Battery Charger in Late 80's Cig
 

In my 88 Bullet there is a battery charger in the engine compartment. It is basicly a big black battery charger. Does anybody know any info about these chargers? Specifically are they trickle chargers or ? Also, my batterys went totally dead because I left the radio on and was wondering how long to leave that charger on so it doesn't cook the batterys.

Thanks