SG vs Capacity

warren

If I understand correctly if you take a battery and fill it with nominal 1.215 SG solution at full charge and take the same identical battery and fill it with 1.265 SG solution at full charge the battery with the higher SG will have a greater Capacity but a shortened life expectancy as compared to the battery with lower SG.

Did I explain this correctly and is it in general true? I seem to remember reading somewhere that a change of 0.015 SG can change the capacity by ~ 5% but can't remember seeing what effect (%) this would have on the expected life.

The reason I am asking is because I have found some 20 yr life expectancy batteries that have been cycled out of a float application after only 5 years and appear to be in excellent shape but are only rated 150 Ahr (8 hour) with 1.215 nominal SG. Would it be feasible to increase the acid % to raise the SG to 1.265 or even to 1.28 at full charge? Anyone have experience or thoughts on this?

Battery specs: 6v, 3 cells, 7 plates per cell, +ve plate thickness 0.28 inch, -ve plate thickness 0.14 inch, 122 pounds wet.

Thanks

tallgirl

Re: SG vs Capacity

All of the plate chemistry and everything is designed as a system. While it's true that more acid is more total capacity, the plates have to be designed properly as well since PbSO4 -- lead sulfate -- is another term in the reactions.

warren

Re: SG vs Capacity

Additional information....the manufacturers website indicates that these batteries could have been optionally ordered with SG of 1.250 which suggests that the battery plate design is sufficient. I suspect going to 1.265 SG would also be possible. As stated before the +ve plates are thick.

The real question I guess I'm asking relates to anyone having experience or info about real life capacity if the SG is increased. If I can get a 15-20% capacity increase it would, I believe, effectively result in a physically smaller battery bank. The increased capacity should result in a smaller DOD % with any daily usage and thus increase the number charge cycles the battery is capable of performing. If the DOD is kept small (20% or less) the battery life may not be reduced too dramatically.

I am more concerned if any real life % capacity gains justify the hassle of modifying & balancing the SG. Battery life isn't a prime concern given the initial cost.

Thanks

niel

Re: SG vs Capacity

it is my opinion that making the electrolyte more acidic will not add more capacity to the battery as from what i understand most battery manufacturers add an excess anyway to account for some losses in the boiling of the electrolyte during the charge process.

RCinFLA

Re: SG vs Capacity

To get maximum battery longevity you need plate reserve. Higher acid concentration allows deeper discharge of the plates.

The more the plates are discharge the more plate mutation occurs during recharge. The ultimate mutation is holes created in the plates.

Cell kinetic equilibrium voltage = SG + 0.845 is pretty accurate above 50% SOC. With SG of 1.300 at full charge, that's almost 12.9v or 6.43v for 6 or 3 cell battery.