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In article <28220@amdcad.AMD.COM> phil@diablo.amd.com () writes:
>I just found a source of 12 V 6 AH sealed no maintenance lead acid
>batteries for $10 each.

New?  How can I get some?

>What is an appropriate fast charge rate?

Usually .2C max. for gel-cells, which is 1.2 A for a 6 AH battery.

>Is it ok to trickle charge them in a burglar alarm application?

Yes.  You will want to use 2.35 V/cell, e.g., 14.1 volts. (Some people like
2.3 V/cell, giving 13.8 volts.)

>What does the discharge curve look like?

At 25 degrees C, the open circuit voltage is about 2.18 V when fully charged
declining linearly to 1.98 V when 10% of capacity is left.

>How should a fast charger know when to turn off?

If you use a regulated supply for your charger, it will shut off by itself as
the battery reaches full charge. A constant 2.35 V/cell charger will recharge a
battery to 90% within 2 hours, and the battery can be left float charging at
2.35 V/cell indefinitely to maintain full charge.  Extremely dead cells may go
above the .2C rate if fed from a 2.35 V/cell charger.  Going above this rate
may cause excessive outgassing, which will eventually dry out the electrolyte.
It is best, therefore, to limit charging current to .2C for gel-cells (which
will result in a longer recharge time).  Wet cells can be recharged at higher
rates.

>Do lead acids have the memory problems of NiCads?

No.  However, it is important to fully recharge lead-acid cells, with 2.35
V/cell optimum for batteries that are cycled once per week or less, and 2.45
V/cell for batteries that are cycled once per day.

>Is it ok to leave them flat?

No.  The minimum voltage allowed during discharge is 1.6 V/cell, and lead-acid
batteries should not be allowed to self-discharge below 1.8 V/cell.  If allowed
to discharge below 1.8 V/cell while in storage, the battery will take longer
than normal to recharge, and the next discharge cycle cannot deliver the rated
capacity.  Subsequent cycles, however, will result in an increase in capacity
to the rated capacity.

If you let a lead-acid battery discharge completely to 0 volts, the electrolyte
turns to water and is therefore not conductive.  You can remedy this, however.
Using a constant-current source, charge the cells at a .1C rate.  This may
require as much as 20 V/cell if the cell is extremely dead.  When the voltage
falls to about 2.5 V/cell, switch to a constant voltage, 2.35 V/cell charge
until charging current levels off (e.g., reaches a minimum).

>What is the best condition to store them in? 

Since they're sealed, humidity is not a concern, and gel-cells may be stored or
used in any position.  As with all batteries, the rate of self-discharge is a
strong function of temperature.  After 5 months of storage at 20 deg C, the
typical battery will have 60% capacity remaining.  At 40 deg C, however, the
battery would be fully discharged after 5 months.

>And out of curiousity, how do the corresponding NiCad systems work?

Standard chargers normally charge at .1C for 14 hours, since this allows a very
simple charging circuit -- batteries can be overcharged at .1C for extended
periods without drastic reductions in life.  Fast chargers usually sense
temperature, starting as high as 4C, and tapering off as the cells get warm.  I
have also heard of fast chargers using temperature compensated voltage
detection and detection of the voltage reduction that occurs after full charge
has occurred.  Nicad batteries in memory backup applications are usually
continuously current trickle-charged at .002C to .1C.  I can tell you more
about nicads if you're interested.

For a cheap charging circuit for lead-acid or nicad batteries, see LB35 in the
National Semiconductor Linear Applications Handbook.

Most of my information comes from "Summary of Battery Chemistries," Maxim
1988/89 Seminar Applications Book, parts of which were quoted above.
Additional information about reviving dead gel-cells came from an article
posted by Jan Steinman to sci.electronics on 9/3/87.
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
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GTE: (503) 627-6920         "The biggest difference between developing a missle
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