The Honeywell 5811 wireless window/door contact uses a CR2032 3V lithium coin cell battery. All you will need is the new battery itself and a smaller sized flat head screwdriver. Go ahead and pop the top off of the 5811 by gently twisting the screw driver on the smaller end of the contact. Once the top has been removed go ahead and dispose the depleted battery and place the new refreshed battery into the same location make sure you place the battery's negative side facing down. The battery should snap right in and once it has go ahead and replace the 5811 face back on until it clicks into place. Now your system should restore the trouble itself.

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