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Re: Uneven Water Loss in Batteries
One battery may be gassing more than the other simply because you've got something unbalanced.
That is, the cabling may not be perfectly symmetrical and same length to both batteries, so one is getting more power and boiling out faster. Or, one may be in a hotter location than the other, getting less airflow or in a position where more heat builds up.
I'd check all the gross environmental issues that can cause uneven heating before screwing around with the acid and looking for a bad battery.