Hi Ante,
That's a unique way to use an ignition system in reverse, complete with a functioning sparkplug.
It may be necessary to connect the battery through a full-wave rectifier, because I think that the antenna will pickup a lot of RF, and the polarity of the static charge may depend on "which way the wind blows".
You could say that a positive wind attracts and carries the electrons from the antenna, leaving it positive. But the wind could instead be negative at times and "dump" some of its extra electrons onto the antenna, leaving it negative.
I think that the "charging current" will be very infrequent, short high current pulses.