The most common secondary or rechargebale battery is the lead-acid battery,where the lead element is used for the elecrodes and an acid used as the electrolyte. This is done in various mixes and in different ways in different kinds of batteries. Nickel-Zinc batteries which contain no toxic material,(Lead is toxic),moderately inexepensive,and with high discharge capability,may soon be a better replacement for lead-acid bateries,however they are newly introduced in the market(2009) and are yet to establish a track record.
Concepts in lead-acid batteries
- Sealed Valve-Regulated Lead Acid(VRLA) (deep-cycle batteries,but a deep cycle battery can also be open-vented,though this is rare)
- Open-vented lead acid (wet cell or flooded cell)
VRLA battery uses an immobilized sulfuric acid electrolyte, reducing the chance of leakage and extending shelf life.VRLA batteries have the electrolyte immobilized, usually by one of two means:
- Gel batteries (or "gel cell") contain a semi-solid electrolyte to prevent spillage.
- Absorbed Glass Mat (AGM) batteries absorb the electrolyte in a special fiberglass matting.
Rechargeable batteries are termed deep cycle when they can be completely discharged and recharged severally (cycled severally,hence the term deep cycle).Most deep cycle batteries can survive up to 400 total discharge and recharge cycle.However,several total discharges can sifgnificantly shorten the life expectancy of a battery. Deep cycle batteries are capable of this several deep discharges because of their thick electrodes,which is usually lead. Lead is a heavy metal,this is why most deep cycle batteries are heavy. Deep discharge of a car battery will shorten its life drastically because it has thin electrode plates which deliver high currents due to their large surfae area, in a short interval of time.Therefore car batteries should not be used with inverters.They will die in a matter of few months because of their thin electrode plates. In general, the thicker the plates, the longer the life of a battery.
Basic battery applications
Batteries are specified for various applications. These include
2.Automotive application as used in cars,trucks,machines that generally draw
large current on starting,but do not need to be continuosly discharged,just for starting.
3.Traction application as used in electric motors as sen in battery electric vehicles(e.g milkfloat)
WARNING! The wrong battery should never be used for an application.This can lead to very undesirable results and damage to equipment. Use only sealed VRLA deep cycle battery for inverter applications.
Industrial batteries come either as open-vented or sealed VRLA
Modern environmentally friendly technology
Need routine maintenance
Must be stored or used in vertical position
Can be stored or used in any orientation
Need a separate room for the battery
Can be used directly in home or office environment
Can require a lot of cabling
Can be used very close to appliance or even inside the appliance
To be used with strict safety requirements
Safe to use since it is self-contained
Due to its huge advantages, sealed VRLA technology is fast-replacing the open-vented battery concept. Majority of car batteries,though not deep cycle are of sealed VRLA technology. Top of Page
Battery Size Selection
Bateries used in inverter systems are deep cycle batteries,which are designed for long discharge cycles of up to 400 depending on the depth of discharge and most have life spans of 3 to 5 years depending on usage.These batteries can actually last up to 10 years if they usually experience low depth of discharge before recharging. The size of the battery bank determines how long your appliances can be powered by the inverter and this depends on the total loads/appliances the inverter powers.
A 12V inverter with one 12V battery has a battery bank, n of one. With two batteries in parallel,it has a battery bank of two and so on. A 24v inverter with two 12V batteries in series has a battery bank of 1.With two batteries in series,connected in parallel to another two in series,the battery bank is 2. More battery bank means more power available. See diagram of battery banks below:
An easy and approximate formular most people use for calculating battery size is given as follows:
Battery Capacity(c) = Total Load(p) * Backup Time Required(t) /[Inverter DC rated voltage(v) * depth of discharge(d) * efficiency of the inverter(e) ],e is usually between 0.70 to 0.95 and d is usually also between 0.70 to 0.95 because deep cycle batteries are never completely discharged when in use.
From above, c = p * t / [v * d * e]
However this formular does not take into account the very important Peukert’s effect resulting from battery internal resistance and current.The higher the battery internal resistance, the higher the losses during charging and discharging, especially at higher currents. This means that the more power a battery delivers(discharges), the lower its capacity in ampere hours(AH). Conversely, if it is drained slower, the AH capacity is higher.In simple terms, a battery of 100AH would deliver 10Amps in 10 hours and would be expected to deiver 20Amps in 5 hours ,but this is not so due to Peukert’s effect. It will actually deliver 20Amps for less than 5 hours due to more battery loss at this higher current.
With consideration of Peukert’s effect,the accurate battery sizing formula for most deep cycle batteries have been used in our inverter sizing and backup time calculators.
You can register to learn more from our world class seminars and trainings.
Most inverters are ‘smart’ and will shut down once discharge depth of battery is within this 70% to 95% of battery capacity. Thus for a 200AH(Ampere Hour) deep cycle battery,actual available capacity is about 140AH to 190AH. Bear in mind that battery life spans are measured in cycles, or how many times they can be discharged and recharged before they will no longer take a full charge.Depth of discharge (D.O.D.) of a battery has a major effect on its life span - discharging only 80% of the total capacity of the battery will typically get you 25% more cycles than 100% discharges, and discharging to only 20% will make the battery last essentially ‘forever’. Most deep cylce batteries can survive up to 400 total discharges. Top of Page
Summary of the steps
- Note the efficiency and D.O.D of your inverter system.(efficiency and depth of discharge are usually between 70 to 90%)
- Note the inverter DC voltage rating(v) e.g 12V or 24V or 48V or 96V and so on.
- Sum the total loads(p) you will run on the inverter
- Choose the minimum hours/time(t) you want to run the loads on the inverter
- To determine the required battery size(c),apply the backup time calculator in reverse manner,which means you will keep increasing the no of batteries or size of battery or both, until you get close to the backup time you require.
Inverter efficiency of 80% and battery depth of discharge of 80% have been assumed in all examples,unless as otherwise stated.
Assuming you want to have about 6 hours on your 1KVA/12V inverter while 1TV,2 fans and 6 light bulbs are turned on,then we have total load as 360W.
The 12V rating of the inverter means that you need 1 or 2 or 3 or 4 e.t.c of 12V battery. So you can keep increasing the number of batteries or the battery size or both.
Deep cycle batteries usually come in 50AmpH, 65AmpH, 100AmpH, 150AmpH,155AmpH, 200AmpH and 260AmpH.
From the Backup Time calculator, 1 of 260AmpH battery provides 7hours of backup time,
2 of 150AmpH battery provides 9.94hours of backup time
and 2 of 100AmpH battery provides 6.63hours of backup time
Thus you select any of the above with due cost considerations,perhaps 2 of 150AmpH battery
If a load of 100W runs for 5hours on a battery,we would expect that a load of 50W would run for 10hours,but this is not so due to Peukert’s effect which is related to the internal resistance and current passing through a battery.In fact,the 50W load would run much longer than 10hours,say about 12 hours due to a lower internal battery power loss. Top of Page
Assuming the TV in example 3 above is turned off as it is when we go to bed at night ,and if the 12V/260AH battery is at full charge, we would now have a total load of 260W and from the backup time calculator,we would have a back up time of 10.68hours compared to 7hours .Therefore shutting down the appliances you do not need could go a long way in increasing available backup time.
It is recommended,for best results to connect together only batteries of same model, ratings and capacity whether in series or in parallel.
We have realised that the arrangement of a 1 to 3KVA inverter system is ideal for most small offices and homes where about 4 to 6 hours of public power supply is available daily for charging the batteries. In fact,somes homes/small offices where the people are so disciplined on the appliances turned on while running on inverter,a 1KVA inverter with 200AmpH battery have been quite adequate,thereby cutting down on the cost of installing the system. Really,where public power supply outage does not last for up to 10 hours in a stretch,having an appropriately selected inverter would ensure there is 24hours power supply to your appliances.
For a 5KVA/96V inverter, with eight 12V/200AH Battery in series(in order to have the 96V rating of the inverter), and for a total load of 1000W, autonomy time or backup time is 21.29hours.
With one 1-horse power AC,one freezer,one Plasm TV and 6 ligth bulbs, Total Load = 746 + 250 + 480 + 6*15 =1566W. Therefore autonomy or backup time is 11.89hours.
12V inverter needs only one 12V battery or more,but all in parallell.
24V inverter needs two 12V batteries in series or four or six or eight batteries and so on,all arranged in parallel of two in series.
48V inverter needs four batteries in series or eight or twelve or sixteen batteries and so on,all arranged in parallel of four in series.
And so on….. But bear in mind that the more batteries you have in parallel,though you will have more backup time, but the system will also take a longer time to charge the batteries fully.
In all the selections above,we have ignored cable losses and other losses which are usually small compared to all the power components considered in the calculations. Top of Page
- Never leave your batteries in a discharged or partially discharged state.The battery should be recharged immediately after discharge.
- Store your batteries in a fully charged state and well ventilated environment.If the batteries are stored for more than 6 months,they must be recharged,since batteries discharge themselves though at a small rate,when not in use.
- Store batteries in a cool dry environment. Hot environment can adversely affect battery life span.
- Avoid short-circuiting battery terminals
- It is wise never to leave battery on charge when you will be away for a long period of time.Unless your equipment and appliances need power supply,you should shut down your charging/inverter system while you are away.
- A hot battery is a potential sign of overcharging,resulting from a faulty inverter or charging system,stop charging and check your equipment. Top of Page