Hello CyberCT,
357mag1, I appreciate your feedback and help.
I'm learning, and think I finally have an idea of what's going on here. Confirm I do or correct me where I don't. I'm still learning and have picked up bits & pieces over the last two years but think I got it:
There are different mah values:
1) Charge capacity of a battery is how much charge it can take from the charger in mah, which will usually be the the highest capacity of the battery.
2) Discharge capacity, which is how much mah a battery can produce for a load. That value will be lower than the charge capacity. Now, for the discharge capacity, the discharge capacity mah values can vary depending on the load or rate of discharge. Let's use an eneloop. The charge capacity might be 2,000 mah. Under a .2 amp load, the battery might produce 1,950 mah. Under .5 amp load, the battery might produce 1,850 mah. Under a 1 amp load, the battery might produce 1,700 mah.
3) The amount of voltage a battery can supply under load. A NIMH battery such as an eneloop is rated at 1.2v. now let's say there is a .5 amp load on the battery. If it produces about 1.15v and 1,850 mah that is better than another AA battery that produces, at .5 amps load, 1.05v but say 1,950 mah? I'm still a little confused about this a little.
Would it be safe to say that a battery with a stated capacity that matches closely its discharge capacity is a good quality battery? Like if a D battery can take 10,000 mah from a charger, but as soon as a load of say 1 amp hits the battery it drops to 8,000 mah capacity, that cell isn't as high quality as a cell that is stated at 9,000 mah capacity but produces 9,000 capacity under load?
When you mentioned that your blue Tenergy cells were around 7,000 mah capacity, you were doing a discharge test of each at what, 4 amps? If you did a 1 amp discharge test should you expect a capacity of more like 9,000 mah?
Just curious, what capacity did the Eneloops show on your C9000 vs the Rayovacs, when you did the discharge?
And which of those two chargers you mentioned do you like better?
A slight correction... Or perhaps a perspective review.
The mAh of a cell refers to its capacity. This capacity is determined during discharge. The discharge capacity is the "official" number. This number is dependent upon a number of things including the discharge rate. When a cell is labeled with a capacity, it was supposed to have been tested according to standard methods that include a standard charge followed by a standard discharge.
In battery testing a standard charge involves charging the cell at a 0.1C charge rate for 16 hours. Following a rest period the cell is then subjected to a standard discharge. The standard discharge rate is 0.2C. This works out to 16 hours of charging, about 1 - 2 hours of rest, and about 5 hours of discharge.
While battery manufacturers test according to these standards, there is no rule that the person making up the label for the cell has to read the results, and there is varying "truth in labeling."
This is why we do testing. If the label says 10000 mAh, and the cell only is capable of delivering 6000 mAh, we know that the label was optimistic. Some manufacturers (Sanyo Eneloop for example) give a minimum capacity for their cells. In this particular case, the Eneloop cells actually test out very close to what they are labeled at.
The amount of charge you put into the cell is important, but it tells you nothing about the capacity of the cell. It does tell you how well the termination method used by the charger works. For example, the amount of charge that goes into a cell depends upon charge rate. When charging at 0.1C, if everything were perfect the cell should be fully charged in 10 hours. However, due to inefficiencies a full charge actually takes 16 hours. If you raise the charge rate to 1.0C, you expect the total charge going into the cell to be about 105 - 110%. If your charger misses its termination, the cell gets hot and the charge number raises much higher. This can actually damage the cell and produce a lower discharge capacity.
So, if you see a cell that has a higher charging capacity it could mean that something is going on with either the charger or the cell and it has been overcharged due to the charge termination signal being missed.
You have a good grasp on the discharge capacity. This is the actual capacity of the cell. The standard load used to determine the capacity is a 5 hour discharge or 0.2C. I like to see how close the labeled capacity is to the cells actual capacity, and then to see what the capacity of the cell is under the loads I plan to use.
The voltage under load varies with the load. The "nominal" voltage of 1.2 volts comes from the voltage seen during the standard discharge. As a cell ages, its ability to hold voltage under load is reduced. This reduced voltage also results in reduced capacity. When using cells in a multi cell application you are better off if the cells are matched to each other. Matching is done on both capacity and voltage under load. Matched cells help reduce the problem of cell reversal when the battery pack is at the end of the discharge cycle.
It is rare to have a cell match its labeled capacity. The exception to this is if the cell has a discharge sheet from the battery manufacturer. For example with AA cells the sheet from Sanyo on the Eneloop cells very closely matches the labeled capacity of the cell. GP cells are a little off on labeling, but the sheets are closer to the actual capacity. The GP 2700 mAh cells while having 2700 mAh on their label have data sheets that show the capacity is closer to 2550 mAh. When you test these cells you find that they come in close to the data sheet capacities. The label in this case is optimistic.
Capacity under load is a good way to start looking at the quality of a cell, but the story goes on from there. Voltage under load is another consideration, as are the number of charge/discharge cycles the cell can handle. I have had some cells that had good capacity and good voltage under load, but they only lasted 20 cycles and then everything tanked and they turned into crap cells. Other cells with less capacity were still going strong after 100 charge/discharge cycles, and in my opinion they were higher quality.
This should help you understand a little more about battery testing and how to interpret the numbers you observe.
Tom
!!!
