Lead Acid Batteries
What is a battery?
A battery is an electricity storage device which can be found in any number of shapes, sizes, voltages and capacities.
When two conducting materials (often dissimilar metals) are immersed in a solution, an electrical potential will exist between them. If connected together through a closed circuit, a current will flow. The value of this potential (or voltage) isdependent on the materials used, giving rise to a whole family of battery types each having benefits and restrictions in use. Examples are:- lead acid, nickel cadmium (Nicad), lithium, silver alkaline.
A battery is simply a number of cells connected together with a given voltage and capacity. The more cells the higher the voltage, the larger the plates the higher the capacity (in general).
Purely for convenience, batteries are made in 12 volt blocks with 6 cells but are also available in 6 volt, 4 volt and
even 2 volt, single cell blocks.
Batteries can be connected in series to achieve whatever voltage is required (add the number of 2 volt
cells) and in parallel to achieve the capacity required (add the capacities of each parallel battery or string of
batteries) For larger systems, a number of series connected strings maybe connected in parallel with each other. This achieves both a higher voltage and capacity.
The lead-acid battery
There are two concepts in lead-acid batteries
1. Sealed or Valve Regulated Lead Acid VRLA
2. Open – Vented
There are three basic applications
1. Industrial
2. Automotive (starter i.e. Cars, commercial vehicles)
3. Traction (Electric motor drive i.e. milkfloat)
ALWAYS USE A BATTERY BEST DESIGNED FOR THE APPLICATION.
Industrial batteries
Industrial batteries are available from two distinct groups with the following features. Note: VRLA have superseded open-vented in many applications
OPEN-VENTED
• Older technology
• Require separate battery room
• Regular routine maintenance
• Separate safety requirements
• Store/use in vertical position
• Can require extensive cabling
VRLA/SEALED
• Environmentally friendly
• Use directly in office environment
• Low maintenance – “Maintenance free”
• Self-contained. Safe
• Store/use in any orientation
• Can be used internal or adjacent to load
• VRLA has in many instances replaced the open-vented type.
Choosing the correct size of battery
As mentioned earlier, batteries come in all shapes and sizes, from types no larger than a shirt button, to a battery system filling an entire room.
To find the size of battery you require you generally need two pieces of information, battery load and back-up times. (Note: other factors may also have an effect).
Battery Load
Whether you power lights, motors, electronic equipment or a toy vehicle your equipment will draw a load in AMPS. If this is unknown then the equipment will have a rating expressed in Watts which may simply be converted to Amps by dividing the value by the normal voltage of the system.
Example, You have chosen NPC for high cycle life and wish to drive a power tool rated at 120 watts 12 volts.
Load current = 120 ÷ 12 = 10 Amps
This is the time you require the battery to support the load described above and is often called Autonomy or discharge time.
Example: To power a cordless electric tool for a total of 3.0 hours before recharging.
With these two pieces of information use our selection graph to plot an intersection point from which you will determine a required size orcapacity in Amp. hours (Ah).
Charging
Correct charging of a VRLA battery is essential in optimising battery performance and life. Although a constant voltage charge should be applied, optimum charging also depends on temperature (Nominally 20°C), charge current (max 1/4 battery capacity) and ripple current (minimum). Two basic categories of charging exist.
Float/Standby
This charging method is used in applications such as emergency back-up when the battery is required only upon mains failure e.g., Alarm Panels, Emergency Lighting, UPS. In each case the battery is continuously on charge and consequently the recommended voltages are slightly lower than cyclic charging so as not to damage the battery. (Float voltage for Yuasa NP range = 2.275 volts/cell). These figures may vary between different battery types.
Cyclic
Cyclic charging is used in applications where the battery is repeatedly discharged then charged, e.g. Portable equipment, Wheel Chairs, Golf trolleys etc.
A higher charging voltage is used but should NEVER be left on indefinitely since it will overcharge and destroy the battery. (Cyclic voltage for Yuasa NP range = 2.4 – 2.5 volts/cell)
For optimum performance always recharge a battery immediately after discharging.
Battery Storage, Care & Maintenance
The Storage or shelf life of a VRLA battery is usually between 12 and 18 months at 20°C starting from a charged condition.
Warning – Never store in a discharged or partially discharged state.
Always store in a dry, clean, cool environment in a fully packaged condition.
If storage of 12 months or longer is required supplementary charging will be required.
Design Life
Float – Each battery type will have a prescribed float design life. Please be aware of this life expectancy and replace the
battery as End-of-Life approaches. Keep a reference or label the battery to show its date of installation to facilitate
replacement at the correct time. Factors other than time may affect the life of the battery and this will be indicated by
a reduction in capacity. The battery should be changed when the capacity is reduced to a level that will prevent it fulfilling its required duty. This may be well in advance of its design life if, for example, the ambient temperature is considerably above 20°C ie 30°C or more.
Cyclic – Each battery suited to cyclic use will reach End-of-Life after a prescribed number of cycles. This number is dependant upon the depth of discharge of each cycle. The deeper the discharge, the less number of cycles to End-of-Life. Depth of discharge is expressed as the percentage of the battery capacity required per duty cycle.
Battery Care
Yuasa batteries are supplied in a charged condition having passed stringent quality checks.
To ensure optimum battery performance and life, it helps to take care of your battery by observing the following:-
WARNING – Never leave a VRLA Battery in a discharged state.
If a battery has an open-circuit voltage lower than its rated value, then sulphation may well be the cause.
When a battery is left in a discharged state or for prolonged periods of storage, lead sulphate crystals begin to form acting as a barrier to recharge and will prevent normal battery operation.
Depending on the degree of sulphation, a battery may be recovered from this condition by constant current charging at a higher voltage with the current limited to one tenth of the battery capacity for a maximum of 12 hours.
Note: The applied voltage will exceed the normal recommendation and so the battery must be monitored (not left unattended) and removed from charge if excess heat is dissipated. The voltage required to “force” this maximum current into the battery will reduce as the battery recovers until normal charging can take place.
In extreme circumstances a battery may never fully recover from sulphation and must therefore be replaced.
Overcharge
Excessive charge voltages will force a high overcharge current into the battery, which will dissipate as heat, and may cause gas emission through the safety valve. Within a short period of time this will corrode the positive plate material and accelerate the battery towards end-of-life.
Under these conditions the heat produced inside the battery can lead to thermal runaway due to the increased electrochemical
reaction within the battery. The battery may swell before failing and will be irrecoverable from this state. This situation is potentially dangerous.
Temperature
The recommended normal operating temperature is 20°C.
HIGH TEMPERATURE will reduce battery service life often quite dramatically. In extreme cases this can cause Thermal Runaway, resulting in high oxygen/hydrogen gas production and battery swelling. Batteries are irrecoverable from this condition and should be replaced.
LOW TEMPERATURES will help to ensure a long service life but batteries used at low temperatures have reduced capacity.
Battery Safety
Batteries are electrically live at all times, take great care NEVER to short-circuit the battery terminals.
High D.C. voltages, are more dangerous than the mains.
Batteries are heavy, take care when lifting and transporting batteries. For weights above 24 kilos, lifting aids should be used.
Do not attempt to remove battery lid or tamper with the battery internal workings. VRLA Batteries are “low-maintenance” requiring no electrolyte top-up or measurement of Specific Gravity.
Disposal/Re-Cycling
Finally, when a battery has reached the end-of-life it must be returned to the point of sale or to a licensed battery dealer for recycling. Please observe the following points.
Do not throw batteries in a bin at end-of-life. VRLA batteries contain substances harmful to the environment so return to your supplier or take to your Council tip for disposal.
Never bury in the ground or incinerate at end-of-life. Batteries contain harmful substances making this unsafe.
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