1400 mAh, about 2 years old.

I tested them in my BC-900 and one has 1400+ mAh, the other one around 1000 mAh. I guess this is because they have been charged/discharged in that dumb charged (i.e. handset).

Anyway, the one with lower capacity seems to have a higher internal impedance (resistance), as soon as I start a discharge, the voltage is lower that for the good cell, and opposite, when I start a charge cycle, the voltage is higher. This seems to indicate to me high internal resistance.

I already tried a refresh cycle on the BC-900 and it gained from 915 to ~1000, but that's it.

I don't really care about it, I am prepared to toss it, but I was wondering if there is anything else I should try before I toss it. More like a learning experiment. I have a 250A power supply, would a high current spike revive it? I seem to recall reading about this with NiCd, what about NiMh?

 

[SilverFox]

Hello Viore,

High impedance is sometimes caused by large crystal growth within the cell. To break down the large crystals, you discharge, then discharge again at a lower draw, then discharge again and a lower draw again. This is followed by a forming charge at 0.1 C for 16 hours. Once all of this is done, you discharge at 0.5 C and charge at 0.5 C and go through 3 - 5 cycles of this.

So, starting with a fully charged cell, discharge at 500 mA down to 0.8 volts, if you can. Follow this with another discharge at 250 mA again down to 0.8 volts. Finally, discharge again at 50 mA.

Now that the discharge is complete, it is time to do the forming charge followed by some charge/discharge cycles.

This works 50 - 70% of the time.

Tom

 

[viorel00]

Hi Tom,

I have the BC-900, and I will try your recipe, except I am gonna start with 500 mA down to 0.9V, pull it out, wait a bit (right?), then discharge again at 250 mA down to 0.9, and again 100 mA down to 0.9 V. BC-900 does not go lower than 0.9 V, and the lowest discharge current is 100 mA.

I will try it over the weekend and let you know if it worked or not.

Thanks

 

[65535]

Cells getting charged and discharged for years die. Sounds like they are getting old you can breath some more life into them not much though.

 

[Handlobraesing]

about 2 years old.

Sounds like end of useful life. Significant increase in impedance is how my phone batteries died too. If you short out each cell with an ammeter (there is about 0.1 ohm, so you won't get much more than 10A, so you're safe) and you should get at LEAST 3A. New cells would push 10-12A. If you don't, the cells are done.

My opinion on the best care of cordless phone is to avoid total discharge and don't make the charging base a home. Leave it off the charger, but let it charge over night every once in a while.

Total discharge leads to polarity reversal.
Chronic trickle charge also leads to damage, especially NiMH.