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Solution

The most obvious cause of a low battery warning is of course that the transmitter battery needs replacing.

The battery required is a CR123 (sometimes labelled CR123/A) type which is a lithium battery and has around 3 V. See following sample image.

 tenergy-cr123a

Note that this is not a rechargeable battery; rechargeable CR123 cells can be found but should not be used because they have a different voltage characteristic and in any event the Davis transmitter circuitry cannot recharge them.