Page 116 - Build Your Own Combat Robot

Chapter 5:
It’s All About Power
NiCads can be stored with or without a charge, without damaging effects. 97
However, it is usually safe to store the batteries in the discharged state.
NiCads have no memory effects when used for this application. Because they
are fully discharged during a combat match, this avoids memory effects.
Following are some disadvantages of NiCad

When stored at 25°C, the NiCad battery loses 1 percent of its charge
per day.
When fully charged, the NiCad will self-discharge to an 80-percent
charge in about three weeks.

Occasional cycling to 80-percent voltage is required to keep the internal
resistance of the battery low. If the robot is noticeably slower, you know
the battery has reached this 80-percent level. It is best to do this every
20 charge cycles or so. During the testing phase, usually the batteries
are repeatedly drained.

NiCads are high-maintenance batteries, requiring careful monitoring,
charging, cycling, and low temperature storage to yield long life.
NiCads have cadmium; and although safely housed in the battery,
cadmium is a toxic element and must be disposed of properly.

Nickel Metal Hydride (NiMH)
The rugged construction of NiMH batteries is well suited for combat robot use.
This is an emerging battery technology that is still seeing constant improvement.
Fast-charge/fast-discharge NiMH packs are required.
The Ahr rating of this battery type is also specified at a 1-hour rate. Multiply by
0.92(see Table 5-1) to convert this 1-hour rate to the 6-minute Ahr rate. For example,
a D-cell NiMH battery pack with a capacity of 6.5Ahr, has a usable 6-minute ca-
pacity of 6Ahr (6 = 0.92 × 6.5Ahr) and can provide a calculated average current of
60 amps (60 = 10 × 6Ahr) for the 6-minute duration. Even so, the specification
data sheets show that for the fast-charge/fast-discharge C-cell or D-cell batteries,
the maximum average current is only about 40 amps, with the peak current limit
of about 100 amps. For higher current draw requirements, it is necessary to parallel
the batteries.
For fast charging, use only a charger designed for NiMH. Using a charger designed
only for NiCads, for example, will usually destroy NiMH batteries. Because this
technology is relatively new, chargers for this type of battery are harder to come
by than NiCad or SLA chargers.