I have a 2003 Mitsubishi Diamante, and I keep two battery boosters in the car "just in case".

My son recently bought a car that had problems starting. He has used both my boosters on his car and claims they are now both broken. He had also, a few times, switched out and used my battery, and now my car won't start.

When the problem started, I was able to charge the "broken" boosters and even though my car would not start on its own, I could at least start it with a booster.

This morning, I went out to the car and saw that he had put a battery charger with missing clamps on my battery. Only the black wires (where the clamp belongs) were connected to the car -- the red side was loose. The charger was blinking zeros and it sparked when I tried to remove the black side wires, so I unplugged it.

At that point, I got my fully charged booster and connected it, red then black, and it instantly drained from 13.3 down to 6.4. The car wouldn't start. I tried again with the other fully charged booster and the same thing happened.

Have my boosters or battery been damaged somehow? Can I fix them?

I know the way he had it connected this morning was wrong, but I can't find any solid info to explain it. I've asked many people why the red cable must be connected before the black, and why it can do damage to your car and battery if this isn't done properly.

3 Answers 3


The story is a bit rambling, but it sounds like your son's car has a serious electrical problem. 3 batteries behaving well for a long time, and all having problems at once is not a coincidence.

Take one of your batteries to a garage (or, better, a battery shop) and have them charge and test it. It is possible to damage a car battery by overloading or overcharging it.

To answer your first question about wether to connect the red or black wire first, the connection order doesn't matter electrically and will cause no damage if done in the other order.

The reason there's a rule about "black first" mainly relates to the fact that batteries have big, bolt-on terminals that usually require tools. If you remove the black wire first, and your wrench touches the car frame, nothing will happen - the black wire is also attached to the car frame. If you remove the red wire first and your wrench touches the car frame, you will have a direct short across the battery which is bad for everyone involved.

Booster / charger cables should be connectged red-red, black-(frame away from the battery). Again, electrically it makes no difference. The final connection will make a spark, and car batteries often leak hydrogen gas which likes to explode. Like the Hindenburg but not as big. Make the last connection away from the battery and it doesn't happen as easily.

  • 1
    ALWAYS Check the battery markings some cars have BLACK positive and WHITE or BROWN (or even red/yellow) negative. Seen this on GM/Vauxhall and some French cars.
    – John U
    Feb 10, 2015 at 16:32

Paul explained the safety reasons, but not the usage issue.

Deep draining a regular battery will kill it, as would over charging or over volting. The smaller batteries in your boosters too. So will letting them sit for too long, batteries do have a shelf life.

Leaving the black ground cable connected will not do anything bad. Leaving it fully connected for too long would (drains them too deeply). Using them as a replacement for a real charger or too much would too. Those battery boosters are meant for the occasional whoops I left lights on overnight not my battery is dead, alternator is crap, major unsolved battery drain issue.

If he is a real mechanic, he should know he needs a proper wall powered charger, and fix the issue his car is having. While it's possible that your boosters just happened to go bad with age, it's not likely. Tell him to stop using your car battery and to replace one of the boosters. You shouldn't need two though.

  • Battery boosters take a very long time to charge. Days.
    – mckenzm
    Aug 1, 2019 at 7:12

If your car's battery has been removed a few times there is a chance that the terminals and/or wires have become loose or damaged. Loose connections could reduce the voltage that is getting to your starter. With a lower voltage getting to the starter motor there may not be enough power to start the car.

You may want to go through each connection point and do some tightening. If you have a simple voltmeter a bad connections can be checked for by placing the voltmeter probes across the suspected connection while someone cranks the engine. A reading approaching or exceeding 0.5v may indicate a problem.

A common car battery that has been fully discharged one or more times can also be severely damaged or completely ruined. For this a voltmeter on the battery might show a reading below 12v even after the car was recently running.

The best way to be sure about the condition of a car battery is to bring it to a shop that has a battery load tester. Many auto parts stores have these now and they usually let you use then free, (basically because they would love to sell you a new battery.)

If you do need a new battery I guess you know where to send the bill.

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