CHAPTER 7 ■ NINE-VOLT BATTERIES
Realistically, many consumer devices stop performing adequately when their 9 V battery falls below 7 V.
Although you’ll usually design your homemade robot to operate at even lower voltages, the power output of
most batteries drops like a rock at the end of their lives. So, even a low-voltage design won’t be able to extend
useful battery life indefinitely.
Referring again to Table 7-1, the Mass column reflects the differences in battery masses due to
differences in chemical makeup and case materials. For example, a plastic case weighs less than a steel or
aluminum case. These differences also suggest a potential opportunity to shave a little weight from a robot
by the choice of battery.
The Capacity column is stated in mAh. The unit “mA” is the abbreviation for milliamps and “h” is the
abbreviation for hours. Basically, the capacity value indicates how much electricity can be circulated for
an hour. If the device uses less electrical current, the battery will last longer than an hour. If the device uses
more electrical current, the battery will last less than an hour.
9 V Battery Recommendations
Recommended
Nickel-Metal Hydride
NiMH (nickel-metal hydride) rechargeable batteries (see Figure 7-8) are preferable for robot
experimentation and testing because you can recharge the batteries hundreds of times.
Figure 7-8. Nickel-metal hydride rechargeable 9 V batteries. Left: low-end 8.4 V 150 mAh. Right: high-end 9.6 V
260 mAh
■ Note Technically, single-use (non-rechargeable) batteries are called primary batteries. Rechargeable
batteries are called secondary batteries.
Unfortunately, rechargeable batteries have a lower run time (total capacity) compared to non-
rechargeable batteries. Also, every day they lose some percentage of their electricity, even when not in use.
However, because you can recharge them, simply top them off immediately before a robotic demonstration.