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I have a tricky question for you. I'm trying to operate a 100w fridge using solar power, a car battery and an inverter. The inverter says that it's 2000w but it most likely seems that its a 700w but whatever. I have 60w of solar panels and a 74Ah car battery. my problem is that when trying to turn on the fridge but the surge power needed to start the compressor motor is not met. Sooo i got thinking that maybe if it could be possible to use a huge capacitor of some kind to start the fridge for that split second when the surge is needed. Could this be done or am i talking bullocks?

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  • \$\begingroup\$ Monitor the battery voltage during compressor start. Charge the battery fully and try again. If it still fails, try a bigger inverter. \$\endgroup\$ – Brian Drummond Jul 22 '16 at 11:44
  • \$\begingroup\$ 74Ah battery ought to be enough, if fully charged. 2kW inverter ought to be enough. Do you have bell wire with a huge drop connecting battery to inverter? The 12v link is easy to overlook, you may need a short bit of 10mm2 cable to get enough grunt (technical term) into the inverter. Monitor your inverter input voltage during attempted start. \$\endgroup\$ – Neil_UK Jul 22 '16 at 11:53
  • \$\begingroup\$ Having a too small inverter, you may be able to cheat. Connect a 500 W lamp in series with the fridge. This will limit the peak power and might not steal too much voltage once started. If you do manage to start it this way, measure the AC voltage across the fridge once running. \$\endgroup\$ – winny Jul 22 '16 at 11:54
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    \$\begingroup\$ Your configuration seems nice for surge current of 100w frigde. Car battery is not suitable for deep discharging and may have already gone bad. \$\endgroup\$ – Ayhan Jul 22 '16 at 12:01
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The starting current of an AC motor is mostly reactive, so a capacitor on the DC side of the inverter in parallel with the battery is a good idea in principle. However, the inverter may already contain as much capacitance as it is designed for. Adding more may just cause the inverter to fail. Using a battery with a higher cranking current rating is also a good idea in principle, but not such a good idea if the inverter is not designed to deliver a high starting current.

You should check to see if the inverter has a motor stating rating. I suspect that a high inverter watt rating may not directly translate to a high motor starting rating because of the reactive nature of the motor starting requirement.

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No, you cannot simply add a capacitor to increase the surge handling capabilities of your inverter. In short, you would need an inverter with a much higher rating, at least one with a verifiable surge rating, or rated to start motor loads.

The compressor in a refrigerator, even a small one, requires a lot of current to start. There really isn't any way around that.

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Compressor loads in particular draw a lot of start-up current, which is why you'll sometimes see lights flicker when a freezer, air-conditioner or fridge turns on.

There is no easy fix for this- if the battery voltage is holding up you need a more suitable inverter (it may not have a higher continuous rating, but it must be able to reliably supply the start-up surge). If the battery is in good shape, most likely the overload protection circuitry of the inverter is shutting the output down.

I've had a similar problem trying to run a pump in an emergency from an inverter and car battery.

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Wow, so many answers... might as well drop my two cents.

  • The big problem with your setup is the use of car battery a.k.a "starter battery". They are specifically designed to deliver high cranking current, at the cost of not being suitable for deep cycling.

The former is a good thing - it should be able to provide enough punch for starting the motor. The latter, however, makes it completely unsuitable for your application (I recall numbers like only 100 cycles lifespan at 50% depth of discharge).

In this respect, deep-discharge batteries (like the ones used in wheelchairs, golf cars etc.) are better fit for your application. Unfortunately they have much lower current ratings.

One obvious solution to this problem is increasing the number of batteries. You can use multiple starter batteries to avoid deep discharge. Or you can use multiple deep-cycle batteries in parallel to increase current capabilities. This seems to be more robust configuration.

  • Despite many dismissive comments Re capacitors, I agree with @charles-cowie that they can be used before the inverter to supplement starting current.

However you need really big capacitor for this. It is just not worth it. Simple adding more batteries in parallel not only provides necessary current but also increases capacity (i.e. running time) and lifespan (by reducing potential depth of discharge). Win-win-win solution.

  • Having said that, the increase of available current by any means is pointless if your inverter cannot handle the surge, as many pointed out before.

There is huge difference between 2000w and 700w. This is the first thing you should figure out, and upgrade an inverter if necessary.

  • Check your wiring.

As trivial as it seems, using inadequate gauge is often the cause of many problems in high-current applications.

  • Finally, as @Andyaka pointed out, your solar panel seems to be under-powered.

Not because it does not provide enough power to charge the battery (you can do that with any current above self-discharge), but because it cannot charge the battery fast enough. Depending on weather conditions your battery has high chance of being left not fully charged at the end of the day. Spoiled food is just a nuisance, replacing batteries every year is a burden.

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It definitely sounds like your inverter is not up to the job. Adding a capacitor is not going to help. I´ve got an old computer UPS I´ve converted for my PV system. It´s rated at 1200W and handles my (oldish) 250W fridge without any problem. Old UPS´s can usually be picked up pretty cheap, as the batteries die and are expensive to replace, not a problem in your case, although as Andy says, you will need to upgrade your system. The maths there just doesn´t work.

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You can't just add capacitors in this instance.

What is key here is that car batteries effectively are a big capacitor. What makes them good at starting cars is that they have a capacitive effect that allows them to deliver a massive "cranking current" that doesn't immediately (chemically) discharge the battery.

How do you know that it isn't working? What is the "symptom" of it not starting.

If you really are determined to do this, there are a couple of options. If the problem is on the AC side you could start it with a variac on the output of the inverter, or find some way to temporally stall the compressor (blocked flow reduces starting load). If the problem is on the DC side, you need to look at the battery. Not all batteries are created equal, and they do have cranking ratings on them (in Amps - the rated peak load for a few seconds). Find a better crank value and it may just work.

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  • \$\begingroup\$ "capacitive effect"?! The high current ability of car batteries is achieved by increasing number of plates and applying lead in sponge-like form. This maximizes surface area and minimizes internal resistance. \$\endgroup\$ – Maple Aug 10 '18 at 17:04
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Could this be done or am i talking bullocks?

  • Fridge power = 100 watts
  • Solar panels = 70 watts

Your battery will run out of juice pretty quickly as far as I can tell whether you use a capacitor anywhere or not. Try fixing the basic energy delivery versus energy consumption problem first.

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    \$\begingroup\$ Fridges do switch on and off according to a thermostat. I could envisage this working, except in the hottest weather... \$\endgroup\$ – Brian Drummond Jul 22 '16 at 11:42
  • \$\begingroup\$ @BrianDrummond 100 watts may be an average of 100 watts with a peak of 200 watts (or more) or, it may be a peak of 100 watts with an average of 50 watts so how do you know which it is to base your comment? \$\endgroup\$ – Andy aka Jul 22 '16 at 11:54
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    \$\begingroup\$ "a 100W fridge" suggests (strongly but I agree : without certainty) a fridge whose motor is rated at 100W. Average would then be lower ... low enough? don't know, but that's not the focus of this question. (Downvote certainly wasn't mine, btw) \$\endgroup\$ – Brian Drummond Jul 22 '16 at 14:02

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