Given that the situation is entirely hypothetical I shall make the following assumptions -
EarthX Main battery was not discharged to cut off voltage
The AGM ignition battery was not damaged by deep discharge
The alternator fault was located and fixed
The engine was started with my jump pack
In general, as far as I can tell, it is not possible to "jump start" a lithium battery such as the ETX-900 as the internal resistance of a deeply discharged Lithium battery will be very very low resulting in such a high charging rate that either the BMS will trip and/or permanent damage will be done to the battery (I have read of fires and worse on reddit). The EarthX documentation says:
If below 13.28V, the battery needs to be charged prior to an engine start attempt with an appropriate charger. Jump starting your battery in this situation is not recommended for it can damage it. Use a plug-in charger to safely and slowly bring the voltage back up to the appropriate level.
The best thing I can think of is that the main battery has enough charge left to power the left ignition, so you can hand-prop the engine.
I would then run at low rpm on ground to charge both batteries. I would monitor battery charge current and voltage and depart when I thought it safe. I do not consider your statement "You have no way of charging it, nor your starter battery" to be valid.
If the main battery were completely discharged, then you would have to disconnect the ground terminal for 60 seconds to reset the BMS so that it would accept a charge. So if there is enough charge in the ignition backup to hand-prop, and hopefully the alternator field will not trip continuously, you can then charge the main battery with the engine at near idle to limit the amount of electrical power generated and start charging the AGM battery (which will take ~22 hours to fully charge). Once the main battery current has started to drop from the initial value after engine start, I would consider flying.
If the alternator keeps working the battery charge state is of no significance.
This is not true - according to what I have discovered so far, a completely dead ETX-900 will try to draw more than 80 Amps, which is much more than the alternator, fuses, and breakers can handle.
I'll fly out from an austere/remote landing site with marginal battery charge rather than run out of gas charging them.
I agree - but it does bring up the question of if we are flying with Lithium main batteries, should we be carrying a solar charger (slower but safe for the batteries) rather than a jump pack?
Yes - This discussion should be moved. It does not belong in a thread about about EarthX STC.
I think that this discussion on the behavior of EarthX and other Lithium main batteries when discharged is on topic. I will be looking into getting one of the
Optimate solar chargers and a small panel to carry instead of a jump pack.
