By Xin Feng [09/07/02]
For a portable fast charger, be sure to get the Kodak K2000 from Kodak's website or the Quest Q2 from Thomas-distributing. These are the only two chargers that are very compact and have four separate circuits capable charging 1 to 4 batteries separately at a time. Almost all other chargers on the market have only one circuit and can charge 2 or 4 batteries only. Few of them, such as the Kodak K1000, have two circuits and charge two batteries in parallel while charging more than 2 at a time.
Charging two batteries in serial is OK, this is the case of 99% of chargers. But, charging in parallel is a very bad thing, because battery's internal resistance is not constant and varies between cells. If you charge two batteries in parallel, one might be still undercharged while the other is already overcharged because their resistance and other characters are different even if they are made in the same group by the same manufacturer. One battery gets more currents than other and reaches full state earlier. If charging stops as soon as one is full, the other is under charged. This result in short battery life between charges, but does not harm the long-term battery life as overcharging is avoided. Overcharging generates lots of heat and dry out batteries.
For example, the Kodak K1000 has two circuits for A and B banks respectively. If you charge 1 battery, fine. If you charge 2, you should put one in A and the other in B, as instructed by the manual. This way, the two get charged separately. If you charge 3 or 4, parallel charging is inevitable. I believe Kodak set the full-charge limit quite high for its K1000 to reduce the chance of undercharging that happened to other chargers in its class, such as the DigiPower DSP2000. However, doing this could put batteries in risk of overcharging. I found my batteries got very hot at the end of charging. Now you know why Kodak has put a thermal-protection in K1000 -- it shuts off the power when overcharging does happen and heats batteries up. Due to these problems related to parallel charging, K1000 has been discontinued and replaced by the K2000. I still miss the small footprint of K1000, but I charge more than 2 batteries at a time very often and cannot accept parallel charging. Olympus too used to sell a 2-circuit charger that has the same form fact of K1000 and discontinued it too. See, nobody likes parallel charging. K2000 is significantly larger and heavier than K1000, 5.3"x3"x1.2" vs. 4.5"x3"x0.9", but, fortunately, the nice folding plugs are remained.
The worst charger I have seen yet is the one from RizCamera. That one has only one circuit and charges 4 batteries all in parallel! You can imagine what a mess that can be. Afraid of overcharging, the manufacturer (in Taiwan) had to be very conservative when set the full-charge limit and thus it never charge my batteries to full. As a matter of fact, far below full -- my 1600mAh batteries can only work 1 hour or less, other than 3-4 if fully charged.
Therefore, be sure to get a charger that has 4 circuits if you need to charge 1 or 3 batteries in addition to 2 or 4. However, these 4-circuit chargers are more expensive and cost $35 to $50 each with four Ni-MH batteries. But they are true 3-hour fast chargers and they never put your batteries in danger. If you charge 2 or 4 only, those that claim to charge only 2 or 4 are OK, but not recommended. In any case, you must avoid those fast chargers that claim to be able to charge 1-4 batteries in 1-3 hours but have only one or two circuits inside. If you do not need fast charging and you charge 2 or 4 batteries only, any cheap 5-hour Ni-Ca (not Ni-MH!) chargers is perfect to charge hi-capacity (1200-1800mAh) Ni-MH batteries in 10-14 hours. The actual charging current of these chargers is about 140mAh, which happens to be the safe amount for overnight charging of Ni-MH. Note: never use these chargers to charge low-capacity (about 650mAh or so) batteries, even they are what the chargers are designed for -- 140mAh is high enough to cause overcharging. If such a 2x (140/65) charging rate is somewhat acceptable for Ni-Ca, it is absolutely unacceptable for Ni-MH unless thermal or auto-off protection provided. Once being overcharged, Ni-MH batteries generate much more heat than Ni-Ca ones. That's why I never see such a cheap 5-hour charger for Ni-MH.
As for batteries, go for Ni-MH which offers double capacity of Ni-Ca. Currently, the best Ni-MH batteries are rated as 1800mAh, while Ni-Ca ones are only 1000-1200mAh. More important, Ni-MH batteries do not have the memory effect and you can charge them anytime no matter whether they are empty or not. With Ni-Ca batteries, you have to charge them only when they are empty (fully discharged). For example, a new 1000mAh Ni-Ca battery can last 10 hours at 100mA current. If you always re-charge it after only 3 hours of use, they'll remember that and soon will serve 3 hours only. A 1000mAh battery then becomes a 300mAh one. This is a big hassle. For example, you are leaving for a trip and you are taking your CD player with you. You know the batteries have been used for about half or so, but you don't want to charge them to full before you leave due to the memory issue. Then, you have to take the charger with you in case the batteries run out in the middle of your trip. Ni-MH batteries do not have this problem and are care-free. Just charge them fully whenever you are ready for a trip. New Ni-MH batteries last even longer than Alkaline ones to serve you an entire trip.
Ni-MH batteries are environment-friendly while Ni-Ca ones are not. Like any other batteries, Ni-MH batteries discharge themselves even not in use and they do that a little faster than others, but not significantly enough to be a problem to most users, considering that you can charge them anytime.
I once developed a charging circuit for a windmill generator and learnt something about batteries and their (over) charging and (over) discharging. This is a very big subject and involves many technical issues. It is impossible for me to talk about them all here without writing a lengthy book. The bottom line is: buy a four-circuit charger and some 1800mAh Ni-MH batteries, both you and your beloved electronics will be happy.
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Xin Feng Company