so I unplugged the USB monitoring cable from the PC to the APC UPS, the PC suddenly turned on normally again, what the hell happened??
That is not normal - even with failing batteries.
It seems several things are happening.
A battery can output its rated voltage but still be faulty - not have enough current behind it to carry a load. This can trick the UPS monitoring circuits in to thinking the battery is still good, but as soon as a load is placed on the UPS, output drops and the connected devices suddenly loose power. It sounds like that is one of your problems there.
Many UPS have setting options in their configuration menus to automatically shut down the UPS when battery charge levels drop below some threshold (typically set by "runtime left" on battery). This is set in the UPS software that runs in Windows. By unplugging the intercommunications cable, the UPS "may" (I'm kinda-sorta guessing here) no longer be receiving the instruction to power off, then decides to restore output power.
The odd part is, the PC turning back on by itself after removing the UPS cable. That is NOT normal.
By default, computers/motherboards have a BIOS Menu setting that keeps the computer "off" once power is restored from a power outages. But this setting can be changed
by the user to tell the computer to automatically boot once power returns. This is typically done for remote "
unattended", mission essential systems. It is so a technician does not need to make a road trip in the middle of the night after a power outage just to boot the computer. Did you change that setting in the BIOS Setup Menu?
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As mentioned above, 3 years is the typical "average" life expectancy of UPS (SLA) batteries. It can actually vary widely from 2 to 5 years or even longer depending on how they are used. If the UPS kicks over to battery frequently, the load is heavy, and the time on battery (depth of discharge) is long, the batteries tend to have a shorter life.
High ambient (room) temperatures can speed up aging too.
SLA batteries do NOT suffer from memory effect so if left unused (or constantly in any charge state), there is not a problem of memory - unlike other battery chemistries.
HOWEVER, the chemical reaction inside these batteries NEVER stops. This is why these SLA batteries (which includes car batteries too) have a "shelf-life". That is, they will grow old and wear out just sitting on the shelf, even if never used.
For that reason, buy your replaces from a place with a rapid turn-over to ensure you get the "freshest" batteries possible.
FTR, I never, as in NEVER EVER buy replacement batteries from the UPS maker. The UPS makers are not battery makers. So they buy from OEMs. The UPS makers then charge outrageous prices for the same batteries you can buy elsewhere. Often, it is the exact same battery that APC, CyberPower, Eaton/Tripp Lite uses that they just slapped their sticker on!
I buy my UPS batteries from
BatteryWholesale,
Battery Mart,
Batteryplex,
RefurbUPS,
Walmart or
Amazon - depending on who has the best price on
that day. Note today one distributor will price high with free shipping and the next day price low but with additional shipping charges added. So be sure to factor in shipping, which can be significant, but sometimes free.
Just make sure you get the same voltage. The amp/hour rating can be higher however, for longer run times, but not lower. For example, it the current cells are 12V 7.2AH, I always go with 12V 9AH. The physical dimensions are normally the same (or close enough) but double check to be sure. And last, make sure you get the correct terminal size. There are inexpensive adapters but typically space inside the UPS battery compartment is too limited.
F1 vs F2 terminal connector size
F1 Terminal – 3/16” (0.187”, 4.8mm) wide
F2 Terminal – 1/4" (0.25”, 6.35mm) wide
If your UPS battery consists of two or more battery cells, they will be strapped in parallel, series or series/parallel configurations to achieve a specific output voltage. This strapping is typically done with one or more small jumper or interconnecting cables. Don't discard those straps (or any plastic framework) the old battery may have used. Save them to use to assemble your new battery. I recommend taking a photo of the battery wiring before disassembly to use as a guide when assembling the new cells into the new UPS battery.
What's this cell vs battery stuff? A single cell can be "a" battery. Or, several cells put together can make up "a" battery. So, for example, if you have a flashlight that uses 4 AA batteries, individually each AA battery is a single cell. Insert the 4 AA cells into the flashlight and now, working together, they have become "one" battery, made up of 4 cells. Clear as mud, huh?
To make it more confusing, standard SLA UPS batteries, like this
12V 9Ah battery actually have 6 cells internally (just like a car battery). So, when this 12V 9Ah battery is used separately, it is a single-cell battery. But strap two of those 12V 9Ah batteries together, they become one 2-cell battery - even though there are 12 internal cells. Totally confused now? No worries. It really does not matter. Most people just call them batteries and that's fine.