wildbadbill

don't really get how the fuse would blow?
Fuses blow in OVER-CURRENT situation,not the direction of current flow.The diodes and rectifiers allow one direction flow only,fuses don't "care".

imp

The alternator and battery are both power sources, voltage sources put another way. They are connected in parallel, along with all the rest of the circuits in the vehicle. That way, either the battery or the alternator can supply the electrical power to the car. The battery supplies it ONLY during starting; once started, the alternator takes over, replacing the power which was drained from the battery during cranking, AND feeding all the rest needed by the car.

Now, connect the battery BACKWARDS and the battery feeds current through the alternator, lots of it, possibly damaging it or the wiring. That does not happen when the battery is properly connected, as the alternator's DIODES block current flow back through the alternator. Diodes allow current flow in only one direction.

Hope this helps you understand. What you say about fuses is true, but when the current flow backwards is extremely high, as in backwards connected battery, fuses DO blow, because the current level exceeds their rating. imp

wildbadbill

I'd get it-the only thing is that would make sense IF the carf was running after the battery was connected backwards.I don't think this was the case-I don't believe you could have started the car at all?
So,the other power source (alternator) was not delivering.So you're looking at the system that would receive positive ground and negative to all instruments,however the total voltage across would be still only 12v battery,just reversed.Current would not flow.Unless it would short thru the alternator-but in this case it would not be just a fuse,but a fried alternator?
Plus,like you.ve said,the alternator diodes would block the current flow thru the alternator.If,true and no current flow-why blown fuse?