How Long UPS Batteries Last
How Long UPS Batteries Last, and Why Four Weak Jars Usually Mean Forty
A larger UPS or telecom string, forty or more individual batteries (jars) in series, lasts 7 to 10 years in service on average. That's our figure from replacing strings in the field, and it's shorter than most data sheets suggest. Know the gap before you put a number in a budget.
A small rack-mount or desktop UPS is a different animal. Those batteries run 3 to 5 years. The rest of this article is about the big strings.
The question usually arrives in a different form anyway. A maintenance report comes back with four of forty jars flagged weak, and somebody asks whether you can replace the four.
Design life, warranty and service life are three different numbers
People who sell batteries for a living mix these up, so let's separate them first.
Design life is an engineering expectation under stated conditions, printed on the data sheet. EnerSys rates its PowerSafe SBS front-terminal line, for example, at more than 10 years of life in float service at 77°F (25°C). It describes a battery held at a controlled temperature on a correctly set charger.
Warranty is the manufacturer's commitment to replace a battery that fails within its term. Across the lines we carry, terms run from one to eight years depending on the series, and we confirm the term in writing with your quote.
Service life is what a string delivers in a real room, on a real charger, with real outages. For larger strings, that's 7 to 10 years. A design life of more than 10 years and a 7-year service life can both be accurate about the same battery.
Why UPS batteries go bad sooner than the data sheet says
Heat is the largest single factor. A string in a warm room ages faster than the same string in a cool one, and manufacturers publish their own figures for how much. We cover what heat does to a string in the companion article on UPS battery temperature.
The charger comes next. A charger holding the wrong float voltage, or set for a different chemistry than the one in the rack, ages every jar in the string at once. It's the item most often left off a maintenance visit, because it's the last thing anyone thinks to check.
Duty matters too. A battery built for float standby, asked to cycle every day on a site with unreliable utility power, ages faster than its data sheet assumes.
Then the weakest jar, which is where the four-or-forty question starts.
Why four weak jars usually mean forty
Replacing the four is the right question to ask, and the answer is usually no. The four flagged jars are the first four to show a problem the whole string has. Here's why.
A string is one circuit. Every jar shares the same charge current, and the string voltage divides across all of them. Old jars develop higher internal resistance, so they take a bigger share of that voltage. Think of the charger's float voltage as a fixed budget split across all forty jars: the old, high-resistance jars grab more than their share, and anything new gets shortchanged.
Put four new jars into that string and they sit at a lower float voltage than they need, next to old jars running hot. The new jars age toward the old ones. You paid for four new batteries. Within a year you have four that behave like the rest of the string.
It works in both directions. The weakest jar sets what the whole string can deliver. Replace it and the string still only performs to the next-weakest.
What happens when you replace only the four
In the field, new jars placed into an aged string get pulled down to the level of the old ones within 8 to 12 months, tops.
The cost shows up in an outage, when the string finally has to carry the load. A string sized to carry the load for 15 minutes can fall to 15 to 30 seconds of runtime when its weakest jars give out. On a site without a generator, that drop is the difference between an orderly shutdown and a hard crash.
Replace four now and the other thirty-six are still coming: a second quote, a second visit, a second maintenance window, and a string running at reduced capacity in between.
Is it worth replacing only the weak jars?
Early in a string's life, when one jar fails and the rest are young, replace the one. That's a defect, and it's often a warranty claim.
Past that, the answer is usually the whole string. Our rule of thumb: one or two years old with one bad jar, replace the jar. Five or six years in with four flagged, that's a whole-string conversation, and we'd rather have it with you before you spend anything.
Get the readings before you decide
Three matter most: float voltage on every individual jar, which tells you more than the string total the UPS reports; conductance or resistance on every jar, compared with that jar's original published specification; and temperature at every post.
You don't need a history of readings for them to mean something. The first set tells you plenty on its own.
Read temperature two ways. If every jar reads hot, check the string's age and the charger before you blame the room. If one jar reads hot and the rest don't, that jar has a problem.
The full list of what a maintenance visit checks, all eight items, is in our article on UPS battery maintenance.
Keep the readings
Keep every set of readings, dated, with the jar positions recorded. They show how the string has aged, they make the next visit's numbers mean more, and they're the first thing anyone asks for when a string fails early.
One thing to be clear about: a maintenance visit doesn't include a discharge or capacity test. If a claim, or your own planning, needs one, it's separate work, and we'll quote it.
We'll tell you straight
Send the string size, its age, the jar part number off the label, and the readings if you have them. We'll tell you straight whether it's four jars or forty, and if it's forty, we'll tell you why before you buy anything.
Written quote in 24 to 48 hours.