Inverter Batteries

A complete guide to what is an inverter battery?

An inverter battery can be any rechargeable or secondary or storage battery (electrochemical power source) like a lead-acid battery, nickel-cadmium battery or Li-ion battery.  Unlike the primary battery which are used in torch cells and wristwatches, we can recharge the storage batteries several hundred times.  The ability to convert chemical energy into electrical energy and deliver it on demand and to accept electrical energy when the battery is charged (and store the electrical energy) are the prime functions of an inverter battery.   Raymond Gaston Planté (1834-1889) invented the lead-acid cell in 1859 in France. T.A. Edison invented the nickel-cadmium battery in the USA.  The most recent Li-ion battery is a collective invention over a period of a few decades.  Notable among the inventors are Prof. John B. Goodenough, Prof. M. Stanley Whittingham, and Dr Akira Yoshino.  The Royal Swedish Academy of Sciences has awarded Prof. John B. Goodenough, Prof. M. Stanley Whittingham, and Dr Akira Yoshino the Nobel Prize in Chemistry 2019, for the development of lithium-ion batteries.

Normally the inverter, which is an electronic device, is connected to the AC mains along with the battery.  When there is a power shutdown, the battery begins to supply the inverter a direct current (DC) (at 12V or higher, depending on the design of the inverter), which is then converted to alternating current (AC) by stepping up the DC voltage to AC voltage of 230 V. It also regulates the voltage, current and frequency. And as soon as the mains power is resumed, the charging circuit wakes up and begins to charge the battery.  The inverters normally do not charge the batteries fully. The maximum charging voltage is limited by the manufacturers and it is in the range of 13.8 V to 14.4 V for a 12 V battery.

What is the difference between an inverter & a rectifier?

The difference between an inverter and a rectifier is the latter converts AC into DC (example, battery charging) and the former DC into AC (home inverters).  Converters/rectifiers are capable of changing the output voltage, for example, from 230 to 110 V AC and vice versa.  It necessitates this because of unique countries using varying mains supply voltage.

What is the difference between UPS & inverters?

 Inverter and Uninterrupted Power Supply (UPS)

The major difference between the inverter and uninterrupted power supply is the switchover time. Switching time is of two types: change over time from mains to back-up and vice versa.   In UPS it is only a few milliseconds (average 8 ms), which one will not realise in practice whereas in inverters, it will be several milliseconds (during which the connected electrical and electronic items would be switched off. When the inverter begins to supply current, all the items will be switched on, for, example, fans and lights (and not the computers, which requires manual switching on).

A UPS is typically used to protect essential hardware such as computers, servers, data centres, telecommunication equipment and other electrical equipment where an unexpected power disruption could cause loss of data or corruption of files. UPS units range in size from units designed to protect a single computer (using, for e.g., 12V/7Ah VRLA battery) to large units powering entire office equipment.  Higher capacity UPS’ use higher voltage and higher capacity systems from 48v to 180v and 40Ah to 100Ah batteries.  Telecom towers use 48v battery bank systems for UPS.

The usage time of most uninterruptible power sources is relatively short (10 to 20 minutes) but sufficient to start a standby diesel generator allowing properly shutting down the protected equipment. UPS also gives protection against mains supply abnormalities like surge, voltage fluctuation, spike, noise, etc.

What are batteries? How do batteries work?

A battery is an electrochemical device that can convert chemical energy kept stored in its active materials to electrical energy with the help of oxidation-reduction reactions. Batteries are classified as primary and secondary batteries, depending on whether the reactions in the cell are reversible or not.

The difference between a primary and secondary cell is that in the primary cell the reaction is irreversible whereas in the secondary cell the reaction is highly reversible to such an extent that almost the same output can be obtained after recharging the secondary cells in the reverse direction. Thus while a primary cell has to be discarded once it is exhausted, the storage cells can be recharged again and again, several times until their capacity falls to 80% of the rated capacity.

The ubiquitous lead-acid battery, still used as a starter battery in cars, was studied by Wilhelm J. Sinsteden as early as 1854 and demonstrated by Gaston Planté in 1859–1860.  The battery has a working principle similar to the voltaic pile exposed to air but was the first so-called secondary battery that could be recharged. The term secondary was derived from early studies by Nicolas Gautherot, who in 1801 observed short secondary currents from disconnected wires used in electrochemical experiments.

The term primary refers to the fact that the source of energy is within the active materials contained in the cell and that the term secondary implies the energy contained in the cell was produced elsewhere. Some authors say the term secondary was derived from early studies by Nicolas Gautherot, who in 1801 observed short secondary currents from disconnected wires used in electrochemical experiments.  Fuel cells although are similar to batteries, the active materials are not stored inside the battery, but are fed into the fuel cell from outside whenever power is required. The fuel cell differs from a battery in that it has the capability of producing electrical energy as long as the active materials are fed to the electrodes.

All batteries are constructed in a broadly similar way and also work in a similar manner. The fundamental unit of a battery is a “cell”.  There is a positive pole and a negative pole visible outside the battery, marked clearly with + or – sign and mostly painted with red and green colour.  Inside each cell of the battery, there are a few positive plates (say “n” number of positive plates) connected to a common bus bar or connector strap.  Likewise, there are a few negative plates (say “n+1” number of negative plates) connected to a common bus bar or connector strap. Separating these positive & negative polarity plates are insulating porous sheets called separators (2n in number), which prevent electronic contact between the opposite polarity plates but allow ions to flow through them.  There is another important component called “electrolyte” which helps in ionic conductions. Usually, it is a liquid electrolytic conductor, either an acid or an alkali.  The valve-regulated lead-acid battery (VRLAB) may also come equipped positive plate with a gelled semi-solid electrolyte or with electrolyte fully absorbed in highly porous absorptive glass mats (AGM) to make the battery Unspillable. The latter type batteries require no periodic addition of water to make up for the loss of water due to electrolysis and are also fitted with a one-way release valve to protect them from a build-up of excessive internal pressures.  If it is a non-aqueous battery like Li-ion battery, the electrolyte will be a mixture of organic liquids or the same may be gelled (gelled electrolyte) or maybe a solid porous membrane (solid electrolyte).

Which inverter battery is best for home application? Flooded flat plate or tubular?

The flat plate battery is inherently a short-lived battery.  Even though the flat plate inverter battery is designed with thicker plates than the ordinary flat plate batteries, the life is poor in comparison with tubular batteries. Tubular batteries offer robust performance, recover quickly from deep discharges & have a very long life.

Hence, the tubular battery is the best inverter battery. Prefer to buy tall tubular batteries instead of short height batteries if space is available.

The SMF batteries are sealed maintenance-free batteries. Also called VRLA Battery it operates on the principle of oxygen recombination chemistry. Read more about VRLA batteries here.

Compared to flooded tubular inverter battery, the cost of the VRLA SMF Battery is more expensive.

SMF batteries should be charged at 14.4 V to compensate for the sulfation occurring inside the VRLA SMF battery while the oxygen cycle is operating and to maintain the battery in the best state of health (SOH).  But most home inverters are designed to charge at 13.8 V.  So the charging would be insufficient and after a few months, the SMF battery may not deliver its original back-up time.

The oxygen cycle process inside any lead-acid battery is an exothermic reaction.  An exothermic reaction generates some amount of heat.  This will tend to reduce the operating life because the heat dissipation property in an SMF battery is not as good as in a flooded battery due to the starved electrolyte design, with exact volume of acid inside the absorbent glass mat separators. The tubular battery have plenty of flooded electrolyte available always keeping it cooler.

Therefore, a flooded tubular battery is best suited for a home inverter.  Here, even though it is flooded battery, because of the low antimony alloy and calcium alloys,  the frequency of topping up is far away from the subsequent top-up.  A properly designed battery used in modern batteries like the Microtex Inverter battery, will not require water addition even after 18 months, though the electrolyte level may go down, it will be within the permitted lower level of electrolyte.

Are tubular gel batteries better than SMF for home inverters? 

By far, the tubular gel battery is the best-suited one for inverter applications, be it home inverter or solar photovoltaic inverter.  Since both the Gel tubular and AGM batteries are of the Valve-regulated type, they should be charged at 14.4 V (for a 12V battery). 

Will VRLA batteries be charged properly with my inverter setting?

However it is not a commonly known fact that most home inverters have a charger setting of 13.8v. Normally, 13.8 V will not be sufficient for maintaining the VRLA battery in the best state of health (SOH).  If there is a provision for boost charge in the inverters, occasional higher voltage (14.4 V) charging would help in prolonging the VRLA battery life by removing the sulfation effects.  Or a bench charge once in 6 months will help in alleviating this problem, even though it may be cumbersome.

How to calculate battery capacity for the inverter? How to calculate battery size for my inverter? what should I do to correctly estimate the load?

For a home inverter, the total power connected to the inverter or UPS will help calculate the capacity of the battery needed.  In addition, the design of the inverter also plays a part; the inverter system voltage is important. For instance, if the inverter uses one number of 12V battery, the capacity of the battery may be 150 Ah.  But if it uses 2 numbers of 12V batteries, the capacity of the battery gets halved.

The parameters required for arriving at the capacity of Inverter batteries are:

  1. Inverter capacity (VA)
  2. DC conversion efficiency (~ 0.90) and
  3. Power factor (cos θ, 0.80).

DC power required = inverter capacity x Cos θ / power factor

  • = 500 *0.8/0.9
  • =  444 W
  • Direct current required for 1 hour =  W/ Mean voltage = A
  • = 444/ (12.2+10.8/2) = 38.6 A
  • Energy  required for 1 hour = 38.6 * 12*1 battery = 444 Wh
  • Energy  required for 3 hour = 38.6 *3* 12*1 battery = 1390 Wh

Therefore the usable battery capacity is 1390 Wh/11.5 V = 120 Ah. One has to understand that this 120Ah is to be delivered over a period of 3 hours, which is equivalent to saying that we want 120 Ah battery at 3 hour rate.

A battery rated 100Ah at 10 h rate can give ~ 72  Ah at 3 hour rate (please refer to table below)

  • So, if we require 120 Ah, then 120/72 *100 = 1.67 *100 =167 Ah battery at 10 h rate.
  • One can select 150 Ah or 180 Ah battery to get a continuous supply of   444 W for a period of 3 hours

If the battery is rated at 20 h, then 15 % extra capacity is to be added to the requirement (conversion factor from10 h to 20 h capacity). 

  • Then the 20 h capacity battery will be 150*1.15 = 173 Ah
  • Then the 20 h capacity battery will be 180*1.15 = 207 Ah

Hence batteries rated at 20 h capacity will be Ah or 200 Ah

How to calculate the load for the inverter?

The foremost point to remember before placing an order for an inverter is to calculate the maximum load for the inverter.  The following can be taken as approximate guidelines:

The following table gives the approximate power consumptions of different electrical gadgets:

Electrical equipment Power consumption (W) Power consumption with power factor, 0.8 included
Tube Light 40 =40/0.8 = 50
Ceiling fan 60 =60/0.8 = 75
Computer 200 =200/0.8 = 250
LED TV 32" 55 =55/0.8 = 70
LED TV 42" 80 =80/0.8 = 100

If one wants to use

  1. 1 Tube light = 50 W
  2. 1 Ceiling fan = 75 W
  3. 1 Computer with 32” LED monitor = 70 W
  4. LED lamps 7W * 8 numbers =56/0.8 = 70 W

The total load = 265 W

The average duration of usage is assumed to be 2 hours.
Current for this Watts = 265/12 = 22 Amperes
Therefore we require = 22 amperes for 2 hours
From the table, we see that
if we require 44 Ah, then 44/63 *100 = 0.7 *100 =70 Ah battery at 10 h rate.
One can select 75 Ah battery to get a continuous supply of   265 W for a period of 2 hours.
The current is then = W required/ V of the system
Ah required = (W/V)*hours for 2 hours
So we have to see the 2-hour capacity.  Normally 2 h capacity = 63 %
[(W/V)*h]*Capacity factor. The capacity factor depends on the hours of usage
[265 W/12 V*hours of usage]/0.63 for 2 hours assuming full usage of 265 W.
[265 W/12 V*hours]/0.72 for 3 hours

For others, please refer to the table below.
Rate of discharge, cut-off voltage and per cent capacity available from a tubular battery (Conventional) [IS: 1651-1991. Reaffirmed 2002

Discharge Rate, hours Final discharge voltage, (Volts/cell) Percentage of capacity (100 at 10h rate)
1 1.6 50
2 1.6 63.3
3 1.7 71.7
4 1.8 78.2
5 1.8 83.3
6 1.8 87.9
7 1.8 91.7
8 1.8 95
9 1.8 97.9
10 1.8 100
20 1.75 115

How to calculate battery back uptime?

This aspect is the reverse of the point discussed just above. We have already got a battery for the inverter. Now we want to know how much backup time it may deliver.

The following points are to be provided or to be assumed:
Battery voltage and capacity (12V/150 Ah10 assumed)
Connected load in Watts (3 tube lights, 2 ceiling fans, 5 Nos. of 7 W LED lamps. Total wattage = 120 +120+35 = 275 W).
Duration to be calculated.
The DC wattage = AC wattage 275/0.8 = 345 W
Current = 345/(12.2+10.8) = 345/11.5= 30 Amperes

By carefully scanning the above Table it can be found out that a 100 Ah battery can deliver about 78.2 % Ah for 4 hours. So 150Ah battery can deliver 150 x 0.782 = 117.3Ah at C4. So 117.3 Ah /30 A = 3.91 hours = 3 h 55 minutes 

How to calculate solar panel battery & inverter size?

A regular or normal inverter is an electronic device that uses switching, control circuits and transformers to convert direct current from a battery to alternating current. This is the basic principle of every inverter.
The inverter takes the DC power from batteries and then converts it to AC power that is used by appliances. The inverter battery and inverter are usually connected to the power connection of home. When the power is available in a network or grid, the batteries are charged and when the power is not available, the inverter switches to the battery mode and allows you to use appliances and other essentials. 

UPS and Battery installation chart

A solar inverter consists of solar photovoltaic (SPV) panels, a charge controller, some switching circuits and batteries and inverters. It has terminals for connecting the battery and solar panels.  The inverter battery is charged from the output of SPV panels when the Sun is bright.  The current generated by an SPV panel fluctuates depending on the solar insolation.  In a solar inverter, the SPV panel produces variable direct current (DC). The inverter converts this direct current into alternating current supplies to the loads in homes. Here, there is no grid-tied mains supply.  This home depends solely on Sun and batteries

Rooftop Solar home

Now it is clear that normal or regular inverter is a simple circuit with a battery and an inverter or UPS. 

Whereas, the Solar photovoltaic  inverter receives DC from solar photovoltaic panels when there is sunshine and stores this energy in the batteries.  On-demand (that is when a bulb or a fan or a TV is switched on), the battery delivers power through the inverter.  Since the solar power produced during sunshine hours is fluctuating (because it depends on the intensity of solar irradiation) there is a charge controller between the SPV panels and the battery.  The SPV panels can also be directly connected to the SPV inverter so that during sunny times a part of the solar power can be used by the loads.

How to calculate the back-up time?

When we say a tube light consumes 40 Watts, it refers only to AC watts, as we are getting only AC supply for our homes. But when we talk about inverter and battery it is DC. To convert AC into DC we have to take into consideration the conversion efficiency, which is approximately 80%. So this 40 W AC bulb will consume 40/0.8 = 50 Watts. Similarly, for fans, 60 W AC = 75 W DC.
Now, without worrying about these calculations, simply
Add the AC power requirements of all appliances and divide by 0.8.
We get the DC power required.
Now, we have to take into consideration the number of 12 V batteries connected to the inverter.
If we divide the value (DC power got in point “a”) by 12 (1 No. Of 12 V battery), we get the DC current to be obtained from the battery.
Now decide about the time of usage of the electrical appliances, say 3 or 4 hours.
Multiply the DC current value obtained in “d” above by 3 or 4. We get the ampere-hours (Ah) required of the battery at 4h rate or C4 rate. Now C4 refers to the capacity obtainable from the battery over a period of 4 hours.
(Note: Do not get confused with the term 4C, which for a battery of 100 Ah capacity, refers to a value of 400. 4C A = 400 amperes current. C stands for capacity and so 4C = 4 *C= 4*100 = 400. But C/4 is different. Its value is 100/4= 25. Likewise, C4 refers to capacity at 4-hour rate, similar to C20 or C10 )
Now, from the table above, find out the capacity of the battery which can deliver the required capacity at 4 h rate.
Worked Examples:
Example 1:
DC power required = 200 W………………….. Point “a”
Current from a 12 V battery = 200/[12.2 +10.8)/2]…. Point “d”
(Watts/Volts = Amperes) = 200/11.5 = 17.4 A.
Usage duration 2 hours. So Ah = 17.4* 2 = 34.8, Say ~ 35 Ah
(Amperes * hours = Ampere hours, A*h = Ah)
Now it is clear that we require 35 Ah at 2-hour rate (C2 rate).
From the table, we find out the 2 h capacity. It is about 63 % of C10 capacity. So divide the Ah value 35 by 0.63, we get the required C10 battery capacity.
Battery C10 Ah capacity = 35/0.63 = 55.6 Ah ≅ 60 Ah at 10 h rate
Battery C20 Ah capacity = 35/0.63 = 55.6 Ah ≅ 55.6*1.15 = 64 Ah at 20 h rate.
We can see that for lower wattages and lower durations, the difference between
C10 and C20 are almost negligible.
Example 2:
DC power required = 600 W………………….. Point “a”
Current from a 12 V battery = 600/[12.2 +10.8)/2]…. Point “d”
(Watts/Volts = Amperes) = 600/11.5 = 52.17 A.
Usage duration, 4 hours. So Ah = 52.17* 4 = 208.68, Say ~ 210 Ah
(Amperes * hours = Ampere hours, A
Now it is clear that we require 210 Ah at 4-hour rate (C4 rate).
From the table, we find out the 4 h capacity. It is about 78.2 % of C10 capacity. So divide the Ah value 208.68 by 0.782. We get the required C10 battery capacity.
Battery C10 Ah capacity = 210/0.782 = 268.5 Ah at 10 h rate.
We can use a 12V/270 Ah battery or two numbers of 12V/135 Ah batteries in parallel.
Battery C20 Ah capacity = 268.5*1.15 = 308.8 Ah at 20 h rate.
We can use a 12V/310 Ah battery or two numbers of 12V/155 Ah batteries in parallel
We can see that for higher watts and longer durations, the difference between
C10 and C20 are significant.

How to calculate solar panel battery & Inverter size? (Off-grid)

The same calculations hold well for solar panel battery, except that we have to take into consideration the no-sun days (also called sunless days or days of autonomy).

Invariably, all solar battery designers take 2 to 5 sunless days.  So the capacity of the solar panel battery required for off-grid Solar Photovoltaic system will always be double or three times the normal household inverter battery capacity.  As the term indicates, no-sun days or days of autonomy means that the Solar Photovoltaic battery can take care of the load even in the absence of sunless or fully rainy days, during which the batteries could not receive the required charge input from Solar Photovoltaic panels. 

Solar inverters will have more than one battery to take care of what is called no-sun days. The Solar panel batteries may be connected in series or parallel or series-parallel fashion, depending on the design of the inverter and its capacity

An extra component in the form of charge regulator is also required. In a solar inverter, the SPV panel produces variable voltage direct current (DC).  The current generated by an SPV panel fluctuates depending on the solar insolation.  A charge controller or charge regulator is basically a voltage and/or current regulator to protect batteries from overcharging. It regulates the voltage and current output from the solar panels going to the battery. Most “12 volts” panels generate 16 to 20 volts. Hence if there is no regulator, the batteries will be damaged from overcharging. Most batteries need around 14 to 14.4 volts to get fully charged in Solar Photovoltaic applications, which is well suited for AGM as well as solar gelled tubular batteries.

How to charge an inverter battery correctly?

The inverter battery gets charged in the inverter system itself. But it is a voltage-limited charge. The charging voltage is prevented from going higher than 13.8 V for a 12V battery.
At this level of charging voltage, the lead sulfate in both the positive and negative plate does not get converted to the respective active material, namely, lead in the negative plate and lead dioxide in the positive plate. The electrolyte stratification may also occur in tall type batteries of the flooded type.
To mitigate or avoid these problems, the inverter battery should receive a full charge (bench charge) once in a year initially and once in six months after 2 years.
During a full charge
All the cells should gas copiously and uniformly.
The charging voltage should reach 2.65 to 2.75 V per cell or 16.0 to 16.5 for a 12 V battery.
The specific gravity must attain a constant value. This point indicates that almost all the lead sulphate in the plates has been converted to the respective active materials. So there is no lead sulphate in the plates and the battery will be able to deliver full capacity. It may please be noted that as the temperature rises towards the end of the charge, the specific gravity value will come down. For example, if the specific gravity measured at a temperature of 45ºC is 1.230, it is actually 1.245 at 30ºC. So, if the specific gravity is required to be 1.240 at 27ºC, its value at 47ºC will be 1.225. So one should not be misled by the lower value of specific gravity at higher temperatures.
While charging several batteries in series, it should be ensured that the source rectifier has sufficient voltage rating. A 12 V battery may require a voltage of 18 to 20 V to take care of the losses in the cables and the resistance offered by the batteries. If it is only 16 V per battery, the current will begin to drop as the battery voltage begins to rise as a result of charging. The extra voltage will take care of this aspect

How do I know if my battery is faulty or if the inverter is not charging my battery?

When the battery is not able to provide the required back-up time during a power cut, we have to locate the fault by measuring the terminal voltage of the battery.  If the voltage is above 12.6v to 12.8v as soon as the battery begins to deliver energy for the fans and lights, it is perfectly all right. After about 10 minutes of a power cut, the terminal voltage value maybe 12.2v or so, depending on the battery capacity and the load.  If it drops to lower than 12V immediately, we have to suspect the battery. In such a situation, the back-up time will be only a few minutes.

Next, we have to measure the specific gravity of the cells, if possible.  If it is near about 1.230, then also it is ok.  If the specific gravity is far lower than 1.230v, it indicates that the battery has not been receiving sufficient charge.  We have to find out whether it is due malfunctioning of the inverter charge circuit or due to sulphation.  This can be done after the power resumes. The voltage should immediately jump to above 12.2V from a value of 11.5V or so. Slowly and regularly, the terminal voltage of the battery should rise to 13.8v or more.  The time taken to reach the 13.8v level will depend on the battery capacity and the charger input amperes.
If the voltage does not rise as described above, it may indicate a faulty charge circuit.  However, if the battery gets warmed up unduly, a short-circuit inside the battery may be a reason.  This has to be decided only in a fully equipped service station by opening the cover and examination of the elements.
It is better if a digital voltmeter is supplied along with the inverter and battery as shown in the photo above.
There is a practical way to decide the culprit. All these can practically be found out by replacing the inverter battery first and then the inverter or the inverter first and battery later.

How many batteries can be connected to my inverter? My dealer asks me to use 4 batteries can I use 2 batteries? what will happen?

The inverter is designed for a particular voltage, say 12V, 24V 48V, 120V, etc.  Most of the home inverters or UPS have 12V design. If you connect more than one battery to this inverter, the electronic circuit will immediately burn and the inverter gets destroyed. So, before connecting the battery, one has to read the nameplate or the instructions given with the inverter.   
If the dealer asks you to connect 4 batteries, then it may be designed for 48V.   If the inverter is designed for 12V, he would have meant to connect them in parallel to increase the back-up time.
If the inverter is designed for 48v, then he may be meaning to connect them in series.  But if you connect 2 batteries only, the inverter will not function.  No damage will occur to the inverter.

How many batteries for a 1KVA inverter? 2 KVA inverter? 10KVA inverter?

Always refer to the inverter manual for connecting the right number of batteries to the inverter. Following information is for reference only:

  • 1 to 1.1 kVA = 12 V (1 Number of 12 V batteries)
  • 1.5 to 2 kVA = 24 V (2 Numbers of 12 V batteries)
  • 7.5 kVA = 120 to 180 V (10 to 15 Numbers of 12 V)
  • 10 kVA to 15 kVA = 180 V to 192 V (15 to 16 Numbers of 12 V batteries)

If you have further questions please feel free to ask us!

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