Build Your Own Combat Robot

FIGURE 5-6
Various sealed lead
acid batteries.
(courtesy of
Hawker batteries)
Sealed Lead Acid
Sizing the Battery
Chapter 5: It’s All About Power 93
If your robot draws an average current of less than 40 amps and has a peak
current less than 100 amps, you can select from SLA, NiCad, or NiMH batteries with
ease. Just size your battery to make sure that the 6-minute rating and peak current
rating is higher than your robot requires.
If your robot will draw an average of more than 40 amps or more than 100 amps
peak, use SLA batteries or parallel packs of NiCad or NiMH batteries. The SLA is
easier, but not necessarily better. Remember, do not mix different types and sizes
of batteries together.
The rugged construction of SLA batteries is well suited for combat robot use. SLA
batteries do not leak and they are a mature battery technology. Figure 5-6 shows
various SLA batteries.
In general, the Ahr rating of the SLA is specified at the 20-hour rate. Multiply by
0.33 (see Table 5-1 for the 0.33 conversion factor) to convert this 20-hour rate to the
6-minute rate. For example, an SLA battery with a capacity of 12Ahr has a usable
6-minute capacity of 4.1Ahr (4.1Ahr = 0.33 × 12Ahr) and will provide an average
current of 41 amps (41 = 10 × 4.1Ahr) for the 6-minute duration. Typical SLA batteries
have a peak current delivery capacity of 10 times its 20-hour capacity. In this
example, the battery can supply a peak current of 120 amps (120 = 10 × 12Ahr).