On Wed, 2 Jul 2014 19:12:02 -0700 (PDT),
wrote:

I work on various alarm systems that use mostly two 12.0 volt
7.0 or 8.0 AH rechargeable lead acid gel cell batteries in series.
Although fifteen or so years ago you could get up to ten years
out of a typical set of Japanese gel cell batteries, these days
it seems that three to five is about all you can expect from
the Chinese knock offs that have flooded the market.

I've similar lifetime problems with UPS batteries. I also deal with
weather stations that use the same gel cells. So, besides the
batteries from China what has changed?

1. It's difficult to find a real gel cell today, even if it's labeled
"gel cell". Most everything is AGM (absorbed glass mat). The gel
cell battery has one advantage over the AGM battery. It has a
constant plate top plate spacing, which makes it better for use where
sulfation is possible. AGM has more surface area, and therefore more
capacity per unit volume. They can also be charged at a higher rate
than gel batteries. AGM can handle vibration better.

2. Sophisticated charging circuits allow fast and safe charging very
close to 100% capacity with temperature compensation. In the bad old
daze, a gel cell charger was just a diode and series resistor, that
would charge to about 80% of capacity, and consider that adequate.
Today, the chargers go much closer to 100%. If overcharged, lifetime
is greatly reduced.

3. Sources for batteries have greatly increase. In the bad old daze,
there were perhaps 4 sources of gel cell batteries. The gel batteries
were fairly close to internally identical and very interchangeable.
Today, there are probably about 30 manufacturers with more appearing
every day. Subtle changes are constantly being introduced to give
each manufacturer a competitive advantage, which introduces divergence
in specs and performance. A clue is simply to weigh the battery,
which is a very rough indication of the amount of lead used. In
general, the junk batteries tend to be light weights.

So, what we have today is a far more critical battery system. The
risk of overcharging and going outside the charge profile is high. The
quality of the batteries are all over the map. The interchangeability
of batteries for both AGM vs gel and for different brands is dubious.
Temperature control and compensation is a bad joke.

If you want long life, you'll need to:
1. Don't charge too quickly.
2. Limit the available choice for substitute batteries.
3. Crank DOWN the charge controller to well below 100% capacity.
4. Don't let the battery discharge below about 50%.
5. Put the temp sensor on the battery, not on the controller
6. Don't use gel batteries in vibration prone environments.

Note that most cheap UPS designs break all these recommendations. They
want to recharge as fast as possible so that if the power dies again,
it's ready. They want as long a runtime as possible for advertising
purposes, so they charge as close to 100% as possible. The low voltage
shutdown point is often as low as 25%. Battery temp sensors are never
on the battery.

Or, think about Li-Ion replacement batteries and a "balance" charger:
http://www.batteryspace.com/li-ion-battery-packs-to-replace-lead-acid.aspx


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558