I recently acquired 32x 9 amp hour 12v cells (the high capacity version of the 7.2ah, 7.5ah)
and 40x 5ah 12v cells.
they were all in 48 and 96 volt battery trays.
all of them with the exception of 1 seem to be in good condition, and they all have resting terminal voltages of 13.0 to 13.2 volts, with 13.1 being the average.
so i'm looking around the internet and i find some info on APC inverters floating the battery at 13.5+ volts, and how bad that is for the battery.
well i did my own measurements and i found a Back-UPS 1500 not only floats the voltage at 13.5-13.6, but it pulls 100 ma from the battery for about 1/10th of a second, and then charges at 50ma for the rest of the time. (continuously btw) so you're looking at about 30-50ma flowing into a 5 amp hour battery (that particular ups is designed for two 7-9 amp hour batteries in series)
anyone think this isn't negligible? i don't know.
so anyhow, turns out sealed lead acid cells are only good for 2-3 years in APC products.. and this is uh, well known
I don't buy the argument that 20 discharges to zero at a 15-30 minute rate can result in exploding battery cases.
so getting back to one of the un touchable subjects here...
pulse charging..
i found a whole bunch of comments posted by John Fetter across various forums over the years.
he brings up several interesting points that i'd not heard elsewhere.
The positive active material is lead-dioxide. Lead-dioxide is a semi-conducting material. When it is housed in a lead-antimony grid, the antimony and dioxide remain firmly in contact. When housed in a lead-calcium grid, an ultra thin oxide layer of a different kind builds up over time between the grid metal and the lead-dioxide, which takes just long enough for the battery to survive a couple of years. Manufacturers add a little tin to try and prevent this problem but it is not very effective.
ultra thin oxide layer between the surfaces of the positive grids and the lead dioxide active material. This is known in the battery trade as “open circuit†or the “antimony-free effectâ€.
As a matter of interest, when the total volume of water that is added over the years reaches the same volume of the electrolyte, your batteries will most likely stop working. I commissioned testing on two sets of Trojan golf cart batteries, two sets of Exide and two sets of Deka golf cart batteries years ago. They all expired after using this amount of water. Two-thirds of them lasted between 40 and 60 percent longer because we had given them something to make them last longer. Their water consumption was slower. I was amazed to find the water consumption so predictive.
Very high magnification (600 to 2000) shows that lead sulfate crystals and lead metal crystals actually grow in completely different areas across the surfaces of negative plates - lead dioxide crystals and lead sulfate crystals likewise grow in different areas on the positive plates.
When a battery is in the process of being discharged, at the negative electrodes, the surface atoms of the lead metal crystals go into solution, then travel almost in contact with the surface towards the lead sulfate areas, get converted into lead sulfate upon arrival and precipitate out, adding to the lead sulfate crystals. The activity at the positives is similar.
so reading through hundreds of other comments i find some interesting things.
the manufactueres tried adding tin to stop the lead-calcium corrosion issue, but it didn't work very well.
then the price of tin jumped 5 fold (its 20 dollars a pound now) and they dropped it altogether.
lead antimony positive plates and lead-calcium negatives could work, if they formed the battery before packaging the cells together
(during forming, antimony is plated out into the negative plates and essentially you get the same water consumption as the lead antimony battery)
antimony prices went up and all the battery manufactuers switched to lead calcium at the same time.
with the exception of industrial batteries.. who's consumers know better.
but getting back to pulse charging, seems it can break through the oxide layer between the lead calcium grid and the lead dioxide active material.. but only on the positive plate, the negative does not have that problem.
so while it appears that in order to get the best life you have to constantly trickle charge..
and then the question comes up.
how much current per amp hour.
seems to me 50ma into a 5 amp cell is too much.
that's a 1/100C rate.
one thing i don't want to argue about is the float voltage.
float voltage changes with age, as the acid content rises with water removal, and then falls when lead sulphate is bound by EDTA (added at manufacture by some?)
while self discharge is strongly temperature dependant, seems to me that you could easily account for this with careful regulation of float current.
this document:
http://www3.alcatel-lucent.com/bstj/vol49-1970/articles/bstj49-7-1321.pdfIs from 1970, and its for lead calcium cells.
It suggests float voltages on the order of 12.9 to 13.2 volts.
Float currents on the order of 45 micro amps per amp hour.
maybe the solution is to just put extra water in the sealed cells and run them as flooded?