Re: [PSUBS-MAILIST] Gasses from batteries

[Subscuba@aol.com]

For what it's worth again,

(I hope this doesn't step on your toes Pat. If I do I apologize in advance)

When I built my sub (wet) I looked at a number of types. The first and lowest 
cost was the flooded lead acid, the kind with caps to add water. The second 
was the sealed Gel Cells and the last was the Absorbent Glass Mat (AGM).

The flooded lead acid are cheapest but offgas during charge and discharge. 
Plus they dump acid when you invert them. Putting them in my sub seemed 
hazardous.

The second was the Gel Cells which are interesting. These are still lead acid 
with the electrolyte immobilized by using silicon dioxide (powered sand). 
This material tends to cake up on the plates. The charging characteristics 
are also changed somewhat with low tolerance for overcharging. The silicon 
dioxide also tends to retard charge carrier mobility which decreases high 
surge capability and increases the output impedance of the cells. 
Nevertheless this is a fairly good chemistry and at moderate cost. High over 
charge rates will degrade the seal integrity and will release gasses so care 
is needed in charging.

The final battery I considered is the costly Absorbent Glass Mat (AGM) 
batteries. This is what is called a starved electrolyte system in which the 
electrolyte is contained in a glass mat between the plates. There is not much 
electrolyte in the glass mats but still enough for high current conduction. 
This cell construction offers the best performance theoretically but is the 
highest cost.

I choose the AGM batteries as the best performance. There is certain things 
to keep in mind when considering the sealed valve regulated lead acid 
batteries.

1) They are not completely sealed, both the AGM and the Gel cells give off 
small amounts of H2 and O2 during charging. The vendors state "Don't charge 
these batteries in a sealed container." They do not give off nearly as much 
gasses as the flooded cells. Try charging them under a Flourinert bath and 
watch the bubbles.

2) Both the Gel Cells and the AGM batteries can be operated upside down. Neat 
for a Sub.

3) The AGM has superior performance but is it cost effective? These are the 
latest craze.

There is a number of things I glossed over such as plate recombination of 
gasses but  this is why I picked what I did.

Good luck,

Ken Martindale