Build Your Own Combat Robot

Chapter 5: It’s All About Power 85
battery will switch polarity. Cell reversal can cause permanent damage to the battery,
which will greatly reduce the charge cycles. Most lead acids will recover well
from a deep discharge (to about 1.5 volts per cell), as long as the discharge was
rapid. Deep discharging a lead acid over a period of days is likely to damage it.
NiCads require an occasional deep discharge (to about .9 volts per cell) to maintain
their full capacity, but going deeper than this risks polarity reversal on the
weaker cells.
The third major killer of rechargeable batteries is shelf life. Even if you follow all
of the appropriate care instructions, most combat robot batteries will require replacement
long before the maximum number of charge cycles is reached. The shelf
life of a typical rechargeable battery is five years when stored at 25° C. If the battery
is stored 10 degrees cooler (15° C), shelf life will increase to 10 years; and if the
battery is stored in a typical refrigerator (5° C), the shelf life will increase to 20 years!
Conversely, if a battery is stored in a hot Arizona garage (average 40° C), shelf life
can be reduced to less than two years. In addition, don’t store below 0° C. Within
reason, store your batteries in the coolest place possible.
Sizing for a 6-Minute Run Time
Choosing to compare battery types at 6-minute run times has many benefits. First,
6 minutes provides some measure of run-time safety margin because generally the
longest fighting competitions can last up to 5 minutes in duration. Sizing to 6 minutes
prevents the deep discharge. In addition, the 6-minute run time is 1/10 th
of an
hour, which makes it easy to calculate the current that the battery can supply for the
6-minute period. To yield the average current that the battery can supply for 6 minutes,
multiply the 6-minute amp hour rating by 10. (Ideally, it makes more sense to
size the battery for the particular competition. For example, BattleBots matches
never run more than 3 minutes and the majority of the matches only last 2 minutes.
The rumbles last 5 minutes, but only a small fraction of the robots make it to
the rumble. In this case, to be a little more aggressive, you could size the battery for
4 minutes and just plan to skip the rumble.)
Except for the NiCad battery type, limited information is available on what
happens when the battery is discharged in a short period of time. Because NiCad
batteries are often used in the hobby radio control market, a lot of information is
available on how they perform for these short run times.
note The information presented here has been gathered from many manufacturers’
data sheets and application notes. From the data sheets and experiments, a special conversion
factor was derived for each battery type. This conversion factor is used to convert the nominal
Ahr rating of each battery type to the 6-minute run-time period (see Table 5-1, later in this
chapter). This allows easy comparison of one battery type to the other for battery capacity.
These factors should be considered “rules of thumb”; for best accuracy, individual battery data
sheets should be consulted and actual experiments with the batteries should be conducted.