I was able to purchase a new 6 volt battery in town today... however it's only rated at 270 CCA ... While the car will turn over and start with this battery, it seems a little sluggish...
I would be interested in hearing what batteries other people are using in terms of CCA capacity.
My previous battery had 650 CCA, but I was unable to get one of that rating that fits.
(I did have to make a battery box to house the new battery, as it is too small for the battery cradle).
Mitch, the small size should have been a clue that your new battery would not be adequate to start your T engine.
Measure the voltage between your battery post and engine with the starter switch depressed and you will have only around 3 volts with your new battery.
A proper battery will provide about 5.5 volts with a good set of cables.
Your new battery was probably made for a small motor cycle.
It would be adequate to test coils with a buzz box to see if they buzz. No buzz equates to a bad coil, so it could tell they were bad, but not if they were good, as random misfires can not be noted or counted with a buzz box. The random mis-fires can only be seen on a Hand Crank Coil Tester or a StroboSpark tester.
650 CCA is about a standard Group 1 battery size and the physical size of all the Group 1 batteries are about the same and will fit a standard battery box.
To add to what Jim said, group one batteries sold today are not as tall as original Ford specified Model T batteries. I found an appropriately cut square piece of one inch thick wood placed in the bottom of the battery carrier brings the level of the top of the battery to where you can easily connect the battery to switch and battery to ground cable.
Ron the Coilman
Seeing as you're in the same part of the world as I am, you should have no problem in getting a 12A or 12B battery - they are used in forklifts. (Despite the part numbers they are 6 volt). Any decent Australian battery agent will recognise those numbers, even the NRMA.
My first battery was a Dunlop 12B which I got from Beaurepaires. It lasted 10 years. Now I have a 12A which I got from Battery World. Provided the wiring is good, etc, and you don't have a dragging clutch, it will start like a 12V car.
The dimensions will fit the battery carrier nicely but do require a piece of wood underneath to bring the terminals up high enough.