What is the best rechargeable battery charger??I am asked this question several times a week. My answer, it depends. What are you going to use the charger for??
Let me share some information that should help you pick a good charger for you needs.
Ground rules. This discussion will deal only with NiMH batteries and chargers.
First let me talk a bit about charge rates. In order to have a well defined knee in the charge so that full charge can be detected by a -dV algorithm it is best to charge NiMH batteries at a sufficient rate. When battery people talk about charge rates they usually throw around a bunch of "C" numbers like .2C or .5C or even 1C. So what is "C" and what does that have to do with the charger you select.
In simple terms you can get "C" by taking the battery capacity in Ah or mAh (explained in a moment) and divide out the time or hours (h). So if we have an AA cell that is rated at 2700 mAh in order to find the 1C rate we divide the mAh by h and get mA (miliamps or thousandths of an amp) So the 1C rate for that cell is 2700 mA or 2.7 A. In theory if you charged that cell at the 1C rate for an hour it would be close to fully charged.
I do not recommend charging at such a high rates for consumer AA and AAA cells. The problem is heat. When the cells are charged at high rates such as 1C or more it is very easy to get too much heat build up in the cell. If it gets so hot that the cell vents gas then it is now on it's way to premature failure. It is possible to charge at those rates but only with the right charger that monitors the batteries for all parameters to prevent any damage to the cells. It would also be best to use active cooling for the cells being charged.
Having said those negative things about fast charging at 1C or higher rates I also have to say that it there is a problem with charging at very low rates. As mentioned above very low rates may not give the indications needed in the charge curve to show when full charge has been reached and the charger will fail to terminate charge properly. In short the charger can miss the -dV charge termination point. So very slow charging is not really a problem with the batteries, it is a problem with the chargers as they may not be able to detect full charge conditions.
So what is the best charge rate?? I recommend chargers that normal charge in the .2C range and fast charge in the .3C to .4C range. Those are rates of charge that are high enough for the charger to work well and not so high as to cause undo heating in the cell being charged. Here is an example. Suppose you have some NiMH AA cells rated at 2500 mAh. The 1C rate for those cells is 2500 mA or 2.5A. The normal charge rate .1C to .2C would be 250 mA to 500 mA. The fast charge rate .3C to .4C would be 750 mA to 1000 mA. In that case a charger that charges at 500 mA would be perfect for normal charging. It would charge those 2500 mAh cells in a little over 5 hours.
Let me summarize. The best charger is one that is computer controlled and has independent channels for each battery being charged. It detects full charge with a -dV algorithm. It has temperature monitors for each battery being charged. It normal charges at .1C to .2C rate or it can fast charge at .3C to .4C rate. It should set appropriate rates for AA or AAA batteries automatically.
Here are some chargers that meet those criteria.
PowerEx MH-C800S. Very good consumer grade charger.
PowerEx MH-C801D. Good professional grade charger, higher rates faster charge times.
PowerEx MH-C808M. Professional grade charges AA, AAA, C, D.
Chargers that can test batteries for capacity.Say you purchase some batteries that are billed as the latest and greatest AA 3000 mAh batteries. How do you know?? Most major brands are very good about rating their batteries, they supply standard data sheets on their web site. However there are manufacturers and retailers that flat out lie about the capacity of batteries. So how can you tell if you are getting ripped off. Get a charger that can actually test batteries for capacity.
First a bit about battery capacity. Capacity rating is usually given in Ah or mAh (Ah - amp hours or mAh - miliamp hours) and is an indication of how much energy is usable from the battery. An example, if we have an AA cell rated at 2500 mAh it means that battery could supply a 2.5 Amp load (2500 mA) for a time of one hour. Or it could supply a 500 mA load for a time of 5 hours. Approximately. So capacity is an indication of how much work you can do with your batteries.
There are chargers that can test your batteries for the actual capacity, as opposed to what is claimed by the manufacturer or retailer. To do this all of these chargers perform the following cycle; put a full charge on the batteries, then perform a controlled discharge while measuring how much energy is obtained from the battery, then perform a full charge again. At the end of the cycle the tested capacity is displayed on the control panel of the charger. If you are serious about getting the best performance from your batteries then this kind of charger is a must have. It is the only way you will know for sure the state of your batteries.
These kind of chargers usually have other functions that are useful such as break-in and recondition cycles. Names and details differ but the ability to test is the same in all of them. Some have better capability and controls than others but the ones listed here will all do the basic job of testing batteries for actual capacity.
Shown in table form with basic parameters.