# Mystery of Lead Acid Battery Self-Discharge Solved

From: Greg Weinfurtner (weinfurtner@ouvaxa.cats.ohiou.edu)
Date: Thu Sep 14 1995 - 11:20:59 EDT

Hi all,
Read this letter in the Oct Electronics Now and thought it would be
of general interest to our group. I scanned it in and the author is listed
at the end of the article!
Here it is:

discharge of a lead-acid battery when it is stored on a cold concrete
surface in this column ("Q&A," Electronics Note, April 1995). All of them
are wrong! The solution to the problem is a simple method for circulating
the electrolyte. My knowledge of battery self discharge under the
conditions discussed in the article is based on several years of work with
submarine storage batteries.
Self discharge of batteries is caused by temperature gradients in
the battery electrolyte and is called electrolyte stratification. When a
battery is stored on a cold surface, the lower parts of the cells are
cooled to a temperature that is lower than the upper parts. That
temperature differential causes a drop of the specific gravity of the
cooler part of the cells at the rate of 0.004 points of gravity per 100° F
of temperature change above or below 80° F. The voltage produced by a cell
is proportional to the specific gravity of the electrolyte. This difference
of specific gravity between the top and bottom of a cell results in a
potential difference that is proportional to the specific gravities of the
two parts of the cell.
An electrical current will flow between any differences in
electrical potentials, so a current will circulate through the cell plates
from the top to the bottom of the cell. Those currents will persist as long
as a temperature differential exists, and they can be enough to discharge
the battery over time, as if it were connected to a constant load.
Devices called percolator tubes inside the cells controlled that condition
in submarine storage batteries (where the steel hull immersed in ocean
water provides a massive heatsink). A stream of air bubbles passed through
the percolator tubes circulated electrolyte from the bottom to the top of
the cells, maintaining constant specific gravity of the electrolyte.
FARNHAM M. CORNIA Toledo, WA

Sounds like he hit it on the nose!

73 de

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* NN N SSSSS 888888 OOOOO Greg Weinfurtner AEE BSS *
* N N N S 8 8 O O Electronic Design Splst *
* N N N SSSSS 888888 O O Ohio University Athens *
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* Canst thou send lightnings *
* Amateur Radio NS8O that they may go and say *
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* weinfurtner@ouvaxa.cats.ohiou.edu Job 38:35 *
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