How long can I reasonably run my T on a 12V motorcycle battery? My mag is on the fritz and I don't want to tamper with it until after the OCF so I'm running fine on battery but I'm worried about it running out of go-go juice.
Look at you battery amp hour. That will tell you how long a fully charged battery should be able to put out the rated amps for 1 hour. If you draw less amps it will last longer than the rating. Depends on your amp draw for the coils, but mine draw about 2.3 amps. Your's may be less. That will affect the time you have to operate.
Kevin, my battery is 230 amp and rated at 19 amp hours. So are you saying your T draws 2.3 amps per hour?
There's something wrong if Ford coils are drawing 2.3A at 12V. The life of the points will be short and the coils will run hotter than normal.
When coils have been set up correctly, the average current is about 1.3A at 6V. Operated at 12V, the current automatically falls to a lot less than this. Something around 800mA if I remember correctly.
Another thing to keep in mind is that when the engine is running, the coils are not drawing current all the time because of the dead space between timer contacts. Therefore, the power drawn from the battery over time is not as high as first thought.
My coils are 6 volt electronic. They actually draw between 2.1 and 2.3. At 12 volts they would draw about half that. Traditional coils draw less. Bill at 19 amp hours you should be fine, just recharge at night to be safe.