Battery-type GPUs

[Avnx EO]

Question.....

Are there a lot of people out there using battery type GPUs for - say light twins or larger? Someone told me there are operators out there who use battery-type GPUs to do the engine start and then count on the aircraft generator to recharge the GPU.

Reason I ask is that with regular GPUs, if the voltage is slightly high (even by half a volt) it can trip off your generator if your generator gets turned switched on without disconnecting the GPU. Many operators I've heard of puposely hold off switching on the generator until the GPU is disconnected for that reason (which comes with its own bunch of problems if your battery is totally flat.) I wanted to set up an aircraft (a light twin) to automatically open the GPU contactor when the generator comes on line. The objection I got was that this would prevent the re-charging of battery type GPUs....... Do people really do this? :confused:

[GoodGrief]

The Voltage of a battery can never be as high as the generator voltage.
24V Batteries have an idle voltage of somewhere between 25 and 27V.give or take a %.
the generator will give 28V
Once you have started your engine a lot of the battery's power will be depleted.
Imagine you are out in the middle of no and the only option you have are two truck batteries. no charger. you need multiple starts on those batteries. now what?
never had any problems

[inmate]

Just a note of clarification.
A Ground Power Unit or GPU is not a Battery Cart.
A Battery cart is just that usually two 12volt batterys tied together supplying at best 24v. whereas a GPU is a power unit that can be regulated and adjusted for both volt and amperage requirement.
If you note in some AFM's they specifically say that when you use a GPU the battery should be in the OFF position however, when you use a battery cart you normally have the battery switch on during the start.
Fly Safe

[Avnx EO]

Sorry for the confusion, inmate.. I'm talking about both the battery cart style external power unit, and the generator style (which is what I meant when I said "regular GPU"). It is the generator stye GPU that tends to play havok with the aircraft generator.

I've seen plenty of generator-style GPUs that have very poor regulation, but even good regulation at a slightly high voltage can trip off the generator Our answer to this problem was to force the external power "off-line" when the generator is switched "on-line"... but this would prevent re-charging a battery-style external cart from the aircraft generator. I'm thinking it's a bad idea to be doing that anyway, and I was looking for confirmation.
Goodgrief seems to indicate he'd be counting on it.... His response seems to suggest that after each start you'd sit there for 15 or 20 minutes burning fuel to re-charge the power cart (You'd have to inorder to get a decent recharge). Assuming the aircraft battery was low as well, the generator would be trying to charge that up too (or do you take the battery off line while charging the battery cart.) The extended run at high current can't be doing great things for the generator, and at say $600-$1000 and operating hour - that's a hell of a price to pay for using a helo as a battery charger. Wow! I wouldn't want to be doing that a lot.

[800]

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
800