The offending car is an eclipse (my gf's) but I figure I can get better help on here than anywhere else... Car was having some issues leading up to about a month ago same as now-- there's not a lot of power to the electronics, slow to crank over, lights will dim when driving for no apparent reason, etc.

A few weeks ago I tested the voltage at the battery, and it was low both when the car was running with lights/ac on and when the car was off. The battery is only a few months old, so I'm thinking alternator not charging properly. I take in the old one to Advance to have them test it, even though I was pretty sure it was the problem, and they can't figure out how to get it on the machine. I go ahead and get a new alternator anyway and put it in. Car was running fine for about 2 weeks (daily driving) and now I'm having similar problems. Windows either won't roll up or do so very slow, radio won't turn on, car won't turn over. Jump start it, and it starts right up, everything working great until I remove the jumpers, then the same symptoms of low power.

With the car on, I hooked up a multimeter to the battery and it's measuring 8.5v - 9v. Turn the car off, and it goes to about 9.50v and slooooowly crept up until about 9.8v at which point I took off the multimeter.

The seems to obviously be electrical, but has anyone had experience with this sort of thing, without me going to advance to have them test the battery and alternator?

 

A good fully charged battery will read 12.8v An alternator should be charging at 13.5 to 14. Use a charger to fully charge the battery and see what happens. Check the cables and terminals especially the grounds.
regards
rikard

 

If the battery is bad would that affect the voltage reading with the car on (alternator running) at the terminals? Or should that read somewhere upwards of 14v with the car on no matter what (even with a bad battery)?

 

Jump started it again after tightening a connection, was getting 13.4v at the battery, so I figured loose connection... took it for a drive to charge the battery up, came home and it's registering at 12.2v... I'm thinking loose connections somewhere

 

12.2, engine running?

 

Yeah... not where it should be, but better than it was

 

12.2 is low, if the engine is off, and way too low if its running. Alternators are designed to keep a good battery charged and will often fail if trying to charge a dead or defective battery. Batteries aren't cheap but their cheaper than alternators.

12.0v 10%
12.2v 25%
12.5v 50%
12.8v 100%

regards
rikard

 

Assuming the battery is dying, will it ruin the (new) alternator to keep the battery on for a bit?

 

Batteries usually don't die until they're at least 3 y.o. Seems like you have a loose connection, probably at one of alternator terminals. Usually battery looses it's capacity with time, not the "voltage keeping ability"(how do you say it in English?), so alternator doesn't get damaged. In your case it doesn't seem like a bad battery to me.

 

You can get a hand held amp meter and hod it over the battery cable to see if the alternator is putting out a charge. You may have a bad battery cable which is not visible to the eye. The sudden drop in voltage tells me that there is a high resistance somewhere-possibly the ground cable which is notorious for corrosion where it connects to the frame of the vehicle. this resistance will force more of a draw from the battery to do what it needs to do. i'd really check the cables. Also connect a volt meter and check the voltage at the battery with the car running- it should show 14.5 volts or so.