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If you are running it on just 12 VDC then that isn't unusual...........but, to be sure I would have the batteries load tested to be sure one or both aren't failing or have an internal short. Even new batteries fail quite frequently. A 12 VDC heating element on this is probably in excess of 300 watts and you can do the math and see how fast that would draw them down. I would install some adequate solar panels so that the batteries are kept at a level to keep the DC voltage over 11 VDC which is about the minimum you need to keep this running. Hope that proves helpful here. Please get back to me if you need more on this as all follow ups are included in our fee.