On 11 Oct 2021 at 8:59, Rob Brockway via Elec-trak wrote:

> One question why would the battery voltage drop as plates lose surface
> area?  I would think that would impact capacity not voltage.

It affects both. 

Again, I'm not an electrochemist, but here's the way I understand it.

In the chemical reaction of discharging the battery, the Pb in the grids
reacts with the SO4 ion in the electrolyte to form PbSO4 (lead sulfate) on
the grids.  As the sulfate ions leave the electrolyte, it reduces the
electrolyte's concentration.  That's why you can measure the charge level
with a hydrometer. 

Charging the battery reverses the chemical reaction, turning the lead
sulfate back into lead, and putting the sulfate ions back into the
electrolyte liquid.  So the electrolyte concentration goes back to what it
was when the battery was fully charged.

Except that it doesn't quite.  A tiny bit of the lead sulfate fails to go
back into the electrolyte, so its concentration is a tiny bit lower. 

This happens because some small amount of the lead sulfate on the grids
crystallizes and becomes resistant to turning back into lead.  You've heard
of sulfation?  That's sulfation, and you can make it faster and worse by
undercharging the battery, or by not charging it right away after using it.

Parenthetically:

Some of the sulfate crystals flake off the grids.  If the battery gets
vibration in normal use, you'll see a grey cloudiness to the electrolyte -
that's the lead sulfate.  It also precipitates out, falling to the bottom
of the cells.  There it piles up and (depending on the battery design) can
eventually reach the grids.  Then you get a shorted cell or high self-
discharge.

Back to capacity and voltage loss. 

The shed lead sulfate can't take part in the charging reaction because no
current is flowing through it.

So the battery has less active material (lead), leading to the loss of 
capacity.

And the electrolyte's maximum concentration becomes lower, which reduces