Hello Mike,
Got your flame proof suit on...
Mike abcd said:
IMHO, I'd definitely discharge them first or better yet, condition them with the refresh mode a couple of cycles or until the reported capacity stabilizes.
If you do a "forming" charge first, the capacity of the cell will stabilize pretty much on the first discharge cycle. With cells that have been "formed" you actually see very little change in capacity during cycling.
It takes 12 hours at .1C to fully charge a fully discharged NIMH cell per Duracell. That's also what I found when I did a quick test. Anything beyond that is overcharging and overcharging, even at .1C, is not good for NiMH cells. A 16 hour charge at .1C is roughly a 30% overcharge and can be a LOT more if the cell isn't fully discharged beforehand. I've found quite a few new cells with substantial initial charge out of the wrapper.
I know popular wisdom is that new cells should be "formed" with a 16 hour .1C charge but I can't find any cell manufacturer who advises it.
I also believe new cells require cycling and won't exhibit stable or full capacity on the first cycle. I've even seen poor cells that require 4-5 cycles such as the Lenmar junk. I just don't believe that you have to abuse them with overcharging based on my experience.
Running for my flameproof suite now...
Mike
Duracell has excellent information. It has long been known that if we only charge to 90% of full capacity, keep the cells cool, and avoid prolonged trickle charging, the cells will last a lot longer. Unfortunately, they don't supply any data to back this up. Their cells are not advertised as being able to run for more cycles using their charging procedure, and if you follow it precisely, you will find that you loose around 10% in capacity.
Let's look at the 12 hour 0.1C charge. With cells that have been formed and cycled and in general are broken in, a 12 hour charge will get you within 95% of full capacity. This is charger dependent. Constant current chargers do better than pulsed chargers. 14 hours gives a little overcharge, but does a better job of redistributing the electrolyte within the cell. 16 hours is considered a "standard" charge according to the battery testing standards. The battery manufacturers use the "standard" charge to rate the capacity of their cells. While they don't come out and directly say so, I suspect Duracell uses the "standard" charge to rate their cells as well.
You have to ask yourself, "If the cell is fully charged at 12 hours, and can be damaged by overcharging beyond that, why does the industry standard call for a 16 hour 0.1C charge?"
Yes, the 16 hour charge does overcharge the cell. However, that little bit of overcharge seems to enable the cell to operate at its fullest potential right away. The next time you charge the cell, it will not develop hot spots, and if you use several cells in a battery pack, you find that they stay in balance better. The 16 hour charge also allows for better balancing of the cells within a battery pack.
The alternative is to do several cycles.
In "normal" use, people won't observe the slight performance improvements that come from cycling, and no one wants to baby sit a "forming" charge. People would be outraged if the manufacturers insisted on them doing a forming charge prior to use. The people may even insist that the manufacturer do the forming charge at the factory before sending the cells out. Sanyo, and others, originally did this along with adding a few cycles to the cell before sending it out. Cost cutting has eliminated this step as being unnecessary for "normal" use.
There are some of us that are "performance" users. We measure run time down to the minute (and sometimes down to the second). We measure light intensity changes with a lux meter and are very interested in getting the best performance from our cells. We are also patient enough to do a 16 hour charge, and have the equipment to dial in the 0.1C charge rate. We also want "fully" charged cells in order to get every minute of run time we can from the charge.
The abuse comes from extended trickle charging. A cell is going to be as full as it can get in 16 hours at 0.1C. If you let it continue to charge for a week, the electrolyte will start to dry up. If you continue the charge for a year, you will see a performance drop.
Duracell offers some interesting graphs. While they are not based on "standard" test parameters, they are very informative.
Let's take a look at they cycle test data for an example. The standard cycle test calls for a 0.1C charge for 16 hours, followed by a 0.2C discharge to 0.9 – 1.0 volts. This is continued until the capacity of the cell is at less than 80% of its original capacity.
Duracell offers a cycle test graph showing the effects of cycle life with charging and discharging temperature. They charge at 0.25C for 3.2 hours. This gives you roughly a 75% full cell. They discharge at 0.25C for 2.4 hours. The interesting part is that they measure capacity every 50 cycles by charging at 0.33C for 5 hours followed by a 1.0C discharge to 1.0 volts. The 0.33C charge for 5 hours works out to a 165% charge. I find it interesting that this is actually a higher overcharge than charging 16 hours at a 0.1C rate. It is also interesting that charging at a higher charge rate is more efficient, so it is a significant overcharge. To get the same results as charging for 16 hours at 0.1C when charging at 0.33C, you only need to charge for around 4 hours.
Fortunately, batteries handle a reasonable amount of abuse and still work well. Duracell offers some excellent advice, and looking at it from a consumers point of view, I agree with it. However, when I want peak performance from my cells, I will consider giving up some "longevity" for improved performance.
Tom