A interesting link I just stumbled upon from chasing a hint madires gave us earlier: hidden service menu
http://www.jjoseph.org/notes/apc_smartups_battery_float_voltage
It's not a "hidden service menu". It's a set of commands for factory calibration of the units that isn't documented unless you have the factory calibration notes. That page (and most of the net) is quite unclear about the float voltage on these units, so here are some real take home facts.
The APC UPS always target the upper end of the float voltage range for the batteries to get maximum runtime at the expense of ultimate battery life. The plethora of reports about APC SmartUPS cooking batteries isn't because APC are deliberately doing anything to cause this, it's because a combination of tolerances drift significantly as the units age and the divider that measures the battery voltage tends to drift low, forcing the charging ASIC to increase the voltage. It seems to happen to most of them, but then those 3G units are all >15 years old now. An occasional service isn't out of the question for a unit that has > 100,000 hours on the clock.
The SmartUPS family comes in 5 basic generations at this point. The most commonly found are G3 (beige SU series) and G4 (black SUA series). Some of the later G3 variants were black also, but they're kind of a half-breed (mostly G3 with some G4ish improvements but not the G4 charging smarts).
Generation 1&2 were configured with trimpots and had RS232 status line reporting. I've never seen one in the flesh, only the service manuals.
Generation 3 (SU) has the ability to report and alter the reported battery voltage in firmware. They have no ability to alter the *real* battery voltage in firmware other than to apply a boost during recharge (and that boost is an on/off thing only). To alter the charge voltage you must alter the resistive divider at the input to the charger ASIC. There is no way to alter the float voltage in firmware. All you are doing by "re-calibrating" the UPS is to change the voltage the UPS *thinks* the battery is at.
G4 -> (SUA) has the ability to alter the reported battery voltage in firmware. The processor uses the reported battery voltage to alter a PWM value to provide trimming to the charger ASIC which does alter the *actual* battery float voltage. This ability is primarily so the UPS can apply temperature compensation to the float voltage, but it has the side affect of being able to change the float voltage by altering what the UPS *thinks* the battery voltage is. As you change the battery voltage constant, the UPS alters the charge voltage to keep it where it thinks it should be based on the battery temperature (NTC hanging in the back of the battery box). There is no way of changing the target setpoint and changing the divider means the UPS isn't accurately reporting the battery voltage, but at the end of the day you are changing the float voltage.
The G4 also encompasses the new G5 (SMX/SMT/...) as the actual UPS back end is relatively the same. There is a second UI processor on those that handles the front panel and the Microlink interface, but if you dig in you'll find that processor is polling an essentially unmodified G4 back end using the same old UPSLink protocol. So if you get between those two you can tweak the UPS like you could with the older generations.
Each generation has a series of sub-generations. I've seen at least 3 variants of the SU series, the latest of which was entirely SMD and with a dedicated FET driver (the original through hole 3G SU series had a discrete FET driver that was awesome at cooking the board on the 48V models). No matter which variant, they all rely on a fixed resistive divider to set the battery voltage.
The reality with the UPS units is the batteries live inside the box with the electronics. That makes the batteries much warmer than they would be in a separate enclosure and *that* is the major compromising factor in their life. I've got a couple of units modified for external batteries, and that takes the battery life from ~3 years to ~5 years. No change in voltages or battery management strategy.
I also modify the SU series to run the fans 24/7 at half speed. That drops the internal battery temperature by about 5C off the bat. The SUA series and up do that out of the box.
As for quality, things certainly started to dive after the Schneider buyout. I don't think there is any dispute of that.