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.

The steps to calculate the required field battery capacity are:

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

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

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

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

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 Manufacturing 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 solution. These typically cost between $50 and $120 and are available in sizes from 40 amp-hours to 120 amp-hours. Be sure to use a deep-cycle battery as regular lead-acid batteries are not designed for discharge on a regular basis. For field battery capacity up to 2880 watt-hours, the lowest cost solution is to use two 6 volt golf car batteries. These typically cost about $140 for a pair and are rated at 6 volts and 220 amp-hours. The Exide E3600 is one readily available battery of this type.

Note: the higher capacity batteries can weigh up to 70 pounds. For example, Exide E3600 weighs 62 pounds. If anticipate problems moving batteries of this weight, then you may choose to use multiple smaller batteries instead.

I do not recommend connecting field batteries in series. If you accidentally connect the batteries incorrectly, it can result in an explosion which will spray sulfuric acid over a wide area. It is safe to connect them in series, however, as a reversed battery will only result in less voltage at the output.