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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Generator for AirCond,  Batts&Inverter for TV
Date: Thu, 07 Dec 2000 03:25:49 -0500

Markus wrote:

> Thanks George (and everyone) for your reply.  Re the UPS: I thought the batt
> & inverter only kick in when the power fails, and that it the meantime there
> would be circuitry to "clean up" the power.  Although a really good power
> bar might do a similar job?    All the things you're running--up to air
> conditioner--I'll need to run. So tell me, what are the specs of the system
> I'll need to match yours?

There are two different kinds of UPS's.  The true UPS contains a
line voltage to DC converter large enough to supply the downstream
load and charge the batteries, a battery bank and an inverter.  They
operate in series.  The converter changes the incoming line to DC,
usually from 24 to 250 volts DC.  The inverter runs on this DC to
produce AC again.  The battery may either float on the DC line or be
connected to it by a high speed switch depending on the type of
charger involved.  When the line goes away, the inverter simply runs
on the battery.  These are the most reliable, least efficient and
most costly.  They completely protect the load from power line
disturbances up to the point where the disturbance blows the UPS

The second type is known as a standby UPS.  This unit contains a
smaller converter just large enough to charge the batteries, an
inverter and some batteries.  During normal operation, the UPS
passes line voltage directly from the input to the output.  When
power fails, circuitry capable of detecting failure within a
fraction of a cycle triggers the inverter on and transfer of the
load to the inverter.  These units typically drop a half cycle in
the transfer.  Cheap standby UPSes just pass the line power
through.  Better ones contain similar circuitry to surge arresters.
The inverter in consumer-grade standby UPS's usually isn't
constructed for continuous operation.

> As for the article that John wrote, it explained why the non-sine wave
> inverter-generators are highly efficient:  the motor can slow right down
> when there's no load. The bad part is that the simulated sine wave of these
> units is good for a fan or coffee maker, but not electronic equipment like
> TV's.

I didn't mean to leave the impression that all electronics won't
work well on modified square wave inverters.  usually only cheap
audio equipment has a problem.  Indeed many electronic gadgets will
work on worse power than motors or microwaves.  Back in the late 80s
when I put my ISP online, I built a true UPS using a large 12 volt
power supply, a bank of marine batteries and a Tripp-lite square
wave inverter.  Not modified square wave.  True square wave.  All
the computer stuff including the laser printers worked just fine.  A
microwave oven would do nothing except light up (not enough peak
voltage.  Audio equipment, of course, buzzed like mad.  Even the
modems worked fine on the square wave power.  A squirrel cage fan
motor overheated badly from all the harmonic energy.

> Here's my conclusion so far:   have one of those small, inexpensive
> generators (or 110v shore power) on one circuit for the AC and the bread
> maker, etc.  AND have a batt-powered inverter system like yours for running
> TVs, Satt receivers, etc.    In which case, I'd run the generator/110V to
> power a charger to charge the batts,  but only when I'm not using delicate
> electronic  equipment.  I'd make sure the generator could produce enough
> power for the charger to really "exercise" the deep cycle batts (see the RV
> FAQ), not that I know how to calculate that for golf cart batts. (The FAQ
> said charge an automotive deep cycle batt at 30-60 amps.)   Do you think the
> above would work?

That should work.

> Final question:  I'm assuming that running the engine alternator for
> charging would be very costly compared to a small generator.  Plus it's
> putting "miles" on the engine.   That's gotta be true, right?     -mh

Been there, done that last weekend. Went dry camping in the
mountains to see some snow.  A couple of weeks ago some punks
unplugged the rig (left the plug in the waterproof housing so it
would look plugged in.) and the batteries ran completely to zero
volts.  I got them to take some charge but the capacity was down a
BUNCH.  Whereas normally we'd get 3 days on the batteries, we didn't
even make it through the night.  I had to run the engine several
times over the weekend and ended up burning a half a tank of gas
(OUCH!)  Of course a smart regulator would cut down on the engine
run time but that's still not a good solution.


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