Aircraft alternators support the electrical load during operation. The system is designed to keep a charged battery charged. The aircraft system (alternator) is not a battery charger. It is a batter "sustainer". Having to use a ground power unit to start an aircraft because the battery is dead and using the aircraft charging system to charge the battery can have the following effects:
- it shortens the life of the alternator by putting full amperage for an extended period of time to charge the battery. It also has the power the aircraft systems at the same time. This is very hard on any alternator and may fail because of the additional load.
- it shortens the battery life due to overheating during high amperage charge.
FAR Part 23.1353 (h) In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continued safe flight and landing. The 30 minute time period includes the time needed for the pilots to recognize the loss of generated power and take appropriate load shedding action.
Jumping an aircraft and then taking off would be a violation of this FAR, the battery must be taken off and charged by a mechanic that can monitor the charging and then test it for the 30 min.