737NG APU Start Issue
 

Just before arriving on stand we attempted to start the APU. Both engine driven generators were online.

The APU was slow to start and the EGT climbed to and stayed at around 900-1000C. We then had a MASTER CAUTION ELEC along with the BAT DISCHARGE light.

The APU uses power from transfer bus 1 to start when AC is available. Why in this case did we get the BAT DISCHARGE light when both generators were online?

Battery charging is inhibited during APU start. If the logic thought the hung APU was still in the start process, then that might be why the battery started to discharge?

With ELEC warning your tech people will likely have done a BITE on the overhead panel to see if there are any messages to begin with. With AC power available the starter should not be using battery power iirc. Possible issue with the start power unit or start control unit.

That or maybe TRU problems?

I'd be interested if/when you find out. I've not seen this myself.

Fursty Ferret, it is quite different to the bus design.

If BAT DISCHARGE light illuminates during an AC power APU start, it's a good idea to call up the engineer. It should only come up during battery APU start.

They did come out and check but found no faults. It is a very strange one. I have never seen that happen before in 10 years.

Sounds like the APU was not acting as if AC power was available. Bad relay somewhere would be my suspicion.

Bad start relay. Should show up on the CDU APU MAINT page under current faults.
Have seen this a few times over the last 10 years, and as stated above, the discharge light should only come on during a pure battery start.