Believe it or not, wiring a car with positive ground is supposed to make them rust less. Doesn’t seem to work very well… my first car, a (positive ground) 1966 MGB came to me with rocker panels completely absent, as though they had vaporized. At the time the car was only 12 year old! The car needed more than positive earth to survive..BMC Corp needed more than the flow of electrons to rustproof their cars.
Around 1967, negative earth became more popular and more common on all cars, especially British ones. Negative is nice because you can power negative ground cigarette lighters, LED lights and stereo systems. Alternators require negative ground too, if you are planning a conversion for more output. Negative ground is more common and less intimidating so it’s a popular conversion for formerly positive ground cars. Thus we now have a mixed fleet, with classic cars (and Frogeyes) set-up for both positive and negative ground. It’s imperative that you learn how to identify with ease which type you have. We find that many people are still quite confused about battery polarity and British cars, so I’ll try to clear things up a little here.
Which way is the Frogeye grounded in the photo above? Can you tell from the photo? Don’t let colored leads and stickers fool you, this is something you want to verify before you create a shower of sparks and potentially do any damage.
All batteries have a marking on the case that identified positive and negative. Sometimes these marks are hard to see when the battery is installed in the car. Let’s presume you can’t see those markings-how can you differentiate the polarity? The larger terminal is the giveaway. Every battery has a larger positive terminal. You can see in the photos below that the positive terminal is slightly larger. So in the photo above, the battery above is wired for negatve ground, with the smaller terminal on the right wired to the firewall for ground and the fatter post wired to the starter solenoid.
Make sure you know before you hook up a charger or a jump, because reversed polarity can melt your charger and/or damage your wife’s car if you jump incorrectly. Whenever you connect another battery to yours (or a charger), always connect negative to negative, and positive to positive. Whether you have positive or negative ground, this rule always holds true.
Once you ID your configuration, mark the terminals with a sharpie to make life easier in the future!