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I have a device (small fridge) that runs on a 12V car battery, but I don't know the current it draws.

I connected a multimeter in series with a normal 12V battery to measure the current it draws. The device sometimes starts with 3-3.2 A and quickly goes down to less 3A and continues to 2.5A.

I know the bigger the current in a power supply the better, but currently, I have a normal mains power supply with an output of 12V and 3A.

Since the device only draws 3.2A for a split second will I be able to use my 12V 3A power supply?

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    \$\begingroup\$ If you have the supply, try it and see. If you don't have the supply, probably it would be better to get one that can deliver 5A or so. \$\endgroup\$
    – mkeith
    Aug 17 '20 at 7:28
  • \$\begingroup\$ Hi #Adrew Towns. Welcome and nice to meet you. Ah, let me see. We better check the fridge catalog and see which model looks like yours. Of course it would be nice if you can give us a link to your fridge product sheet. (1) AliExpress 12V Car Fridge Catalog: aliexpress.com/w/wholesale-car-fridge-12v.html. Even if we don't have any datasheet, we can still make a guess and see what type of power if required. \$\endgroup\$
    – tlfong01
    Aug 17 '20 at 7:37
  • \$\begingroup\$ I found the following cheap cheap 10L 12V 45W US$30 mini fridge looking nice. (1) AliExpress Portable 10L 12V 45W Mini Cooling Warming Refrigerators Fridge Freezer Cooler Travel Warmer For Auto Home Office Outdoor Picnic Travel - US$30 fr.aliexpress.com/item/…. Now let us do some calculation. Current = Power / Voltage, / to continue, ... \$\endgroup\$
    – tlfong01
    Aug 17 '20 at 7:47
  • \$\begingroup\$ / ... so I = 45W / 12V ~= 4A. I guess 45W is the average power, and usually the starting current might be a little bit large, perhaps 5~6A, then current goes down when settling down. So I think 12V 6A should be good enough. Cheers. \$\endgroup\$
    – tlfong01
    Aug 17 '20 at 7:48
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It depends on how fridge and power supply react to overcurrent (power supply) or voltage drop (fridge).

The high peak current is due to the start-up current of the motor in the compressor. It will start fine with a lower current, it will just take a little bit longer. So as long as the fridge does not have any monitoring that switches the compressor off at input voltage drops you will be fine (from the fridges perspective).

The power supply has to handle this as well. If the output voltage just drops until it can deliver enough current, then this will work. If the output of the power supply switches off immediately in case of over current, then you will have a supply that constantly turns on and switches off again.

As you can see there are a lot of ifs in this answer. This is because we neither know how your fridge reacts, nor what your power supply will do. If you are sufficiantly sure that the power supply has some kind of protection (so it won't blow up) you can just test it. The fridge will take no harm.

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