Generally, most starter motors and generators operate on turbine engines that work alongwith 30 volts without causing any damage. The highest amount of amperage draw is felt at the moment when the starter is activated. The amperage figure could climb up to 1000 amps on certain gas turbine start up engines.
As the startup continues, requirements in terms of amperage drop down throughout the activation cycle. A battery would need to be in good state by this time in order to minimize a voltage drop as a considerable load is applied during the start up. Usually as a load is applied to a battery, the voltage would tend to drop if the load is increased. In order to effectively start turbines, the voltage drop would need to be kept to a minimum.
Lower battery voltage levels will exude a lesser amount of amperage. Certain starter motors might even pull batteries down to 15 volts at the first phase during the activation of start up. Most engines manufacturers would recommend a volt of around 28.5 at start if this is available in order to limit any voltage drop to a minimum.
In order to achieve this, external ground power units (GPU) are used. In this particular case, it can be beneficial to the turbine engine as 20% of the energy is sent to the starter motor which makes the start more rapid and generates less heat. This increases the turbine life.