I'm guessing, so shoot me down if I'm wrong…. but my experience has been that early battery technology tended to be low on amperage – so if you wanted more power going up in voltage made more sense.

The net result was that 'standard' voltages were developed for various items – small motors and bulbs were 1.5 – 3v, mid-range 4.5-6v and powerful items 12v. You will find batteries matching these voltages from the 1940s.

12 can deliver more power, but the extra cells are heavier. And the advent of Ni-Cads started to mean that you didn't need so large a voltage to push amps around the circuit. So you could use fewer cells and make the battery lighter – 7.2v became the standard for R/C cars where weight was nan issue.

Nowadays the difference is probably minimal (though 12v will always provide more power, and it comes down to what voltage equipment you can buy easily. If the motor you want is available in 7.2v, use that. Though I suppose that 12v incandescent bulbs are always going to produce more light…

Let the corrections begin…!

Edited By Dodgy Geezer on 29/03/2016 22:42:10

Because of its use in the automotive industry post WW2, 12 volts has become a standard for batteries and things that they power. The two sides feed on each other. Different battery chemistries give different voltages per cell. With dry batteries, NiCads and Pb batteries, 6 and 12 could easily be hit. When the usefulness of NiCad became practical, 6 of C cells in one pack ([email protected]=7.2v) was very convenient, and, for a lot of uses, became the norm.

A lot of battery specifications mention a figure for "Watt-hours per kilogram". Whatever the voltage, a given battery of a stated size will hold just so much power. Lead acid is near the bottom of the pile, but in many applications this is unimportant. In a heavy boat that is not a speedster, it does the job very well and is easy to look after.

Any RC boat needs a lower voltage supply to power the radio which could be a separate battery of the right voltage or derived via a BEC, either one integrated into the ESC or a separate one. There might be some separate ones available to derive 3 volts for your lights from whatever voltage you use, but beware of the current if using filament lamps – they are thirsty.

While you can run 12 volt motors on 7.2 volts (albeit with a performance drop) you can't go the other way without risking the motor which might not be able to handle the extra power that it will take.

In the days when getting adequate amounts of power out of a battery was difficult, it made a lot of sense to use a motor-bike or car battery in a big tug or battleship. You needed the ballast, the batteries and chargers were available and cheap (compared to specialist model accumulators!), and you had oodles of power available.

The only down-side was the weight. And as we all get older, heavy ships are difficult items to move around. Now that power is readily available in light-weight LiPo packages, perhaps there is something to be said for building big boats lighter, and using water ballast, filled at the poolside, to obtain sailing trim. Transport, launching and storage would be easier!