I use small bench supplies because I have a lot of small bench
supplies. You set it up for 13.8VDC, and the current limit cuts in if
the battery wants more. Gell cells don't like excessive voltage, so I
avoid the cheaper car battery chargers. My bench suplies range from 1A
to 8A, no where near as heavy duty as a car battery charger.

You could buy a "cyclic" charger if you want faster charging. For
instance, for the np7-12, you could hit it with 14.5V to 15V at
1.75Amps to charge it. I personally don't like doing this, and rather
just set up the float voltage (13.5 to 13.8V). I only mention the
np7-12 because I have one next to me. The one I use in the field is
much larger, and unfortunately not handy. [I put it in a West Marine
battery carrier, so it's all locked up so to speak.] Note that you can
float a SLA (sealed lead acid) at the float voltage forever. That is
how you use the battery in backup mode.

The charge time at the float voltage won't be linear. As the battery
charges, it draws less current. If you want to quickly charge the SLA,
you can buy a cyclic charger. As I said, I use my infrequently, so the
bench supply works for me. http://www.xantrex.com/ has chargers meant
for SLA, but I still suggest getting two batteries and just floating
one on a good bench charger (HP, Labda, Sorensonmetc.] The only problem
with a bench supply is some jerk could crank up the voltage.

You can drain the SLA down to 10.5V (i.e. 1.75V/cell). In the field, I
use a cheap Harbor Freight DVM. No use getting a good meter all
scratched up.

I don't like the idea of using car batteries for this application. The
outgassing can ruin electronics or your lungs. SLA batteries are safe
enough to install inside electronics gear. I've use noise measuring
gear that would disconnect from the mains when making a measurement,
using an internal SLA to power the electronics.

bpnjensen wrote:
M...sushi.com wrote:

I suggest getting a gell cell to avoid battery acid spills or fumes. I
only use mine when in the field, so I don't have a set up as you
suggest. I charge my battery from a bench supply using 13.8v as a
float voltage.

Thanks, also to RHF and Cuhulin. I just took a look at these gel cells
on a couple of websites, incluidng the one recommended by RHF and some
others. This, of course, raises more questions :-)

How big a charger must one have? Must the current available on the
charger be some minimum fraction of the amp-hours of the battery, or
can one use a smaller charger and just leave it on longer?

Also, is the charge time a simple proportion between the charger
current and the battery amp-hour value? i.e., to recharge an 18
amp-hour battery with a 10-amp charger, the charge time is 1.8 hours -
?

Is it OK to drain a gel cell until it loses almost all of its charge?
Or is there some minimum level at which one must stop (25%, 50% or
etc)?

Finally, once the gel cell battery is fully charged, how does one know,
and will it hurt if the charger is kept on without checking? Does a
charger have a device to shut off when full charge is reached?

Thanks,
Bruce