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?
Alternator Charging 15+ Volts
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Mad Mart
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Alternator Charging 15+ Volts
Sprintless for the first time in 35+ years. 
... Still Sprintless.
Engines, Gearboxes, Overdrives etc. rebuilt. PM me.
2012 Porsche Boxster 981 S
... Still Sprintless.Engines, Gearboxes, Overdrives etc. rebuilt. PM me.
2012 Porsche Boxster 981 S
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.
Current Heaps: 1968 Austin 1100, 1974 Dolomite Sprint, 1974 MGB GT, 1985 BMW E28 520i, 2000 Porsche Boxster, 2002 Clio 172 and a boring 2010 Audi A4 that keeps the wife happy!
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Mad Mart
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Re: Alternator Charging 15+ Volts
The multimeter is fine, it's a UNI-T UT61E. Not the most expensive but not a cheapy one.
Sprintless for the first time in 35+ years. ... Still Sprintless.
Engines, Gearboxes, Overdrives etc. rebuilt. PM me.
2012 Porsche Boxster 981 S
... Still Sprintless.Engines, Gearboxes, Overdrives etc. rebuilt. PM me.
2012 Porsche Boxster 981 S
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...
Current fleet: '75 Sprint, '73 1850, Daihatsu Fourtrak, Honda CG125, Yamaha Fazer 600, Shetland 570 (yes it's a boat!)
Past fleet: Triumph 2000, Lancia Beta Coupe, BL Mini Clubman, Austin Metro, Vauxhall Cavalier MK1 & MK2, Renault 18 D, Rover 216 GSI, Honda Accord (most expensive car purchase, hated, made out of magnetic metal as only car I've ever been crashed into...4 times), BMW 318, Golf GTi MK3 16v x 3
Past fleet: Triumph 2000, Lancia Beta Coupe, BL Mini Clubman, Austin Metro, Vauxhall Cavalier MK1 & MK2, Renault 18 D, Rover 216 GSI, Honda Accord (most expensive car purchase, hated, made out of magnetic metal as only car I've ever been crashed into...4 times), BMW 318, Golf GTi MK3 16v x 3
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.
Current Heaps: 1968 Austin 1100, 1974 Dolomite Sprint, 1974 MGB GT, 1985 BMW E28 520i, 2000 Porsche Boxster, 2002 Clio 172 and a boring 2010 Audi A4 that keeps the wife happy!
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MIG Wielder
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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.
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.
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cleverusername
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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.