Tue, 10/01/2017 - 11:46
Submitted by editor
Every now and then, we are all reminded to test our smoke alarms at home and make sure that the battery has not run down. This is a simple check which takes a few seconds and could potentially be a life saver.
Now consider a UPS system (with one or multiple strings of VRLA/SLA type back-up batteries) located in an environment where the critical load it supports may be for an operating theatre in a hospital special care baby unit or a server room controlling traffic systems for a flight control tower. If the back-up batteries to this UPS were to fail on a mains blackout, the UPS would undoubtedly fail and drop its critical load. The consequence of this occurring could potentially be considered life-threatening. Regular UPS maintenance is always recommended (however old your UPS installation may be) and more often than not, a ‘good UPS service provider’ will undoubtedly include a UPS battery test as part of its maintenance. That is all very well and good, but even if a UPS battery test result shows up as being ‘GOOD’, this does not always guarantee the batteries will be sufficiently resilient for supporting its UPS at the full load autonomy as it was originally designed for. There are several reasons for this: 1. The UPS battery test duration may only be for 1 minute or a few minutes. In reality, your UPS may need to support the load for a longer period on mains failure. A deep battery test (where the UPS is set up to carry out a battery test for a much longer duration) is a possible option but most UPS systems do not offer ‘controlled deep battery tests’ and will drop the load during such a test if there is an issue with the batteries. The risk of dropping the load under these conditions is like forcing the UPS (whilst protecting a critical load) to operate on its back up battery bank by simply manually opening the mains input breaker to the UPS – very risky indeed! 2. One or more ‘bad’ battery blocks (usually with high internal impedance) will often, under sufficient load conditions drag a whole string of batteries down and the UPS will not be supported. The UPS may detect and ‘see’ the DC circuit from the battery string as being OPEN CIRCUIT. Quite often, a Battery Fuses Open alarm on a UPS display is a tail-tail sign that one or more battery blocks have become open circuit. As an analogy to this scenario, consider a metal chain with one of its links broken. The chain will of course be considerably weakened and break in half across the broken link if you were to apply a sufficient pulling force (or load) across it! 3. An increase in the UPS’s output load or changes in ambient temperature between maintenance visits could also affect the performance of the batteries when they are placed under load or test. Annual PM visits (i.e. one maintenance visit per year) are certainly riskier with the end user unknowingly compromising their critical load protection. There have been many incidences where an air conditioning unit located in a UPS battery room has failed over the course of twelve months and gone unnoticed. With no climate control in place, the room temperature can often reach as high as 40° C. The damage to the UPS batteries when exposed to such high temperatures (even for short periods of time) is irreversible and in most cases, can cause other problems (leakage and swelling etc.). If the UPS is in the same location as its batteries, the heat output from the UPS can also contribute to a rising ambient temperature and battery degradation. UPS battery management is essential with the UPS system being totally dependent on its battery backup if it is to perform as required. 4. The maximum lifespan of a UPS VRLA/SLA type battery will vary. Most manufacturers will quote a max. battery life based on optimum environmental conditions. The reality is that most installations are far from perfect when it comes to battery environments. The operating temperature has a significant effect on battery life and performance. At low temperatures, the performance (capacity) of the battery is reduced; at high temperatures, the battery ages more rapidly and reaches the end of life more quickly.
5. If the UPS Charger Float Voltage is not configured correctly (or the UPS charger is faulty), this may also cause irreversible internal damage to its batteries. What can be done to maximise UPS autonomy whilst on battery back-up power? 1. By checking (or monitoring) the impedance of each battery block, any bad blocks (i.e. Batteries that have a high impedance) can be identified and replaced ahead of any imminent failure. PPSPower maintenances are very thorough and include a full battery impedance test (in addition to carrying out a UPS self-battery test). The greater the frequency between PM visits, the lower the risk of having a failing UPS system when you need it. 2. UPS battery installations should ideally be in designated UPS battery rooms with adequate ventilation and suitable air conditioning units to boot. Daily round robin checks will eliminate the risk of any failing aircon units going unnoticed (or if they are linked to a BMS system - even better!). 3. Introduce sufficient air flow and climate conditioning in areas where the UPS and its batteries are in the same room. The air conditioner is required more for controlling the battery environment than for the UPS. Even with aircon units set to 20° C, the ambient temperature around the batteries can typically rise to 26° C and above with heat contribution from the UPS, servers and other systems typically found in an MER. One data centre in Bedfordshire went as far as fitting an array of small AC powered fans on the front doors of each battery cabinet as a desperate attempt to keep the UPS batteries cool.
1. Battery strings installed on open racks always fair better, are more accessible for servicing and inspection purposes than one’s built inside a UPS cabinet or reside in a battery cabinet which restricts air flow and traps heat. 2. If you are protecting a critical load with a single UPS (i.e. with no redundancy), opt for a battery bank comprising multiple battery strings configured in parallel (provided of course that your UPS can accept this kind of config.). The cost mounts up but at least should one string fail, the remaining battery strings will hold-up the UPS on mains failure. 3. If your critical load is being supported by a parallel UPS system, opt for each UPS module or unit having its own string (or strings) of batteries as opposed to a common, single bank of batteries. If one the UPS modules fails (or its corresponding battery string(s) fail), the remaining UPS units will still provide back-up on mains failure. 4. On ageing batteries (or for installations where there has been a known history of environmental temperature issues), it may be worth considering carrying out a battery load bank test. This involves placing the UPS on External Bypass (which means your load could be subjected to any flaws in the mains supply) or continue operating the UPS with the ‘load not protected’ for the duration of the load bank test. If the site has back-up Diesel Generators installed, it would probably make good sense to supply the load via the UPS on generator power rather than relying on the supply from national grid. However, there are risks involved in switching supplies. The reason is that if the UPS has faulty batteries in the first place, during the UPS feed changeover (i.e. from raw mains to generator supply), there will be a 'break period' (usually less than 20 seconds) that needs to be bridged by the UPS operating on its batteries. If there is an issue with the UPS batteries, the UPS may drop the load before the generator starts-up and stabilises. Therefore, the supply to your critical load will not be a seamless one. To sum up, a UPS system will only be fit for purpose if its batteries are in good working order. Without its DC power backup, a UPS system simply acts as a glorified mains.