Focus Fanatics - View Single Post - WEIRD Battery light on, but no problems...?
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Old 09-09-2010, 02:29 PM   #19
Adventure-Biker
Focus Rookie
 
Join Date: Oct 2007
Fan#: 51271
Location: Florida
What I Drive: 2003 Black ZTW with all the options

Posts: 23
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The battery light in my car came on while driving. After I turned the car off I noticed that the battery light came on each time about 15 seconds after starting the engine. I checked the charging voltage and amperage with a digital mulitimeter- charging system was working but rather weak. The next day I fully charged the battery and took the car out for a drive...the battery light was on the entire time and the car completely died with a dead battery about 30-40 minutes after driving. All this while my multimeter was showing alternator output (however low and weak).

I decided to upgrade and ordered a new Bosch hi-output replacement (220 amp) for $249.00 shipped from an eBay seller since I live in a hot climate, run the air conditioner, and also tend to run a power inverter and other electrical accessories inside the car. I installed the alternator myself- took me about 3hrs.

Battery light no longer comes on. Charging voltage is a solid 14.2-14.4 volts off-idle. I didn't check the charging amperage because it would probably fry my multimeter.

But now I have a new "problem". This new Bosch hi-output alternator seems to have to spin at a higher RPM before it kicks in and actually charges. My car idles at about 700RPM and I am getting less than 12.5v at the battery. If I increase engine RPM to 900 then charging voltage immediately jumps to a solid 14.2-14.4v. The Bosch alt has the same diameter pulley as the OEM alt I removed, so alternator shaft speed is exactly the same.

I mostly drive my car in the lower RPM ranges and also do a lot of relatively shorter distances in city (ie. stop and go driving), so this poses a problem - my battery now is usually discharging considering the way I drive, unless I hit the open road or something (which I don't do too often).

I never had this low rpm charging issue with the stock alternator :(
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