NiMh Self Discharge Test

[SilverFox]

I had some extra Energizer 2500 mAh cells, and decided to charge them up and let them sit for a while to study their self discharge rate.

I have some sort term data that indicates that the current NiMh cells self discharge at a rate less than the 1% per day that is commonly stated. Using observations based on short term self discharge rates, I have come up with a rate of around 0.7% per day. That means that starting with a cell that has rested after charging, you should have 100% the first day, 99.3% the second day, 98.6% the third day, 97.91% the fourth day, and so on.

This rate is reasonably accurate for the first two months, but I was interested to see if it holds up for longer periods of time.

Keep in mind that there is an initial loss from hot off the charger, to resting. I am ignoring this initial loss.

I charged the Energizer 2500 mAh cells up and put them into room temperature storage. 6 months later, I ran a discharge test at 0.5 amps to see how much capacity was remaining. I then charged the cells back up and did another discharge test at 0.5 amps (allowing the cells to rest for 30 minutes after charging) to see what full capacity is, and compared the remaining capacity with the full capacity.

All discharge tests were down to 1.0 volts per cell.

Cell 1 - 43% remaining after 6 months
Cell 2 - 37%
Cell 3 - 39%
Cell 4 - 36%

This gives me an average of around 37% remaining capacity after 6 months of room temperature storage.

My short term tests indicate that the remaining capacity after 180 days should be about 29%. This could indicate that the self discharge rate slows down the longer the storage time.

The variation in remaining capacity is also interesting.

Tom

Edited to correct the self discharge % sequence. The rate was correct at 0.7% per day.

 

[wptski]

Tom:

Nice work! :thumbsup:

 

[chimo]

Tom, I'm guessing you mean 0.7% above. Your text shows 0.7% but the example is calc'd at 7%. :)

Paul

 

[wptski]

Tom, did you mean 0.7% or 7% above? :)

Paul

Paul:

You read that closer than me! There's a mistake in the math.

 

[Bogus1]

Thanks once again Tom for you work, forsight and patience. It's good to see this info.

 

[chimo]

BTW, thanks for doing this, Tom! I find your battery posts very informative. :goodjob:

Paul

 

[Handlobraesing]

Can you post the result for both coulombic and energy capacity? I understand that midpoint voltage depletes with storage, so even if the coulombic capacity sees no loss, the stored energy is reduced.

 

[SilverFox]

Hello Paul,

The self discharge rate was correct at 0.7% per day, however my sequence showing the rate was wrong. I have corrected that.

I seem to be having a bad number day... :)

Tom

 

[Archangel]

The discharge rate must slow down within a few days, because following 7% a day, 29% will be hit day 17.

My short term tests indicate that the remaining capacity after 180 days should be about 29%. This could indicate that the self discharge rate slows down the longer the storage time.

 

[paulr]

I saw some other post indicating a rapid self-discharge for the first few days, then much slower. 40% remaining after 6 months of uniform exponential decay would be 1-0.4**(1/180) = 0.5% per day. I think more likely, 10-20% is lost in the first few days and then there's much slower discharge.

 

[Brighteyez]

Isidor Buchman had some helpful information regarding self-discharge with Nickel-based batteries at http://www.batteryuniversity.com/parttwo-32.htm

I saw some other post indicating a rapid self-discharge for the first few days, then much slower. 40% remaining after 6 months of uniform exponential decay would be 1-0.4**(1/180) = 0.5% per day. I think more likely, 10-20% is lost in the first few days and then there's much slower discharge.

 

[SilverFox]

Hello Archangel,

I screwed up the sequence numbers. I have edited the first post to reflect the correct sequence numbers.

Tom

 

[Archangel]

Cool. The more i thought about it, the more i figured you were right the first time.

 

[SilverFox]

Hello Handlobraesing,

35% of the original watt hours were left after the 6 months of storage.

Tom

 

[Handlobraesing]

Hello Handlobraesing,

35% of the original watt hours were left after the 6 months of storage.

Tom

On which one of the samples, or is this the mean value?

 

[SilverFox]

Hello Handlobraesing,

That is an average value.

Tom

 

[LEDcandle]

Hi Tom, great work! Could I ask what's the resting voltage of the NiMh that was left there for 6 mths? (35% capacity left) Still above 1.2v? An NiMh's resting voltage doesn't indicate it's life like a Li-on right?

My NiMh batteries come off the charger at ard 1.4-1.43, then within the same day drop to about 1.36, and in another day or so, drop to as low as 1.29-1.3 and seems to stay there so far. I guess this is normal since they are actually 1.2v batts?

I read your post in another thread that if their resting voltage heads towards 1.2v too fast, they are 'dead'. How fast is fast and how close to 1.2v is close? Thanks!! :D

 

[Al]

I believe the self-discharge rate for rechargeables is a PERCENTAGE, thus giving the impression over time that the rate of discharge is slowing down, i.e: 3 percent of 100 (fully charged cell) will appear more than 3 percent of 50 (half-charged cell).

 

[SilverFox]

Hello LEDcandle,

These cells were all around 1.25 open circuit volts.

When you first remove a cell from the charger, the voltage will drop as the cell cools down. Once it has cooled down, the voltage should stabilize. If you continue to see the voltage drop like it did during the first few hours, there is something wrong with your cell.

Tom

 

[SilverFox]

Hello Al,

A point of clarification:

The self discharge rate is a percentage of remaining capacity, as your example pointed out.

Tom