battery voltage

tom12

HI my battery's are showing weak under load test used a toaster type load tester  6.36v,6.46v when i load tested,  will adding acid bring em back ive always kept them above 6.30v they are the duracel 6v type oldest is from 2013 read 6.36v last charged  11-22-21 on charge on 11-28-21 , 2014 6.46v last charged 11-22-21 on charge 11-28, 6.42v  , 2016 6.52v, 6.50v last charged 11-21-21 not load tested there not all connected together they are 2 6v batterys each in series to make 12v. The water is covering the plates. I noticed that when I added water  to  just below the caps the battery is weaker and as the battery water dropped battery got stronger is it ok to just keep the water just  above the plates but not touching the bottom of the fill caps both when battery voltage is at 6.18v. thank you in advance

RCinFLA

Eight years on that type battery is pretty good.

Most likely issue is positive plate grid corrosion which increases cell resistance.  Charging bubbles are hydrogen from negative plates and oxygen from positive plate.  Being bathed in oxygen from constant charging accelerates grid corrosion.  Battery will still have good capacity at low current but cannot be loaded with moderate to high current without significant terminal voltage drop.

Adding acid is generally not a good thing to do and if issue is positive grid corrosion it will not benefit performance.  

If you have low acid concentratiion its from sulfation where sulfuric acid is converted to lead sulfate on plate surface, then hardens due to not being recharged for some time.  The unrechargable crystalized lead sulfate blocks plate surface and ties up sulfur that cannot be converted back to sulfuric acid in electrolyte.

Surfpath

8 years is really good for 6v Duracell’s. You have obviously been kind to them.
The usual prescription for generally younger batteries is to try a ‘dose’ of equalization. But, given what RC said above, this may not be advisable.
Like your tester..they are probably now toast.