When designing a battery backed dc power system, it pays to remember that all forms of battery wear are cumulative on the plate structure and active materials. Because of this, lead acid batteries often do not meet the expectations of the user and can fail prematurely. It's easy in such cases to put the blame on poor quality batteries, but quite often the reason for untimely battery failure is a poorly designed or controlled charging system. A good battery charger with specific key features will ensure the expected life of the lead acid battery is obtained in practice.>
By keeping in mind and
avoiding the common causes of premature battery failure discussed below you can
expect to get the full rated life from your backup batteries. With any system,
however, you should also regularly test the backup battery to ensure that is
healthy enough to provide the necessary power when called
upon.
What damages batteries
and how to avoid these factors.
Deep discharge - Batteries should
never be allowed to completely discharge and remain connected to the load. If a
battery is discharged to below 1.8volts/cell and left connected to the load for
more than say 15 minutes, the cell with the least capacity discharges first and
suffers reverse charging as load current flows through it. This is extremely
damaging to that cell. Batteries typically will recover from this perhaps only
up to about five times.
The solution is to use a charger/system with an end-of-discharge battery and load disconnect relay (adjustable set point and self re-setting preferred). The relay will disconnect the battery from the load before damage can occur.
Battery short circuit - When the battery is short circuited for more than 15 seconds it can be quite badly damaged and the user can no longer be assured of a long and reliable float life.
Damage to the battery from a short circuit can easily be avoided by protecting the battery using fuses, circuit breakers or electronic methods, such as current-limiting the output.
High float or boost voltage - A battery can be damaged if held at an inaccurate float voltage (often caused by temperature variations in the battery) or boost voltage for any 'length of time' (say more than a week) i.e. it is gassed and vented.
Using a good quality, well regulated charger with accurate temperature compensation of float and boost voltages will avoid this problem.
High ripple current - A problem can occur when the battery and charger are left connected to a fluctuating load for several months and the load or charger produces a high alternating ripple current in the battery.
Again, use a good quality charger with a suitably low output impedance and fast transient response capabilities, thus ensuring accurate response to load changes without loading the battery. This is particularly important in installations with potentially 'dirty' mains supplies, such as in factory which use a number of variable speed drives. If in doubt, you should conduct an analysis of the mains supply to the charger to see if there is likely to be a problem.
Time - When the battery has been through more than 100 discharge cycles or equivalent and has spent 1.5 years or more on float charge it may be approaching the end of its useful life. This number of discharge cycles combined with the long float service life will have worn the battery plates sufficiently to warrant the removal of the battery from the critical application. At this point the user may wish to test the battery to destruction in order to gain valuable data and experience for reference at other similar sites/applications.
By keeping the preceding points in mind you should be able to design or purchase a reliable backup power system. By their very nature, backup power systems come into use only occasionally and sometimes in critical or emergency situations. It is important to ensure that the system is reliable and will always work when called upon. Choosing an intelligent and well designed charger that includes circuitry to avoid the common battery problems mentioned above will go a long way to ensuring the long term reliability of any system.