May I suggest you check your Rotorcraft Flight Manual (RFM) as this will always list the procedure for conducting an External Power Start.
The RFM will also nominate the minimum and maximum current and voltage requirements for the external power unit (whether it be a GPU or a battery cart).
If you are not operating remotely, and using a battery cart, how about you place it on charge after the start. This will make sure it has enough capacity for the next start.
I would be suprised to find that the Generator installed on the aircraft by the manufacturer to recharge the aircraft starting battery would be suitable OR capable of recharging a Battery Cart.
Use the KISS system (Keep It Simple S%#@$*)
Have fun
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