A story of some degraded NiMH cells

[Mr Happy]

In another thread I mentioned that I happened to obtain some 2700 mAh that measured exactly zero on a voltmeter. I am in the process of examining them to see what they can do, starting with a break-in cycle on the C9000.

Others have mentioned that they have experienced missed terminations with 2700 mAh cells and so my investigations may shed some light on that.

The cells I have are labelled "Targus 2700 mAh". I do not know the original manufacturer, but I suspect it might be GP.

To begin the story, the cells are currently at about 3100 mAh of charge supplied in the first part of the break-in cycle. I used an entered capacity of 2500 mAh here rather than 2700 mAh to be conservative with unknown cells, so the charge current is 250 mA.

The voltage behaviour has been interesting. Almost immediately after charging was underway the voltage had risen to 1.39. It remained at 1.39 V for hours without increasing, remaining there after 1000 mAh of charging when I had to leave the charger for a while. Now at 3100 mAh, the voltages are 1.43/1.43/1.41/1.42. Clearly these cells would not be terminating on the C9000's max voltage (1.47 V) if they were undergoing a normal charge. Also, it doesn't much look like there would be a detectable -dV signal at this point. These cells are very likely candidates for missed termination and overcharging.

After the break-in cycle finishes I will see what capacity they register and then I will experiment with normal charging at different rates to see what happens.

To be continued...

 

[45/70]

I can't say this for sure, as I haven't documented it or anything, but as I recall, when cells are charged at such a slow rate, they are not as likely to reach a voltage as high as 1.47 Volt. I generally don't charge at slow rates, except for forming or break in, but you may have something there.

Dave

 

[Mr Happy]

I can't say this for sure, as I haven't documented it or anything, but as I recall, when cells are charged at such a slow rate, they are not as likely to reach a voltage as high as 1.47 Volt. I generally don't charge at slow rates, except for forming or break in, but you may have something there.

I have observed my eneloops to reach 1.47 V on a forming charge, but eneloops are special.

The missed terminations reported here were typically at low rates like 400 mA on a 2700 mAh cell. If maxV is not reached then termination will have to rely on other methods like -dV or 0 dV and there may be a lack of detectable signal for those too.

 

[MarioJP]

Is that bad if a cell hits to 0 volts?? Can this cause permanent damage?

 

[clintb]

Is that bad if a cell hits to 0 volts?? Can this cause permanent damage?

Depends on the chemistry. In the context of this post, which is NiMh (NiCd also), it also depends. As a single cell, it is not a problem if the cell hits 0V, but NiMh should not be left at that state for long periods of time. NiCd, on the other hand, and from what I remember, can be left at 0V for longer periods (think: longer than 1 month). Basically, you're getting into the area dubbed "Storage Queens". In a multi-celled pack, 0V is a bad thing; just don't go there.

 

[MarioJP]

Also are Nickel Metal Hydride are forgiving if overcharged, or has the damage already been done??.

How much overcharge does it take for a cell to become paperweight?

 

[45/70]

Also are Nickel Metal Hydride are forgiving if overcharged, or has the damage already been done??.

How much overcharge does it take for a cell to become paperweight?

MarioJP, I think I'm starting to see why you collect paperweights! :nana:

I think a lot of the questions you have, could be answered here. This guy is with Cadex. He's written a really good unbiased (they make battery analyzers and chargers, not batteries) collection of information regarding rechargeable cells. You don't have to read it all at once, take a couple days, it's easier to digest that way. There's a lot of good info there and this site is often referenced here on CPF.

Dave

 

[Mr Happy]

Is that bad if a cell hits to 0 volts?? Can this cause permanent damage?

Yes, it's generally not a good sign if an NiMH cell reads 0 volts when you receive it. Some bad and irreversible things can happen to the chemistry inside the cell under those conditions. Therefore I am interested to see how these cells turn out.

Also are Nickel Metal Hydride are forgiving if overcharged, or has the damage already been done??.

How much overcharge does it take for a cell to become paperweight?

NiMH can take some overcharge, but not a lot. When they are overcharged oxygen gas is produced at one of the electrodes which raises the internal pressure in the cell and at the same time the cell starts to get hot. If the overcharge is moderate the oxygen gas recombines with hydrogen via a special catalyst and produces water. This relieves the pressure and the cell recovers. NiMH cells are usually designed to withstand a continuous 0.1C overcharge, and can withstand short periods of higher rate overcharge.

If you exceed the overcharging limits the pressure inside the cell becomes too much and it vents the excess oxygen (and steam) out of the safety valve. This causes a permanent loss of electrolyte and thus permanent degredation of the cell.

However, even if you don't exceed the pressure limits the cell may still get too hot and temperature itself can cause damage.

Overcharging damage tends to be gradual and cumulative. A cell won't necessarily become a paperweight after one event, but keep doing it and the cell will eventually be toast.

 

[MarioJP]

So this means if you buy brand new cells in they read 0 volts. These cells are pretty much damaged by the time you start using them??

One of the new cells reads 0 after I got them.

 

[Mr Happy]

So this means if you buy brand new cells in they read 0 volts. These cells are pretty much damaged by the time you start using them??

Yes it could mean that. That's what I plan to find out with these cells.

It's best if new cells read over 1 V when you get them, preferably 1.2 V or more. They should also be freshly manufactured, less than six months old.

 

[TakeTheActive]

So this means if you buy brand new cells in they read 0 volts. These cells are pretty much damaged by the time you start using them??

IMO, if you buy BRAND NEW CELLS and they read 0VDC O.C., you should immediately return them. :thumbsdow

 

[MarioJP]

well that's definitely sucks lol

 

[Mr Happy]

Well these old and dead Targus 2700 mAh cells are not looking too promising. A break-in cycle on the C9000 at 250 mA charge, 500 mA discharge gave capacities of 1749/1839/1547/1668. A second 500 mA discharge after the break-in gave 1795/1857/1716/1712. Based on the voltages during discharge they seem to have a slightly high internal resistance (c.f. charging temperature below).

They have just finished a normal charge at a rate of 1600 mA. For each cell the charge finished after about 2100 mAh charge supplied with a cell voltage of about 1.43 V and a maximum cell temperature of 65°C

I'll discharge them after they have rested a bit, but I am guessing they will still measure around 1800 mAh.

I don't know if these cells were closer to 2700 mAh when they were new, but sitting unused for so long has not done them any favours. Actually I wonder if sitting in the charger drained them right down...there was no insulation to to isolate the cells from the charger contacts.

 

[MarioJP]

So when do you know when it is time to replace the battery. Does this depends many factors??

 

[Mr Happy]

So when do you know when it is time to replace the battery. Does this depends many factors??

It depends to some extent on what you are willing to accept. But an oft-quoted rule is that you replace the battery when the capacity has decreased to 80% of the design capacity. In the case of 2700 mAh cells, 80% would be 2160 mAh, so the cells I have are essentially in the "crap" category and not worth keeping. This is especially true since they got so hot in charging and probably would not terminate reliably when charged at a lower and cooler rate.

 

[45/70]

In addition to Mr H's 80% rule, I like to add, if the cell won't hold a high enough voltage under load, and if self discharge becomes a problem.

My 3 cents. (That's worth 50% more than 2 cents )

Dave