JOOI, What did you reset the charging voltage to on that APC SmartUPS? Presumably it would have originally been set to 27.6v. Did you throttle it all the way back to 27.0v?
BTW, I've never seen APC batteries so badly cooked as that. More typically ime, they may swell slightly in the beam, never developing a noticeable hump on the top. That battery pack is in an even worse state than the set of 3 NP7s that got cooked by my UPSonic600, an ancient pure sinwave inverter type - it has a Honeywell "PAT" sticker with test dates of December 1993 and 1994 stuck on the back panel!
None of my APC UPSes have been modern enough to be blessed by anything other than a dumb charging circuit (voltage regulated to 13.8v per 6 cell's worth of battery pack). Similarly for that monster Upsonic600 with its 3x12v7AH SLA 36v battery pack (a slide in tray, optionally filled with two bank's worth of battery packs) and an ancient Emerson 30 with a pair of 7AH SLAs.
I've long since retired the SmartUPS700 simply because of its unconscionably high maintenance consumption of 20W (virtually none of which goes into keeping the battery pack charged). I think I retired the venerable Upsonic600 just prior to that when it started to cook its second lot of batteries after just another 2 or 3 years of service.
The APC BackUPS500 that I bought brand new in the box at a radioham rally some 15 years back is on its second (single) NP7. I haven't tested that in anger since the initial half hour commissioning test of the 50 odd watts test load of my FreeNAS (now NAS4Free) box (left halted during the POST to avoid FS corruption if the test had exhausted the UPS battery before I could manually restore the mains supply). I've not really had a chance to safely put it to the test during the past 4 or 5 years, so it's anyone's guess as to the condition of the battery now.
I presume the battery is still be in good condition since the total power consumption looks to be just the normal 3 watts in excess of the base load of the NAS box. I keep a UK version of the Kill-A-Watt meter permanently in line to monitor total consumption of UPS plus NAS box just to keep an eye on this. The last time I finally got around to checking the UPS out to discover the battery had failed, I'd been observing total power consumption figures of 58 or more watts. The knackered UPS battery proved to be the cause of the mysterious rise in energy consumption, initially misattributed to the NAS box itself.
I suppose I aught to take the opportunity to properly test the battery and to update the boot image file and pull the now finally redundant 5yo 3TB tiddler out of the box (I finally finished my 2 year "Convert mpg to mkv to free up disk space" project just a week or two back). I think that after some 3 years of uptime, interrupted by a fortnight's break just over a year back, it's high time I did a spot of maintenance. I'll be able to achieve three goals for this next planned downtime, boot image update, UPS battery test and elimination of the retired HDD and its 7 or 8 watts of loading (should reduce the server idle load down from 51 to 45 watts or less, all three remaining drives spinning - I don't utilise any spin down power saving).
Anyway, it isn't only APC who use the 13.8v SLA float charging standard, it's pretty well every make of UPS. They don't care for extending the battery life unduly by choosing a less corrosive 13.5 v when the higher voltage offers the cheapest way to achieve the maximum autonomy from a new set of batteries.
The one or two year warranties typically exclude the battery pack since they treat it as a 'mere consumable' whose life depends on the number of outages it may have had to deal with during the woefully short warranty period. As long as the batteries can survive the initial one or two years without obvious signs of failure after handling maybe just one or two short lived outages, that's good enough for the manufacturers who are only too keen to supply replacement packs at premium prices every 3 to 5 years.
The modern UPSes might include improved SLA charge management algorithms these days, I just don't know whether any of the manufacturers are looking to gain a marketing advantage over the competition in regard of battery pack service life. One way to improve battery pack life would be to recharge to 13.8v then drop back to 13.5v with a monthly boost back to 13.8v for say 6 to 12 hours at a time.
I've kept a spare 12A SLA in good enough condition by solar panel charging alone for just a week or so per year to jump start my wife's Y reg 1.6 litre automatic Astra a year or two after buying the battery 2nd hand from a flea market stall some 5 or 6 years ago. It proved to be nearly flat when I got it home. Just 11.99v open circuit on my own DMMs as opposed to some 12.5v on a borrowed meter from another stall holder at the flea market - presumably, a meter with a low battery warning indicator I'd overlooked in my haste to get a reading before parting with my fiver.
I've checked it just now and I'm seeing a reading of 12.65v some 6 months after its summertime solar charging stint. A brief test with an H3 55W halogen headlamp capsule shows it can still provide power (it's not a case of high resistance normal voltage failure symptoms that these batteries can land up developing). Normally, I'd see post fully charged resting voltages around the 12.78v mark for most of the remaining 11 1/2 months of the year that it sits on my office window ledge in splendid isolation awaiting its next annual refreshing charge.
The fact that a charging regime of just once per year to 13.8v (peaking to 14.6v for short periods of an hour or so) has been enough to keep this battery in good condition, rather reinforces my theory that 13.5v is more than sufficient to keep the battery from sulphating at, presumably, some small reduction in effective usable capacity from a float charge state of 13.8v.
The choice of voltage level for continuous float charging of a LA battery is a compromise between a lower limit that defines an acceptable sulphation rate and an upper limit that defines an acceptable corrosion rate (along with water loss). I've no doubt that the float charge voltage also effects the maximum usable capacity as well so it's not too surprising that the maximum voltage is chosen to 'improve autonomy bragging rights' by the UPS manufacturers at the expense of a life shortened to a mere 2 or 3 years when it becomes the customer's problem (and their chance to sell premium priced battery packs - a win, win for the manufacturers - the inkjet manufacturers weren't the first to profit from the consumables market, just the first to do so so outrageously).
Johnny B Good
Johnny B Good