There is often some confusion over battery design life and the manufacturers warranty period on batteries. With some batteries having a “design life” of 5 or 10 years it can lead to questions being raised if the batteries don’t last the full design life period and need replacing earlier than expected. It’s important not to confuse the two as the design life and warranty are quite different.
All batteries have a design life that meets the parameters required for Eurobat Classiffication. This means that if normal operating conditions are met consistently then the battery should last the period that it is designed for. Service life is strongly related to the working conditions of the battery, however, in practice normal operating conditions are not consistently met.
The following outlines factors effecting service life:
- Ambient temperature:- The environmental factor which most severely affects the life of VRLA batteries is temperature. Laboratory and field experience is showing that the best rule for calculating the effect of temperature, is that life is halved for each 10C rise in average operating temperature. This means that if a battery has a life of 10 years at 20C it will have a life of only 5 years when operated continuously at 30C. If the temperature fluctuates between these two levels the life will be based on a time weighted average of the fluctuations in the temperature. This generally is the main cause of failure of VRLA batteries not meeting their specified design life regardless of manufacturer.
- Float charge ripple:- Excessive ripple on the D.C. supply across a battery has the effect of reducing life and performance. It is recommended therefore, that voltage regulation across the system including the load, but without the battery connected, under steady state conditions, shall be better than +/- 1% through 5% to 100% load. Transient and other ripple type excursions can be accommodated provided that, with the battery disconnected but the load connected, the system peak to peak voltage, including the regulation limits, falls within +/- 2.5% of the recommended float voltage of the battery. Under no circumstances should the current flowing through the battery when it is operating under float conditions, reverse into the discharge mode.
- Float stabilisation ripple:-This form of ripple arises where the demands of the load are out of phase with the capabilities of the rectifier, and the battery is used to stabilise the system. Some static UPS systems behave in this manner, and the condition is more like shallow cycling. In these circumstances, normal battery characteristics no longer apply, and the manufacturer should provide the optimum operational conditions.
- Deep discharging:- It is recommended that at the discretion of the user, low voltage disconnect features should be used in connected equipment. It is however recognised that there may be circumstances, particularly for system safety reasons, where the requirements for maximum performance would preclude the use of a low voltage disconnect feature. In such circumstances it may be necessary to replace the battery following such a discharge.
- Failed Batteries:- Not replacing a failed battery in service at the time of discovery is likely to put a lot more stress on the full string of batteries, meaning the battery string could be receiving an overcharge or undercharge from the UPS. Long term this will have a detrimental effect on the batteries and shorten the service life.
So to summarise, with so many environmental factors and conditions that are beyond the user or manufacturers control effecting the rate at which a battery degrades, the design life of a battery is only a guide or an indication as to how long the battery might last. Whilst it is reasonable to expect a 10 year design life battery to stay in service longer than a 5 year design life battery, in reality the constantly fluctuating service environment that the battery operates in means that the true service life and the design life of the battery are never the same.
