Have you got a digital multimeter? You can get a cheap one for around $20, I bought mine on-line, forget where though, Dick Smith should have one. You can use it for checking battery condition etc, so it is money well spent.
With the MM, turn to 20 volts, open the coolant top, put the red + in the coolant, don't touch the sides, and the black - on the neg battery terminal. Should read .3V or less (mine is .09). If it is more than .3V, you probably have electrolyis happening . Do this with engine off, and again with engine running with your normal power goodies running, AC, lights, radio.
Electrolysis is caused by some electrical gear grounding through the coolant, ie not earthed properly.
Let us know what your reading is.
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