s60tyn

Hey guys, have any of you's suffered problems with alternators and not charging the battery? I replaced the battery 2 weeks ago and this morning it was flat. Jumped it and checked the voltage and it was reading just over 13 but when the cooling fan came on it jumped up to over 14 then when the fan stopped it went back to 13? Also when I put my lights and heater on it jumped back up to over 14?? Any ideas?( 05 wrx )

Djgyro

Sounds strange s60tyn, the voltage shouldn't increase when the fan kicks in! Usually without significant load, a decent alternator should supply 14.1 - 14.3v
As with all 'flat-battery' problems there are 3 causes: battery, alternator or something drawing current when it shouldn't be.
You have eliminated battery so the next step will be to eliminate the 'something drawing current'.
You need to start with a charged battery. You can test for that by checking battery voltage with the engine stopped and the main beams on. You should expect 11.7-12.2volts. You could put the battery on another vehicle and let it charge up first.
If you disconnect the multi-plug on the alternator, the battery light will come on in the dash and the battery should read 12 volts or something in that order (ie battery voltage with a small draw from your ecu). If its below 11.5v and is dropping as you look at it without lights or fan on then you have another consumer drawing power.
I suspect that isn't that case though and its actually the charge regulator on the alternator and you may have to have the complete unit serviced.

alcazar

iTrader: (2)

AFAIK, Scoob alternators are a "smart charge design", so that they produce NO charge unless there is demand from the system.

SO yes, as demand increases, so will the voltage.

Many people run with the fans on lowest setting in the car, just to get it to trickle charge.

Midlife......

iTrader: (2)

Many people also run with the sidelights on to "force" the alternator to charge the battery......

Shaun

s70rjw

iTrader: (2)

I've just been through the same with my 05 WRX. I replaced the battery as a precaution in December as I've had the car 5 years and it was still on the original battery. A few weeks ago I went to work and when I returned to it it was flat. I do a lot of short journeys but always have the fan on and the heated rear screen. Had the battery checked...fine. Alternator checked... fine...all fuses pulled and no change in the drain. As Midlife advises, drive with your sidelights ( or if you want to impress people refer to them as daytime running lights ) on. My car has been fine since. I reckon my new battery wasn't fully charged and the 4 months of short journeys with little or no alternator help, finally lost all its charge. 20 mins on dipped beams was enough for mine to come back to life. Bob

Maz

iTrader: (34)

'New' batteries can spend months on shelves leading to sulphation. As others have already explained the Impreza has a smart charging system that only charges under load.Therefore a brand new battery can often go flat in a matter of weeks and sometimes days! Follow the above advice and you will be okay.