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Date:         Wed, 14 Feb 2007 14:27:15 -0700
Reply-To:     Richard A Jones <jones@COLORADO.EDU>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Richard A Jones <jones@COLORADO.EDU>
Subject:      Re: Batteries don't die, they get murdered
Comments: cc: Michael Diehr <md03@XOCHI.COM>
In-Reply-To:  <70713694-09C7-4D8E-AE55-AB4B2CDB6563@xochi.com>
Content-Type: text/plain; charset=ISO-8859-1

> my optima rarely charges to above about 70% ish (12.4v) > even after a full day of driving. Voltage when driving measured is > about 13.5V at the battery.

The voltage while driving is being governed by the voltage regulator and doesn't represent the state of charge of the battery.

A fully charged wet-cell 12V battery will measure 12.73V at 80 degrees F at rest--which means no load or charging for some period of time--6-24 hours. In a car, this probably means disconnected, too, because of stereo memory, clock, etc.

But the "80 degrees" is the gotcha. Battery voltage increases with a decrease in temperature. 0.28V/cell/10 degrees F. So for a (6-cell) 12V battery, that's .168V/10 degrees. So a 50 degree battery would read 13.2V at rest, fully charged. (12.73 + .5)

As battery temperature decreases the charging voltage has to increase, too, to fully charge the battery, by the same .028V per cell per 10 degrees.

Here are two good sheets on this: http://www.trojan-battery.com/Tech-Support/BatteryMaintenance/Charging.aspx http://www.trojan-battery.com/Tech-Support/BatteryMaintenance/Testing.aspx

Richard Boulder


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