Hi Crab,
External GPU recharges batteries much slower than onboard Generators. GPU recharges at about 75Amps and it rapidly ramps down. Aircraft Generators push 200+Amps into the batteries after start.
I used to use GPU recharging where we had hangar power for the aircraft, but not a GPU on the line. Top up the batteries before pushing out to the line to make sure every last Volt was in there!
One thing that catches many people out is when they use a battery cart (as opposed to a diesel GPU.
Once the battery cart voltage drops below the aircraft battery voltage (like during a start because the 139 sucks so much out of the batteries), the aircraft battery will try to charge the battery cart as well as try to start the engine. You're better off not using a battery cart at all. Diesel GPU or nothing.
There is no diode or reverse current relay on the External Power circuit.
out of interest, what are peoples thoughts about this?
There have been concerns on other types that allowing a rapid charging with the GPU
- following a low batt voltage situation for example - could create conditions for thermal runaway in the battery.