I was thinking about this...
And I decided that if i was going to attempt this, I'd have an analog circuit, using a voltage divider, that way when the voltage rises high enough (depending on the 2 resistor values), the base of your transistor will switch on..
12v --- 30k ~~~~ . ~~~~10k ---0v
.
.___________ base will rise to 3v, when the solar panel reaches 12v, the base of the resistor will be 3volts.
Collector/Emitter begin to flow, which can turn on the 85 safely, using this voltage divider, the solar panel has to reach 12v.
Change the values to suit your specific needs, but any reason why this would not work?