I'm wondering about testing single cells, and wondering what to use for my standard peak threshold?

More importantly I want repeatable tests and fair comparisons. I'm wondering what determines when a nimh cell is "full"? If I use 5 mv for all cells are they full charged? or does something very terrible happen at 6 mv 7 or 8 mv?

I guess you can get the point to my question? when comparing cells, I could test the same cell 6 times and get 6 different capacities? which one is the true capacity?

The only ways I know iof are to use a standard temp , but I run into the same dilemma. The cell that I let charge to 135 is going to appear to have more capacity than the one I stop at 125 right? The other thing I can think of is to trickle the cell at a safe rate for too long a time and then I would know the cell is "full" but then is it possible to "overfill" it ?

A cell that is overcharged will only appear to have a greater capacity when it is being charged...when you discharge it, the truth comes out.

It is very difficult to charge a single cell using mv thresholds...false peaking is common. 5mv is too small for a single cell...I would use 10. 5 per cell or less is great when you have 6 cells, with some losing voltage, others staying put, and others dropping. For one cell, it's far too low.

However, the best way to charge a single cell is temperature. If you have a charger with a temp sensor, set your mv threshold level to the highest it will go (or turn it off if you can) and stop charging the cell at 120F or so. With a single cell, temperature is your best guide.

Also, a single cell cannot stand high amperage discharges for any length of time. When a Turbo Matcher matches cells, it does 4 at a time. I believe this is a great flaw with the concept of cell matching, but that's a topic for another day. For example, if you take a fully charged GP3300 cell and try to discharge it at 20amps, it will drop below .9v very quickly, yiedling very few mah. The same cell in a set of 4 or more will yield full capacity under that load.

I have found that the most stable way to test cells is to charge then at 2 amps until 120F, then discharge them at 1.5 amps. I use this technique to match cells in damaged packs based on capacity. I always get the same results (+/- 30mah), no matter how many times I do this method. The same cannot be said of other methods I have tried.

All of the above is assuming you are talking about sub c cells. I know very little about all other types.

Yes, I was talking subc nimh. Very interesting what you say about 1 cell vs. 4 or more I wonder why that is. I have very little experience with testing and matching cells, but the experience I do have uses similar specs as you say. I think I used 2.4 amp charge and then whatever discharge the triton wouild go to.

This method gave me repeatable results, but under real world amp draw would always show the same cells getting hot, telling me that I need to discharge much higher amps when testing/matching. I've decided to get a CBA, but have yet to decide on a routine for chargeing and finding the "full mark"

From what you're saying it seems best to whatch the cell (I have an idea what it should be) and then use about 120. Thanks for the info, very informative.

Anyone else have any to add?

Which cells are getting hot? The cells in the middle of a pack should be hotter than those on the edges, sinc they are surrounded by other hot cells instead of cool refreshing air.

If you really want to match cells, you will need a charger /discharger that can monitor each cell individually, like a TurboMatcher or a Futaba CDR-5000, both of which are very expensive. That way you can cycle banks of cells yet still get results for each one.

With the Futaba, you will have to rig some kind of thing (like a plasitc board with four sets of connected terminals) into which you can pop at least four cells, then attach its monitoring harness and cycle away. You will be able to get average votlage and capacity for each cell at up to 40 amp discharge rates (which is way too much...30 should do it).

Be aware however, that discharing cells at 30a damages them. Typically, the numbers you see (run time) on matched cells represent the runtime it had during its first few 30a discharge cycles. You would be lucky to ever get that kind of run time out of them again.

This is why matched cells are a tradeoff...you get the voltage performance, but the run time is a little less. However, this is an academic difference...you won't notice it when you run your car, as the difference can be as little as 30 seconds.