CHAPTER 7 ■ NINE-VOLT BATTERIESWhen purchasing NiMH 9 V-size batteries, look for the rated voltage and the rated capacity (mAh). For
most uses, you want to purchase 8.4 V-rated batteries rather than 7.2 V-rated ones (most manufacturers have
discontinued making these). You may want to have a 9.6 V-rated battery available to add a little more speed
or pushing power in competitions. But, be aware that the 11.2 V fresh voltage of these batteries may be too
high for some robots and may not work in many battery chargers.
A higher-capacity (mAh) battery outlasts a lower-capacity battery of similar chemistry. It also retains
a higher voltage for a longer period of time during usage. All things being equal, a robot using a 260 mAh
battery would continue operating 73% longer than if it were using a 150 mAh battery.
Retaining Charge When Not in Use
The newest NiMH batteries are much better at retaining their charge when not in use (see Figure 7-9).
As such, you can use them immediately upon taking them out the package, because much of the
manufacturer’s charge has been retained.
The batteries are advantageous to the robot builder, as you can have a stock of nearly charged batteries
without constantly keeping them in the charger.
Lithium-Polymer
Li-poly (lithium-polymer) rechargeable batteries (see Figure 7-10) are similar to li-ion (lithium-ion) batteries
used in laptops. You can recharge them hundreds of times, they retain their charge better than NiMH, are
lightweight, can provide high current, and have capacities that nearly match alkaline batteries. As such,
I switched from NiMH to lithium-polymer during robot competitions.
Figure 7-9. “Ready-to-use” rechargeable batteries