VRLA Battery Maintenance, Ways To Obtain The Best Service Life.
This paper discusses techniques that will improve the service life and reliability of VRLA batteries. There are many facets that influence this and here we look at the most important items.
By improving the service life the reliability will be enhance. In a standby emergency battery system it is more important to improve the reliability rather than the service life. There are some parameters that will marginally reduce the service life but will enhance the reliability. Some care needs to be applied to obtain the correct balance.
It is imperative to start right if a long life and reliability are to be expected from the battery. To this end, competent persons should only carry out the installation. This is particularly important from a Health & Safety point. In this “time conscious world” that we live in we continually look to faster ways of carrying out tasks but this must not be at the expense of reliability. A VRLA battery is often an expensive item and the last line of defence and it is important to safeguard the Health & Safety and investment from the start. Without compromise, the following procedures should be followed at the installation stage: -
a) Upon receipt, move the consignment to a safe and secure location.
b) The storage area should be dry with an ambient temperature between +5ºC and 20ºC. The storage time is halved for every 10ºC increase in temperature. Consult the supplier for the maximum storage time that should not be exceeded.
c) When the battery is being installed, all individual cell or monobloc (unit) voltages should be measured and examine them for damage before they are installed into the enclosure or on the battery stand. Look for abnormalities such as low voltages and consult your supplier if in doubt about any values.
d) At the installation stage, ensure that the terminal connection faces are clean, free from corrosion and not damaged. Some manufacturers recommend a small quantity of corrosion resistant grease be applied to the terminals prior to assembly. Contact your supplier for further details. The units can now be positioned and connected. It is important to tighten bolts or nuts to the correct torque setting. Fires have been known to start because of incorrectly tightened connections.
e) Still isolated from the charger, but fully connected measure and record the open circuit voltages of all units. This is the “base” number used in the future. There is now a definitive voltage for the unit position in the string and its voltage that can be cross related in the future. Check that the typical unit voltage multiplied by the number of units reasonably matches the total string voltage. A 54 cell battery of VRLA cells may have a typical voltage of 2.13Vpc x 54 cells = 115V. If one cell is reverse connected, a voltage nearer 2.13Vpc x 52 = 111V will be measured.
Clearly a differential of 4V indicates a problem. The polarity should be checked if more than 2V difference is found between the typical unit voltage x the number of units and the voltage measured across the battery terminals.
f) The battery can now de connected to the charging source and all individual unit voltages should be measured and recorded typically about 1h after connecting to the charger. Also measure and record the charging voltage and if possible, the charging current. It is also a good idea to measure and record the typical unit temperature and ambient temperature round the battery.
g) It is recommended that a test discharge be carried out on the complete battery system before putting into service. This will not only prove that the battery can supply its designed power but will also prove that the charger is operating correctly when recharging.
It is important to set the float voltage to the correct value in accordance with the manufacturer’s recommendations for the battery temperature.
Significantly, the battery temperature may not be the same as the ambient temperature. If air conditioning is used to control the room temperature air circulation should be considered. It has been shown that in some instances, air conditioning may result in a temperature gradient across the battery by up to 5ºC and in some extreme cases more than this. An actual example of this was a 3-string battery system installed on a 6-tier stand where the bottom tier was 7ºC lower than the top tier. In this case the only solution was to revise the air conditioning. No adjustment of the float voltage could compensate for this difference. Typically, the temperature across the battery should be within ±2ºC of the average.
The charging source must be capable of maintaining the correct voltage within the recommended limits and preferably with temperature compensation. Ripple current should also be within the manufacturers limits. It has been shown that batteries fitted to some static UPS machines will “wear out” sooner than those connected to a typical “telecoms smoothed” charging system.
An overcharged battery will “wear out” faster and have a shorter life. However, the effects of undercharging may be even more dramatic because an undercharged battery may not supply the required power in the event of a critical supply incident and it may not be fully recharged following a discharge.
Float voltages should be checked at regular intervals and modern charging equipment often has this “built in” with the facility to send an alarm via the building management system.
The operating temperature has a significant effect on service life. Typically, for every 10ºC increase in battery temperature, the service life will be halved.
Temperature compensation of the charging voltage will mitigate some of the life lost if the temperature is greater than the optimum. However, when the battery temperature exceeds about 50ºC, depending on battery type, a significant reduction may take place, which the charging voltage may not be capable of countering. These include container-softening leading to loss of element compression because of swelling and dry out.
At the other end of the temperature spectrum, when the battery temperature is below 0ºC, the relationship between voltage compensation and temperature will not be linear and significant undercharging can result.
It is tempting to set the float voltage marginally low in the assumption that this will enhance the life because the charging current will be lower. Providing the voltage is set to within the manufacturers parameters, this is acceptable. However, a lower that recommended float voltage could lead to severe undercharging that will reduce life. Taken further, if the voltage is too low, the battery may not fully recharge after a discharge, leaving the available standby time lower than the design. Taken to the extreme, for short duration standby batteries, this can lead to no available protection because the voltage will collapse as soon as it is connected to the load.
It has often been claimed that the installation of battery monitoring will increase the life of the battery. In part this is true because faults may be detected before they become critical and suspect units may be replaced and the life of the system may be extended. The most important point in this subject is that the reliability will be enhanced.
Battery monitoring may be permanently connected or “hand held” and there are several different types available. Some measure the internal resistance whilst others are more sophisticated and measure impedance or conductance. This paper makes no recommendation for the type that should be used.
Battery manufacturers have their own recommendations for servicing batteries made by them and below are typical of what can be found in their literature. In all cases, the user should consult the supplier for specific requirements.
Monthly Service Recommendations.
Ensure that the battery float charge voltage is within the recommended limits for the operating temperature. Record the voltage and date. See Additional Information below.
Three Monthly Inspection and Records.
Ensure that the battery float charge voltage is within the recommended limits for the operating temperature. Record the voltage and date. See Additional Information below. With the battery connected to the charger operating in the normal float charge mode, measure and record all unit voltages, temperature, and as far as possible the charging current.
Six Monthly Service Recommendations.
In addition to the Three Monthly Service Recommendations detailed above it is further recommended to carry out the following: -
If the battery has been subjected to vibration, check the unit connectors, inter-row, inter-tier and end connections for correct torque tightness.
Ensure the battery is in a clean and dry condition. For cleaning and drying, use only soft cotton cloths moistened is a solution of soap and water.
Examine all units for abnormalities.
Record all service requirements that have been carried out along with any abnormalities. Report all information to the person responsible for the battery and emphasise any abnormalities and recommendations.
Extended Period Service Recommendations
After a minimum of six months of satisfactory monthly inspections, the frequency of monthly requirements may be extended to three monthly.
Similarly, after six months (2 service operations) of satisfactory operation, the three monthly service recommendations may be extended to six months. The six monthly service recommendations may be extended to annually after one year of satisfactory operation.