You don't apply the rated voltage, you apply the rated current. That method alone is only used by the very cheapest "dumb" chargers, it's unreliable and will damage your batteries over time.
Those chargers deliver only a set current, like 200mA, and you have to calculate how many hours your 2000mA battery needs to be full. That method only works when your battery is completely discharged, otherwise you wouldn't know how much capacity is left and you couldn't calculate the needed time.
Those dumb chargers continue with the rated current even when the battery is full. A full battery can't store more current than its rating and will heat up when the charging continues (which isn't healthy for the battery).
- Some of the lower priced chargers are built with a simple timed charge. For instance, a charger may say that it charges AA NiMH batteries in 6 hours...so it charges the batteries for 6 hours and then shuts off. If a battery has a higher capacity than the example (i.e., 2000mAh versus 1600mAh), and the battery is fully discharged, you are likely not going to have a fully charged battery at the end of the cycle.
The smarter chargers offer more options to care for your battery.
- Most chargers will switch to a trickle charge once the charger determines that the battery is done charging, or once the charger's timer completes. The purpose of this trickle charge is to keep the battery topped off to the optimum capacity. This is standard in most chargers, but is still something you should look for.
- Minus Delta V
Chargers that test for -dV monitor the voltage over the cycle of the charge. When a battery is fully charged there is a drop in voltage (hence 'minus delta v'). When this is detected the charger will either switch off or switch to a trickle charge, depending on the specifications of the charger.
- Some chargers monitor the temperature of the batteries as they are being charged. This is a basic safety measure to prevent overcharging and is typically included in fast rate chargers.
- Channels
The most common number of channels you will see in AA & AAA chargers is 2 or 4 channels. A 4 place 2 channel charger treats each set of 2 batteries as a single unit. It is important to charge and discharge each 'pair' of batteries together, since the voltage is measured as a total voltage for the pair and so you can easily over charge a battery if you attempt to charge two batteries at different states of discharge. A 4 channel charger will charge each cell independently for complete charging.
- Conditioner
Some chargers are built to charge both Nicad and NiMH batteries. You will find that some of these chargers have a 'conditioner' designed to remove the 'memory effect' in NiCad batteries. This conditioner basically discharges and recharges the cells a couple of times to remove the voltage depression that NiCad batteries sometimes develop.
- "burp charging"
That method stops the charging periodical for a few millisecs and dischargers the battery during that time. This is to minimize the developing of gases inside the battery while charging with high currents. It's mainly used for NiCd high-current batteries that RC-fans often use.
- CCS (Computerized Charging System), check
BTI CCS for more detailed info. They can identify each battery type and charge them correctly, they can even handle the rechargable alkaline cells.
