Shape of battery capacity-life curve as a decisive decision-making parameter

The end of useful battery life is normally defined as the point at which battery capacity has reduced ca. 20 % from the nominal value. This aging is typically taken into account by multiplying the back-up time value by a factor of 1.25. The effect of capacity reduction on back-up time depends, however, on the lenght of the discharge time. If the discharge time is in the order of hours, then the 20 % reduction of capacity results in ca. 20 % reduction of back-up time. If the discharge time is in the order of 10 minutes then the effect could be ca. 50 % reduction of back-up time. This means that a 25 % increase in back-up lime is not sufficient in this case. The shape of battery capacity-life curve is more important than people normally think. Their thinking is affected by the experience gained from the use of flooded batteries: The actual capacity of a battery is often higjer than nominal after the use of 10 yrs and the reduction is very gradual even after this. In the case of VR-batteries the shape of capacity-life curve can be quite different from that of flooded batteries. The rale of capacity reduction can increase along the life and be very steep at the end of life. This raises a question whether the aging can be taken into account feasibly at all. It is also evident that the monitoring of battery condition will be more and more essential to be able to replace the batteries in time to avoid unexpected system shut downs. This paper discusses the matters referred above more in detail from the user's viewpoint and suggest actions to be taken to avoid fatal consequences.