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Alternator Charging 15+ Volts
http://forum.triumphdolomite.co.uk/viewtopic.php?t=33578
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Author:  Mad Mart [ Sat Feb 03, 2018 11:38 am ]
Post subject:  Alternator Charging 15+ Volts

I replaced the battery in my Boxster last week as it was rather old and not holding its charge. I checked in the week and did a few tests:-

The battery was at 12.4V, engine cold, not running.

Charging at 15.4V at idle and 2000 rpm, nothing turned on.

Fluctuating between 14.2V & 15.2V at idle with lights (including fogs), aircon, heated seats, hrw, blower fan at full & stereo/sat nav on.

Steady 15.2V at 2000 rpm, with all the above on.

I've just duplicated the tests this morning and the battery was still sitting at 12.4V (engine cold, not running).

12.4V is a bit low. Would you say that 15+ Volts charging is a bit high?

Author:  SiC [ Sat Feb 03, 2018 12:55 pm ]
Post subject:  Re: Alternator Charging 15+ Volts

Sounds a bit high side. Usually max charge voltage is around 14.8v - even for AGM batteries (which iirc these have). What multimeter are you using? Do you have an accurate source to test it against? Cheapy meters aren't known for accuracy and go way out of spec if the battery in them starts to go flat.

Author:  Mad Mart [ Sat Feb 03, 2018 1:49 pm ]
Post subject:  Re: Alternator Charging 15+ Volts

The multimeter is fine, it's a UNI-T UT61E. Not the most expensive but not a cheapy one.

Author:  Galileo [ Sat Feb 03, 2018 2:43 pm ]
Post subject:  Re: Alternator Charging 15+ Volts

AGM battery? I've read somewhere before about Porsche and apparently correctly reading 15.2v when charging, if I remember where I'll post it...

Author:  SiC [ Sat Feb 03, 2018 3:43 pm ]
Post subject:  Re: Alternator Charging 15+ Volts

I'm pretty certain it should be an AGM battery. I had a quick look at a couple of manufacturers technical datasheets and they all said 14.8v max.

Author:  MIG Wielder [ Mon Feb 05, 2018 6:56 pm ]
Post subject:  Re: Alternator Charging 15+ Volts

Hi Mart, If this was a Dolomite / MGB charging system I would also agree that >15 V is a bit on the high side. Can you get hold of an official workshop manual to check ? In which case are the battery and the alternator Porsche specific parts if those numbers are O.K.
If they are not correct ( and it was B.L. ) it would be interesting to check the "sense" terminal and its wiring on the back of the alternator if it has one. ( If the sense terminal drops off the alternator defaults to an internal sense mode ). I'm guess a bit here as I'm not familiar with Porsche equipment.
Tony.

Author:  cleverusername [ Mon Feb 12, 2018 10:34 am ]
Post subject:  Re: Alternator Charging 15+ Volts

You're voltage regulator has gone, it needs replacing, it is the only thing I can think of that would lead to overcharging like that.

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