Sealed Maintenance Free batteries are widely used where minimal maintenance and space requirements are the main considerations. In UPS systems, it is used because of its maintenance free and eco friendly features. SMF batteries do not require water and acid top up and are totally maintenance free. There is no water loss during the charge- discharge cycle and the battery works on the oxygen recombination principle. SMF battery is also called as Valve Regulated Lead Acid or VRLA battery.
The important features of SMF battery are
1. It is compact and can be oriented in any position like vertical, horizontal etc.
2. There is no fume or smell from the battery since the acid is in gel form.
3. The battery can be installed in the equipment prior to transportation since there is no leakage or water spill during transportation.
4. More economical than any other forms of battery.
Based on the use, the SMF battery is classified into three types of batteries.
Types of SMF Batteries
1. 6 Volt 4.5 Ah type
These are used in Emergency light applications and telecom instruments. These are small sized with 4.5 Ampere hour current capability.
Emergency Lamp 6V battery
2. 12 Volt 7.2 Ah type
This type is used in offline UPS and can deliver much current.
12 Volt UPS Battery
3. Big type
Big type SMF battery is used in Online UPS.
New forms of SMF batteries are being designed to get better performance and cost effective. Some of the specially designed batteries include Deep discharge battery for longer cyclic life, high discharge battery, Front access battery to monitor the terminal voltage, Gel type battery for longer cyclic life and temperature tolerance etc.
Long Cyclic life
SMF battery is totally maintenance free and has longer cyclic life, thus compensating the initial higher cost. One of the important aspects of battery life in relation to the use of equipment is the cut off voltage set in the equipment. For example in inverter systems using high current batteries like 150Ah, a cut off voltage less than 10.5 volts can shorten the life of the battery. If the cut off voltage is set as 11.5 volts, it will increase the battery life since the battery is not entering into the deep discharge state.
In the SMF battery, the chemical recombination occurs in the Positive plate as per the following equation
H2O = ½ O2 + 2H+ + 2e-
In the Negative plate, the reaction occurs as
2Pb + O2 = 2PbO
PbO + H2SO4 = PbSO4 + H2O
PbSO4+2e- + 2H+ =Pb + H2SO4
During the chemical reaction, oxygen evolved from the positive plate recombines at the negative plate and suppress the formation of hydrogen. This prevents gas accumulation inside the battery. So SMF Battery is clean and Eco-friendly. Moreover there is no free acid or electrolyte in the battery, so that the battery can be placed in any position. There is an Absorbed Glass Mat or AGM inside the battery that holds the acid. The acid is thus in an immobilized form as gel. In order to avoid excess pressure and battery explosion, vent valves are provided to release the pressure inside the battery.
How to optimize the SMF battery performance?
In order to increase the life of battery, it is necessary to monitor the battery performance constantly. Here are some tips to optimize the battery performance.
1. The SMF battery should be placed in well ventilated area to avoid high temperature environment. With each 10 degree rise in air temperature, the float life reduces to half. If the battery is installed in equipment, it is necessary to isolate it from heat generating components like transformer.
2. In order to prevent deep discharge, it is necessary to set the cut off voltage to 11.5 volts in 12 volt battery and 4.5 in 6 volt battery.
3. It is necessary to prevent no load battery drain. The additional components of the equipment such as sensors, displays and indicators consume power in the no load condition. So if the equipment is not provided with automatic charging, this power loss leads to deep discharge.
4. Boost cum float charging is suitable for SMF battery. If the power failure is frequent, the battery should be charged immediately to acquire full charge before the next power failure.
5. If there is any problem such as low backup time, battery heating, low terminal voltage even after continuous charging develops, the battery should be attended immediately.
6. If the equipment is not using for long time, it should be charged once in every week. If the battery is allowed to charge and discharge frequently, its life can be extended to 3-5 years.