I called for the power truck and he came along and connected the power cable, when I clicked on the EXT PWR switch I got a nice loud thump and the battery voltage jumped to 28V. However very soon after that, first one then the other battery voltage fell back to about 27V indicating little or no power from the GPU being delivered and engines could not be started. So, start up on APU generator and all will be well (you may have to toggle the battery switches off-on).