Windspeed,
SLA are often called gell cells.
My 1kw UPS use 12v 12ah SLA. They last about 5 years.
Interestingly the ones cycled seem to last longer. Go figure.
For testing I keep a couple loose batteries (not in UPS). The weaker ones.
Noticed printed on the side: charge to 14.00v, cycled 14.50v.
So charged to 14.5v. after messing with the higher charging a couple times, battery seemed to have more AH, tested and confirmed went from 6ah-ish to just shy of 10ah? (Battery rated at 12ah)
Accidentally went to 15v recently, battery was warm, in the short term did not seem to harm it any. Did not hear bubbling noise... Still has 10ah capacity.
The UPS charges to 13.78 to 13.89 (measured several units, None make it to 14v). Based on the battery it should be charged to 14.5v if cycled.
Must have 4 or more brands of SLA, not all list the 14.5v. Your millage may vary.
Consider charging to a higher voltage, I think of it as a SLA equalizing charge.
Your batteries are failing, what do you have to loose?
I keep my old SLA, make good weights to hold things down when doing my wood working projects. Couple recently retired batteries will be tested out with the reverse charging recently posted here.
How has your Banki project come along?
Have fun,
Scott.