Build Your Own Combat Robot

FIGURE 5-2
12-volt battery
types compared to
capacity versus
run time.
Capacity (amp hours)
10
draw most of the current out of the battery in the given run times shown in the figure.
Notice the fairly steep slope for both of the SLA batteries and how both the NiCad
and NiMH are almost flat. The physics of each battery technology will determine the
shape of these curves. This curve is repeatable between the various battery manufacturers,
so capacity can be predicted for various run times. This is true even for the
steep slope of the SLA batteries.
To determine the 6-minute run-time capacity for a battery, look at the 1/10 th hour
(or 6-minute) run time data in Figure 5-2. The average current the battery can deliver
for 6 continuous minutes will be 10 times this 6-minute run-time value. In Figure 5-2,
you will see that the discharge rates for the 17Ahr SLA and the 6.5Ahr NiMH batteries
are nearly identical at approximately 6Ahr; thus, the average current these
batteries can deliver during the 6 minutes is 10 times this value, or 60 amps. For
the 12Ahr SLA and the 4.4Ahr NiCad, the 6-minute run-time capacity is about
4Ahr, so these two batteries can deliver on average about 40 amps for 6 minutes.
Voltage Stability
9
8
7
6
5
4
3
2
1
6-minute run time
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Run time (hours)
SLA 17.5AHr
SLA 12AHr
N:Cad4.5AHr N:MH 6.5AHr
Chapter 5: It’s All About Power 87
Figure 5-3 shows the voltage supplied by the various battery types for the 6-minute
run time. This graph assumes that your robot will try to drain the battery in 6 minutes.
Only three curves are shown, as these graphs are normalized for the 6-minute
run times. Both of the 17Ahr and 12Ahr batteries will see nearly the same type of
voltage change when both are discharged to the same level in 6 minutes. Notice
how stable the voltage is out to 5 minutes and that the voltage starts to drop off
rapidly after 5 minutes for all battery types. The NiMH voltage discharge is flat