Flat battery
 

My battery keeps going flat...A quick jump will fix it (temporarily) Have had the battery tested and is comes up fine, so does this mean possibly the alternator is on its way out?


There doesn't seem to be much of a correlation between how long the car is off before it dies. Sometimes if it sits for a week it will be flat, other times it will sit for a few hours and be flat - and I mean REAL flat - not even enough to power the stereo.


The alt guage sits around 13-14, sometimes just below 13 if its idling in gear (its an auto)

Is a Z28 with stock LT1

To test the alt start the car then disconnect the battery. If the car stays running then alt is good. Does this only happen when the car's sitting, running or both?

never unhook the battery when the engine is running. it can overload the alternator and burn it out. i am going to say the battery is not happy and is a goner. battery can only take a couple times being drawn down to totally dead before it fails. the cause of that is the problem. check the center console light and make sure its turning off when the button is pushed. same with glove box. also if you have an amp try unhooking it.

Easier way to check if the alternator is doing it's job is to check voltage at the battery with the engine running. Personally, sounds like you got a drain off the battery. Craby is right though, standard lead acid batteries can only be drained so far and recharged before their life is ruined. AGM and Deep Cycle batteries are better suited for being drained down.

Grasshopper, have your alternator tested. It doesn't sound like it's your alternator but you can at least eliminate it as the cause.

It doesn't sound like a battery problem either. It sounds like you have a power draw that is sucking the power out of the battery when your car is parked.

There isn't enough in information to support any conclusion but do you have a dome light that say's on?

Have your alternator checked. The diodes act like a gate valve, only allowing power to flow in one direction. If that is bad, power can flow backwards when the engine is off and drain the battery, and yet, the alternator could still charge the battery. I've had a bad one once where the alternator was even making a high pitched whine/whistle after the engine was shut off, and the battery would be drained the next morning. I was also getting crusty build up on the battery terminals that returned a day or two after cleaning it off.

Cheers for your responses.


Is there some way I can test the alternator myself with a multimeter? Or do I need to take it to a mechanic?


I was thinking battery draw as well, and have checked all interior lighting and cannot see anything staying on. Don't have an amp or any fancy electronics, just a basic aftermarket stereo.


Initially I thought it was just a bad battery as it is quite old, however Multiple mechanics have told me not to replace it as it tests fine.

I'm not aware of anyway you can test it yourself. If you can take off the alternator, you can have it tested at NAPA or most any parts store.

Ok thanks. I'll get it tested and see if that's the issue..I'm tempted to just replace the battery since its quite old, but don't want to throw money away.

Use your multimeter set to volts, test your battery with nothing turn on. It should be between 12.6 volts and 13.2 volts. Note the voltage (lets say 12.8 volts). Now start the car with everything turn off except the engine. The voltage should be higher than the base 12.8 volts. (14 ish) Now start turning everything on. Lights, blower, AC radio Note the voltage. If the power has not fallen all the way back to or below 12.8 then you alternator is outputting decent amps even at idle. If for some reason your car is idling very low the alternator may not be able to keep up but at normal idle speed it should

Good luck.

Volts with engine off - 12.4

Volts with engine on nothing running - 13.8

Volts with engine on stereo and ac on - 13.6.

Volts with engine on, stereo and ac and headlamps on high beam - 13.4

I noticed it drop to around 12.6 when the other fan kicked in, and then back to 13.4 when it turned off

Idles fine.

What do you reckon? Replace old battery?

Checked all inside lights and can't see any obvious drains. Everything is basically stock anyway except the stereo (stock speakers) just head unit changed.

bump :(

it sounds like the fan motor is drawing to many amps or alternator is not working as well as it should. does revving the engine change the volts when fan and stuff is on? i would take alternator and or fan motor in and have it tested at a local electrical shop.

I don't think its the fan as the voltage wont drop that low if I don't have head lights and stereo on, I guess it makes sense it be the alternator since mechanic has said battery is fine.


Will get the alternator tested and hope that's the issue... Then I have to try find the part here in New Zealand D:

for the boat we have a shop that does electric motors, starters and alternators and i can usually get fixed for way less than a new one. never tried taking an auto part in...