The open circuit voltage of a battery goes up as temperature goes up. The
required charging voltage goes down as temperature goes up.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Richard A Jones
Sent: Wednesday, February 14, 2007 4:27 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Batteries don't die, they get murdered

> 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