On Mon, 19 Jul 2004 22:28:32 -0400, George wrote:

I bought a power inverter Friday from Radio Shack. It is rated at 140 watts,
115 volts, continuous duty. It should run an air pump (6 watts) for quite a
while. When the power starts gettng critical, a light comes on, and it shuts
down. Then I can just start the car, let the battery re-charge, and turn it
back on. I'm not sure how long it will run the pump before the battery gets too
low, but it is a better solution, I think than the UPS. But I still have the
UPS if I need it.

Just some comments but these are based on UK cars and batteries but
should give you some idea:

Assuming the inverter is 100% efficient 6 watts at 12 volts is 0.5
amps. My battery is 60 amp/hours so will run the pump for 120 hours
(60/0.5).

When the light comes on you may not have enough juice in the battery to
start the car, it could well need 100 amps surge to get the engine
turning over. Car batteries are not designed for deep discharge and
that will damage the battery.

My car (2.5 litre) uses 0.7 imperial gallons an hour on tickover and if
you have a 100 amp alternator it will in theory take 36 minutes to
fully charge your battery. Unfortunately it doesn't work like that
since you can't put much more than 6 amps into the battery for long
without it getting hot and gassing. As the charge in the battery rises
the charge rate falls and it takes something like 5 hours to fully
charge.

I would suggest you run the car for 20 minutes every day to keep the
battery topped up and then you shouldn't be draining the battery much
so you will be able to start the engine and avoid damaging the battery.

Also if the battery is over three years old there is a good chance it's
worn out and even the low current for the pump will flatten it
overnight but if this was the case you'd probably find it hard to start
if it had been left for a couple of days. That's based on average use
and no one is really average :-)

--
Regards - Rodney Pont
The from address exists but is mostly dumped,
please send any emails to the address below
e-mail ngpsm4 (at) infohitsystems (dot) ltd (dot) uk