Question about charging NiMH

Hello,
I use NiMH rechargeables for many devices from MP3 player to flashlights since 1 year and I noticed something when charging batteries. I often test the batteries voltage with a multimeter after charging and I noticed that the final voltage of batteries incrased after many charge cycles. At beginning, with Duracell 1800mAh it was 1.43V and now it's around 1.48V. Same thing is happening with Energizer 2500mAh. Around 1.4V at the beginning and I have now 1 battery I test around 1.45V after charging. Is it related to internal resistance decreasing?

WildChild said:

Hello,
I use NiMH rechargeables for many devices from MP3 player to flashlights since 1 year and I noticed something when charging batteries. I often test the batteries voltage with a multimeter after charging and I noticed that the final voltage of batteries incrased after many charge cycles. At beginning, with Duracell 1800mAh it was 1.43V and now it's around 1.48V. Same thing is happening with Energizer 2500mAh. Around 1.4V at the beginning and I have now 1 battery I test around 1.45V after charging. Is it related to internal resistance decreasing?

Click to expand...

It is actually related to an INCREASE of the internal resistance. This leads to an increased overvoltage during charging and subsequent higher OCV.

What is OCV?

open circut voltage.
btw most nimh need 10-50 deep cycles to come up to full capacity.

So, this increase in OCV shows the incrase in capacity of the cell?

Thanks

WildChild said:

So, this increase in OCV shows the incrase in capacity of the cell?

Thanks

Click to expand...

Usually not. When the cells are ageing their capacity is gradually decreasing (how quickly depends on how they've been charged). Parallel to this their internal resistance is increasing. This leads to a higher voltage during charging. If you measure the Open Circuit Voltage (OCV) shortly after charging, they will show higher OCV compared to new cells but their capacity will be lower.
During discharge this higher internal resistance leads to a lower viltage under identical load. Say,a new cell will plateau at 1.25-1.2V under C/3 load. Older cells will do it at 1.2-1.15 V. This is a bit simplified but I hope you get the picture.

Hello Blr,

I believe that Snakebite was referring to the Sanyo information that indicates that at a 1C charge using a - delta voltage charge termination value of 10 mV and discharging at 1C, they are seeing an increase in capacity over the first 150 cycles of the cells 500 cycle life.

This information is referenced in Figure 2 of this paper.

It would seem that if the capacity of the cell is increasing over the first 150 cycles (at a 1C discharge rate), the internal resistance of the cell should be decreasing during those cycles as well...

Tom

SilverFox.
If you look at Fig16 in the same document you will see that the peak voltage increases during charging and there is a measurable difference between cycle#1 and cycle #150.
At the same time from Fig15 it can be seen that the internal resistance is increasing during these first 150 cycles, although there is a increase in discharge capacity.

blr2005 said:

Usually not. When the cells are ageing their capacity is gradually decreasing (how quickly depends on how they've been charged). Parallel to this their internal resistance is increasing. This leads to a higher voltage during charging. If you measure the Open Circuit Voltage (OCV) shortly after charging, they will show higher OCV compared to new cells but their capacity will be lower.
During discharge this higher internal resistance leads to a lower viltage under identical load. Say,a new cell will plateau at 1.25-1.2V under C/3 load. Older cells will do it at 1.2-1.15 V. This is a bit simplified but I hope you get the picture.

Click to expand...

But the cells are not much used. The Duracell ones (bought 11 months ago) have about 15 charge cycles and the Energizer ones (bought 4 months ago) about 6 charge cycles...