24 replies to this topic

[mk=john]

Hi
I have a Mk1 998 cooper, pre verto. The electrics are positive earth. If I want to jump start it, can I connect the battery terminals to a negative earth car, by connecting positive to positive, and negative to negative on both batteries?

Thanks

[bert998]

can you not bump it? might be a bit safer... :S

[minidaves]

yes but make sure cars are a distance apart like 12inch's

dave

[mk=john]

Ok, but electrically, is it safe to jump start a positive earthed car from negative, ru sure?

[AlexM]

Personally I think it sounds a touch sketchy.......

[Bounce]

It'll be perfectly fine. but like said above, make sure they are a good distance apart.

Red to red, Black to black.

[nev_payne]

positive earth? i didn't realize any minis were even wired that way

[Feeble]

So what would happen if they weren't far enough apart, just out of interest..?

[Wiggy]

I'd assume the shells would short out on each other, because one shell is positively earthed, one negatively.

[Bounce]

yep, destroying paint, metal and possibily causing a fire!

[dklawson]

For goodness sake....

Even when the cars are of the same polarity you don't let them touch... it's just not done. As mentioned above you can arc through things like bumpers and damage them. 12" gap? Sure, but just because it will let you walk between the cars... not to prevent arcing. Your car's 20,000 volt coil can only throw an arc about 1/4", your 12 volt battery is much more limited.

There's nothing magic about the polarity of the cars. Yes... pos to pos, neg to neg, cars not touching. As with other jump starts, don't make your last connection to a battery terminal, make it to an earth point on the car with the dead battery. The intent is to prevent any sparks near either battery when the last connection is made.

By bump start to you mean pushing or rolling? I'd never push a Mini with another vehicle as its bumpers are too flimsy. Roll starting will work but only if there's enough power in the battery to operate the ignition coil.

As for the surprise about positive ground cars... most LBCs were positive ground until the mid-1960s. There were even a few (very few) alternator equipped cars that were positive ground. However, in general you can assume that a car with an alternator is negative ground by default.

[miniboo]

DK FYI over here we call bump starting getting a few mates to push you along at a reasonable speed and then trying to bump it into life with the clutch and second gear

[Total Richie]

Quick lesson in electricity.

To get Electrical arc or spark, you require voltage and a 12 V car battery just does not have the voltage to do anything.

If you jump started a positive earth and a negative earth vehicle and somehow you managed to earth both chassis together (which would require direct contact of metal between both vehicles), what would happen, you would create a direct short to the vehicle which running and charging its own battery, now because a car battery does have one thing that's amps, more than likely you would end up melting your jump leads, if the jump leads were heavy duty and you left them in place for a while the battery will its start to melt and if you were really unlucky to battery cells could come into contact with each other and it would explode, spraying hot battery acid everywhere.

[dklawson]

Thanks for the clarification on terms.

[Bounce]

Cheers for that Richie. We hadn't already got to that conclusion already...


And i dont think the battery would melt. Yes the electrolite would get warm, maybe even boil, but the battery itself wouldnt melt. Plus they are only 33A/h batteries, with a direct short they will be out of charge very fast!

Edited by Bounce, 30 October 2006 - 05:39 PM.