When the temperature display in the engel stops working.
It will normally flash db on the LED.

These are some of the steps required to change the battery.

Before you start, turn off the unit and turn off the power supply to the unit.

Step 1.
Remove the lid and undo the 4 screws holding the hinges to the unit, being careful not to let them drop down into the compressor compartment.

Step 2.
Undo the 2 screws at the top back on the unit, and again being careful not to drop the screw or washer into the compressor compartment.

Step 3.
Gently lift cover and roll forward to rest on on top of the unit.

Step 4.
Now using a flat blade screwdriver or a 5 cent piece placed in the slot, gently turn the the cap anti clockwise about an eighth of a turn and remove.

Step 5.
With your hand over the opening of where the battery is, using your other hand gently tilt the cover over till the battery falls into your hand.

Step 6.
Now with the old battery removed, gently place the new battery in the hole being careful that the flat side of the battery is facing up.

Step 7.

Replace the cap and turn clockwise with the 5 cent piece or flat blade screwdriver.

Step 8.
Place cover back on the unit being careful that the overlap is right way around. (it can be a bit tricky)

Step 9.
Replace the screw on the back and the front with the hinges, do not tighten until all screws are in place.

Step 10.Replace Lid, turn on power and then turn on the unit to the desired setting.

Another tip
We had to improvise when we left our 240 volt lead at home, what we did was use a 3 pin lead for a computer which can be bought in just about any town around the country. ( For us it was better to pay $15 than $42 for the one with the decal on the packet ).
The only difference we could see was that the computer lead wasn't at right angle to the plug but it didn't matter for us as there was plenty of room to get the plug into the socket on the engel.