Why does a cell's apaprent capacity increase just as high self-discharge sets in?

[Turbo DV8]

Why does a cell's apparent capacity increase just as high self-discharge sets in?

I don't think this is my imagination. As I have been identifying the increasing number of my ROV Hybrid AAA cells which are succumbing to high self-discharge, I have also noted that when tested, the cells apparent capacity in TEST mode on the BC-900 increases also, compared to when new. This is along with the expected larger jump in voltage when inserted in the charger, and lower voltage sag under discharge load, indicative of higher internal impedance. But why would the tested capacity increase when compared to when new, then discharge rapidly upon sitting?

 

[Bullzeyebill]

In your example you talk of lower sagging as a result of higher impedance. Quite the opposite, lower impedance would result in less sagging of the cell under load.

Are you talking about the drop off in voltage that occurs just after charging? Some of these LSD AA, and AAA cells can reach 1.50 volts at full charge, but they will always drop fairly quickly to around 1.42 volts or so, sometimes lower. This is a normal occurrance with NiMh cells, high capacity, or the lower capacity LSD's.

 

[VidPro]

Mabey some of it has to do with the termination alogrythm of the charger, a "loose" battery will not terminate as quickly, so it will overcharge more, therin destroying it more still.
so mabey that is why? it gets more charge, as it loosens up (high resistance) and that overcharging also doesnt help it none.

 

[45/70]

I don't mean to PYO, Turbo, but you aren't reading the mAh's that are input into the cells, are you?

In my experience, cells that are experiencing high self discharge have increasingly less capacity than good cells. On the other hand, when charging these cells, they do take in more mAh to achieve a full charge, due to internal losses within the cell, caused by separator damage.

Dave

 

[SilverFox]

Hello Turbo DV8,

I believe in the test mode the BC-900 charges the cell, then discharges it to determine capacity. There is no time delay between charging and discharging to let the cell cool off.

As cells age, their internal resistance increases. This means that they will heat up more during charging. The RC people have discovered that hot cells perform better.

You may find different results if you separated the charge from the discharge by a time that allows the cell to cool off after the charge.

Tom

 

[Turbo DV8]

In your example you talk of lower sagging as a result of higher impedance. Quite the opposite, lower impedance would result in less sagging of the cell under load... Are you talking about the drop off in voltage that occurs just after charging?

By "lower" I meant the voltage sags lower with higher impedance, not to a "lesser degree."