Car batteries generally have a larger (at least twice as big) capacity than the battery in your power pack. Of course these larger batteries would be a pain to lug around with you.

Because of the design of a car starter motor, it draws a lot of current when standing still or rotating slowly. So multiple start attempt will drain the battery quickly. The harder the starter motor runs, the more back-emf is generated and the higher the apparent resistance of the motor becomes, which reduces the current used. The reason for this design is that these kinds of motors generate the most torque when standing still, which is exactly what you want in a starter.

Charge indicators on batteries usually measure the voltage of the battery. An almost depleted bettery might still show a sufficient voltage when no load is applied to it. With a good voltage meter you will see the voltage drop like a stone when you connect a load to an almost depleted battery.

BTW, how are your solar panels working out?

The generation of Back-EMF is a new concept for me. Reminds me of how much I don't know about Lenz's law of electromagnetism. One of these days I'll try to get my mind around it.

Maybe I can get a more accurate reading of how much energy is left in the power pack if I push the test button while running the built in compressor? That should be a sufficient load. I'll have to give that a try.

The solar cells are on the to do list. I haven't quite figured out the best location to mount them. I'm concerned about a sudden storm and think we need to build something to hold them in place with some sort of motorized tilting feature. Mainly got them last year to take advantage of the sale price and tax rebates.

Thank you to whichever politician is responsible for trying to motivate people to save energy through these kinds of incentives.