A Better 7.2 Volt NiCd battery Charger

Kenny Ham
Vice President, KinAestheticWind.

Here at KinAestheticWind we are dedicated to alternative energy and the products they power. One field of great interest to us is rechargeable power storage devices, like super conductors and batteries.

One particular technology that we are interested in is the continued development of the nickel cadmium (NiCd) batteries, commonly called Ni Cads. NiCd's are rechargeable batteries that use nickel and cadmium for the electrodes see here [wikipedia.org]. Although, NiCd batteries do not have the energy density of newer battery types such as the nickel-metal hydride (Ni-MH) and lithium ion (Li-ion), the NiCd battery is still widely used. NiCd's are cheap, reliable, and can with withstand sudden high discharge rates which makes these batteries idea for use in economy eclectic cars.

Because of their usefulness, we decided to use our resources to build a better NiCd charger for hobby battery packs You often see these NiCd hobby battery packs used in remote controlled cars (below)

The primary advantage of NiCd batteries, is that they can be charged a minimum of 100 times. For this reason, NiCd battery packs are used by robotics designers and other electronics professionals because they are effective and cheap.

NiCd hobby battery packs come in many types, but the most common is the 7.2 volt variety. In fact, while working for an organization called (BEST Robotics) we used over 80 7.2volt NiCd batteries alone (see below).

When we first started examining 7.2volt NiCd battery packs, we noticed there seemed to be wide performance fluxuations in the battery packs we were using. Specifically it seemed that the run time of the same battery pack varied wildly from charge to charge. After running a few basic tests to determine what was causing these differences in run time, the source of the problem was found. It turned out that it was the battery chargers we were using to charge the battery packs that were giving us trouble.

This charger problem starts with the fact that most chargers sold for these hobby battery packs take five or more hours to charge the battery. However, do good job of fully charging the battery pack without overcharging it, and we noticed that when a battery was charged on a 5 hour charger it seemed to give the most run time. This type of charger sells for about $15.00 or less new.

However, myself and most other battery users do not want to devote several hours for charging a battery pack, and quick chargers are a must have item.

As quick battery chargers go, the most popular model encountered was the 15 minute crank timer charger. This charger requires the crank timer be manually turned to get the correct amount of charge time and sells for about $30.00 new.

However, after studying the charger we noticed several problems with this type of charger. You never know precisely how much time a battery needs to become fully charged. Therefore, when using this charger battery packs often have less run time because the charging time was miscalculated by the user. The opposite problem is even worse. You never know if the battery is being over charged. There have been a few times when I put a fully charged battery on the charger by mistake and then fully charged it again. This happens to be an excellent way to kill a battery pack. This type of charger is also potentially dangerous since many first time users (especially, freshmen students) often assume that if one crank of the charger puts power in the battery, then five cranks of the charger will give it much more power. Often the result is a plume of smoke from a dangerously overcharged battery.

A step up from the crank charger is the peak charger (often know as a Delta V charger). It chargers a battery until is sees a slight drop in the battery's voltage, at which time it shuts off. This type of charger sells for about $40.00 new.

Our studies showed that while peak chargers require less guesswork than crank chargers. Most peak chargers however, can be complicated to use because the users must select different charging options. These multiple options are a significant drawback to first-time users who are unable to figure out which buttons to press. Additionally, these chargers are still bulky and often end up in a tangled mess due to their alligator clips. Most alarmingly, a combination of pressing the wrong buttons and leaving the battery pack on a charger for several hours or more can produce a fire hazard. In fact I have seen other people reduce these chargers to a pile of dangerously charred rubble that almost caused a fire.

When looking around at the different 7.2volt NiCd chargers, we found there are good quality chargers on the market that are quick, compact, reliable, and safe to use. However, these chargers are expensive and out of the price range for most hobby battery users. The charger shown in Photo 7 costs over $120.00 and many are priced higher than this one. This high cost is a particular drawback to school classes that use hobby battery packs and thus need several chargers available to students at a time.

The above chargers are just four of the dozens of hobby battery chargers available on the market today. Through much interned searching, we have found that the available chargers for 7.2 volt NiCd battery packs either take too long to charge a battery, require a significant amount of guess work in the charging, are potentially dangerous battery cookers, or are out of many people's / school's price range.

Once we had determined that a cheap and effective battery charger could not be found, we began to looking how to creating our own NiCd battery charger. The result of this effort was that we the creation of a compact, powerful, safe, and quick 7.2votl NiCd charger that could be made with off the self components for about $20 each, or about $15.00 each when made in bulk.

Our charger design, the Black Box charger, is a DeWALT model DW9116 NiCd battery charger.

DeWalt battery charger (left) Black Box charger (right)

Click here to learn how to build our Black Box Charger