Dead PC680 then lots of radio noise (help)

Sunday AM I fired up the RV-6 with no apparent issues and went to a cheap gas stop about 20 minute flight. After getting fuel, battery was dead, prop moved about 20 degrees, then nothing. I jumped it off and it everything seemed fine. About 5 mins from my home field I started getting a lot of radio noise, my KX 155 screen went blank, my garmin 250 XL was going blank then coming back on, and loud noise in headsets, even after I turned off both radios. The amp meter looked normal, just right of 0. I landed and put it in the hangar. Voltage at that point was around 8 volts.

I ordered a new battery, but my question is, do you think I fried any of my electrical equipment? Shouldn't the alternator keep everything at 14 volts, and keep the system working properly. Not an electrical expert. I have a new battery coming and will find out Thursday, just thought I would pose the problem to the forum and see if anyone has experienced this problem. I am not the builder and I am not 100% sure how everything is wired.

Thanks.

Geeman said:

Shouldn't the alternator keep everything at 14 volts, and keep the system working properly.

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Not if the alternator died....probably why your battery voltage went down.

That is why jumping off a "dead" battery then immediately flying is not recommended...this is a good way to begin a flight with no alternator....

Pull the alternator and have it checked before springing for the new battery.

What was your system voltage *while the engine was running*?

FWIW, an ammeter is of very little use if you know how it's wired, and less than no use if you don't know how it's wired. (There are several ways, and each monitors different 'stuff'.)

The important thing is to have ~14 volts when the engine is running; otherwise the battery isn't being charged. [edit: less than 14V while running says you have a charging system failure, as Sam noted.] A fully charged lead-acid battery will be around 12.5V with no load on it (and not being charged).

Low voltage will not damage electronics. *Overvoltage* can damage electronics, but with modern avionics typically designed to operate on 12V or 24V, OV damage in a 12V system is unlikely. Damage due to voltage *spikes* (transients) is possible, but again, unlikely in a typical installation.

If you have 14V with engine running, you could well have a bad battery (easy to check with a capacity tester by you or an auto parts store). But you could also have poor or corroded contacts anywhere in the charging circuit, including the battery terminals.

As strange as it sounds, you can sometimes start an engine through corrosion on battery terminals but be unable to actually charge the battery through the corrosion. So you crank the engine, see 'normal' 14V on your voltmeter, but the battery never recharges. You might get multiple starts (each normal start uses maybe 5-10% of the available energy in a starting battery) before seeing symptoms of low voltage or a failure to crank the engine.

Charlie

Good comments. To sum up:
- You had a failure that caused your battery to be fully discharge. Finding that failure is the first step. If the alternator has an output breaker, look to see if it is tripped. If not, pull the alternator and take it to a shop for bench testing. If the alternator proves good, look elsewhere. Note - standard 60 amp alternators should have a 70amp or so output breaker. A standard 60 amp alternator will put out more than 60 amps if it is trying to resurrect a flat battery.
- Consider your battery dead. Replace it. If you are able to breath life into it have it serve in your lawn tractor of choice. As you caused damage you do not know the battery's Amp-Hour capacity.
- No, your alternator will not function without a battery. I also suspect whatever the alternator was putting out was the source of your radio noise.
- As discussed, jump starting and launching with a dead battery is invitation for an "unforeseen incident". If the battery is dead, charge it before flying away. If it does not hold a charge because you abused it (the reason why it is dead), replace it before next flight and fix whatever caused it to go flat.
- Your radios should be fine but do a thorough check out to be sure.

Carl

Can only speculate without testing...

Loose or dirty battery connections (especially negative side) would give all the OP post symptoms-> static progressing to intermittent avionics, starter drag, unable to maintain voltage in flight. Boosting at fuel stop possible loosened the battery connections more? Did you try re-charging battery after the flight? Did the battery refuse to accept charge from the charger confirming the dead battery theory? Start diagnosis simply (loose connections) before condemning the whole charging system.

Fuse not breaker

Carl Froehlich said:

Good comments. To sum up:
- You had a failure that caused your battery to be fully discharge. Finding that failure is the first step. If the alternator has an output breaker, look to see if it is tripped. If not, pull the alternator and take it to a shop for bench testing. If the alternator proves good, look elsewhere. Note - standard 60 amp alternators should have a 70amp or so output breaker. A standard 60 amp alternator will put out more than 60 amps if it is trying to resurrect a flat battery.
- ....
Carl

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Note that a lot (most?) RVs have been built with a firewall mounted fuse for the alternator output rather than a circuit breaker.

Pic shamelessly stolen from the web -

Caution; thread drift....

Contrary to popular belief, that fuse (circuit breaker, fusible link, etc) in the alternator line is actually there to protect the wire from the *battery*; not the alternator from damage. The alternator wire should be sized to handle the max current (plus reasonable margin) that the alternator can deliver. Then the wire should be protected from shorts, because of the much greater current delivery possible from the battery.

Note at which end of the wire the protection resides.

Carl is absolutely right about what alternators can deliver relative to their ratings, and that the wire protection should be greater than the max output possible. (And it should be obvious that the wire should be able to handle the alternator's max output, as well.)

Charlie

Geeman said:

I am not the builder and I am not 100% sure how everything is wired.

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Are you saying that you received no drawings of electrical schematics when you purchased the aircraft.