My favorite tool is a portable 12v
jumper battery/compressor. I use it to adjust my suspension leveling air bags mostly. But last summer travelling in the middle-of-nowhere Montana (read: no cell coverage) all the F-150 dashboard lights went out but the engine kept running for a while, then it too gave out. Thought is was a belt or fuse, but not. I attached the jumper cables to the truck battery and low and behold, all the bells a whistles came on: dead battery. I was able to place the jumper (still connected to the battery) next to the motor and closed the hood. I the drove the final 25 miles to Lincoln, MT where a local garage diagnosed the problem as a failed alternator and replaced it, I might add, quicker and cheaper than a Ford dealer in Missoula would have.