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Date:         Wed, 31 Aug 2011 14:35:20 -0600
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: Battery Sages
In-Reply-To:  <201108312004.ALV28655@Colorado.EDU>
Content-Type: text/plain; charset=ISO-8859-1

> Next day I put my digi-meter on the batt >> and got 12.96v reading..Poke the starter and it cranks very weakly for a >> very short time. Back to the multi meter and I see 12.4 volts.

Something to keep in mind is that to use battery voltage as a measure of battery charge, the electrolite must be uniform. That isn't the case during or just after a charge. The acid level is higher close to the plates. Standard battery recommendation is to wait several hours, preferably 24, and then measure the voltage.

See II at http://www.trojanbattery.com/BatteryMaintenance/Testing.aspx

When I pull the battery maintainer off my aux batteries, the voltage will be 13.3 or so, falling steadily over several hours to 12.6 or so.

Another factor often overlooked is temperature. This is seldom mentioned. "All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes (negative temperature coefficient - lower voltage at higher temperature)." This from: http://en.wikipedia.org/wiki/Automotive_battery

This page also has "After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more accurate reading."

Richard


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