Hello Joypog,
Trying to recover damaged cells is a labor of love. If your time is worth anything at all, it is better just to recycle the cells and purchase new ones.
However, some of us are easily entertained, and between flashlight purchases, we like to play with batteries...
It seems that almost all of cell degradation has to do with separator failure, in some manner or other. When starting with new cells, the separator may not be fully saturated, or equally saturated with electrolyte, and we do a 16 hour 0.1C charge to evenly distribute the electrolyte.
In constant use, the final "nooks and crannies" of the separator are finally saturated with electrolyte and the cell capacity will peak. Then we start a downward slope, in capacity, until the cell doesn't hold enough capacity to meet specifications.
Keep in mind that this downward slope is natural, and the cell eventually wears out. This is considered normal wear and tear. If your cells are worn out, there is nothing that you can do to bring them back. However, if some "premature" aging has taken place due to something like extended storage, then you may be able to restore life to your cells.
Your original question asked if there was a best charge/discharge rate to use when trying to restore life to your "abused" cells. I will turn this around and say that there is a technique that is used that involves different charge and discharge rates.
Before we get started on this, let me stand up on my soap box and say that the best way to restore your cells health is to make sure they never get sick in the first place. Batteries perform best when they are used. Storage tends to be hard on batteries, so limit your "extra" purchases and try to use all of your cells as much as you can.
It is good to have some extra's on hand, so let's look at how to take care of them. When you get new cells, check the open circuit voltage. Any that are below 1.0 volts (1.2 volts is better) should be returned and replaced with newer stock. Next you should do a 16 hour charge at 0.1C and a discharge at 0.2C to determine the cells capacity. This tested capacity should be in close agreement with the manufacturers data sheet capacity. If you can't find a data sheet, then you are on your own and will have to simply note the tested capacity. Keep in mind that tested capacity is often different from labeled capacity.
Any cells that fall short of the manufacturers specifications should be sent back and replaced.
Finally, the best way to store NiMh cells is to discharge them (at 0.5C or 1.0C) down to 1.0 volt and store them discharged. After 30 days of storage you should exercise the cell by running a charge/discharge cycle on them. This will keep your cells in top condition and they will be ready to enter into service when needed.
The low self discharge cells are a little different and we are still learning about them. They originally didn't benefit from a standard charge, but now it seems that they do. Since nothing is lost in doing this break in or forming charge, I think it should be done on them as well.
In storage, we have seem some increase in internal resistance when they are stored completely charged. This seems to be reversible, but I am not running some tests storing the cells at 50%, instead of fully charged.
The low self discharge rate means that the cells are more stable and a monthly charge/discharge cycle is not needed. I have found that if you stretch this out to a year, the cells need a few cycles to come up to peak performance. I am now recommending running a charge/discharge cycle every 6 months, or if you want better performance from your cells, every 3 months.
Cells should be charged and discharged in the 0.5 - 1.0C range and stored in a cool location, but they don't have to be refrigerated.
OK, I'll hop off the soap box now...
To try and restore some life to old cells you need to reverse any damage or deficiencies that have occurred within the separator in the cell. Any damage that has occurred to the electrodes is most likely permanent and can't be reversed.
Start by discharging the cell at the highest rate you can up to a maximum rate of 1C. At the end of the discharge, let the cell rest for an hour, then repeat the discharge at half the rate originally used. Once again let the cell rest for an hour, then keep discharging at rates reduced in steps until you get down to the lowest rate you can discharge at.
The low voltage cut off for all this discharging is 0.9 volts, although sometimes I will go a little lower during the very last discharge cycle. If your set up doesn't allow for discharging below 0.9 volts, you can take the cell, after all of your discharging, and put it into a single cell light and run it for a short period of time watching the light. If the beam is steady, things are progressing well. If the beam flickers, more work will be necessary.
We now have a completely discharged cell. The next step is to do a 16 hour 0.1C charge. If your charger doesn't let you do this, charge at the lowest rate and set a timer. Attend to the charge and if it terminates early, stop the timer, let the cell cool down, and start the charge again. If it terminates again, you have done as much as you can do, but it is better if you can charge for the full 16 hours.
Note the temperature of the cell during this standard charge. Warmer cells indicate more damage within the cell, and also indicate that more work will be necessary to try to restore them.
The next step is to do a discharge. Once again I am looking for a discharge rate in the range of 0.5 - 1.0C. You can note the capacity during this discharge and see if you have any improvement. At this point I generally do 2 additional discharges, but it depends on what the cell is telling me.
Next the cell needs to be cycled 3 - 5 times.
Now it is time to once again do the standard charge (16 hours at 0.1C) followed by the standard discharge (0.2C) to check capacity. If you are not within 80% of the cells original capacity, it is time to recycle the cell.
Finally we need to check the cells internal resistance. During a 0.5C discharge, the mid point voltage of the cell should be around 1.2 volts.
As you can see, the process is a little involved and can extend over several days. You can run different variations of this, but I still insist that the best way to restore a cell is to keep it from getting in that condition in the first place.
Tom
