In article ,
steve wrote:
SLA - sealed lead acid - which use a gel as the electrolyte - should be
charged with a maximum voltage of 13.8. They can't gas in the same way
as a liquid electrolyte type can, so mustn't be overcharged.

Agreed. Charging gel-cells at over rated voltage kills the capacity.
The battery will still have proper voltage, but if you do a capacity
test, it will be drastically less than rated. We see this all the
time on ATC-601's. Aircraft mechanics are told to keep equipment on
chargers at all times, which is fine for old NiCd packs, but these
charge at 14 volts and constanly come in with ruined packs.

Yes. The accepted way is to charge at constant voltage of 13.8. This, of
course, takes more time than blasting a non sealed liquid electrolyte type

They
won't last more than 5 minutes at rated capacity. I've been told that
the cause is because they can't "vent", as you say. They seem to work
perfectly as a replacement for lead-acid, as in they charge in exactly
the same manner, but they do lose their capacity if charged at too
high a voltage too often. As Daniel Sofie says, they put them in dirt
bikes, which makes sense.

Or anywhere where they might be overturned. So called sealed no
maintenance liquid ones still have a vent and can leak if overturned.

They charging system charges the batteries
just as it should, but I can guarentee that it won't last as long as a
lead acid would in its place if it wasn't vibrated and jarred to
death. The capacity loss is just an acceptable sacrifice for no
battery acid spilling. Possibly a bold statement, just my thoughts.
I bet the gel-cells still last at least a year in that situation, but
they probably have to be replaced prematurely due to capacity loss.
But, there is no perfect battery pack, and gel-cells are definantely
nice.

The big snag for auto use is that they can't be re-charged so quickly
without shortening their life. But for many other applications they're
still quite competitive.

I'm a sound engineer and will use them where I've got a reasonably static
location where I can't easily get mains. One car sized one will run all my
gear all day with capacity to spare. And re-charge overnight. Ni-Cads,
etc, are of course used where weight and space are a consideration, but
the overall running costs of the much higher initial price per Ah - and
the lack of large capacity ones at a sensible price, so more changes
during the day - make the ancient technology of lead acid still attractive.
nice.

--
*A 'jiffy' is an actual unit of time for 1/100th of a second.

Dave Plowman London SW
To e-mail, change noise into sound.