A quick and easy test for your battery, fully charge it and connect it to a 12v car bulb, if the bulb goes out or starts to dim within seconds its dying under load, if it holds bright for a few minutes chances are its good. The bulb will draw far more than the detector ever will.
If you sell it you will lose out a lot on its value it having a problem, even though it could be something quite simple.
If you sell it you will lose out a lot on its value it having a problem, even though it could be something quite simple.