Long life Lithium Thionyl Chloride Battery, AA Size, ER14505. Most of our sensors use this battery. Use caution in buying these from local suppliers as they do not sell very many and their inventory may be old. While this battery has a very long (15-20 year) shelf life the issue is that it "hibernates" if it is not in use. Once it hibernates you have to wake it up before it can deliver sufficient current to properly power our sensors. To wake the battery up connect it to an ammeter and draw at least 100 mA, or in a pinch you can insert it into our sensor reverse polarity (aka backwards) for 10 seconds which forces the high current draw needed to wake the battery. We use a lot of these batteries so our stock is always fresh.