"Rodney Pont" wrote in message
news:atcfzvasbuvgflfgrzfygqhx.i15kar0.pminews@ihs1 ...
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).

The problem with that is that the pump that the inverter is powering is 6 watts
at 115 volts, not 12 volts. But I assume that you are referring to the current
drawn from the battery.

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.

The inverter is designed not to discharge the battery to the extent that the car
will not start. My car battery, by the way has 400 amps of cranking power.

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.

I would likely run it much more often, and longer than that. And there is no
need to run the air pump continuously anyway. I would likely run it only for a
few hours in the day, and then run it for a much longer period at night, since
the O2 demand is greater at night. I have a telescope with an electronic clock
drive (equitorial). It runs on 115 volts, about 5 watts. I used it the other
night, connecting it to the new inverter through the cigarette lighter on my
car. I was able to use it most of the night without it discharging my battery
to the point where the car wouldn't start.

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 :-)

New battery. Having said that, you are correct, since hopefully, we won't have
another outage like that one for a long time. But you never know.

--
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