As said earlier the operators wanted to be able to tow without the use of the APU or other external power source.
Well, they got that, OK, Whilst completely overlooking the fact that their "system" allowed the aircraft to be reduced to an unairworthy state.
Withy the best wil in the world, you can't expect a tug-driver to understand the ins and outs of a lithium battery's charge-discharge characteristics. they're MOVERS not scientists!...The hundreds of posts on here, by a predominantly intelligent membership, prove that the assumption that unskilled personell could be let loose on a fragile system,was deeply flawed.
In a former life, I made, and later serviced, Medical Equipment, including premature-Baby Incubators, Respirators, oxygen tents and and various pumps.
A major part of the designer's efforts , was to make everything "idiot-proof"
We had a saying....
Make it idiot-proof and it'll be ALMOST Nurse-proof.
No matter how well something is placarded, complacency and familiarity will ignore it. safety and protection need to be built-in to pre-empt misuse.
my suggestion of a "bypass" emergency-reserve switch (on the flight-deck)
addresses that issue....
Why Boeing didn't employ two discharge limits - one for ground operations and one for emergencies in the air - is due to it adding complexity to the fault tree and the necessity to take into account the possibility that the ground discharge limit activates during an emergency, denying the flight crew power and possibly directly leading to an accident.
....And a burning battery or a U/s over-discharged one doesn't add complexity to the Aircrew's day???? -
Come on! this would be a very minor mod , in the big scheme of things.
The plane had too much design-input from smartass Graduates and bean-counters and not enough from experienced aeronautical and systems engineers. and it shows!