787 Batteries and Chargers - Part 1

[Chris Scott]

Quotes from bsieker:

(1) “The only thing I found on this is that the APU battery is designed for two consecutive start attempts, after which a 5 minute cooling period must be observed before a third starting attempt. I don't know if that means 5 minutes from then on after every start attempt, or if a maximum of 3 attempts is permitted, and if the limiting factor is only the battery or perhaps also the starter motor.”

It sounds a very familiar, conventional limitation, typical of APU starting limitations since the 1960s, where the batteries have been lead-acid or NiCd. My understanding was always that it was to stop the starter overheating, and/or time for unburned (sorry, pun not intended!) fuel to drain away. But within these factors, an unwritten battery-protection measure may have been contained, as you seem to be suggesting might be the case with these Li-Ion ones.

As you know better than I, the peak battery load on starter engagement is massive (just as it is on a car battery during engine start). The A320 uses its 2 main (NiCd) batteries for APU start if no external power is available. In the 1990s, when starting the APU using external power, I was able to monitor the two TR loads. Initially, they would be off-scale but, IIRC, they reappeared as the current was falling through 350 amps (each). So the peak current delivered by the TRs was in excess of 700 amps. But the main (NiCd) battery was also contributing, so this suggests a peak starter load of the order of 1000 amps.

(When the A320 entered service, it only employed one TR for APU start. This led to an embarrassing incident on a demonstration flight for the French President, during which the relevant TR failed during APU start, resulting in considerable DC load-shedding.)

(2) “(Starting from other sources, the starter motor used can be alternated, but the APU battery can only power the right starter motor.)”

I’ve never used an APU that had more than one starter motor, AFAIK. When you say the “right” starter motor, do you mean “R/H” or “correct”? (The English word “right” can be ambiguous, which is why many of us old pilots replace it with “starb’d”.) Are there two APU starter motors on the B787?