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MG MGA - 61 MGA battery hook up

Had to replace both 6 volt batteries. I drew a diagram of the original connection. The red cable was grounded to the battery holding frame behind the drivers seat and connected to the positive post on the battery in that frame. There was a red cable about 14-16 long also connected to that post and to the positive post of the battery behind the passenger seat. The black cable coming up from the passenger side of the car connected to the negative post of the battery behind the passenger seat and a 14-16 cable connected to that post and the negative post of the battery behind the passenger seat. I am ready to connect new batteries and everyone says this is wrong. I know how they were when I took them out. Were they connected wrong? How should they be connected?

Vicki Moncrief

Assuming your car is still connected with a positive earth. The red on battery one is connected to the frame, or earth. The negative of this battery is connected to the positive of battery 2, and the negative of this battery goes to the starter motor, or the long black cable going forward. Hope this helps.
Remember, battery 1, + (red)to earth, (black) - to + (red) on battery 2, then - (black) to starter.
Regards, Don
Donald J Walker

Are you sure you had two 6v batteries, not two 12v batteries?

The way you describe it, the batteries were connected in parallel, which is what you would do if they were 12v batteries.

For 6v batteries, they should be connected in series, the way Don explained.
Dave O'Neill 2

First off, ignore the cable colors. Cables can be any color, most commonly black, but many replacement cables will be red (occasionally some other color). Cable color does NOT indicate polarity of the battery connection.

Next note that the positive battery terminal post will be larger diameter than the negative post. This was originally intended to "reduce" the probability of connecting batteries the wrong way around.

Original cable grounding point was a single bolt hole on the frame welded bracket above and slightly inboard of the center exhaust hanger.

If you intend to use two 6-volt batteries and positive earth (as original), then see here:

Two 6-volt batteries connected in series makes 12-volts, as the electrical system is (and always was) a 12-volt system.

These days it is common to substitute a single 12-volt battery in place of two 6-volt batteries. IN this case simply omit (delete) one of the batteries from the electrical diagram, and call the remaining battery 12-volts.

If you want to change the system to negative earth (to use an alternator or any other modern electronic equipment), then simply reverse orientation of the batteries (or battery). For a new assembly, or any time you change a generator or change system polarity, you should polarize the generator before running the engine(very simple, refer to the Workshop Manual).

In any of these cases there will be a single cable connection on each battery post, never two cables on one post.

The only time you might have two cables on each battery post would be if you were installing two 12-volt batteries to be connected in parallel for net 12-volt system. This can double the storage capacity and double the cranking current ability without changing system voltage. This is NOT recommended, because an internal short in one battery will destroy the other battery, most likely with sudden catastrophic failure (explosion). If you intend to do this, then do install a 500 amp fuse between the batteries so that one battery cannot discharge rapidly through the other battery.

This thread was discussed on 23/12/2017

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