Servo - May 2009

PHOTO 3. A cordless drill battery pack marked to show polarity.

PHOTO 4. A battery contact clip with warning light and connector.

PHOTO 5.

A 7.2V, 1,800 mAh pack made of AA batteries.

metal clip that picks up the two contacts (Photo 2). Check the battery pack with a voltmeter to see which side is positive and then mark the positive and negative contacts with strips of colored tape (Photo 3). Color code the wires to the clip: red to positive, black to negative. The drill handle was keyed so that you could only insert the battery one way around, but that was lost when you took it apart. You must get the polarity right. Mistakes are very expensive — wrong polarity will instantly destroy your speed controllers. Double-check the polarity every time you install the batteries in your bot.

Nothing beats being very careful but you can make yourself a warning system, as well. Connect an LED across your battery clip, making sure you have the right value of load resistor for the LED so that it lights normally. Then, solder it to the clip backwards so it does not light when the polarity is correct (Photo 4). You should always have your main switch off or link out for safety when you are installing your batteries. So if the LED lights, you can quickly reverse the battery connection with no harm done because the connection to the speed control was not completed.

Another good way to get cheap power is to put individual AA or AAA cells in a battery holder. Household chargers and batteries are familiar, available, and inexpensive and they will get

you going for very little if you get the batteries on sale and borrow a charger. Recharging takes time — you have to take all the cells out and charge them individually. Don’t forget to wrap some tape around your pack so when your robot hits its opponent at full speed your batteries don’t pop out of the pack. Photo 5 shows a 7.2V, 1,800 mAh pack made of AA batteries. Homemade battery packs are very convenient for development because if the voltage is not quite right for your motors you can vary it by adding or subtracting cells. Photo 6 shows a Beetleweight powered by eight AA cells. This battery pack is actually excessive. It can run the machine for 30 minutes — far longer than you need for combat — although it is great for practice. Note the aluminium bar holding the batteries down so they can’t pop out. If you have more time than money, go for the homemade battery packs.

In the search for cheap power, you might be tempted to buy a few alkaline batteries to power your machine. Don’t! Alkaline cells

actually have a better energy density than NiCads or NiMH but they deliver their current very slowly. They can’t deliver the high peak currents a fighting robot needs and your machine’s speed in the arena will be at a crawl compared to what it would be with rechargables. Not to mention that replacing alkalines will eat up money.

At some point, you may want to move away from cheap power to proper model batteries for energy density or convenience. Photo 7 shows a 7.2V, 1,900 mAh battery used in model cars. These need a special charger. The one shown in Photo 7 is a field charger which operates off a 12V battery, chargers that you plug into a wall socket are equally common.

The cheapest chargers require that you calculate the charging current and time, and then unplug the battery when the time is up. They tend to charge slowly and so need several hours to bring your battery to full charge. There are relatively cheap fast chargers with mechanical timers but you need to work out the correct charging time.