INVERTER Battery Buying Guide
Inverter battery plays an important role in determining the performance of the inverter. So you should check the battery specifications too when deciding on the inverter to purchase or when the old battery becomes unusable.
INVERTER BATTERY SIZE CALCULATIONS
Generally, the size of the battery is determined by the wattage load and the projected runtime. Using a battery that is not strong enough can cause the inverter not to power up and may lead to discharging issues that could permanently damage the battery.
The formula mentioned below can be used to calculate the battery size:
Battery size = Inverter rating in watts ÷ input voltage x usage time (hrs)
e.g. 300W ÷ 12V x 5 hours = 125 Amp
Some other formulas that you should know are:
- To Convert AMPS to WATTS:
WATTS = AMPS X 120 (AC voltage)
The result is a ballpark figure of wattage required for continuous load of device.
- To calculate approximate Startup Load:
Starting Load = WATTS X 2
The result is a ballpark figure of wattage required for starting load of most appliances.
Devices like air conditioners, refrigerators, freezers and pumps may have a start up surge of 3 to 7 times the continuous rating.
- Formula to convert AC Watts to DC Amps:
DC Amps = AC Watts divided by 12 x 1.1
- Continuous rating:
Maximum combined wattage + 20%.
AMPERE HOUR OR AH RATING
Ampere Hour is a measurement of electrical capacity the amount of energy the battery will store. Current multiplied by time in hours equals ampere-hours. This is the most important measure to be looked at when choosing a battery for an inverter.
A current of 50
amps for one hour would be 50 AH at the 1hr rate; a current of 30 amps for 5 hours would be 150 AH at the 5hr rate.
AH ratings vary with temperature, and also with the rate of discharge. For example, a battery rated at 100 AH at the 6-hour rate would be rated at about 135 AH at the 48-hour rate.
The ratings during actual use can change due to effects of temperature and internal resistance.
INVERTER BATTERY DISCHARGE RATE
C10 is the available capacity of the battery when discharged over 10 hrs. C20 is for 20Hrs and so on. These indicate how many amperes the battery will supply for 10 and 20 hours without dropping below a certain voltage. So, lesser the discarge rate, the lesser electricity you will be able to draw for the designated period.
TYPES OF INVERTER BATTERY
Lead-acid batteries are the oldest type of rechargeable battery. Most of the inverters batteries are lead acids battery of different types.
These batteries need maintenance and distilled water to be filled periodically. The traditional engine start and traction style battery. Liquid electrolyte is
free to move in the cell compartment User has access to the individual cells and can add distilled water as the battery dries out. Popular uses are engine starting and deep cycle designs.
Deep cycle battery
The deep cycle batteries are specifically designed to be less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged like uninterruptible power supplies. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging.
They are also known as traction batteries or golf cart type. They should be used in order to be able to handle the repeated discharge/charge cycles and hence are more suitable for applications.
Based on Design
a.) Pure Plante
The Plante positive plate batteries can provide long life but at a relatively high expense than flat plate or tubular batteries and hence find less use nowadays.
b.) Flat Plate
The positive plate is a rugged lead alloy grid which is filled with a specially compounded paste active material
c.) Tubular batteries
The positive plate is composed of a series of parallel polyster tubes filled with lead oxide, this prevents 'Shedding'. They are recommended for back up power for UPS and Inverters where environmental conditions are tough and high ambient temperatures are common. These are capable of long hours of backup.
These batteries are generally maintenance free. This term can refer to a number of different constructions, including only a slight modification to the flooded style. Although user does not have access to the cell compartments, the internal structure is still basically the same as a flooded battery. The only difference is that the manufacturer has ensured that a sufficient amount of acid in the battery to sustain the chemical reaction under normal use throughout the battery warranty period.
a.) VRLA battery
Valve Regulated Lead Acid battery - this is a sealed battery. The valve regulating mechanism allows for a safe escape of hydrogen and oxygen gasses during charging and hence no seperate room is needed for keeping the battery.
b.) AGM battery
Absorbed Glass Matt battery - a sealed battery.
Newer sealed battery with "Absorbed Glass Mats", or AGM between the plates. These batteries have all the advantages of gelled batteries, but can take much more abuse.
These are also called "starved electrolyte" or "dry" batteries because the glass mat is only 95% saturated instead of being fully soaked, which means that they do not leak acid even if the casing is broken.
Generally AGM batteries have about 20% less capacity, cost about two times more, and have a shorter cycle life than comparable flooded lead acid batteries. However, AGM batteries do not need watering, are safer (no acid spilling out), can be placed in a variety of positions, have a slower self-discharge characteristic, and are more efficient in charging and discharging than flooded batteries. AGM batteries are more for light cycling applications where power cut duration are low.
c.) Gel battery
The gel cell is similar to the AGM style because the electrolyte is suspended, but different because technically the AGM battery is still considered to be a wet cell. The electrolyte in a GEL cell has a silica additive that causes it to set up or stiffen. The recharge voltages on this type of cell are lower than the other styles of lead acid battery. Probably the most sensitive cell in terms of adverse reactions to over-voltage charging. Gel Batteries are best used in VERY DEEP cycle applications and may last a bit longer in hot weather applications. If the incorrect battery charger is used on a Gel Cell battery poor performance and premature failure is certain.
COMMON MISTAKES WITH LEAD ACID BATTERIES
Undercharging: Generally caused by not allowing the charger to restore the battery to full state of charge after use. Continually operating the battery in a partial state of charge, or storing the battery in discharged state results in the formation of lead sulfate compounds on the plates. This condition is known as sulfation. Both of these conditions reduce the battery's performance and may cause premature battery failure. Undercharging will also cause stratification.
Overcharging: Continuous charging causes accelerated corrosion of the positive plates, excessive water consumption, and in some cases, damaging temperatures within a lead acid battery. Deep cycle batteries should be charged after each discharge of more than 50% of the batteries rated capacity, and/or after prolonged storage of 45 days or more.
Under watering: In deep cycle, lead acid batteries water is lost during the charging process. If the electrolyte level drops below the tops of the plates, irreparable damage may occur. Water levels should be checked and maintained routinely.
Over-watering: Excessive watering of a battery results in additional dilution of the electrolyte, resulting in reduced battery performance. Additionally, watering the battery before charging may result in electrolyte overflow and unnecessary additional maintenance.
HIGH OR LOW D.C. VOLTAGE?
Higher-voltage systems tend to be more efficient and put a lower load on the batteries. Factors other than the battery enter into the system's overall efficiency.
TEMPERATURE FOR CHARGING BATTERIES
When charging lead acid batteries, the temperature should not exceed 52oC intermittently or 45oC. At this point the battery should be taken off charge and allowed to cool before resuming the charge process.
CHECKING IF BATTERY IS BAD
To determine if the battery system is experiencing a problem, fully charge the batteries then shut off the charger and remove all electrical loads. Allow each battery in the system to stand on open-circuit for about one hour. Measure the voltage of each battery. If the battery voltage spread exceeds .30 volts for a 12 volt battery, a problem is indicated. Battery voltage alone does not confirm a problem. When the voltage spread indicates a problem, confirmation is accomplished by taking electrolyte specific gravity readings using a hydrometer. If the specific gravity readings show a spread greater than .030 (30 points), give the batteries equalization charge i.e slow constant current charge @ 3% of the battery capacity in amperes.