The light meter? You mean the 3 or 4 LED meter labeled "full" "1/2", etc?
That meter is worthless for determining much of anything.
When a battery is going bad it uses more water. If you've been noticing a pattern of water use and it increases that's a bad sign. Also, it will lose capacity and not last as long as it used too. There really isn't any way to know "This battery is 80% gone and needs to be replaced in the next 50 days". It's just not that exact.