NiCad Batteries

Many adaptive devices use NiCads for their power. I've read more than one article regarding NiCad batteries and I don't want to plagiarize or be repetitive. However, there still seems to be some confusion regarding the charging of NiCad batteries and this article is written to eliminate some of the confusion.

NiCad batteries have two lives. When discussing NiCads you must explain which life you are talking about. The first life is, "How long will the batteries deliver power to my device before going dead?" The second life is, "How long before they must be replaced?"

The answer to the first question is dependant upon the batteries and the device they are powering. Batteries are rated in milliamp hours. This rating is power delivered multiplied by hours. An analogy to this rating would be¾my body can run one mile in eight minutes or eight body-minutes. Therefore (theoretically) I should also be able to run one-half mile in four minutes or even fifty miles in 400 minutes. As a matter of fact, I cannot run fifty miles but the point is this: milliamp hours are almost linear. If your device uses twice as much power as my device, my device will last twice as long as your device on a full charge if we both use the same batteries. Other factors such as temperature are also important but I'm not getting into all the possible factors.

What this all boils down to is this¾if you are buying NiCad batteries from a store, you should find out what the milliamp hour rating is for the batteries. Bargain batteries may not be such a bargain if their milliamp hour rating is low. I just opened a battery catalog and can find double A batteries rated anywhere from 650 to 800 milliamp hours. If you draw 800 milliamps from 800 milliamp hour batteries, your device will run for one hour. If you draw 400 milliamps from the same batteries your device will run for two hours, and so on.

To make this more confusing, NiCad batteries have a memory. If the batteries in your device are capable of running eight hours on a full charge, but you repeatedly stop the device after four hours and then recharge the batteries, after a while your batteries will only be capable of delivering power for four hours. NiCads "remember" how much is asked of them. The best way to use NiCads is to always run them all the way down followed by a full recharging. By the way, this memory problem is temporary. Repeatedly fully charging and discharging will bring the battery back to its maximum capacity after a period of time.

Another factor is age. The milliamp hour rating is for new batteries. As time passes the capacity will decrease. Fortunately, modern NiCads keep most of their capacity until just before they fail. Once the capacity starts to decrease, it's time to buy new batteries. Just like my body and running - age matters.

The second life is related to the number of recharges. NiCads can typically be recharged a few hundred times (200 to 500). No one will tie this number down because it is totally dependant on the user, the device, and the environment it's stored and used in. It's analogous to two drivers¾the first driver slows the car gradually when coming to a stop sign and the second driver continues at full speed until the last moment and then slams on the brakes. The second driver will have to change the brakes sooner. In any event, if you have to recharge your batteries daily you may have to replace your batteries after the first year.

If you need your device daily there is little you can do to lengthen this life. If you have the option of operating your device on either NiCads or plugging it into the wall (what we engineer types refer to as "AC"), plugging it into the wall will lengthen this life. Of course they will last longer if you don't use them. This isn't quite true however; read on.

In the real world, most devices don't draw a constant amount of current. Tape recorders draw a lot when rewinding and fastforwarding but much less when just playing. Microprocessors have "sleep" modes which cause them to draw much less power when they are not computing. People forget and leave batteries dead for weeks at a time and then find they don't hold a charge like they used to. Operating in cold temperatures and storing in hot temperatures both cause problems.

If you leave your device on and it doesn't have a circuit to protect against it, the batteries may get damaged. If NiCad batteries are completely shorted out, individual cells may act as if they are plugged in backwards and then the whole battery pack won't deliver the proper voltage. When I advise you to run batteries completely, I am referring to the power level your device considers as run down. Some devices beep or illuminate a "battery low" light at this level.

My advice is this -- when using NiCads let them run down completely prior to recharging. If you occasionally break this rule, it's not the end of the world, but don't make it a habit. Also, if you have the option of using either AC or batteries, use the AC. But (and this is a big but) you should run the batteries completely down a minimum of once per month or they will "remember" that they don't have to deliver any power -- and they won't.

This article was based on one originally appearing in July 1995 "Voice of Vision". Dan Weirich has given his permission to share it with our readership.

*Ed Salisbury, DATI AT Specialist in Kent County, adds: For any system using a laptop computer, a deep cycle battery charge is recommended along with an extra battery or two. A deep cycle charger discharges the battery completely before charging thus helping to avoid the memory effect. For systems that have easily accessible batteries, it is a good idea to have an extra charged battery on hand that can be used while the other battery is charging, especially if a deep cycle charger is being used. County, adds: For any system using a laptop computer, a deep cycle battery charge is recommended along with an extra battery or two. A deep cycle charger discharges the battery completely before charging thus helping to avoid the memory effect. For systems that have easily accessible batteries, it is a good idea to have an extra charged battery on hand that can be used while the other battery is charging, especially if a deep cycle charger is being used.