Hello Bmoorhouse,
The capacity going into a cell has no value, other than as a gross indication of where you are in the charging process. If you are charging a 2000 mAh cell, and the capacity going into the cell is showing 1500 mAh, you are not quite done charging yet. On the other hand, if it reads 3000 mAh it probably means that your charger had missed the termination.
There has been some effort to try to find a correlation between the amount of charge input to battery performance, but that correlation is not readily apparent.
The RC chargers often use maximum capacity as a safety cut off. If you are charging 3300 mAh Sub C cells, you would select 5000 mAh as a maximum charge put in. When your charger misses the end of charge signal, it will finally terminate on the maximum quantity you selected.
There are several consumer chargers that also utilize this for charge termination, but it is a fixed number and not user adjustable. I have mentioned this to William, but do not know if it can be implemented into the C9000. People who are charging high capacity cells on their BC-900 chargers at low charge rates, are probably terminating on the 3000 mAh capacity built into the charger, rather than the charger actually seeing an end of charge signal.
Your last question is difficult to comment on. We need some common definitions, but they are illusive.
Let's gain some perspective and embrace the fact that cells die. They die while in storage and they die while in use. If you use your cells, they are being damaged every time you charge or discharge them. The idea is to minimize the damage.
We look to the battery manufacturers to give us an idea of life expectancy. The battery manufacturers look for standard procedures to test to. Now we have a basis to start from.
The standard charge is a 16 hour, constant current charge at a rate of 0.1 C. The standard discharge for capacity involves a 0.2 C constant current discharge rate and a cut off voltage of 1.0 volt per cell. The standard for determining cycle life involves measuring the number of standard charge/standard discharge cycles it takes before the capacity drops below 80% of its initial standard capacity.
If you want to realize the advertised cycle life from your cells, follow the standard charge and standard discharge guidelines.
There is no standard for fast charging.
The battery manufactures realized that there are some things that happen during fast charging. The cell temperature can increase, the voltage can stabilize or drop, and the internal can pressure increases. The charger manufacturers approached their chip suppliers to provide them with a controller that would enable them to charge at faster rates.
As cell chemistry changed from NiCd to NiMh, it was observed that the voltage related end of charge signal was enhanced at faster charge rates. Now we have a recommended charge rate of 0.5 C to 1.0 C, with most of the charging data being given for 1.0 C charge rates.
Two problems arise. We find ourselves in a situation where we using the cells differently from how they are rated according to the standards and while we have an idea of what is going on at a 1 C charge rate, we have little information on what an overcharge is at lower charge rates, and what the effects of such an overcharge is.
That last statement is not entirely true. The Swedish did a study comparing a 30% overcharge at 1 C. Their study revealed that cycle life dropped from roughly 500 cycles to roughly 100 cycles when the cell was charged at a 1 C rate to a 30% overcharge.
They went on to find that at a lower charge rate of 0.3 C, a 30% overcharge dropped the cycle life down to roughly 225 cycles.
We could probably conclude that charging at higher rates results in higher cell temperatures, and the higher temperatures caused the reduction in cycle life. However, no temperature data was taken during the testing.
Unfortunately, they did not continue the test to show the effects of a 10% or 20% overcharge. We can speculate that it should be less than a 30% overcharge, but that is about it. Their testing was done with 1800 mAh Duracell cells.
We need a definition of a full charge at a fast charge rate to answer your question.
One definition could be that a full charge occurs when the cell temperature raises 20 F above ambient when charging at 68 F. Another could be that a full charge occurs when the rate of temperature rise is 1.8 F per minute over a window of a few minutes. Another is to check for a drop in voltage during the charge. With this metric, the sensitivity needs to be determined. You will get different results using a 3 mV drop than when using a 10 mV drop. Another would be to first discharge the cell, then time the charge going back in, of course the right amount of time varies with the charge rate.
One interesting thought is to assume that the charge and discharge characteristics are similar, do a standard charge followed by a discharge at the rate we want to charge at. Now we have a capacity for that rate and can factor in the charger efficiencies and come to what would be necessary for a full charge.
How do you get the most from your cells? Use your cells until you notice increased recycle times in your flash. Charge them at a 0.1 C rate for 10 hours, then pull them off the charger. Do not expect the charger to terminate the charge. Set a timer and pull them after the time is up. Every 20 cycles, use the Break In function.
You will have to anticipate your needs with this method and may need to have a few extra sets of cells around for the times when you are waiting for a charge to complete. You will have to balance storage times with use times. I don't believe in leaving cells on the charger, but you may be able to get away with it with normal cells. If you use a low self discharge cell, don't leave it on the charger for any extended length of time.
Let me know how it works out…
Tom
