soay, I agree with you in principle. If you would have some sort of "low battery" warning, most of us would not take off until it disappeared, or grab the abnormal/emergency checklist when in the air. However... I think this is very hard to implement in practice. After all, a voltmeter across the battery would show the *alternator* voltage (about 14 or 28 volts) once the alternators are running and connected to the battery, not the disconnected-battery voltage.
I'm not an electronics engineer but I believe you need some very sophisticated equipment to determine the state of a battery that's being charged. (OTOH - my laptop battery can show the % charged even when charging, so it is doable.)
B2N2 - I think you just gave the best advice which works in any case, regardless of the type of aircraft. After a GPU-assisted start, make sure you run the engines for at least five, maybe even ten minutes to get a minimal charge in the batteries before taking off. (On the other hand, don't certain types of alternators require a minimum RPM before supplying the 14/28 volts required to charge the battery?)