Recently we have seen a number of questions around the starting current of induction motors. Perhaps starting fridges are closer to most of us.

While load testing my Mecer lithium dropins I thought I might as well shed some llight with an actual test as opposed to the normal text book statement of 6 times the full load current.

Fridge: Samsung bar fridge with 90W motor. Power used in the middle of the cooling cycle before the thermostat switches off 70-87W.

Starting current as shown on the picture. A video was taken as a multimeter and the human eye is normally too slow to capture the peak.

Voltage: 12.5V during surge. 13.1V while running

Current peak: 98.6A for 1232.5W. This is about 15 times running power.

Power used is about 700-800Wh per 24hrs.

This also explains why people battle to start house type fridges with a smaller than 1.5kW pure sine wave inverter. Also at risk are those using small 0.5C lithium batteries for medium sized  non inverter fridges.

 

 

Peak current during starting.

Edited October 4, 20223 yr by Scorp007

Thanks. Good information, clearly put.

The other problem with fridges is that they are always turned on. So out of all the appliances in your house, the fridge can be the dripping tap. We've twice had a fridge repaired and afterwards it ran nearly all of the time. Maybe this is down to no-name brand spares (or no spares, so the repair guy just bridges some of the circuitry). 

Anyway, this can take a chunk out of your battery overnight. I figure the most recent instance was costing me about 15% per night. That's the difference between 60% and 45% DOD in the early morning, and my system is set so that when there's grid it will not discharge the batteries past 40% DOD so that I always have something in hand in case of outage. So in the early AM the batteries would hit 40% and the inverter would start drawing from the grid, and so I got unhappy.

If I DIDN'T have solar, my back pocket would start to feel the pain. A couple of units extra each day soon adds up. Plus if that thing is humming every time I or one of the dogs wants a pee in the night, I have one more thing to dwell upon whilst I'm laying awake. More unhappiness.

Edited October 4, 20223 yr by Bobster.
clarity

1 minute ago, Bobster. said:

I figure the most recent instance was costing me about 15% per night. That's the difference between 60% and 45% DOD in the early morning

If I'd had a 5kWh battery, this excess useage would have been enough to take the battery all the way down. So then I'd be very unhappy and would make rude observations to the dogs in the middle of the night. Plus if Eskom decided to load shed at 6:00...

21 minutes ago, Bobster. said:

If I'd had a 5kWh battery, this excess useage would have been enough to take the battery all the way down. So then I'd be very unhappy and would make rude observations to the dogs in the middle of the night. Plus if Eskom decided to load shed at 6:00...

Using the bar fridge as my trusty Kelvinator was busy packing up. I thought let me regas after over 25yrs. While it was working 80% after the gas no dice. After trying 3 regasses on a previous fridge I will rather get a new one.

This fridge was using 3kwh per day and on par with my heat pump use. These 2 account for 60% of my consumption.

Also this old mommy was the reason why it had to stay on the grid and not part of my loads on the inverter during the night.

On 2022/10/04 at 4:22 PM, Scorp007 said:

Using the bar fridge as my trusty Kelvinator was busy packing up. I thought let me regas after over 25yrs. While it was working 80% after the gas no dice. After trying 3 regasses on a previous fridge I will rather get a new one.

This fridge was using 3kwh per day and on par with my heat pump use. These 2 account for 60% of my consumption.

Also this old mommy was the reason why it had to stay on the grid and not part of my loads on the inverter during the night.

Just a further update. The 2 Mecer 200Ah bought 2nd hand 3 months ago did perform much better than my gut feeling. I stopped discharging at a 7A load when they reached 12.5V. My guess is quite a bit of power still left in them but 12.5V is my own set limit for discharging. Final power used I allowed for 15% losses as I measured the AC output. Due to the high 98A peak starting current I rate these batteries quite good. Now time will tell on cycles. They were used on an inverter before I bought them.

All figures are kWh and not Wh as per the picture below.

 

Edited November 8, 20223 yr by Scorp007
Wh vs kWh

I have 2 fridges and a upright freezer, the tasmota sampling is per minute, so the instantaneous power is averaged out.

kWh used over 24h

Edited October 6, 20223 yr by system32

Is there anything simple and unobtrusive one can do to make the fridge soft start? Like plug it into a plug similar to those anti surge plugs etc

Edited October 7, 20223 yr by Lee2

13 minutes ago, Lee2 said:

Is there anything simple and unobtrusive one can do to make the fridge soft start? Like plug it into a plug similar to those anti surge plugs etc

Unfortunately, the only way to get rid of the surge requirement is to replace the fridge with one that has an inverter compressor.

4 minutes ago, P1000 said:

Unfortunately, the only way to get rid of the surge requirement is to replace the fridge with one that has an inverter compressor.

Or get someone who knows his stuff to wire in a inverter start type of compressor. I have not seen it done but I am sure it can be done.

Looking at the low power used for the bar fridge in this topic I find it hard to understand why some people would go and pay upwards of R6000 to have a DC compressor installed. Yes one will save in smaller battery as the DC has more start torque and lower start current but give me the AC and inverter any day.