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high surge current

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Posted by Steven on January 3, 2005, 4:03 am
One of the reasons I have the fridge I do is that it does not consume a
tremendous amount of power. The "energyguide" when I bought it says it
only consumes $8 worth of electricity in a year's time.   My "watts Up"
meter indicates that it only consumes 53 watts while running.  The
problem however is that it's startup current is close to 2000 watts!  My
  SIMA 1000 watt/2000 surge inverter does not have enough kick to "turn
over" the compressor, and I really don't want to hafta buy an even
bigger inverter just to start the fridge.

  Is there any modifications I can do to the hookup to make it work?  I
was thinking of getting one of those big 1 farad capaictors like  the
kids use for the ghetto-blaster cars and connect that inline with the 12
volt side of the inverter.  The other thought was to install a slightly
smaller capacitor on the compressor of the fridge that will surge less.
  I am thinking though that this is a problem with the 12 volt side,
since I am right at the inverter's limit.

The inverter currently has "gator clip" connections on 2 gauge wire tha
sort of resembles jumper cables.  I am also thinking of just changing
out the gator clips with regular battery terminal clamps.  Do any of you
all think that might help?

Steven W

Posted by Robert Morein on January 3, 2005, 7:09 am

That is a very intriguing idea. It might also protect AGM batteries from
surge caused degradation.

 The other thought was to install a slightly

That might fry the compressor motor.
Any stalled motor is in danger of frying.

You should definitely change out the gator clips.
Large cables, with solid terminations, will help the inverter approach the
design surge performance.

Posted by Morten on January 3, 2005, 11:09 am


Is this a Sinewave inverter or a Modified Sinewavw inverter?

Good idea on paper but when the capacitors have been discharged they will
cause an even bigger charge current from the batteries than they would
normally, unless you put a resistor online with them to minimize the charge

I would go for a smaller capacitor on the compressor and then monitor it for
a while to see how large the startup current is with the smaller capacitor

Another thing that may help is to add a 'Zero Crossover Solid State relay'
that little device contain a couple of SCR's that is triggered only when the
line voltage is very lov (2-5 volt ) and because of this there is virtually
no noise from them, ie. no full voltage turn-ons if you happened to trigger
the start relay at max line voltage (remember the linevoltage is AC so
changes from full linevoltage to 0 to full linevoltage100 or 120 time a
second and this switch on current creates a lot of harmonic distortion on
the powerlines...

Yes that's correct, that why any motor that can stall should have a fuse
inline with the hot wire to cut out if the current gets to large for some
time, ie. the motor has stalled. Stall current is quite high compared to max
running current 5-8 times depending on motor, so if the fuse has been
selected correctly is will protect against stalled motor and let the motor
runn at full power without blowing...

Many inverters can deliver more startup power for very short periods without
to much trouble, have you tried to start your compressor on the inverter and
see if it can kick it over?

Do it anyway, there will be some quite large currents flowing from the
batteries when the inverter is running at max (2000 Watt / 12 Volt / 85% =
196.07 Amps (The 85 % is the assumed effiency of the inverter at max power
might be a little high on better inverters)) so to minimize any losses in
the system it's vital to keep the resistance of the cables and connectors as
low as possible, and it's more secure with battery connectors than crocodile

Try the next size smaller capacitors on the compressor and monitor it...


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Posted by Robert Morein on January 3, 2005, 4:32 pm



I don't see your reasoning.
I think Steven and I are thinking of paralleling a capacitor with the
battery, not cutting off the battery for starting, which, I agree, would be
a bad idea.
At start, the current might be split between the two devices according to
the internal resistance of both devices.
Depending upon whether the battery has a surface charge, ie., fully floated,
this is variable.
The rate at which the capacitor is recharged would be much smaller than
discharge, because it would be proportional to the delta-E between capacitor
and battery.

Posted by Morten on January 3, 2005, 5:43 pm



When you connect a large capacitor accros a battery and draw a large current
the capacitor and the battery will 'work together' and deliver the current
to the device, but when the large current draw disapears, the battery will
have to charge the capacitor again and this charge current will only be
limited by the internal resistance of the battery and could be very high

Capacitors are great for reducing ripple and ac noice on a dc powerline, but
I'm not so sure that the benefits would warrent the extra cost when
connected in parallel with a battery, but I might be wrong...

I still think it woll be better to use a larger battery or two in parallel
effectively halving the internal resistance of the batteries and halving the
current draw from each individual battery.


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