Build Your Own Combat Robot

88 Build Your Own Combat Robot
FIGURE 5-3
12-volt battery
types compared to
voltage during
6-minute discharge.
and even increases slightly as it warms during discharge. Both the SLA and the
NiCad have slowly fading voltage curves.
If you are familiar with traditional battery literature, you would not expect the
NiCad’s voltage to fade. This is due to the high discharge rate and the increase of
series resistance in the cells. Of particular interest is the fact that throughout the
discharge, the SLA battery voltage is above the other two battery types. Why
would this be the case? The reason for this is that all the SLA battery types have the
lowest internal resistance, hence they have the lowest internal voltage drop.
Voltage Stability for Peak Currents The preceding section brings up a good point.
What happens to the battery voltage when one tries to draw various amounts of
current from the battery? Figure 5-4 shows how the internal voltage losses increase
as the current demand increases.
The voltages shown on these graphs use Ohms Law. The formula is
voltage loss = (internal resistance of the battery) × (current draw)
note Remember that for the NiCad and NiMH packs, the internal resistance of each
cell is added together. For 10 cells, then, the total internal resistance is 10 times the internal
resistance of 1 cell.
Figure 5-4 should provide an intuitive feel for what is happening inside the batteries.
It shows the relationships for the various battery types using batteries of similar
6-minute capacities. Notice the voltage loss in the NiCad pack when trying to