I have set up a peltier module with an aluminium heat sink, cooling fan, 12v, 7ah battery supply and thermal paste for the purpose of cooling water. At first, it performed well. It dropped down to 3.5°c in 2 hours. However, after the 2 hour mark, it had suddenly started increasing in temperature. I didn't move it or change the room temperature at all. I decided to change the cooling fan to a liquid coolant, which didn't work either. I bought another set up, but this time I noticed the peltier module and cooling fan would only perform seperately when tied to the battery unlike before when both performed well even when the wires were tied together. So I brought back the previous set up, and tested it with the battery again. The same problem occurred. Can anyone explain why this is happening and what I should do to get it to perform again?
It is possible that there is a short circuit somewhere between the heat sink, the peltier slab, and the fan housing. Heat sink grease is electrically nonconductive but if a direct path gets formed through it then the device would stop working. I suggest you take the entire thing apart, clean it up, and test the fan by itself, the fan with the heatsink, the peltier slab by itself, and the slab with contact with the heat sink.