Specifications
If you must go to a flying field in order to fly, then you will probably need a field battery. It is not
recommended to charge larger batteries from your car's battery because automotive batteries
are not designed to be discharged on a regular basis.
7.4.4.1. Calculating field battery capacity
The steps to calculate the required field battery capacity are:
1. Calculate the number of watt-hours required per charge
The number of watt-hours required per charge is the voltage of the battery multipled by
the amp-hour capacity of the battery.
For example, a ten-cell 2400 maH battery pack has:
(1.2v per cell) x (10 cells) x (2.4 amp-hours) = 28.8 watt-hours of capacity
2. Multiply by the number of charges
If you want to charge a battery of this size about six times every trip to this field, this will
require:
6 * 28.8 = 172.8 watt-hours of capacity
3. Add fudge factor for charger inefficiency
Battery chargers are not perfect at charging batteries (due to switching power supply and
other losses), so add about 20% to account for this.
In this case, this would be:
172.8 watt-hours * 1.2 = 207.36 watt-hours of capacity
4. Add extra capacity to avoid completely discharging field battery
Lead-acid batteries will last longer if you do not completely discharge them at every use.
Therefore I recommend adding at least 20% to your expected required capacity to avoid
fully discharging the field battery.
If you anticipate flying extremely frequently (more than twice a week) then you should add
at least 50% to your required capacity.
In this case, this would be:
207.36 * 1.2 = 248 watt-hours of capacity
7.4.4.1. Choosing a field battery
The most common type of field battery is some type of deep-cycle lead-acid battery, due to its
low cost and availability. For field battery capacities up to 240 watt-hours, a car "jumpstart"
package is a very convenient source of power. One suitable package is the Vector Manufactur-
ing VEC012 jumpstart system. This retails for $50 and includes a 12 volt 19 amp-hour sealed
lead-acid battery and a built-in AC charger.
For field battery capacities up to 1440 watt-hours, a marine-style deep-cycle battery is a good
Support Equipment
57