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So I live off grid and have a 12v solar system with an array of batteries and a 12v 45amp MPPT controler. I also have a 1000w inverter for things such as powering my laptop and things.

I have recently bought a fridge however I am having some issues powering the device as when the compressor starts it will surge at around 750w. My MPPT controller does not go this high (and I have not been able to find any that will) and so the compressor will not start correctly and just make clicking noises while the inverter complains that it does not have enough power.

I have tried removing the MPPT from the circuit and just having the batteries go directly to the inverter which seems to work fine however it means I will have no undercurrent control over the batteries. Ideally I would like everything to go though the MPPT controller.

I am looking into soft start devices such as this one to slow the start up draw of the compressor however I am not sure if this will allow the compressor to start correctly.

I was wondering if anyone has any ideas or solutions to to this problem.

Thanks!

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  • \$\begingroup\$ Start surge is 500% or so, you underestimated the power and there is no easy solution. Yes this will relieve your MPPT controller of failure and your fridge motor may overheat momentarily with longer start times, but probably work. \$\endgroup\$ Feb 18 '18 at 1:21
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    \$\begingroup\$ It may be a bit late to mention this, but there are fridges that are intended to operate from 12 volt batteries - intended for RV and marine use. \$\endgroup\$ Feb 18 '18 at 1:27
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    \$\begingroup\$ there are some that operate on propane \$\endgroup\$
    – jsotola
    Feb 18 '18 at 1:30
  • \$\begingroup\$ contact the device manufacturer ... i am not certain that any of the motors listed on the web page contain a start capacitor ... your fridge probably has a start capacitor \$\endgroup\$
    – jsotola
    Feb 18 '18 at 1:34
  • \$\begingroup\$ Maybe you could have a smallish supercapacitor bank between the MPPT and the inverter, depending on how long the start surge is (not sure how the MPPT would feel about that though) \$\endgroup\$
    – BeB00
    Feb 18 '18 at 3:25
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The inverter itself if it is thought for offgrid application should have the undervoltage protection. So you can connect the battery directly to the inverter. This is how it is normally done in off-grid solar appplications and the MPPT charge regulator is only for the DC loads and regulating overvoltage while charging. Otherwise you would need to always oversize the charge regulator.enter image description here

See an offgrid circuit schematic of one of my projects

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The high startup draw of the fridge is not caused by inrush current or motor startup current, it is caused by mechanical load on the compressor as it gets the refrigerant moving. If you try to limit inrush current, the compressor will stall. Nothing short of running it off a (expensive) VFD will help.

You will need to get a different fridge. Either get a 12V fridge (which will have a similar startup draw, but your batteries can handle it) or get an inverter compressor refrigerator (pricey, but efficient).

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The battery should take care of the surge. This does mean you need to connect the high current loads directly to the battery, and that does mean, as you say, that you lose the undervoltage protection.

I would be looking at a high current solid-state relay, controlled from the charge controller's switched output, if you can find one capable of handling the high current load. That way, when the charge controller shuts down its switched supply, the relay disconnects the fridge too.

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