Battery Analyzer for RC Power
because the wattage levels could be exceeded.
RC packs are
assembled using multiple single battery cells
that nominally produce
1.2 volts at full charge.
The number of cells in a
pack depends on the
application. A typical
RC car battery will contain six cells and produce 7.2 volts,
while a model boat application may have 12 cells supplying 14. 4 volts of RC pleasure. Hobbyists can easily design
power packs to meet any power requirement by simply
assembling the proper number and type of cells to satisfy
the need. This test-load is only designed for a 7.2 volt six-cell RC pack. At 7.2 volts, there will be a 12 amp current
producing about 86 watts of wasted energy. The two 50
watt resistors are not rated to handle much more power
than this. The test-load will have to be carefully designed to
handle the power being dissipated if higher current levels
are to be analyzed. If higher voltage packs are used, the
test-load will require design changes that limit the maximum voltage to the data board to five volts. Adding resistors in series will divide the test voltage into safe levels for
the interface board to process. The PC software will also
need to be changed to support a different voltage division
other than the current design of one-half.
The current output of batteries designed for RC applications is incredible! A high performance electric boat
may draw between 40-50 amps during a race, but battery
life is reduced. A lower, 30 amp level is what high-perform-ance battery manufacturers recommend as the norm for
battery longevity and has become the standard performance test-spec when comparing batteries from different
Batteries are rated in how much current can be supplied for one hour or milliamp hours (mAh). A typical
1,000 mAh RC battery can supply one ampere of current
for one hour, or 10 amperes of current for six minutes.
High capacity battery cells in the 3,000 mAh range will
usually have their specifications advertised in the form of
a label placed on each cell (Figure 3). The discharge-time,
capacity, internal resistance, average voltage, and other
info that appear on the label are typically done at a 30
amp discharge rate. These statistics are important factors
when choosing a race-winning power source.
It could be argued that a 12 amp test-load is not
enough to fully exercise a battery pack used for racing. It
is possible that a battery will test fine at a 12 amp rate, but
fail when tested using a 30 amp rate. I chose a 12 amp rate
so that a wider range of battery capacities can safely be
tested. A 1,500 mAh sport battery will not last long if
subjected to several high-current test sessions.
There is always an explosion hazard when charging
and discharging batteries that is reduced when operating
Figure 3. Label with battery cell
Figure 4. Interface board.
at lower power levels. High current problems are still
easily detected if a history is kept about the battery. A new
battery should be tested and the results be kept as a baseline for future tests. Test data done during the life cycle of
the battery can be compared to the baseline test report.
Any significant change in performance from the baseline
is a good indicator that the battery will most likely fail
during racing conditions.