battery charge causing fuse failures?

prkaye · #1 08-06-2010, 08:42 AM

I've had two different items on my battery bus experience fuse failures. First was the tach transducer, which I have since re-wired and moved to the main bus. Today before going flying (2nd flight) I discovered I had no alternator current, and found that the 5A fuse on my alternator field wire was blown. This fuse was in the Battery bus. Since I had flown once and done several engine runs with no problems, I identified that the one thing I had done between the two flights that might have caused this is that I put my charger on my battery. Is it possible for a battery charger to induce some transient current on the battery bus large enough to blow a 5A fuse??

Kevin Horton · #2 08-06-2010, 09:25 AM

Charging the battery should not cause fuses to blow. The fuse should only blow if there is an excessive current in the wire between the fuse and the device that the fuse is supplying. I see a few posibilities:

Fuse too small, such that normal loads are high enough to blow the fuse.
A short on the wire between the fuse and the device. Perhaps you inadvertently shorted a wire when working on something else. Maybe something is moving around and shorting an unprotected wire. I would closely inspect the wiring looking for potential issues.
Defective fuses.
Defective charger that puts out a very high voltage, which then causes high current in wires connected from the buses to the devices. I would measure the bus voltage when the charger was connected.

Bob Axsom · #3 08-06-2010, 10:57 AM

I disconnect and usually remove the battery for charging. I believe it is a good idea to isolate the battery from all of the circuits and do it independently. Chargers should be thought of as an energy source with all of the potential for damage associated with an energy source, in-rush current, switching transients, unknown aircraft circuit state down stream, etc.

Bob Axsom

Bavafa · #4 08-06-2010, 12:05 PM

Not knowing much about your set up, it is hard to say? I have a 5A fuse (circuit breaker) on my B&C Alt field circuit and vast majority of times will connect my trickle charge to the battery and have never had it to pop.

az_gila · #5 08-06-2010, 02:39 PM

Quote:

Originally Posted by prkaye

I've had two different items on my battery bus experience fuse failures. First was the tach transducer, which I have since re-wired and moved to the main bus. Today before going flying (2nd flight) I discovered I had no alternator current, and found that the 5A fuse on my alternator field wire was blown. This fuse was in the Battery bus. Since I had flown once and done several engine runs with no problems, I identified that the one thing I had done between the two flights that might have caused this is that I put my charger on my battery. Is it possible for a battery charger to induce some transient current on the battery bus large enough to blow a 5A fuse??

...is directly connected to the battery, and the master is off, then your master contactor should isolate the circuits you mention from anything happening at the charger/battery interface.

Do you have a master contactor?
...and is it isolating the two circuits you mention with the master switch off?

prkaye · #6 08-06-2010, 04:35 PM

Gil, you're absolutely right. But I may have turned the master switch on at some point while the charger was still connected, in order to check something on my EFIS/EMS. I'm not sure if I did that or not while charging on Wed.
I've found some people on other threads that have experienced the same intermittent failure with the 5A alternator field fuse, with the plane power alternator. In some cases it seems like the cause was not identified. I may run a 5A breaker instead of the fuse. In the mean time, I'll just keep an eye on it and see if it happens again.