I bought a second hand 6/12 volt 6 amp charger. Never tried it on a battery. Hadn't tried it on a battery. I have 2 T's and one set of coils. Put coils in one I hadn't cranked in a while, grounded the charger and hooked the positive up to the battery post on the coil box. Plugged charger in and it hummed. Wanted to see if all the coils buzzed before trying to start it. One didn't I took it out put it in and all 4 buzzed. I unplugged the charger several times while doing all this. Put in gas, plugged in charger was going to start on battery, well charger, there is no battery. No sound. I tried to hook the charger up to my lawnmower and sparks flew from the terminals with the charger unplugged. I don't know how that could have damaged it but it doesn't work. On a positive note the car cranked on mag. I can take the charger back but I feel like I fried it somehow.
I have made a few mistakes in connecting my cheep 6 and 12 volt battery charger.
Normally if I wait a few minutes it does a reset and works out
It is usually connecting the wrong terminal
Those old 6/12 6 amp chargers were ferroresonant chargers and nearly indestructible. My experience is that only two things will kill them ... a direct short for length of time, or rust/corrosion.
The output however is not DC but a half rectified AC. They do not do well substituting for a 12V battery. Doubt if coils will work well on them without a battery in the circuit to smooth out the pulses.
Its possible that the coils with their make and break primaries put out a voltage spike that destroyed the rectifier in the charger, especially without a battery in the circuit. No way to tell for sure, but possible.
It can and it did fry the six button rectifiers in my wheel charger. Fortunately I was able to purchase a replacement rectifier from the manufacturer. In mine, it was a ceramic assembly that looked like an asterisk (*).
I now test coils under a spark plug load and only with a battery.
I plugged it in while ago, nothing. It cost $15. I was at the store with my friend anyway and bought it. It will cost me $10 in gas to take it back so I'll just get another one sometime and not do that again. Live and learn. I let somebody borrow my good one and it got ruined. Won't do that again either.
My new el cheapo 6v/12v charger has a fuse mounted inside. Slot looks like those where you would expect to see battery trays. Perhaps you have one??
Worth a look, I'll take it apart tomorrow. I used to use one of those jump boxes to see if the coils were buzzing but I don't know what I did with it.
I think Bud has it right, Model T coils can produce a voltage spike in excess of 250V when the points open which can destroy the rectifier diodes in the battery charger without the battery connected; the battery acts as a filter to suppress the voltage spikes. You can test them using an meter with diode test mode. A good diode should read low resistance with the leads connected one way across the diode and a high resistance with the leads connected to the diode the other way. The diode will typically fail short so it will likely read low resistance both ways indicating it is bad. Good Luck with it!