This guide explains how a battery backup works, how to specify an inverter and how to size a battery bank for backup applications.

2. How to build a battery backup
© MathyMpassyIsinki 2016
Mathy is an accomplished off grid energy solutions business and technical sales professional technically competent in the
applications and installation of off grid photovoltaic systems, as well as in hybrid energy solutions combining photovoltaic
and diesel generators.
In addition to providing solutions design, customers development and project management services to the off grid solar
industry, Mathy contributes to better energy solutions through training, with the vision of building a better future for under-
served communities in off grid locations

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1. How does a backup works
A power backup is a standby power system which supplies electricity to a load in the event of main
power failure. A battery based power backup system is made of two main components:
 A battery bank storing energy and;
 An inverter converting battery bank DC electricity into AC, inverters used for backup systems
usually have inbuilt battery chargers for charging the battery from the mains power source.
A separate battery charger is required when the inverter does not have one inbuilt. This
guide will focus on backup systems using inverter chargers.
Battery based backup systems are an alternative to generators. Their advantage over generators
are:
 Silent and non polluting operations
 Instantaneous change over when mains power fails or come back
 Low running cost; just limited to the necessary power to charge the batteries.
Their main drawback compared to generator is that they only supply power for a predetermined
amount of time and, the backup time decreases as the load increases. Therefore, the battery bank
should be properly sized to meet the requirements.
In a backup system, energy is stored within a battery bank for use during periods of mains failure.
It is recommended to use deep cycle batteries.Deep cycle batteries have the capacity of being
charged and discharged hundreds of times before they wear out. Lead-acid batteries are the most
used.
Car batteries which are also lead-acid type are not recommended. However, because they are
cheap, they are sometimes used for small backup applications but, their operational life is likely
to be short: their thin plate do not allow them withstanding the deep discharge experienced by
batteries in backup applications.

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On the other hand, the inverter must provide enough power to start and run the connected load.
To maintain continuous power supply to the load, backup systems are installed between the mains
AC power supply and the load.
Battery Bank
Inverter

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2. Battery bank
Lead acid batteries are made of separate 2-volt compartments known as cells. The cells are filled
with sulfuric acid, serving as electrolyte, and, inside of each cell is a series of thick, parallel lead
plates. Insulators prevent short circuit between the plates and partition walls separate each cell
preventing sulfuric acid to flow from one cell to the next.To build up the voltage, the cells are
wired in series. Electricity can flow out or in the battery through the negative and positive
terminal.
All the cells are encased in a heavy-duty plastic case.
Lead acid batteries can be of two types: flooded or sealed. They work in two directions;
converting electrical energy into chemical energy at the charging and reconverting chemical
energy into electrical energy at the discharge. The discharge, off course corresponds to when
batteries supplies power to the load.
The thickness of plates allows multiple deep discharges of a deep cycle battery. Charging and
discharging being the cycle.

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Unlike flooded batteries which require to be filled before installing them, sealed batteries are
factory filled; they are delivered ready for use.
Sealed lead acid batteries are also known as maintenance free batteries because fluid level never
need to be checked and they never need to be filled with water.
Large applications use flooded tubular plate lead-acid batteries; sealed lead acid batteries are
used in smaller applications. Their main advantages over flooded batteries are that they don’t
require maintenance, they are less affected by lower temperature, they don’t release gases and
they charge faster. However, they are more expensive, store less electricity and due to their
hermetic design; there is risk of breaking down due to lack of internal heat dissipation.
Two types of sealed lead acid batteries are available: AGM standing for absorbed glass mat and
gel cellbatteries.
Flooded lead acid batteries on the other hand are less expensive, they store more electricity,
and they have longer lifespan and can be rejuvenated after they have been left deep discharged
for a long period of time.

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A system might require more than what a single battery can supply. This might be the
voltage or the amp-hour storage capacity.
Batteries are wired in series to build up the voltage. In a series connection, the positive terminal
of the first battery is connected to the negative of the following battery.
Installers need to connect as many batteries as required to reach the system voltage. System
voltage itself is dictated by the size of the load and its usage time. These two factors determine
the energy consumption;
 Bellow 1kWh, recommended system voltage is of 12Vdc;
 Between 1kWh and 3kWh, recommended system voltage is of 24Vdc;
 Above 3kWh, system voltage should be of 48Vdc

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The battery bank amps-hour capacity determines the backup time or how long the system would
run with a given load. Amps-hour are build up by connecting batteries in parallel. In a parallel
connection, all the positive terminals are connected together and, the negative, separate from
the positives are also connected together.
In practice, battery banks are often wired in combining series and parallel connections.

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3. Sizingthebatterybank
The System design is closely linked to the appliances the system will have to supply and how they
are used. A load estimate is necessary before sizing the battery bank.
Load estimate consist in listing the appliances that will be connected to the system and, record
on the load estimate worksheet the quantity of each type of appliances, its wattage and the
hours it will be in case of main power failure. As a result from the load estimate, the power and
energy demand are known.
Watt rating of equipment can be found on its name plate either directly or by multiplying the
volt and amperes rating which are provided when the watts are not.
Watt-hours are obtained by multiplying watt by usage time.
This step of the design process is the occasion for the designer to detect energy waste and
propose energy efficient alternatives. The goal of energy efficiency is to reduce energy
consumption without affecting users comfort. It offers the possibility of reducing the size of the
system and therefore its cost. Lower energy consumption comes with low consumption
equipment or a smarter use.
Battery bank sizing is critical:
 An under sizing the battery bank will reduce the system’s autonomy and;
 An oversized bank dramatically impact on the cost

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To determine the battery bank amps-hour capacity, you first need to determine the energy
consumption on the batteries by converting the energy needs from watt-hours toamps-hours by
dividing the watt-hours from the load assessment by the system voltage also determined from the
load assessment:
 12V for energy demand bellow 1kWh
 24V between 1kWh and 3kWh
 48V above 3kWh
After converting watt-hours (Wh) into amps-hour (Ah), you will have to de-rate for inverter
efficiency and batteries Depth of Discharge (DOD). Manufacturers DOD vary from 50 to 80% and,
accepted inverters efficiency is of 90%. For de-rating for DOD and inverter efficiency, the math is
to divide the Ah by DOD and efficiency values; in doing so, what you get is the amps-hour capacity
of your battery bank.
The final battery bank configuration will depend on the Ah and voltage of the available batteries.
Here, the number of batteries in series string and the number of parallel string should be
determined. We call string the number of batteries in series:
 The number of batteries in single string is calculated by dividing system voltage by battery
voltage and,
 The number of parallel string is calculated by dividing the battery bank Ah capacity by the
Ah capacity of the available battery.
 The number of batteries forming the bank is calculated by multiplying the number of
batteries in a single string by the number of parallel strings previously calculated.
In summarizing, the battery bank sizing goes through 9 steps:
(a) Determine the share of energy to be supplied by the batteries
(b) Determine the DC system voltage. Typically 24 or48V
(c) Calculate the consumption in amps-hour on the battery bank by dividing the watt-hours
obtained in (a) by the volts obtained in(b)
(d) De-rate the amps-hour obtained in (c) for inverter efficiency. 90% generally accepted
(e) Determine the battery bank’s capacity in amps-hour by de-rating amps-hour obtained in (d)
for battery depth of discharge(DOD)

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(f) Select the voltage and amps-hour of the single battery which will be used in wiring the
battery bank
(g) Determine the number of batteries in series string by dividing the system voltage
determined in (b) by the single battery voltage specified in(f)
(h) Determine the number of batteries in parallel by dividing the amps-hours obtained in (e) by
the amps-hour in(f)
(i) To obtain the number of batteries, multiply the number of batteries in series string (g) by the
numbers of batteries in parallel(h)
An example will make it more understandable:
(a) Let’s assume 7kWh to be supplied by a battery bank. 7kWh could be converted in7000Wh
(b) If system voltage is 48V
(c) The consumption on amps-hour on the battery bank will be calculated by dividing
7000Wh (a) by 48V (b) we obtain 7000Wh ÷ 48V = 145.8amps-hour
(d) In de-rating for 90% inverter efficiency, we obtain 145.8Ah ÷ 0.9=162Ah
(e) To determine the battery bank capacity, If DOD is of 80%, for example, we de-rate by
dividing 162Ah (d) by 80% or 0.8; we obtain 162Ah ÷ 0.8 = 202.5 as the battery
bankcapacity
(f) If we decide to use 12V 100Ahbatteries
(g) We get the number of parallel strings by dividing 202.5Ah (e) by 100Ah (6) 202.5Ah ÷
100Ah =2

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(h) We get the number of batteries in a series stringby dividing 48V (b) by 12V (6) 48V ÷ 12V = 4
(i) By multiplying 2 (7) by 4 (8) we can determine that our battery bank should be of 8
batteries of 12V 100Ah wired in 2 parallel strings of 4batteries.
4. From DC to AC: The inverter
Inverters come in 3 basics output types: square wave; modified square wave and sine
wave. Modified square wave inverters generally have good surge and continuous capability and
are usually cheaper than sine wave types. However, some appliances, such as audio equipment,
television and fans can suffer because of the output wave shape. Sine wave inverters often
provide a better quality power than the grid supply.
The inverter is first selected based on its capacity of supplying enough power to handle the peak
load. Other factors which will influence the specification are:
 System voltage which should correspond to inverter input voltage
 Inverter’s output voltage should correspond to the load nominal voltage. 240Volts in
Europe and Africa and 120V in USA.
 The inverter should maintain a frequency of 50Hz in Africa and Europe and 60Hz in USA
 The inverter should have enough surge capacity to start motors. Most inverters are able to
exceed their rated power for limited period of time. That is necessary to power electric
motors which can, at starting, draw up to five times their rated power.
 In case a generator is present, the inverter’s inbuilt control mechanism should allow
starting and switching of the generator based on predetermined parameters which
generally include battery state of charge, PV array power output presence or absence and
load level.
 The inbuilt battery charger must provide enough current to charge the battery bank within
the specified amount of time; the charging time is determined by dividing the battery bank
Ah capacity by the inbuilt inverter’s charger current.
 In case one single inverter is not sufficient to supply the peak load, check up to how much
stacked inverters could increase the power output and meet the demand.Stacking is an
option of some inverters to be connected together and work as a single unit.
Best practice in charging the inverter requires to de-rate the peak load to account for inverter
efficiency.

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5. Wiring the system
When wiring the system, you should make sure that the batteries are installed in restricted area;
either a dedicated room or an enclosure depending on the size of your system. The battery area
must be well ventilated and all the wires protected from mechanical damages.

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If using flooded batteries, provision must be made to contain any spilled electrolyte. Batteries
should not be in contact with the floor.
In preventing spark ignition and minimizing the risk of explosion, all battery interconnect and
terminal must be protected accidental short circuit; all crimps lugs must be fitted using the
appropriate tool.
In preventing excessive current from the batteries, overcurrent protection is to be provided in
each battery output conductor except where one side of the battery bank is earthed, in which
case, only the unearthed conductor requires overcurrent protection.Additionally, a main
protection giving the ability to readily isolate the battery bank must be provided.
The inverter should be mounted outside the battery enclosure and, no exposed live part is
allowed.
All the cables used in the installation should be securely fixed in place to minimize any movement.
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