One of the units is now making a humming sound. Maybe its just charging the batteries or should I be worried?
The batteries should have come with a near-full charge, meaning they should only need a few hours to top off. Once fully charged, and if not operating on battery, there would be no need for the fan to be running. So I doubt it is the fan, but suspect transformer plates or coil windings are vibrating. That's not uncommon. You would need to open up the UPS case completely to determine which.
But first, assuming it has had time to fully charge the batteries, feel all around the UPS case. It should be cool or only slightly warm. Compare it to the non-humming unit. If you feel some very warm or hot spots, and you are certain the batteries have had time to fully charge, the UPS maybe faulty.
If these APCs have a USB port, you can connect it to your computer and install
APC PowerChute UPS monitoring software (its free). It will tell you the battery/charge status.
If no excessive heat issues noticed, I would turn off the UPS and unplug it from the wall, then open it up. So if able, open the UPS and position it so you can power it on while open. But WARNING! There are exposed and
deadly voltages in there! Hopefully, you will be able to see the fan and if you can, gently and
momentarily touch the center hub of the fan. If you can reach it safely with your finger, fine. If not, a wooden dowel or Popsicle/glue stick will do. Nothing metal. This will temporarily slow down the rotation speed of the fan. If it is the fan making the noise, the pitch of the fan noise will change as it slows down and speeds up again. If it is not the fan making the noise, the sound of the noise will remain constant.
If the humming noise is coming from a transformer or coil, there may be nothing you can do about it. However, it can last for many years without worry. In some cases, you can apply some epoxy resin or hot glue to the transformer plates or coil windings to stop them from vibrating.
You might also make sure your wall outlets are wired properly - for peace of mind if nothing else. Every home and every computer user should have access to a
AC Outlet Tester to ensure the wall outlet is properly wired
and grounded to Earth ground. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets (outlets near water) too. These testers can be found for your type and voltage outlet, foreign or domestic, (like
this one for the UK) at most home improvement stores, or even the electrical department at Wal-Mart. Use it to test all the outlets in the home and if a fault is shown, have it fixed by a qualified electrician.
But question is are the batteries I bought comparable to what APC puts in their units?
Yes. As I noted before, I NEVER EVER buy replacement batteries from the UPS maker. When you own a Ford F150, are you going to buy a replacement battery from the Ford Dealer? Or from Walmart, or AutoZone? APC just slaps a label on the them and jacks up the price. Other than that, they are the same.
lead batteries are called "starter" batteries for a reason,
Oh? As a tech, I've been using UPSs to support computer and other critical communications systems for over 35 years. I never heard or referred to SLA batteries used in UPS and other battery backup devices called "starter" batteries. Got a link for that?
Yes, SLA UPS batteries use the same battery technologies as car batteries, which are definitely used to "start" engines. But they typically are called "storage" batteries as they sit there holding a charge until needed. But for sure, as the car is parked, the battery is still being used to keep all sorts of electronics in the car "alive" - including the security system, clock, the computer itself, and more.
These type batteries are also commonly used in electric start gas powered lawn mowers and similar equipment, but still, I have never seen a LiPo (lithium polymer) battery pack capable of supplying the necessary 24VDC needed to provide several 100 watts of 120VAC power needed to support a PC and monitor for several minutes. Got a link for one of those too?
Also, as seen
here, LiPo batteries can cost more than typical SLA cells. So not sure of the economical feasibility for using them here
ESPECIALLY when the primary reason for using a good UPS with AVR is for the automatic voltage regulation capability - which regularly uses the batteries to "boost" the output voltage during low voltage anomalies (dips, sags and brownouts) without kicking over to full battery power.
same for charge, always keep the topped of/recharge as soon as possible.
I really don't understand this statement. It suggests an unfamiliarity with how a UPS works. For sure, as soon as grid power is restored, the charging circuits of the UPS kick in and and top off the charge for the UPS batteries.