Outer Limits 4x4 Board

I purchased a 50 litre car fridge freezer called a reefer thats made by a company called refrigeration research and up till now it has performed brilliantly.I noticed on the weekend that it was using alot of power from the new battery I just fitted and that it would run for around 20 seconds then cut out for the same amount of time then cut in again.I measured the voltage at the plug and it still had 12.1 volts so the low voltage cut out shouldn't have been working. Has anybody had any experience with this type of fridge or maybe an idea what the cause of the problem could be.
Thankyou in advance.

Did you measure the voltage while the battery was under load (fridge motor running)???

12.1 volts is a bit on the low side for a lead acid battery under no load, and you may well be tripping the low voltage cutout on the fridge under load.

Could also be corrosion in the connections to the battery, or from the circuit used to charge the battery.

Big pic is it could be a battery not being charged, corroded connections over time, or the battery is on its way out. This does not discount the possibility that the fridge is bad, but let's start with the easy stuff first.

Try running the fridge on the 240V power supply if you have one and see if you can replicate the problemwith the fridge by leaving the battery out of the picture.

See if you can get an ammeter inline to the charging side of the battery to see if it is being charged fully.

Clean all contacts and connections to both sides of the battery.

Put a spare headlight globe across the battery and measure the voltage across it.

Now start the vehicle and see if the voltage increases to at least 14.2 volts (normal charging voltage).

Let us know how you go.....

Thankyou for a very helpful reply I put the fridge on 240 volts earlier today and it ran perfectly for 2 hours then went and put it in the car on the 2nd battery and it has run fine for over an hour with no cutting out and is slowly getting colder. The battery was reading 12.35 volts with the fridge motor running but it was still a little down from yesterday I suspect. The car is a landcruiser with a 1hz and I replaced the alternator with a new 120 amp bosch unit a few years ago and when testing the alternator it is only charging at 12.7 volts at idle and at 1800 rpm Im getting 13.8 volts on the main battery and 13.75 on the fridge battery. I replaced the battery 2 days ago with an Amaron 80 amphour deep cycle and cleaned all the terminals when fitting the battery.

I have found the problem and has been fixed, I was checking the voltage at the plug that goes into the fridge and when I looked I noticed that the contacts inside the plug had very slightly opened up due to heat and were causing a bad connection.