[time-nuts] OT Gel Cell question

[Neville Michie]

Hi,

Lead acid cells have lead supports carrying lead oxide and lead metal active material 
in an electrolyte of sulphuric acid.
When they discharge, the sulphuric acid electrolyte is reacted with the oxides and metal
to form lead sulphate and the concentration of the acid falls, that is why garages 
used to check batteries with a hydrometer to measure the electrolyte concentration.
At the same time the terminal voltage drops and the internal resistance rises,
when the concentration of the electrolyte gets very low, the lead sulphate becomes 
soluble and will re-deposit all over the battery. With gel cells the electrolyte can 
be completely absorbed making the battery resistance infinitely high.
If you can get some current to flow, you may be lucky enough to get the battery to 
reform some electrolyte, conduct some more, and eventually charge. 
However, when flat the lead sulphate dissolves and redeposits all over the battery,
and when recharged will convert back to lead and lead oxide, often most inconveniently
bridging the plates to a short circuit.
The lesson is to not let the battery ever get flat.

Lead acid batteries have some very good features.
The terminal voltage rises as the concentration of the acid increases. So a constant voltage will
charge a cell, and current stops flowing when the electrolyte reaches its proper concentration.
The catch is, when you have a battery of several cells, if one cell gets weak, the others will be overcharged
causing gassing and over concentration of the electrolyte.
There is a judicious voltage that causes an acceptably low rate of gassing 
(the oxygen hydrogen catalytically recombining) that will keep the charges of cells equalised.
But it only takes one total discharge event to cause enough leakage in one cell
to bring about failure.

Lead acid batteries are also environmentally excellent.
They consist of nothing but pure lead and sulphuric acid and water.
Sulphuric acid is not volatile so you can make batteries out of old batteries
forever, recycling the acid, lead and water.
If made on a large scale they are also very efficient (99.9% +) electrically.

cheers,
Neville Michie