jayhawk316

Last night my wife drove the rx on a quick errand. She ran into the grocery store for a couple minutes. Upon returning to the car, it was completely dead. No power locks, lights, etc. I was able to jump it and get it home. While driving home the headlights flickered when hitting the gas. Tried to start it this morning and it is completely dead again. Any ideas?

oguitar

My guess is the battery is dead. How old is it?

bob2200

This probably eliminates the alternator. I agree, probably the battery. FYI, I just got a new one at Costco for $60 plus sales tax.

lexina

Might as well test the alternator for the proper charging voltage. Chances is the battery, but not a bad idea to check the alternator.

jayhawk316

I'll have to check on the battery age when I get home. I'm curious why the flickering headlights probably eliminates the alternator. Everything I've read online suggests that flickering headlights is the alternator. I'm very new with DIY auto maintenance, so if the answer is obvious, keep that in mind.

bob2200

The alternator (and it's diodes) puts out pulsating direct current (DC) at a higher voltage than the nominal voltage of the battery. The battery acts as a filter, filtering the pulsating DC to constant DC and keeping the voltage constant. The evidence that the headlights brighten with increased RPM suggests that the fanbelt isn't slipping and that the alternator has output. A burned out alternator diode would reduce charging, but a properly working battery should work to not let the voltage vary dramatically and short-term with engine RPM. Note that I am a retired electrical engineer and not an auto mechanic, so this is somewhat a theoretical analysis.

jayhawk316

Gotcha...thank you for the explanation.

Just jumped it again and it died after driving about 10 feet. The battery is an Interstate 85 month from Nov. of 2005.

maxSteel

The battery is there to start the engine and power accessories with the engine off.

Once the engine is running the alternator provides all the power needed for the engine, accessories, and battery recharging. The voltage regulator keeps the voltage level steady across changes in load and engine rpm's.

If you jump start it and it dies after disconnecting the boost car/battery charger the alternator and/or voltage regulator are malfunctioning.

If you were able to jump start it and the engine kept running after disconnecting the boost vehicle the alternator is generating power. If the headlights are surging and sagging though the voltage regulator is probably bad.

You can connect a voltmeter to the battery and watch the voltage level as you vary the engine rpm's and the load (turn the headlights on and off). The level should be steady around 14.2 volts.

Any place that load tests/changes batteries will usually check the charging system for free.

carguy07

"If you jump start it and it dies after disconnecting the boost car/battery charger the alternator and/or voltage regulator are malfunctioning."


I would not go that far. In theory that is correct, but in real life many cars will die without the battery to lean on. Jump a car with a dead battery and remove the cables and it may stall and that may not mean anything is wrong other than a dead battery.

oguitar

If the battery is from 2005 I am sure it is the battery. Batteries only last like 4 to 5 years. Alternators last normally a lot longer.

code58

Bob the EE is essentially right. If the battery has developed an open circuit (they do, but not very often) it MAY not continue to run after removing the JC's, but if you can REMOVE the battery from the car after jumping it, it should run even with an open circuit. I have read that it's not a good idea to remove the battery after starting, whereas it didn't use to seem to hurt it. My guess is the alternator, because the car and lights and accessories DO run off the alternators charge, NOT the battery. As stated, the battery's job is to regulate the the alternators charge to a constant, well regulated flow of juice for all that it feeds, not primarily to SUPPLY that need. (and OBVIOUSLY to start the car so that the alternator can then take over the job)

jayhawk316

Turned out to be a bad battery with a shorted cell. After describing the problem, the technician thought it was the alternator. The battery was tested and turned out to be bad. After replacing the battery, the alternator was tested and good.

Thanks for all the input.

code58

Guess we have the answer as to whether an alternator can run the car when a cell has taken the proverbial "dirt nap". The tip-off that it was an "open cell" or "shorted cell" was the fact that it started fine to go to the store and did nothing when she came out. Some got it right and some of us missed it. That was what I was referring to when I said it's not a very common failure (in all my years of driving (a lot)), I've never had it happen to me, but it does happen.