I'm not A-1 Battery. This site is not for this kind of questions, but I will give you an answer. Clean the cable end and the battery post very thoroughly and put baking soda on the cable and battery post. You may also put pennies around the post. I would use the baking soda first.
i wonder if there is an other kind of metal somewhere in the electrical system thats causing electrolysis? inside the alternator or even a light bulb in a turn signal. somewhere ? Was there ever a LITTLE wire coming off the the ground cable ? I've seen those. maybe a second ground ?
Try using baking soda and water to clean the terminals. After that purchase some battery terminal protection spray and apply that onto the battery. If that dont help you may have a acid leak on your battery.
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