Build Your Own Combat Robot

Chapter 5: It’s All About Power 95
■ Because the acid in the SLA battery will attack the plates of the battery
when discharged, it must always be stored in a charged state and must be
periodically recharged when in storage. If stored uncharged for an extended
period of time, the battery will die.
Which SLA Manufacturer Is the Best?
Most SLA batteries have similar capacity performance. Even so, the Hawker
brand (formerly Gates) stands out as the best SLA battery manufacturer. Cyclon,
Genesis, and Odyssey batteries can be 1.5-hour fast charged (or faster), can be repeatedly
fully drained with little battery degradation (down to 9 volts), have the
lowest shelf leakage of the SLA lineup, can supply three times more peak current
than other batteries with similar Ahr ratings, and have good shelf life.
The SLA battery manufacturer to avoid is Panasonic. Many of the Panasonic
brand SLA batteries have built-in thermal cutoff switches (a safety feature), making
fast, high-current discharge impossible. The Power Sonic brand seems to have a
good price/performance value. For the largest robots, the Optima battery brand is
great. Optima is a good battery, but the 12-volt Optima weighs almost 40 pounds.
Are SLA batteries too heavy to have a competitive advantage? Not at all. Electric
wheelchairs, golf carts, even electric racing go-karts and boats use SLA batteries. If
your robot requires high sustain currents or high peak currents, the SLA battery
may have the best performance.
Nickel Cadmium (NiCad)
The rugged construction of NiCad batteries is well suited for combat robot use.
Though NiCads are a mature battery technology, they are still seeing incremental
improvements in price and performance. Fast-charge/fast-discharge NiCads are
required for competition applications.
The Ahr rating for this battery type is specified at the 1-hour discharge rate. To determine
the 6-minute, run-time capacity, multiply the 1-hour capacity rating by 0.9
(see Table 5-1). Sometimes, even with a fast-discharge NiCad, this 6-minute discharge
rate is higher than a NiCad’s datasheets will allow. For example, a D-cell
NiCad battery pack with a capacity of 5Ahr has a usable 6-minute capacity of
4.5Ahr (4.5 = 0.9 × 5Ahr) and will provide an average current of 45 amps (45 = 10 ×
4.5A) for the 6-minute duration. Even so, a typical fast-charge/fast-discharge C-cell
or D-cell NiCad datasheet will show only an average drain of 35 to 40 amps, with
short duration (less than 100 milliseconds) peak currents of 100 to 130 amps. For
higher current draw, you need to parallel multiple battery packs together or run outside
the manufacturer’s recommendations.
Fast charging is accomplished by applying the current equal to the Ahr rating of
the battery for about 1.5 hours. Charge must be terminated when the battery
starts to heat up, when the battery voltage begins to decline, or some combination