Ford cars before the 1919 model year did not have starters. With no electric starter there was theoretically no need for a battery. Electric headlights, introduced in 1915, were powered by the magneto. But Ford was realistic enough to know that in the real world not every magneto stays in top condition, and sometimes might not supply enough current to the coils for easy starting. So while the Model T was sold without a battery, the ignition switch was designed to use a battery for starting if the owner wanted to install one.

Batteries used in pre-starter Fords, being aftermarket accessories, came in a variety of shapes and sizes. Some folks used a full sized storage battery kept in a box on the running board. Many used the dry cell batteries that powered telephones of that period. They were small enough to be hidden somewhere inside the car, they supplied enough current to buzz the coils, and they were cheap.

A full size battery takes up a lot of space, especially in a roadster.

A hundred years later reality still applies. Not all Model T magnetos are up to the challenge of easy starting, so most owners want batteries in their Fords. And they still face the choice of which battery to use. Automotive storage batteries are just as large as they were in the Model T era, and fitting one into a T in a convenient location is a problem if you don't have one of those running board boxes. Dry cell phone batteries can still be had, but are produced in much smaller numbers now and are therefore much more expensive than they used to be. Most folks today use small sealed batteries easily found online. Some use twelve volts so they can run some modern accessories, but six a volt battery will have no trouble buzzing your coils.

A small sealed battery, easily found online, fits in much better.

Inside the frame rail is one place for hiding a small battery.

A non-starter model T comes without a generator of course, but the battery still needs to be charged. Some just put a charger or a tender on their battery between drives. With the battery being used only to power the coils for starting, and the switch turned to MAG for running, a single charge will last a long time. Others install some modern geegaw like a belt-driven alternator. But there's a cheap, easy, and unbotrusive way to keep the batterey charged by the magneto while the car is running. All it requires is a bulb and socket, some wire, a diode, and a home-made bracket to hold everything.

Magneto battery charger operating in a 1915 Ford runabout.

There are several diodes that will work. The one I was given for my charger is from MIC:

Others are:
NTE 5812HC     and     1N5402 200v at 3 amps (Radio Shack p/n 276-1143).

Be very careful which way you connect the diode. The end with the silver stripe is the cathode and should be connected to the 1156 bulb. The other end is the anode and connects to the magneto. (The most convenient place for me was at the MAG terminal of the coil box.) Installing the diode backwards will send battery current into the magneto and discharge the magnets. Fully recharging them will require removng the engine/transmission and disassembling the transmission and magneto.