Replacing bad cells in a working set

Sub_Umbra · Jul 5, 2004

When I buy rechargeable cells I usually buy extras in case of accidents or whatever. This hasn't happened yet, but I'm sure that if I live long enough it will.

Let's say I bought some NiMH cells new for a device and at that time I also bought some extras in case I needed to replace one at a later date. Suppose that after breaking them in and using them for 6 months I dropped one and it was wiped out. Even if I had an identical new cell to replace it with, how would I do it? Would I have to 'exercise' the new cell somehow before I used it, or could I just drop it in with the old set?

Even though I used NiMH as an example, I am definitely looking for the answer to the same problem if it occurred with Nicads too, as I use both.

I know that at least some of you have gone through this already.

Thanks

eluminator · Jul 6, 2004

New cells often don't charge completely when charged in a "smart" charger. They somehow fool the charger into thinking they are charged fully when they are not. A few charge/discharge cycles will fix this.

I always use a CCrane charger when I have new cells. This charger will show you which cells are charged fully and which ones aren't. The key is to analyze each cell and look at the voltage. A good cell will show 1.45 volts just after charging, when the charger switches from "topoff" to "trickle". This voltage slowly drops to around 1.39 volts in 24 hours or so. New cells, and over-discharged cells will often show a voltage from 1.25 to 1.35 volts.

This charger also has a discharge function which makes it easy to condition the cell so it works as it should.

Because this charger doesn't charge each cell independently, but rather in parallel, it's a good idea to analyze new cells and only charge together those cells that have a voltage that is similar.

Once a cell has been broken in, any "smart" charger will do the job, until the cell has been damaged by discharging it too much. Once again the C Crane will show which cell has been damaged by over-discharging, and will allow you to recondition it. Sometimes the cell can't be brought back to normal and should be discarded.

Cells that have been discharged too far act like new cells. They trick the smart chargers into thinking the cells are fully charged when it fact the voltage is lower than it should be.

There is no such thing as "identical" cells. There are always minor differences in the capacity of cells. When you use several cells connected in series in a device, one will become exhausted before the others. If you start with several healthy cells and discharge them until most are at about 1.0 volts, you will see that one of them will be at a lower voltage. That one is just about exhausted. If you are lucky, it will read around .7 volts and will probably be good. If it's .5 volts, it may or may not need to undergo a charge/discharge cycle to make it normal. I've had them show as little as .1 volts and still be capable of being brought back to normal with several charge/discharge cycles.

To put it another way, buying a bunch of new cells, charging them in a "smart" charger and using them without measuring them, is a recipe for disaster. You can easily end up with a cell that is destroyed because it was never charged very much, but the other cells were charged fully.

It would be wise to recharge when the cells are at 1.1 volts or even 1.2 volts.

Personally I would replace the entire set when one goes bad if they are AA cells, but probably replacing one with an unused one from the same batch would work okay. It should be analyzed and conditioned if necessary, for best results. I would suggest you get a battery analyzer/conditioner. Did I mention the C.Crane? /ubbthreads/images/graemlins/smile.gif

D cells are so expensive I would try replacing one, because of the cost of replacing all of them.

Canuke · Jul 9, 2004

Well, that likely explains what happened with my 1850mAh Energizers... one of them seems to have died. I now have three cells that show 1.27 volts and easily run my X1 to full brightness, while a fourth cell shows .9V and runs the X1 dimly. These are all used in a digital camera, which doesn't turn on with these cells in this condition.

That cell first gave a trouble sign earlier by causing the charger to flash its light (instead of being "off" for the other three) after charging, now shows .9V. I tried moving it to another cell, and it charged. I marked both the odd cell and its charger slot in case something else happened, and here we are.

These are new, have only been through about 3 or 4 cycles so far, and the bulk of said discharging was self-discharge (I haven't been using the digital camera nearly as much as I originally thought).

Is it salvageable? or should I replace it? If I replace it, how should I try to match the three existing ones with the new one in terms of charge level?

These are the 15 minute Energizers with matching charger, btw...

eluminator · Jul 10, 2004

I'm not sure what the flashing light means in your charger. I guess it means the charger thought it was bad and unusable. It worked the second time you tried to charge it. That's why smart chargers ought to be called stupid chargers. They are okay for cells in good condition, but otherwise they aren't much help.

I don't know if it is salvageable. I would run it through a few charge/discharge cycles and see if the voltage comes up to normal. That's easy for me to do, as I have the C Crane. I can only pity the rest of you /ubbthreads/images/graemlins/smile.gif

If the voltage looks good, the real test is to use it in series with the good cells. When the other cells have been discharged as low as you care to go, if that cell has a voltage that is similar, then it's okay.

RussH · Jul 14, 2004

I mix & max NiMH AAs frequently since I have about 40 of them and several devices that use 1-4 cells. My radio (Sangean 909) quits when battery voltage gets below 1.1v per cell, and this is what I use to break in a new set(s) of batteries. You could have one cell get too low but it hasn't happened yet. Normally the battery closest to the negative end is a little lower (about .05v) than the two middle batteries and the cell on the positive end is a little higher.