If you are a user or potential user of Valve Regulated Lead Acid (VRLA) batteries this article may be of some interest to you. matt jordan of yuasa answers some of the questions often asked by installers when considering the use of a VRLA product in applications such as Telecommunications, Stand-by UPS, Fire and Security, Mobility and Leisure markets. By following these recommendations, service life and performance of the product will be enhanced
What precisely is a Valve Regulated Lead Acid Battery?
VRLA batteries have been designed to maximise the use of gas recombination technology and can be used in modern office environments, removing the need for expensive purpose built battery rooms etc. They utilise the latest cell plate technology with each cell comprising a number of positive and negative, lead calcium alloy plates, which are filled with either lead dioxide (positive) or spongy lead (negative) active materials. Unlike motor car batteries the electrolyte (Sulphuric Acid) is trapped in a gel substance or, in the case of a Yuasa product, suspended in absorbent glass matting (AGM), which is located between positive and negative plates. Each cell has a voltage of 2V. Therefore, a 2, 6 or 12V battery will comprise one, three or six cells respectively. The battery container and lid are commonly manufactured from ABS (Acrylonitrile-Butadiene-Styrene) which are welded together to form a high integrity leak-proof seal. The container lid contains a number of low-pressure valves, which are designed to release excess gas and reseal automatically in the event of the internal gas pressure rising to unacceptable levels. A VRLA battery effectively recombines 99% of the gas generated in normal use at 20°C.
How can I do a simple battery sizing?
To carry out the most basic battery sizing for a portable tool for example, you must establish: (a) DC output voltage of the tool i.e. 6, 12, 24V etc. (b) load (amps) of the tool and (c) length of time (autonomy) the tool needs to be supported by the battery i.e. battery back up time. Note. If the tool is rated in watts, simply convert to amps by dividing the watts by the nominal voltage of the equipment. Determine if there are any physical constraints that may affect your selection, such as: application; environmental; temperature or dimensional restraints. Then, using Figure 1, which denotes a series of graphs based on time against current (amps) for a selection of Yuasa NP batteries, a battery selection can be made. NB. Battery sizes are normally calculated in the UK based on an ambient temperature of 20 to 25°C. At higher temperatures, the capacity of a battery increases while life expectancy decreases and, conversely, at lower temperatures, the capacity decreases.
Is it possible to increase the DC voltage and capacity (Ah) by connecting VRLA batteries together?
Voltage can be increased by connecting the positive terminal of one battery to the negative terminal of an adjacent battery. Therefore, connecting four 12V 7Ah batteries together in this manner will effectively raise the voltage across the battery from 12 to 48V. Battery capacity will remain at 7Ah. This is commonly termed ‘series’ connection. Capacity can be increased by connecting the positive terminal of one battery to the positive battery of the next. Negative terminals could be connected together in a similar manner. Therefore, connecting four 12V 7Ah batteries will effectively raise the capacity available across the batteries from 7 to 28Ah. Battery voltage will remain at 12V. This is commonly termed ‘parallel’ connection.
The connection of the batteries in series and parallel will increase both capacity and voltage. Note. It is recommended that consultation with the battery manufacturer be made before connecting “mixed” product types in series or parallel.
What does the term 20-hour and 10-hour mean?
Battery manufacturers rate capacities (Ah) against specific times (hours) to a specified end of discharge voltage. For example, a Yuasa NP product is rated at the 20-hour rate. Therefore, the NP7-12 VRLA battery will support a constant load of approximately 350mA (7/20) for 20 hours to an end voltage of 1.75V per cell (VPC). It will not support a constant load of 7A per hour for 20 hours. Discharging the battery over one hour will reduce the efficiency of the battery to approximately 60% of the battery’s rated capacity (4.2Ah in the case of a 7Ah battery) and will therefore support a constant load of 4200mA for this period.
What do the terms eg. ‘2C’ or ‘2CA’ mean in relation to a battery manufacturer’s published documentation?
Battery manufacturers often publish data that refers to the ‘C’ or ‘CA’ rates. Discharge curves being a typical example. Both terms refer to current (Amps) in relation to the capacity (C) of the battery. Therefore, for a Yuasa NP7-12 battery ‘2C’ equates to a current of 14A (2 x 7) and ‘1C’ or ‘C’ for the same battery relates to 7 Amps etc.
What do the terms ‘Standby’ and ‘Cyclic’ applications mean?
Standby refers to battery applications designed to support AC mains failure, such as emergency back up for alarm panels, emergency lighting, UPS systems etc.
Cyclic refers to battery applications where the battery is repeatedly charged then discharged, independently, from the charging source. Typical applications are portable equipment, wheel chairs and electric golf trolleys etc.
Can I use a standard motor car charger on my VRLA battery?
We do not recommend permanent use of car battery chargers, unless the charger has a VRLA setting. To maximise the service life and performance of a VRLA battery, it is essential the correct charging regime based on the specific type of application be used. Car battery chargers charge at a higher voltage than we would normally recommend for standby applications and, if these higher voltages are left continuously connected to a VRLA battery, it will cause irreversible damage. It is also recommended VRLA batteries be recharged after use. Never leave a VRLA battery in discharged state.
What is the correct charging regime?
This depends on the type of application ie. ‘standby’ or ‘cyclic’. In both types of applications, constant voltage charging is recommended. For standby applications, using the Yuasa NP range of batteries, the voltage of the charger should be set to give a constant voltage of 2.275V per cell at 20°C ambient. This relates to a value of 13.65V for a 12V block (6 x 2.275V).
For cyclic applications a higher constant charging voltage is used but should NEVER be left on indefinitely since it will overcharge and eventually destroy the battery. The voltage of the charger should be set to a level of between 2.4 to 2.5V per cell at 20°C ambient. This relates to a voltage of between 14.4 to 15V for a 12V block.
For standby or float we recommend that the current be limited to a maximum of 0.25CA i.e. 25% of the capacity of the battery in Amps. Typically for an NP7-12 this value would be 1.75A.
What sort of service life can I expect from my VRLA battery?
For standby applications, battery manufacturers produce a range of batteries that are designed to give a pre-determined service life, usually 5, 10, and 15 years if used as per the manufacturer’s specification.
Towards the end of service life, battery capacity will reduce resulting in reduced run times. Operating the battery at temperatures above ambient 20 to 25°C for prolonged periods will shorten the service life of the battery.
For cyclic applications the service life of batteries depends on (a) quantity of cycles of charge and discharge and (b) the depth of discharge as expressed as a percentage of the capacity required per duty cycle. Most VRLA products can be cycled.
What can I do if I need support on a VRLA product?
If unsure of any safety or operational aspects associated with the use of VRLA batteries, always check with the supplier or original manufacturer.