Rechargable batteries

There are many types of rechargeable batteries, and some act more consistently than others. NiCD (Nickel Cadmium) batteries are significantly more problematic than most other types of modern batteries due to memory effect of the chemicals in the battery: If the majority of the material in the battery doesn’t change state completely in a full charge and discharge cycle, the material that didn’t change state loses its ability to change state altogether, which reduces the effective capacity of the battery.

For modern batteries, such as NiMH, Li-ION, and Li-POL, the memory effects of the materials are virtually eliminated. However, consider how a battery is constructed: It’s a container filled with chemicals that has only a (+) and a (-) connection. Obviously, there’s no way for a cell phone for other device to do chemical analysis on the battery chemicals to determine its ability to hold a charge over time, and no battery lasts forever. Every battery loses some of its ability recharge a small amount over time, with every charge and discharge cycle. How fast a battery deteriorates is dependent on how fast it’s charged, discharged, and the other environmental conditions. To make a battery “intelligent” so that it can properly communicate with a cell phone (for example) to indicate its charge level, a circuit is added to the battery called “HDQ” (short for High-speed-DQ), and this circuit has been programmed with a curve that provides a guesstimation of how long a battery will last based on its use. The circuit monitors the amount of current being pulled from the battery (rate of discharge), the battery temperature, the battery recharge rate and voltage. The circuit takes these variables and calculates the battery life based on these factors and the battery’s factory capacity. As the battery ages, the curve adjusts accordingly, gradually reducing the effective capacity of the battery.

That’s why your cell phone acts better than the other batteries you mentioned: It’s got some smarts that your other batteries don’t have. It’s also made out of more advanced materials. Rechargeable AA batteries don’t have HDQ circuits, and there’s no circuits built-in to them to regulate the charge rate of the battery to boost its life expectancy.

As for chargers, most are based on voltage, but all batteries have an internal resistance that causes them to self-discharge over time, some faster than others.

A dead battery presents a lower voltage to the charger. For example, a cell phone battery has a nominal rating of 3.7v, but it actually charges to 4.2v. If you measure a fully charged cell phone battery, you should see around 4.2 volts if the battery is fresh. As it ages, the “fully charged” voltage will drop. 3.7v typically indicates a nearly “dead” battery, and since modern batteries are susceptible to permanent damage from over-discharging, that HDQ circuit mentioned above also keeps the mobile device from over-discharging the battery.

When you charge a smart battery, the charger can ask the battery about its usage characteristics and charges it accordingly. However, most chargers just base the charge rate on the voltage present in the battery, so a 3.7 volt battery will get charged until it reached 4.2v, and then a timer & measurement process kicks in on the charger to replenish the current over time, based on the amount of current being pulled from the charger. After that current draw is very low, the battery is considered fully charged and the charger stops applying current to the battery.

“Dumb” chargers usually charge over a time period, but some will do both: Time and voltage; so it may start charging for several minutes and then turn off, indicating a full charge.

It just depends on the design of the battery and the charger.

Hope that helps!

Sonartech