Riello UPS technical services manager Jason Yates explores the often unsung heroes of UPS systems – the batteries – and outlines the steps site operators should take to reduce the risk of premature failure.
Without a well-maintained battery system that’ll perform as and when required, an uninterruptible power supply is practically useless.
Despite the rise of alternatives such as lithium-ion, the majority of UPS systems today still use sealed lead-acid (SLA) batteries, also known as valve regulated lead-acid (VRLA).
Such cells are big and heavy because of their low energy-to-weight and energy-to-volume ratios. But they do deliver high surge currents. That means they’re ideal to provide instant backup during a mains failure or to power up a generator.
Difference between ‘design life’ and ‘service life’
Most SLA batteries have either a five or 10-year design life. In effect, a 10-year battery will last for 10 years, assuming perfect operating conditions.
However, there’s no such thing as “perfect” conditions. There are too many factors, such as operational temperatures, that impact on a battery’s lifespan.
That’s why EUROBAT (Association of European Automotive and Industrial Battery Manufacturers) guidelines state a battery reaches its end of service life when capacity falls below 80% of the original figure.
It’s recommended you replace 10-year design life batteries in years seven or eight. This takes into account all the factors that can reduce lifespan and provides a safe enough margin for potential failure.
There’s an equivalent performance drop-off in five-year design life batteries, which is why they should be replaced in year three or four.
UPS batteries tend to follow the well-known “bathtub curve of failure”, namely there’s a relatively high but decreasing number of early failures caused by manufacturing defects, then a constant low level of random failures during the normal UPS working life. Finally, there’s a dramatic increase – known as wear-out – towards the end of working life.
Batteries can account for more than 98% of critical UPS failures at the latter stage.
Factors affecting battery lifespan
High temperatures tend to be the most common cause of premature UPS battery failure. The higher the temperature, the quicker the chemical reaction, which increases water loss and corrosion. SLA battery capacity is based on optimum operating temperatures of 20-25oC. Service life will halve for every 10oC temperature increase. Note that short-term temperature fluctuations have little effect on battery lifespan.
Frequency and depth of charge
Each battery has a finite number of discharge and recharge cycles. Each discharge slightly reduces the battery’s capacity. Partial discharges have less of an impact than full ones.
Overcharging batteries above the manufacturer’s recommendations produces excessive hydrogen and oxygen gases, which will cause batteries to dry out. Undercharging can result in sulphate crystals forming on the plates. These harden over time and reduce battery capacity.
AC ripple generated by the UPS rectifier, charger or inverter is a big cause of overheating, which speeds up the deterioration of the battery poles and eventually leads to premature failure.
Poor storage of unused batteries
Even unused UPS batteries automatically discharge small amounts of energy. So if you’re holding batteries for a prolonged period before use, it’s recommended to top them up every few months and store them at a maximum temperature of 10oC.
Incorrect battery application
UPS batteries deliver high rates of energy for a short time, typically a few minutes. Other batteries, for example backing up switchgear or telecoms, can provide several hours’ autonomy. Using a battery designed for telecoms with a UPS system won’t perform as well as one manufactured specifically for that task.
Common conditions that limit battery lifespan
Several conditions will reduce battery life. First of these is grid corrosion, which occurs in a battery that has been in service longer than its expected lifespan. Normal chemical reactions in the battery cause corrosion, such as shedding lead from the plates.
Sulphation is common in stop-start battery applications like a UPS. It happens when the battery doesn’t get a complete charge and results in sulphate crystals forming at the plate terminals and within the electrolyte. The condition increases internal resistance, leading to a longer charging cycle. As long as the degree of sulphation isn’t too high, you can recover batteries by charging them at a higher current for around 12 hours.
Dry out or water loss is caused when overcharging increases the concentration of acid in the electrolyte and leads to a decline in capacity over time.
Another condition is known as thermal runaway, where an increase in battery temperature, in turn, releases energy that causes a further rise in temperature. That’s because the heat inside the battery can’t escape through the safety vents of the sealed cells, causing increased temperature around the outside of the battery, causing a similar knock-on effect within the battery.
These take place when the paste on the positive electrode becomes porous and leads to a loss of contact between the electrode and the grid.
Inaccuracy during the initial battery design process, such as poor alignment of the separators and plates, can result in a final condition, known as top mossing. That’s where a crystalline moss forms, which can result in the cell self-discharging.
Steps to prevent premature battery failure
The good news is there are several steps you can take to maximise the service life of UPS batteries. Proactive battery maintenance, monitoring and testing are paramount.
At the bare minimum, batteries should be manually monitored at the individual cell level at least annually. Basic physical tests cover the inspection of terminals for corrosion, looking for any leaks, cracks or swelling, and tightening inter-cell connections.
More advanced support comes in the form of battery monitoring systems, which record measurements such as the number of cycles, float voltages and cell temperatures.
There are also several types of test which can help determine whether a battery is approaching its end of service life.
Firstly, impedance testing is a non-intrusive method to build up a “history” of each cell which makes it easier to identify any signs of weakness. It doesn’t require batteries to be taken offline, but it only gives a broad indication of their general condition.
Also known as load bank testing, this is much more thorough as it tests the batteries under normal and peak load conditions. It does require the batteries to be taken out of service, at the worst-case for several days, but usually for less than 24 hours.
Partial discharge testing
This offers something of a middle ground. This discharges the batteries by up to 80%, reducing their availability, but they should be fully available again within eight hours. If there’s an emergency meaning the UPS needs to run off batteries, it still has 20% capacity to fall back on.
Battery care systems
Most modern UPS systems now incorporate battery care systems that automatically test the cells at regular intervals, protect against ripple currents and offer a range of recharge methods.
What else can you do?
Try and keep UPS batteries at their optimum operating temperature of 20-25oC. This might mean installing air conditioning if the batteries are internal to the UPS or stored in the same room. Of course, with larger installations, you could house the batteries in a separate temperature-controlled room, while the UPS and other IT equipment runs in a separate space, often at a higher temperature.