I have jump started straingers vehicles in the past, but am much more cautious now days.
The last time I jump started a straingers vehicle was last winter with snow on the ground at night. They had a very old truck with bad paint and rust, and it was a young couple with kids in the vehicle and bags full of groceries. He had the hood up and as I walked by him to go to my car I saw that the battery was a top terminal with very corroded terminals and it looked cruddy. I figured it was old. I asked him if he needed a jump and he said yes. I told him I have a very good set of jumper cables but they are in my garage back home, wait ten to fifteen minutes for me to go home get rid of my groceries and come back with the cables. I did, connected it up and it fired right up. I think they were surprise in the first place that I offered to help, because they were of a different race. Anyhow, after it started he offered me a five for helping out and I told him to keep it, your going to need it to buy a new battery.
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Someone posted about being careful that the person looking for help is not someone looking to use you stopping to rob you, or car-jack you. I had not thought of that, but now days it is something to keep in mind.
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One thing I always do when I jump start a vehicle is turn off the engine of the good vehicle and leave it off. You do not want to take the small chance of damaging your alternator by over-loading it with the load of the bad vehicle trying to start.
Sometimes if the bad vehicle has a completely dead battery, and jumping it will not work, you can get it to work by charging the bad battery for fifteen minutes with the vehicles connected and running the good vehicle. However before I attempt this I always have the owner of the bad vehicle give me every set of keys to the bad vehicle so there is no way they can try cranking the engine on the bad vehicle while the good vehicle is running. Then after fifteen minutes I turn off the good vehicle, give them there keys back, and let them try to start it. I usually works.
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One time back in the 1970's I was going to roads end junk yard and the one lane dirt road to it had a big earth mover sitting sideways in the middle of the road with a stalled engine. I had a long set of 12 gauge jumper wires with me that I sometimes used to charge my grandmothers little garden tractor. The battery for the earth mover was on the side of it, and it was only one 12 Volt battery that was very big, and I though my cars battery was way too small, and my wires were way too light a gauge but charging the big battery might work. I told the driver that it would take a while, but I could connect my car to the earth mover and charge the battery and it might start. After about 20 minutes of charging I disconnected it and they tried it and it started. They moved it, and I was then able to get to the junk yard.
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One of the reasons I am cautious about jumping any vehicle is that I have seen a bad battery explode as soon as the cables were connected, and rain acid on the person doing the jump start, and he said he had the polarity correct. Fortunately for me I was far away from it when it happened.
One of the things to keep in mind is where are you going to run to to get under a shower or turn on a hose if you get sprayed with acid.
The last time I jump started something there was a young boy who wanted to get in close and watch. I told him to get far away, because there is a very small chance the bad battery will explode and scatter acid everywhere around it. I have seen it happen.