Really good question. I am awaiting delivery of my first ebike so this is all new to me. I know they make car chargers for the LiPo batteries that remote control cars, etc. use but those are much smaller batteries. How about using an inverter, those devices that convert your car's 12v DC to 110 AC?
Bosch actually makes three chargers for their batteries, the standard 4A unit, a smaller 2A unit, and a 2A unit with a 12V input that does not appear to be sold in the US. Given the cost of Bosch stuff, you'd be much better off with a cheap inverter and a standard charger. The amp rating refers to the 36V output not the 120V input.
4A x 36V = 144W, in theory a 250W inverter would work as long as it didn't fry your car's electrical system assuming the charger wasn't terribly inefficient. You'd need to be on a car circuit with a 20A fuse to be safe.
I plugged my Bosch 4A charger into a generic 250W inverter plugged into my truck and it charged, but the inverter got pretty hot, so it was definitely working hard.
BTW, don't try to charge your dead ebike battery off your car battery without the car running. Your car battery will probably be dragged down to where the car won't start.
You can get powerpacks like the expensive "Goal Zero" ones with built in inverters and enough amp hours to charge your ebike battery. I found a closeout Schumacher Storm 700 that will give me about two 60% charges on my Bosch 500wH battery. I needed something like that for my camper so I bought it for that purpose, but it does work great for the ebike as well.
Modified sine wave inverters and Li ion batteries do not mix all that well. You will decrease the life of both the battery and inverter. Full sine wave is not a problem. Most inverter manufactures include some information about the suitability of their inverters for charging Li Ion.
Since an alternator is a three phase AC current unit that converts to DC it would make sense for someone to build a unit that takes off the AC, converts to single phase, steps up voltage and regulates to 60 cycle with plugin for bikes, tools etc. or steps up and converts directly to DC at higher voltage. I will have to think about that.
I use to have a welder than ran off the alternator and converted to DC for welding and charging the vehicle at a lower 13.6 volts.
My Schumacher SBP1 700W power unit claims to have a 700W pure sine wave inverter built in, and I have not seen any issues in charging my Bosch batteries with it using the Bosch charger plugged into the AC outlets. I really like the thing, but it weighs about 60lb and is quite bulky, so it just lives in my camper.