The real reason the MGB had two six-volt batteries

January 1, 2016 | By Staff


I am puzzled by your November answer about MGB ballast resistors. You state that using twin six-volt batteries, as in the early MGB, would produce “amperage from that setup (which) is roughly twice that of a single-battery 12-volt system.” But the two six-volt batteries were wired in series producing twelve volts for system usage. The total cranking amps or the amp-hour rating of the two batteries together may be greater than that of a single 12-volt battery, merely because of the total size of the cells and their plates.

But I would have thought the available amperage draw at any point in the system would be the same whether using one 12-volt or two series-linked six-volt batteries. The amperage would depend on the work any device was asked todo with the 12 volts of “push” available, from whatever configuration that was supplied. Admittedly, I’m a theologian, not an engineer, so I may have this all wrong; but I would enjoy clarification of the matter. Thank you.


You are correct on every count. A well-known British car restorer and parts supplier had furnished me the information but it is incorrect. After receiving your inquiry I did an afternoon’s research on the subject, and finally found the answer to why the early MGB had two six-volt batteries instead of one 12-volt battery. One 12-volt battery that could supply the juice put out by two six-volt batteries would have been large back then, but extra amps were not the main concern. Another explanation I found in several places was that the engineers who designed the MGB were worried about weight distribution and handling, but that can’t be right either, and makes little sense when you think about it.

For one thing, the handling would not have been affected enough by the weight of a battery to make any difference for racing purposes because the driver would have more than counterbalanced it. And if two people were in the car, there could easily have been a 100 pound-plus weight bias either way due to the passengers. Imagine a 250-pound person in either seat and 110-pound person in the other, or vice versa.

It turned out that one of the car’s chief designers was over six feet tall and had long legs and insisted they make the car comfortable for him to drive, so they put two small six-volt batteries behind the seats in order to provide for longer seat runners. The source for this information was a book by an engineer named Don Hayter who worked on the original MGB design, and wrote about his experiences. I can vouch for the fact that an MGB will accommodate a six-footer nicely because I had an MGB, and am a six-footer.