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      01-23-2013, 08:51 AM   #9
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doughboy's Avatar
United Kingdom

Drives: 2016 M6 Gran Coupe LCI
Join Date: Jan 2009
Location: UK - Midlands

iTrader: (2)

Think this way.

A car lead-acid battery is made up of six 2volt cells in series making 12 volts (all wet lead acid cells are 2 volts)

So it will be 12v, or just above, when not charging (6x2v = 12).

Lead acid cells need to be pushed above 2.25V to charge, so 6 x 2.25 = 13.5V

So it must be over 13.5 volts when running or else it won't charge properly.

Engine running volts shows the alternator is doing its job, but engine-off no-load voltage is not a very good indicator of battery condition, you need current/load to see how the volts drop to see if the battery is shagged.
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