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Correct battery jumping

Correct Battery Jumping

Can someone assist...? I am the owner of a 150 DHC, that sometimes needs a little help to start. Hence, I want to connect the terminals from the battery of my ordinary car to the termainals of the battery in the XK150. I have been given the advice to disconnect the terminals on the ' donating' battery from the leads of that car, before I hook them up to the jag's battey terminals ( or rather one lead to the batter terminal and one to the engine block). This, since the Jag has a positive earth, and the ordinary car a negative earth ( as most car have nowadays, I guess ). Another guy I met said this had no relevance. So, the question is, does it mater that the battreries have different earth when I intend to connect them? If yes, would it also be a problem if the cars touch each other bumper to bumper)? - Claes Goran Boregard

Claes, You are right to be concerned about touching the bumpers together after the jumper cables are hooked up. This will result with big sparks and welded bumpers. But as long as you keep the cars apart there is no problem jumping a car with negative earth to a car with positive earth. The simple guideline is you must connect positive to positive and negative to negative. This means on your car connect the positive (earth) terminal to the donor car's positive terminal, and connect your negative terminal to the donor car's negative terminal (earth). - Regards, Dick White

I agree no relevance, positive to positive, negative to negative in ALL cases. A direct battery connection to avoid confusion is preferable. If you need to use the engine block on the Jag as one terminal, it is the positive terminal. So hook positive on your donor car to the engine block of the Jag and negative on the donor car to the negative side of the Jag battery or the starter solenoid. This assumes, (As you stated.), your Jag has NOT been converted to - ground! <So, the question is, does it mater that the batteries have different earth when I intend to connect them?> No, the batteries do not see earth or whatever. The battery only sees a current path. <If yes, would it also be a problem if the cars touch each other bumper to bumper?> THIS MATTERS!!, here you are connecting negative to positive. When you connect the batteries correctly, +to+, -to- and then touch bumpers or other chassis parts you are shorting both batteries through this new +to- connection. BAAAAD....VERY BAAAADDD!! Rule #1 POSITIVE TO POSITIVE, NEGATIVE TO NEGATIVE, ALWAYS. You may get a small spark when you do the hook up but if Rule #1 is followed there won't be a problem. Rule #2 Remove the cables from the donor battery first. The charging battery (The one needing help.) will generate a small amount of hydrogen during the process. To be on the safe side, always remove the cables from the donor car first so if there is a spark it will not be near the hydrogen. Batteries have been known, in rare instances, to explode as the hydrogen ignites due to the spark from removing the jumper cables on the charging battery first. Not a high risk item, but caution is not a bad thing. - Cheers, and happy jumping, Ken Boetzer

Just a comment on this point. I have heard that there is a possibility of blowing the diodes in the assisting cars alternator when jump starting another vehicle. I have jump started other cars many times with the donor battery connected and also disconnected (only need to disconnect one of the battery leads to isolate the donor circuit) with no problems. Is this a rumor or are there conditions where the diodes can be reversed biased above their ratings? - Regards; Andy Leavitt

After years of dual battery drain, and jumps from my Morris Minor, I elected to rewire for a single 12 volt battery, first on the left (far) side >from the starter and solenoid, then on the near side. The near side wiring resulted in an immediate improvement , and quick starting is now the rule rather than the exception. This would obviously not impress the judges if that is the goal , but it sure eliminated a constant pain in the you know what. - Bill Burke 150DHC 838839

Yes, you will get a considerable spark when the bumpers meet. In the old days....so I hear ;-)...you could jump a car from the wrong end (i.e. nose-in in a parking space) by linking both jumper cables for one connection and running the bumpers together to make the ground. Jim Warren

Don't know about the diodes but you can burn out the alternator in a 1985 Toyota if you leave the engine running while boosting another car. The toyota apparently has nothing to limit the alternator current output when it sees the low resistance of another battery to charge plus the other car's starter motor and the alternator starts to smoke after about 5 seconds. Some other cars might do this too but I've never heard on another example. - Bruce Cunningham

You bet your bippy it matters if they touch! Keep them separate and you won't have a problem as long as you put + to + and - to - . Not frame to frame. Connect the "like terminals" on the batteries together. - Dan B

I blew a diode on a car once, but I don't recall which car it was. After that I decided that I would always connect batteries with the engine off, and try to jump start that way. I haven't wrecked an electrical system component since. If the battery on the receiver car was so weak it still didn't start, then I would make the difficult decision of whether to start the car to charge the battery, or forget jumping the car. - "Mark 1" Mark Stephenson

Aside from the possibility of their butting bumpers, the concern is to not make connections with the last connection on a battery because they generate hydrogen which is explosive. Make the last connection to the ground at probably the motor or other point away from the battery but well connected to it! - Regards; Michael Rogers

Hi Andy Leavitt & all -- Some more comments: (1) Every set of jumper cables I've seen/used has RED for one wire at both ends, BLACK for the other at both ends. I simply use RED for negative, BLACK for positive, no problems. (2) If your car has the twin-6v batteries, make SURE you connect positive to the positive battery post that runs to the frame (positive ground), and negative to the negative battery post that runs to the starter solenoid. This way, the twin 6v batteries act as a single 12V. (3) DON'T let the cars touch each other in any way; this avoids current passing anywhere except battery to battery. It also avoids metal/chrome/paint damage caused by current flow; it also avoids fire risk if things melt down. I hope this helps -- Larry Martz [USA, Calif.]

As long as the positive pole on one battery gets connected to the positive pole on the other battery, it doesn't matter what is connected to each battery. While it is usually good advice to make the second connection to the frame of the vehicle to avois creating a spark near the battery (fumes from the battery are explosive and can be ignited by a spark), this could get confusing with reversed polarities. The frame on the XK is connected to the positive pole of the battery while the frame on the modern car is connected to the negative pole on its battery. Just think of the frame as an extension of the battery terminal in each case because that's what it is. You must not let the vehicles actually touch each other while the batteries are so connected as this would be equivalent to shorting out both batteries at the same time. A fire could easily result. Twelve volts with almost no resistance creates extremely high current that will turn a jumper cable red in a very short time. - Bruce Cunningham

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