Turns out that this is a fairly serious bug in the sim itself, but I hadn't known about it before Someone-Who-Shall-Remain-Nameless (hereinafter called 'SWSRN') pushed me into doing some research on what was going on. The following is extracted from the updated Manual Part 2 which is coming with the patch:

 

4. There is a long-standing bug in the battery part of the simulator which causes the battery to discharge in under twenty minutes as soon as it is put under load. The bug has been there since FS2000; Microsoft say they ‘improved it’ in FS2004 but it has not been fixed or ‘improved’ since by either Microsoft or Lockheed-Martin. There are three possible solutions:

1.   1.Start the engines as quickly as possible but definately in under ten minutes from battery on

2.   Get a registered version of FSUIPC and set 'Magic Battery' to On

3.   Use Doug Dawson's 'battery charger' gauge (https://www.douglassdawson.ca/).

If you use either option (2) or (3) then internal code will take over and calculate battery discharge time. If you decide to go for option (3) then set the dll call into the [VCockpit] section of the panel.cfg file. See Doug’s documentation for further details.

 

I will not be providing any specific fix or recommendation as the choice is down to the user and because it is a bug in the sim, not the B-314. Many of you will already have a registered version of FSUIPC; for those of you that do not have FSUIPC and are familiar with editing the panel.cfg file then Doug's gauge may be the best solution.

Ah... ignore that. Two calls to SimConnect and eleven lines of code. No more battery drain bug.