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Good day all.

So I've been studying up like a ferret, my head's jam packed - numbers going up, numbers going down. And now my noggin's turned into 'slap-pap'. I have a few very basic (read dof) questions, hoping someone can set me straight...

 

  1. How does one calculate the 'surge draw' from a fridge? (Spec sheet only tells me consumption per annum and 24hr-period)
  2. Can I literally attach (plug) a cable from the wall socket into the inverter to charge my battery? (when I want, and this to avoid wiring into DB)
  3. Can I also feed out a cable from the inverter to an adaptor off of which I'm to run a laptop, fridge?
  4. And does this cable have to be of certain spec or can "normal" extension lead do?

 

I came across this statement from "Solar Shop" website Re: Pylontech batteries:

"Transformer based inverters or Victron Inverters specifically specify a 100% continuous discharge current from the batteries. As an example for a 5kva Victron which is 4kW the battery requirement would be 4000W/48V = 83A continuous discharge from the batteries. In this case a full 3x US3000 or 4x US2000 batteries would be required."

The webpage also started by saying with 'high frequency inverters'  the 'batteries supported continuous discharge current can be 70% of the inverters capacity', eg "5000W/48V = 105A...the supported continuous battery discharge current can be 73.5A" (ie 105-30%)

 

  1. With this in mind and having seen comments here and there across many platforms of "Inverters destroying batteries" or "sucking them dry" could someone please explain this? What's going on here?
  2. I mean, does it mean your batteries/pack always have to be massive? (Meaning you cannot initially invest in a larger inverter?).
  3. And where on the various data-sheets is this nugget of info hidden?

 

Or perhaps a link to an explanation. My Goggle searches are proving fruitless.

 

As I said, not the brightest questions but thanks nonetheless.

Regards.

 

 

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I'll take a stab at question 1.

Best way to calculate start current (also called inrush current or surge current) is to actually measure it with a suitable electrical tester. The only problem is that testers that can accurately give you a start current reading are expensive and outside the reach of DIY'ers so you'd need to either hire one or find an electrician who's got one.

We also use a rule of thumb where we allow that the start current could be up to 5x the run current for a single phase motor and you can measure the run current using a cheap and cheerful clamp meter or one of those kill-a-watt type power testers.

 

123.jpg.cec53eb5722d9367c0c4347c0fc345b3.jpg

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23 hours ago, Red Rubber Gloves said:
  • How does one calculate the 'surge draw' from a fridge? (Spec sheet only tells me consumption per annum and 24hr-period)
  • Can I literally attach (plug) a cable from the wall socket into the inverter to charge my battery? (when I want, and this to avoid wiring into DB)
  • Can I also feed out a cable from the inverter to an adaptor off of which I'm to run a laptop, fridge?
  • And does this cable have to be of certain spec or can "normal" extension lead do?

1. Most inverters can handle quite large surges(of overcurrent) for short times. What inverter are we ralking about? Having a spike for a couple of seconds is normal for inverters (unless you are looking at fong-kong inverters). A fridge migt pull 1KW for 5-10 seconds and then settle on 130W

2. Do keep in mind that wall sockets are generally behind a 20A or 30A breaker. This will limit how much power the charger can draw. Some inverters will have a grid connection for battery charging. Hybrid inverters might export power if not correctly setup. If the inverter has a generator input then this might work great. I guess it depends on the inverter.

3. Yes, this will work fine. The earth is floating in this instance since nothing bonds it (unless the inverter bonds it internally??). Normally you would take output current through a circuit breaker of 20A to disconnect the load when overloaded. If something overloads it and there is not circuit breaker it might cause a fire. Personally I would feel more at ease with a circuit breaker. I would suggest you maybe get something with over current protection, something like this https://midas.co.za/chargepro-extension-lead-reel-25metres.html

4. Normal extension leads should be fine for about 16A that is about 3-3.5KW, if they are cheap chinese ones... who knows what they will handle or not handle.

 

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i have a 5 kW sunsynk with ONE 4.8 kWh battery, have most of my high load appliances, geysers, pool pump are on non-essential, just the other day i was running 8.9 kW total load, 5kW was coming from solar/battery and the rest from grid, so, no, you don't have to start off with massive ( expensive ) battery bank

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  • 2 weeks later...

Hi @iiznh

Thanks for the full response, some good tips there, and apologies for my own tardiness.

On 2022/03/23 at 10:59 AM, iiznh said:

What inverter are we ralking about?

With regards to to your question, I'm looking at one of the Victron inverters, Multiplus, Phoenix or such. Though I have been swaying about quite a bit. Still looking and in planning phase.

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Modern home fridges hardly draw any electricity and only have a small compressor pump motor in them.

This is a 3/4 fridge freeze - uses about 55W when running - Basic Defy - not sure of model - purchased about 5 years ago.
image.thumb.png.af606fc9c45796f4ed43242bd7c99b86.png

This is a full size fridge uses about 58w when running - AEG/ElectroLux SANTO 269l - purchased about 12 years agoimage.thumb.png.1222a2892a0a3f17b6eb9992c24b7ff5.png

This is a f
ull size upright freezer uses about 90w when running -AEG EletroLux ARCTIS 232l - purchased about 12 years ago
image.thumb.png.340d555ccdca3ccbeadcd806f0ef60cb.png

Measurements were done with an UltraLink Smart Plug flashed with tasmota.

Edited by system32
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4 hours ago, system32 said:

Modern home fridges hardly draw any electricity and only have a small compressor pump motor in them.

This is a 3/4 fridge freeze - uses about 55W when running.
image.thumb.png.af606fc9c45796f4ed43242bd7c99b86.png

This is a full size fridge uses about 58w when running.image.thumb.png.1222a2892a0a3f17b6eb9992c24b7ff5.png

This is a f
ull size upright freezer uses about 90w when running:
image.thumb.png.340d555ccdca3ccbeadcd806f0ef60cb.png

Measurements were done with an UltraLink Smart Plug flashed with tasmota.

Perhaps one has to mention only for A+ or better units. There are new units still drawing like 180+W normally class B.

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From the spec sheet:

My fridge is rated A.

It's a combi-fridge, 230L (157L/73L- fridge/freezer)

It's energy consumption (EN153) per 24hr (A/A+) 0.857

Energy consumption per year 313

 

I'm guessing those are kWh.

It clicks on for about 15min every 30min or so. These graphs posted by @system32 make sense to me, I mean, if i divide those figures (857W) into the day and consider the on/off periods it kinda adds up. Of course an actual test would be accurate. Still, I doubted my understanding till i saw those graphs. Thanks @system32.

 

Fridge is only about 2yrs old.

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Yes the figures are spot on. I was merely mentioning the class as the cheaper modern fridges can be class B and they use quite a bit more.

I knowingly bought a new chest freezer for the caravan and only 97L and being class B uses 90W and runs more than 16h out of 24h. This based on the power actually used on a Sonoff.

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Generally speaking the fridges/freezers that have better energy ratings will have a similar sized compressor to the units with the poorer energy ratings. The difference comes with the length of time the compressor will be running where the unit with a good energy rating might be running 25-30% of the time whereas the poor energy rated unit could be running 50-60% of the time. Obviously this wouldn't have any effect on the start/inrush current or the run current and hence wouldn't have any effect on the inverter required for back-up power, it would only effect the battery capacity required.

Edited by Marv
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I have also compared the actual compressor fitted by make and model and found even with the same unit one finds very different power used stats. A number of the latest offerings have the compressor behind a panel. With the above in mind I found the lower rating units seems to have thicker insulation which should help power used but as said not the starting current.

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