Feature UPS & Standby Power Taking care of battery installations
Neglected batteries can contribute significantly to UPS failure so, with that in mind, monitoring battery installations makes sound commercial sense. Here, Mike Smith, UPS sales manager at Finning Power Systems, offers some tips on how to get the best from your batteries Below: the
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PS manufacturers have claimed that at least 40% of all UPS power failures are caused by bat- teries that have been neglected, and since batteries can fail in as little as two days, careful and continuous bat- tery monitoring is just good sense. The most common method of support- ing the load in static UPS installations is parallel string, valve regulated battery arrangements. For higher power loads, they provide a more cost efficient means of providing the necessary capacity. Therefore, it is advisable to put mea- sures in place to enable continuous monitoring of the battery installation rather than relying solely on routine checks when scheduled preventative maintenance takes place. In this way, any issues which could present an operational risk and potentially result in power being unavailable to support the load, can be rectified early on.
Check design life
Battery design life is the estimated life determined under laboratory condi- tions and will be quoted at a standard operating temperature of 20°C. Typically design life is structured into four main groups. These range from a three to five year life expectancy for a standard commercial battery used to power smaller, emergency equipment, through to 12 years and longer for use in applications with much higher power and safety requirements. As a result, service life is affected by and related to the working conditions of the battery and can vary depending on the installation conditions. Alongside operating temperature therefore, other
factors to take into account include deep discharge and charger settings, in particular the ripple factor of the charge or float current. It’s also important to calculate bat- tery capacity based on end of life per- formance, when the actual capacity has reached 80% of the nominal capacity. Apply a 125% factor for age in the initial battery size calculation to ensure the duty cycle can be achieved.
Ambient temperature
Consider the ambient temperature of the battery room and size batteries accordingly. Operating valve regulated batteries at temperatures higher than 20°C will reduce life expectancy. For example, operating routinely in very hot environments of 40°C can reduce battery life to as much as a quarter of the design life at 20°C.
Standby batteries electrically ‘float’ across the DC supply in parallel with the rectifier and the load in order to provide uninterrupted power to the system, and manufacturers will state the recommended float voltage limits (defined in IEC 60896-2). In instances where ambient temperatures are ele- vated for prolonged periods, float volt- age compensation is recommended.
Run a discharge test
In order to determine actual battery capacity, discharge tests will need to be carried out. Testing will provide trend data to help identify any developing battery problems and assist with for- ward planning for battery replacement. UPS systems often offer a recharge mode with a 2-phase charge cycle.
selection and monitoring of battery installations is crucial to UPS performance
Step one will provide a constant cur- rent limited to one tenth of the battery capacity for a ten hour discharge. Step two provides a contact voltage at the maximum permissible value where the charge current regularly decreases and reaches the floating value.
Continually monitor As mentioned earlier, consider invest- ing in a system that will monitor each battery block continually, to provide status updates of any internal faults or electrical variations. Many UPS sys- tems, such as the range of Caterpillar UPS solutions available from Finning Power Systems, come as standard with a battery monitoring system. Such systems provide a high level of system safety, enable accurate mea- surement of battery back-up time and help to optimise service life. In partic- ular, monitoring battery health daily can help save on the cost of mainte- nance or replacement and prevent oth- erwise healthy batteries from being disposed of unnecessarily.
Essential functions to consider as part of a battery monitoring system should include accurate measurement of the real back-up time remaining (taking into account the age of the battery, the tem- perature and the load level), automatic estimation of remaining battery life, a battery test to detect battery function faults early on, temperature compen- sated charging to optimise battery life and an automatic battery discharge test at adjustable time intervals.
Install and maintain Finally, always ensure that the battery arrangement has been installed and commissioned correctly, taking into account the manufacturer’s recommen- dations as well as national codes of prac- tice for safe installation and operation. In conclusion, power failure is simply not an option. Standby electri- cal installations are a major invest- ment and, by virtue of the fact that they need to support the load in situa- tions where mains power is unavail- able, operators need to have confidence that they will perform whenever they are required. Careful selection and continual monitoring of UPS battery installa- tions will go a long way to providing the power resilience needed to sup- port business continuity.
Fining Power Systems
www.finning.co.uk T: 01543 465 165
Electrical Engineering JUNE 2013
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