Why use an external source of power to start a turbine engine?
A starting GPU has more power than the installed certified aircraft battery. Period.
Aircraft batteries of the lead acid type have a big weight disadvantage. Thus a minimally sized battery is used but is only effective in the 1st half of its lifespan in starting an engine quickly. Dangers of using a weak battery are hot starts, hung starts, slow spool-up time, excessive voltage drop and long recharge times
This is especially pronounced at low temperatures.
A starting GPU is better able to handle the initial high current spike as the compressor begins to spin up. The friction loads and compressor inertia push lead acid batteries to their limits which lead to degradation of the lead plates inside.
Many commercial turbine engine operators must replace their lead acid starting batteries each year as they become less and less effective at starting the engine.
Certified aircraft batteries are very expensive to replace, so to extend their usefulness external GPU power is used.
Hot Start Issues:
a. Turbine inlet temperature exceeding the specified maximum
b. Acceleration time from start of rotation to stabilized RPM exceeds the specified time limit. This results in insufficient air flow to cool the combustion flame away from the metal surfaces and will cause the exhaust temperature to exceed limits leading to engine damage.
To prevent / avoid Hot Starts:
Hot Starts occur both in turbine and piston engines.
 Piston engines with fuel injection are most at risk of a hot start when trying to start after a recent shutdown. Residual engine heat from the cylinders and exhaust pipes especially greatly increases the temperature under the cowling.
Fuel inside the injectors and exposed fuel lines next to the engine will start to vaporize causing a condition known as vapor lock which creates a pressurized bubble inside the line stopping the fuel flow. If the fuel pump is too hot it also will develop a gas pocket inside and fail to move any fuel. The resulting loss of fuel to the cylinders leads to long cranking times, leading to an overheated starter and battery depletion. Repeated attempts to start a hot engine with a vapor lock will burn out the starter and drain and overheat the battery, damaging it as well.
Wrapping high temp exhaust tape around the exhaust pipes will greatly reduce the temperature under the cowling as well as opening the oil access door.
Turbine engines develop hot starts when there is insufficient airflow around the combustion chamber. So it is paramount that the compressor spins up rapidly to develop sufficient air flow before adding the fuel. Lead acid aircraft batteries struggle to provide ample power to spin the compressor fast enough and quick enough for a cool start. Older portable GPU's made from lead acid batteries suffer from the same weaknesses as the aircraft battery and should not be used, besides being too heavy to carry around. The coolest turbine starts let the battery crank the Ng as high as possible, about 15% or higher before introducing the fuel. These minimums set a higher bar for any starting battery to meet especially when starting in freezing temps or high altitude airports.
Jump Starting an Airplane:
The best solution is to use external power in the form of a small lightweight handheld GPU. The most powerful, smallest and lightest and fastest starting GPU's in the world are made by Aerolithium at half the weight of the competition.
A note about jump starting an aircraft:
Although a portable start GPU can be considered a ' jump starter for aircraft ' the definition should not be confused with jump starting a vehicle.
Jump starting a vehicle is done when the battery is very weak, in poor condition, very old, won't hold a charge, dead... If an aircraft battery were in the same condition, the airplane would be unairworthy and must have a new battery installed.
Portable Start GPU's exist only to relieve a well functioning battery of the stress of starting the engine, not as a crutch for a dead battery. Flying with a severely undercharged battery is unsafe as the alternator is a poor substitute for a battery charger.