Originally Posted by Schwa

Confusion...

Your vehicle has two motors that act like alternators in various situations. They both charge or drain the high voltage battery, and the high voltage battery is monitored and maintained by the computer so that it never dips under ~40% charge and never gets charged more than ~80% so that it will last a long time. The small battery is charged by a DC to DC converter (some people mistakenly call this an inverter) that is capable of at least 100 amps of 12v current into the accessory system.

I think what Ford did is made some kind of "manual on" button for the DC-DC converter (or a second small "emergency" DC-DC converter) so that if your accessory battery (12v) dies you'll be able to charge it up manually from the big HV battery so that the computer (ECU) can turn on when you go to start the vehicle. The engine itself is spun up by the HV battery, so you don't need much charge on the 12v battery to get it started, but I think the manual (quoted by someone else) says to push the jumpstart button and then wait 8 minutes or so, then try to start it.