Started writing a comprehensive answer, interrupted by some phone calls, finally finished it...and then accidentally clicked to another page and lost it. Arghh!!! So this answer may not be quite as complete/well-written.

TL;DR Normal for a UPS with a Low Battery - Replace the Battery or the UPS

A typical uninterruptible power supply (UPS), unless it is a true dual conversion UPS often has this problem as the battery ages. Note that age is primarily "charge cycles" and not calendar time.

A typical UPS works in two modes:

• Good AC Power

UPS filters (basically the same as a good-quality surge protector) incoming power and passes it on the powered equipment. At the same time, the UPS monitors the battery (meaning: checks voltage) and charges it as needed.

• Bad AC Power

This includes:

• Low voltage (a.k.a., brownout)
• High voltage (unusual in most places except if you have a serious utility or electrical system problem such as a lost neutral)
• Power outage

When any of these problems is detected, the UPS disconnects the powered equipment from incoming AC power and turns on an inverter and other circuitry to draw power from the battery (typically 12V DC) and convert it to 120V AC power for the equipment. This switchover happens very fast - typically 4 ms to 12 ms. Compare that to a single AC cycle in the US (60 Hz) of 16 ms. The powered equipment doesn't know anything has happened.

The problem is when the battery starts to wear out. Despite the metaphors used when describing battery level, it is not a simple level like checking the amount of water (or gas or oil or whatever) in a tank. Battery charge is checked by reading the voltage. It isn't 12V = 100%, 6V = 50%, 0V = 0%. In fact, the range tends to be rather small.

For example, this article and chart shows the numbers for lead-acid batteries, with a range of 11.63V to 12.89V. Lead-acid batteries are quite common in consumer-grade UPSes. Lithium-ion batteries have a different scale, but the same general concept applies.

A good UPS will tell you when the battery can't hold enough of a charge any more, with some sort of indicator light or other display or beeping. However, it is actually hard to get this right, so sometimes the problem isn't found until it is too late. People also tend to not like beeping, so many UPSes deliberately do not beep to tell you there is a low battery.

If the battery is low, whether you (or the UPS control circuitry) "knows" about that or not, when it tries to switch and draw full power from the battery to convert to AC to power your equipment, it will fail very quickly. The result is exactly the problem you have found. It is actually worse than no UPS at all! Why?

If you have no UPS then the computer (or router or whatever) will rely on its own power supply to ride through very short outages. A typical example is when a thunderstorm comes through and the lights blink for a fraction of a second. The UPS will attempt to switch as fast as possible - less than one AC cycle, but then fail due to insufficient battery power. On the other hand, a computer power supply is typically designed to handle at least one cycle, often more, before causing a reboot or other problems. At least in my area a storm will often cause power outages that are so short that many (but usually not all) digital clocks (microwave, clock radio, etc.) will not even need to be reset, even though there was a visible outage for a fraction of a second.

Brownouts are another issue, in two ways. Many computer power supplies can now handle a wide voltage range - e.g., 100V to 250V for a single design to work worldwide. A UPS will typically treat anything outside a relatively narrow range - e.g., 105V - 125V - as a problem, so a dip to 102V might cause a problem for the UPS but not cause a problem for a computer connected directly to AC power. In addition, large loads such as a laser printer printing or a refrigerator compressor startup can cause enough of a voltage drop that a UPS will detect a brownout, and these repeated UPS cycles can wear out a battery very quickly. I had problems of that sort until I finally had my electrical service upgraded a year ago, which made it very easy for my electrician to split the main basement circuit - previously there just weren't any spare circuits.One circuit was simply running so much equipment that the critical loads on a UPS were subject (every couple of years, now hopefully longer) to the UPS battery wearing out due to frequent cycling.

A note about UPS/battery replacement. Unlike cell phones, laptops and many other devices, most UPSes have easily replaceable batteries. There is a good reason for that. Cell phones, laptops and similar devices are as likely to have other problems (physical damage, other components wearing out) or simply become functionally obsolete before the batteries wear out. Manufacturers like non-user-replaceable batteries because it encourages device replacement rather than battery replacement, but much of the time the user replaces the device anyway, even if the battery is perfectly good.

But with UPSes, it is quite common for a good UPS to last through 2 or 3 or even more battery replacements. The rest of the UPS is basically a surge protector, charge controller and an inverter - relatively simple components with no operating system upgrades or cell phone network changes to worry about. You don't have to get the battery from the UPS manufacturer. I generally go to Batteries Plus, formerly Batteries and Bulbs but there are other retailers as well. Be careful with mail-order for batteries, as returning the old batteries for recycling is much more of a hassle, though at least in my area Batteries Plus will take batteries for recycling even if you don't buy new batteries there, and I often bring them batteries from old UPSes if, for example, I replaced the UPS rather than just replaced the batteries.

posted 9 months ago

CC BY-SA 4.0

9mo ago

manassehkatz‭ staff

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