Do You Test Your UPS Batteries?

30/06/2015

By Eco Power Supplies

We’ve all heard the old adage of “buy cheap, buy twice” but how many data centre managers regard the batteries within their UPS systems as a parachute or safety rope? The fact is that the uninterruptible power supplies used to protect IT facilities and the critical power paths to which their servers are connected, are only as good as the batteries within them.

Many data centres classify themselves into one of four tiers for resilience. Tier 1 is the lowest and Tier 4 the highest with multiple redundancy. A Tier 1 facility will have a standalone UPS and battery system. A Tier 4 facility will have a two mains power supplies (A and B), each protected by a parallel UPS, battery and standby power generator systems.

Within any of the Tiers and UPS configurations, the batteries will be the weakest link.

Batteries, like any other consumable item vary in quality. All too often UPS systems are purchased on price and technical specifications such as footprint, operating efficiency, kVA/kW output and battery runtime. Very few clients ask which brand of battery is used. Fewer still specify an end of life runtime value or ask for battery testing to be included within an annual preventative maintenance visit.

Some battery brands are well known such as Yuasa and Fiamm and are the mainstay of the UPS industry. They may be manufactured in the USA, UK, Europe or the Far East. Even when supplied from a branded supplier quality and consistency cannot always be guaranteed 100%.

Furthermore, cheap UPS batteries are always available from less well known Far East-based manufacturers giving some UPS suppliers the potential of a price advantage when pitching for a new UPS system but one that a client could find vanishing rather quickly.

Most UPS batteries have a 5 or ten-year design life with replacement expectancy around years 3-4 or 7-8 respectively. The simple fact is that batteries age whether they are used or not. The generation of electrical power in a battery is down to a chemical process and interaction between positive and negative plates (normally made from Lead) and an electrolyte (Hydrosulfuric Acid as a gel).

Batteries self-discharge with no electrical charging system in place and cannot be left in storage for longer than 6 months. In use batteries require a 20-25 degree Centigrade ambient environment ensure they meet their performance and battery life specifications. Interestingly, the higher the temperature the better the performance but the quicker design life falls. For every 1-degree rise about 30, design life halves. Battery performance also improves from day one of an installation after 2-3 battery charge/discharge cycles but the overall distribution curve is non-linear and falls off the nearer the battery is to its design life.

Now UPS manufacturers, as well as investing in improving operational efficiency, footprints and prices also promote improved battery management functionality. This generally means a temperature compensating charge, sleep-mode in their charging function (allowing the UPS batteries to self-discharge to a “safe” defined level) and complex algorithms that monitor automated battery tests.

The fact though is that 80% of UPS failures are down to a failed battery set and one that the internal battery testing was unable to identify. A side note here is to always connect your UPS to a monitoring solution, whether this is via SNMP, SMS modem alerts or a building management system (BMS). There is nothing worse than hearing of a UPS in a basement or plant room that was alarming (reporting a failed battery test for example) and no action being taken because no one was monitoring the UPS or heard the audible alarm.

The algorithms used within UPS battery testing only tests the overall battery set health and DC voltage. Most UPS battery sets have one or more battery strings that can themselves have up to 40 individual batteries.

How can an overall test identify one or two failing batteries and why does this matter? The lowest performing battery in the string could fail, split and leak acid, either reducing the overall battery string performance or removing its availability. The only time this becomes obvious is when inverter has to call on the battery for its DC power; when the mains power supply has failed or become too erratic to operate with. If the instantaneous demand can be met the DC voltage collapses and the inverter powers down, crashing the critical load.

Battery monitoring systems can provide a solution but they can be expensive to install. A battery monitor is connected to each battery and wired to a central monitoring station. Some systems can operate over WiFi but most data centres prefer to avoid this. Another option is individual battery testing. Two schools of thought here include: impedance and conductance testing.

As batteries age, their internal resistance (referred to as impedance) rises and sulfation can occur. The increased resistance to the flow of electrons between the plates leads to heat. Batteries are exothermic and generate heat. A visual inspection of UPS batteries approaching their end of life will always reveal buckling of their plastic cases due to the high internal heat generated. A white deposit around the terminals is another indication of the electrolyte gel “weeping” along the terminal posts.

Impedance testers are hand-held devices that a UPS engineer can use to test batteries during a battery inspection or as part of a wider preventative UPS maintenance visit. The main issue with impedance is that the test tells the engineer whether a battery has failed or not. It does not provide an indication as to the overall health of a battery.

Conductance battery testing is becoming more popular and is recommended by leading telecoms companies. These organisations rely heavily not just on their UPS systems but also DC rectifiers and their connected batteries. A lot of telecoms equipment is not AC powered but runs from a 48Vdc supply. Conductance testing is preferred in this type of environment because it more accurately measures overall battery health by comparing the “conductibility” of a battery and comparing this to a known Siemens value.

Conductance testing actually measures the ability of a battery to conduct a current. Scientific tests have provide that at low frequencies, battery conductance is an indicator of battery health and demonstrates a linear correlation to a battery’s timed discharge-capacity. Trending conductance provides a reliable predictor of battery end-of-life.

Hand-held conductance testers are easy to use and can test an individual battery within 30 seconds or less. They provide visual LED indication of a good connection of the device to battery terminals and audible indication of a successful test. Each battery has a unique identifier and is labelled with battery one string one from the positive terminal. The process allows a UPS engineer to quickly and safely move along a battery set, whether in a cabinet or on an open or closed stand.

The taken results can then be compared to known Siemens values from the UPS manufacturers. Note: if they cannot provide a Siemens value for their batteries they should probably not be used within a critical power environment. If the batteries were conductance tested on day one of installation, a truer trend of battery performance can then be plotted. Either way, whether using battery manufacturer’s data or baseline data from the UPS installation itself, Conductance testing tells you the overall health of the battery, far ahead of the impedance level rising.

The benefits of conductance testing are that the process allows for the planned replacement of UPS batteries. It may be possible to extend UPS battery life knowing that the batteries are healthy and this has an environmental and financial impact. It may be possible to only replace one or two batteries if these are known to be failing. Alternatively, if a UPS is upgraded to a new one for efficiency, footprint or warranty improvements, it may be possible to reconfigure existing batteries rather than invest in a new battery set.

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