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101 renewable - dummy guide to lead acid batteries

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Last Updated
7th of January, 2020

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This is a dummy guide to lead-acid batteries in PV installations. Handle with care!

Solar energy as the main source of thermal and electrical energy:~

Almost all of the energy we use today in a way or another derives from solar energy. The sun can be described as an enormous fusion reactor that sends huge amounts of energy into space. A tiny part of that energy but for us the humans, still an enormous amount, reaches the surface of the earth. Since the beginning of time, man has known how to use the thermal energy of the solar rays in order to fulfill his most urgent needs: prepare the daily food.

Sun power conversion into electricity:~

Electricity extracted from the sun radiations at earth level called sun power harvesting is the result of different conversion methods and it has been possible to convert solar energy directly into electrical energy with the help of photovoltaic modules. But since the sun does not shine all the time it is necessary to store the electrical energy we harvest from the sun. This is made possible by using electrical batteries, from where electrical power can be drawn at any time during the day or in emergency situations. One of the most used in such a situation is the lead-acid battery.

Lead-acid batteries:~

Ever since the invention of the starter engine for motor cars, the lead-acid battery has been a commodity available in almost every part of the world. A starter battery for cars is made to withstand very high loads during short periods of time when the motor engine starter runs and brings the petrol engine to the optimal conditions to ignite.

What affects this type of batteries is the process of aging.

In normal operation of a storage battery there are four major reasons for the aging process:
>_ deep discharge (this gives irreversible sulphation);
>_ overcharge (this increases the corrosion velocity);
>_ low electrolyte level exposing the electrodes to air (this reduces the capacity permanently and increases the corrosion velocity);
>_ high battery temperature (this increases the corrosion velocity).

As an example, the battery life is shortened dramatically if the battery is left in a deeply discharged condition for a long period of time (more than a few days). This situation can occur if the load is too large compared to the energy delivered from the photovoltaic module and the controller does not have a low state of charge warning or does not offer a disconnect function.

In such an instance the battery shall be completely disconnected from the load or recharged by other means until the battery is fully charged again. A good controller will avoid such kind of situations and warn the user if the load is too big to handle. The lead-acid battery can of course also be made suitable for other applications than cars. To serve as a buffer battery in a photovoltaic power system there is no need for high current discharges or rapid charges. On the other hand, a battery should have a higher capacity to avoid this type of situation. This does not mean that a starter battery cannot be used in a photovoltaic system. It works very well for a shorter time ( in the range of 1-5 years) but the economics of the system is less favorable due to the fact that the numbers of charge/discharge cycles obtained from a starter battery are lower than that of a battery designed for photovoltaic systems.

The electric battery is a “container” for electricity:~

An electric battery is built of one or more cells. Each cell includes an electrolyte and two electrodes, one positive and one negative. Between both electrodes, a voltage is generated. The energy is stored in chemical form in the active masses of the electrodes. When a battery is connected to an external load a chemical reaction takes place between the electrodes using the electrolyte mass between them. The chemical energy is converted to electrical energy and current flows from the positive electrode through the load back into the negative electrode.

The batteries can be classified into two main types:
>_ Primary Batteries, which cannot be recharged (standard batteries for the radio, electric torches, etc.);
>_ Secondary Batteries, which can be recharged again and again.

In this article, we will deal with the second type of rechargeable batteries since periodical we need to store the electrical energy supplied by photovoltaic cells. The battery system we will describe here is the open or vented lead-acid battery but there are also other systems on the market. For instance, more advanced "sealed or valve-regulated" lead-acid batteries, alkaline batteries of nickel-iron or nickel-cadmium type. These batteries usually have a longer lifetime but are also much more expensive than the open lead-acid battery.

Characteristics of the open (or vented) lead-acid battery is that the small amounts of hydrogen and oxygen produced at the electrodes during battery operation can be vented to the atmosphere through small holes at the top of the battery. In a sealed (or valve regulated) battery, a special catalyzer arrangement inside the battery is used to recombine the hydrogen and oxygen back into the water. One advantage of the sealed battery is therefore that no loss of water will occur during normal operation, whereas water has to be added at regular intervals in an open or vented lead-acid battery. By using a modern PV charge controller the water loss can be minimized and the need to top up water can be reduced to once or twice a year.

SLA deep Cycle Battery Section

A closer look inside a lead-acid battery:~

Design of a positive electrode for a lead-acid battery:~

This is the most important element of a battery: the electrode. There are different designs of this type of electrode depending on what type of application the battery is addressing. Posted grid plates are primarily used for automotive applications like SLI (starting, lighting, and ignition) battery type used in conventional cars. The power capability of this type of battery is very high while the deep discharge capability is poor. Except for the SLI application, batteries with positive pasted plates are used in many applications because of their low price. As a carrier, pasted grid plates have a lead grid into which the active material is pasted. The grid is both conductor and mechanical carrier of the active mass. The corrosion stability over longer periods of time is indeterminable; thus use in stationary applications is only possible with reservation. If still, an SLI battery is going to be used in a PV system, choose a truck battery. They have thicker plates than a car battery, almost of the same thickness as special solar batteries. This will extend the battery life in a PV system significantly compared to a car battery.

Tubular plates are often used for traction batteries, i.e. for electric industrial or road vehicles. The main feature is a fairly high specific energy per volume and good capability for deep discharge. The charging time is medium, 5 - 10 hours. In tubular plates, where a lead spine is surrounded by a highly porous, plastic tube, the active mass is located between the lead spine and the tube. The high current capability of this type of electrode is nevertheless limited since one cannot reduce the dimension of the tubes. The normal tube diameter is 8 mm (discharge time 3 - 10 h), which can be reduced to 6 mm for specific higher power applications (discharge time 1 - 3 h).

Rod plates are used in batteries for lighter traction as well as for some stationary applications. Rod plates consist of vertically arranged rods. The active mass surrounds the rods and is completely enveloped in a pocket. The lead rods correspond largely to the spines of the tubular plate. Because of the construction, the utilization of the active material is high, so is also the high current capability.

Trojan Section

Design of a negative electrode:~

The negative electrodes in all types of lead-acid batteries are of a pasted grid plate design.


The lead-acid battery electrolyte is a solution of sulphuric acid in water. The specific gravity of the acid in a fully charged battery is 1.20 - 1.30 g/cm3 depending on the type. Stationary batteries have a lower concentration than traction batteries. A typical value for a fully charged SLI car battery is 1.28 g/cm3. The acid is participating in the reactions as its sulphation ions are consumed. As a result, the specific gravity is decreasing when the battery is discharged. The gravity is traditionally used for measuring the battery state of charge. But readings can be very misleading (too low values) especially during the slow charge without mixing of the electrolyte in a PV system.

During the charging process, strong acids are created on the electrode surface. During overcharge, the acid in the cell is stirred by the gas evolution, which helps the concentration equalization. If the charge rate is low and the overcharge limited, it happens that the strong (and heavy) acid does not mix with the bulk electrolyte, but accumulates at the bottom of the cell. This is often the case for tall cells (>200 mm plate height). The time for equalization will be several days. Such strong acid increases the corrosion of the electrodes, which decreases the battery life. As the acid sample is taken at the top level of the electrolyte very wrong results (too low state of charge values) can be derived if mixing has not occurred. This is most often the case in a PV system. Therefore acid density measurements in a PV system during charge is often misleading (gives too low values on the state of charge). The electrolyte is a strong acid. When working with a battery, safety glasses should always be worn. Rubber gloves should be used when working directly with the acid. Droplets or mist of acid will destroy many materials used for clothing. This is often not observed until the clothes are washed.

The concentration of the acid has to be adjusted on a regular basis by the addition of deionized water. This is automatically done when topping up the electrolyte level with the water. The acid is never leaving the battery except for very long overcharge periods when electrolyte mist can carry acid away. If the electrolyte level is too low, the acid concentration is accordingly higher, which increases the corrosion. Especially during the end of the charging process water will be separated into hydrogen and oxygen gas at the electrodes. These gasses will leave the battery in an open or vented battery. This gives a water loss that has to be replaced by distilled water at regular intervals.

A water loss of at least 0.5-1 liter per year can occur even in a small stand-alone system. A good controller can minimize this loss but a check of electrolyte level every half year is a minimum recommendation. If the water level is not adjusted in time parts of the battery plates will be above the electrolyte surface and exposed to air. In time those parts will become permanently damaged.

The water loss also has the result that the acid strength will be increased and the corrosion of the electrodes will accelerate. In cold climates, the electrolyte may freeze if the battery has a too low state of charge. This can seriously damage the battery. In the case of a stratified electrolyte, the risk of freezing is even higher. A good controller with the automatic low state of charge warning and disconnect will reduce the risk of freezing significantly. If the controller has a built-in equalization stage this will further reduce the risk of freezing.

Cell cases made of molded plastics:~

The first thing you see when you have a lead-acid battery in front of you is the case. Historically, rubber was found to be a suitable material for cell cases and covers. They were easily formed to the right dimensions and the cells were easily sealed. However, the material was quite brittle, especially at low temperatures, which made these cells sensitive to mechanical abuse. Most cell cases manufactured today are made from injection-molded plastics. Such cases are form stable and quite resistant to mechanical abuse. The cost for tooling is very high which requires quite large production volumes for each size of the battery to lower component costs. Some cell case materials may have dimensional instability. If the cell walls are bulging, the electrolyte level will deviate from predetermined values. To avoid this situation, such cells should be put into a frame, which facilitates the dimension integrity.

On top of each cell, there is a plug that in the case of a vented battery has small holes that will release gasses but will prevent acid to come out. In a sealed or valve-regulated battery, the plug contains a catalyzer and a safety valve that will release overpressure in the case of catalyzer failure or too high charging rate.

Battery boxes for small systems:~

For small systems, an extra battery box covering also the wiring and connectors on top of the battery can be recommended. This box will also take care of water and electrolyte droplets that otherwise may destroy the floor. When a battery box is used the battery can be placed indoors more freely closer to the load. Still, the requirement for good ventilation in the room should not be forgotten.

Standardized terminals and connectors:~

The positive and negative terminals are part of the cell cover. There are standard configurations with a conical layout for standardized connection cables and different layouts for special connectors. In a monoblock (e.g. a 12 V) battery, the series connection between the individual 2V cells is made inside the case, penetrating the walls between the cells. If a metal object is dropped onto the battery so that the terminals are connected, the battery is shorted which can end up in an explosion. Use only insulated tools when working with batteries. Do not wear watches with metal bracelets. When connections are made, protect the terminals by insulating material like rubber, plastics or wood to prevent shortcuts between the terminals. It is very important to keep the terminals clean from oxides to avoid voltage drop due to the higher resistance between terminal and connector. Clean terminals will also help to reduce creep currents between the terminals. Otherwise, this will increase the rate of self-discharge.

What type of battery should be chosen?~

The chart presents an overview of the advantages and limitations of different types of lead-acid batteries concerning their use in PV systems.

For a typical small PV system (10Wp to 1kWp) both the initial investment cost and the life cycle cost has to be kept low and the following battery types can be recommended:
>_ (1) Solar Batteries,
>_ (2) Leisure/Lighting,
>_ (3) SLI truck batteries.

SLI car batteries should be avoided due to the short lifetime of a PV system. In practice of course also the local availability of batteries will decide. Therefore SLI truck batteries can be the best option in some developing countries with no other batteries than SLI´s available. It has also been shown that local factories for SLI batteries can be changed to also produce modified SLI batteries with thicker plates that have a significantly better performance in small PV systems.

For professional use, the more advanced and expensive batteries are often more cost-effective in the long run. Their lifetime is longer, the maintenance requirement is sometimes lower and the performance higher, concerning frequent deep cycling. Therefore a smaller battery bank in nominal Ah can be used for the same load and the number of exchanges during the system lifetime is lower. The main disadvantage is the higher initial investment costs.

In professional applications comprising daily discharging and recharging, the traction battery design is preferred. A semi-traction design with pasted plates can be acceptable if the lead grid is stable enough. The stationary battery design is recommended for applications with random discharge/charge cycles but where a reliable power source is needed for emergency alarm systems, etc. Stationary batteries of the Planet type are not recommended for PV systems as the lifetime will be very short due to the special charging conditions in a PV system (Ref. 2). They are also the very expensive voltage of a battery that can be compared to the height difference in meters between the surface of a hydropower dam and the turbine outlet. The current in Amperes can be compared to the water flow in m3 per second.

If you connect a voltmeter over the terminals of a 6-cell mono-block lead-acid battery at rest, it will show about 12-13 volts. During charge, up to 15 volts may be acceptable and during very rapid discharge down to 9 volts can be normal. The theoretical voltage of a lead-acid battery cell depends on the chemical reactions inside it. Under standard conditions, it is 1.93 V or 11.6V for a 6-cell mono-block battery. In practice, 2.0 V is used as a reference value for a single cell. This is called the nominal voltage. According to this, a 6-cell battery is referred to as a 12 V battery.

The higher the discharge rate - the lower the battery voltage:~

In reality, the voltage varies with the operating conditions. During high power discharge, low values as 1.5 V per cell (9 volts in a 12V battery) are acceptable as cut-off voltage, but normally the voltage should stay above 1.8 V per cell (10.8 volts for a 12V battery) during the time battery is being discharged. At the lower cut-off voltages, there is a risk of permanent damages to the electrodes. The battery manufacturers often provide curves in their datasheets showing the voltage/current relationship for their products.

Series Connections

Higher voltage with series connection:~

For some purposes, one might need a higher voltage than that of a single battery. It is then possible to connect batteries in series. This means that you connect the negative terminal of one battery to the positive terminal of another battery. If you have connected three 12 V batteries in series you will have 36 V between the positive terminal of the first battery and the negative terminal of the last battery. This way you can connect several batteries in series and the total voltage will be the sum of the voltages of the batteries. The capacity in Ah will be the same as for a single battery but of course, the energy content in Wh (Wh = V * Ah) will increase the number of batteries.

Bulk Charge Stage Gel AGM Flooded
Max Current  30% of 20hr  30% of 20hr  30% of 20hr 
(Mono-block Battery Design) Rate  Rate  Rate 
Max Current  20% of 6hr  20% of 6hr  20% of 6hr 
(Single Cell Battery Design) Rate  Rate  Rate 
Absorption (Regulation) Stage
Constant Voltage 2.35 - 2.40vpc 2.30 - 2.35vpc 2.40 - 2.45vpc
Float Charge
Constant Voltage 2.25 - 2.30 
2.25 - 2.30 
2.30 - 2.35
Equalize Charge 
Constant Voltage
2.40 - 2.45 
2.35 - 2.40
2.50 - 2.55

Battery capacity:~

Battery capacity is the amount of electricity stored. Capacity is the measure of the amount of current that can be stored and withdrawn from a battery. The unit for capacity in ampere-hours (Ah). The battery capacity can be compared to the volume of water stored in a hydropower dam. The voltage is comparable to the height difference in the power station as mentioned above. In the same way, as we talked about a theoretical voltage there is a theoretical value of the capacity of a battery depending on several factors. In lead-acid batteries, there are three active components, the positive electrode active material, the negative electrode active material, and the electrolyte. One of these substances will limit capacity. When one of the active substances is consumed the battery voltage will collapse and the battery is discharged. Most often, the positive electrode material is limited in a new battery. The amount of that material will, as a result, determine the capacity.

Again it is practical to use an approximate value of the capacity of a battery called the nominal capacity. The nominal capacity of a battery is a measure given by the manufacturer for the capacity guaranteed to be reached when a new battery is discharged according to a standardized test procedure. For starter batteries (SLI), the battery is discharged 20 hours with a constant current down to a predetermined cut-off voltage. This current is called I20 and the corresponding capacity value is denoted C20.

Capacity can differ from battery to battery:~

The capacity available to the user might differ substantially from the nominal value. Multiple parameters will influence capacity such as temperature, the previous charge, the time spent after charge, the age of the battery, the current profile, the cut-off voltage, etc.

Design your system based on 80% of nominal battery capacity:~

It is also important to understand that the nominal capacity relates to the capacity obtained from a new battery. The end of life is defined as the point where the capacity has declined to 80 % of the nominal value. When designing a battery installation, one must take into account how the capacity decreases from a new battery to the end of life. It is important that the battery can fulfill its duty even when the capacity has decreased.

In a PV system, an extra margin is also recommended as the recharge of the battery can take several days or weeks under some parts of the year with low solar radiation. A rule of thumb is, therefore, to size the battery for only 50% discharge under the worst conditions. This will extend the battery life significantly in a PV system, as the probability for deep discharge will be smaller.

How to size the battery for a certain system and load:~

As an example, one can take a small 12V PV solar home system with 2 lamps of 11W used 5 hours a day and a 15W Television set used 3 hours per day. First calculate the daily energy use or load in the system: The daily energy use will be < Number of appliances * Power consumption * operating per day >. Example with data from above: 2 * 11W * 5h + 1 * 15W * 3h = 155 Wh per day.

Then calculate the design energy content of a suitable battery available locally: For example, a 12V / 75Ah battery contains nominally about 1 kWh. More exactly 75Ah * 12V = 900Wh if fully discharged. According to the rules given above only 80% of the capacity can be counted on in the long run.

Furthermore, to extend the battery life only 50% of that should be discharged each day at the most. Totally only 0.8 * 0.5 = 0.4 or 40% of the nominal energy content should be base for sizing. In this case 40% of 900Wh = 900 * 0.4 = 360Wh. The design energy content of this battery is 360Wh or 0.36 kWh.

Calculate the number of batteries needed: Normally the sizing of a small system is done so that the battery has enough energy for 3-7 days energy use without the sunshine. This is also called autonomy, In this case, one 12V / 75 Ah battery has an energy content of about 2 days use (360Wh / 155Wh/day = 2.3 days). At latitudes close to the equator (– 40 latitude) with the more even annual distribution of the solar energy, 2 batteries should be enough in this example giving (2 * 2.3 = 4.6 days autonomy.

At latitudes higher than 40, 3-4 batteries can be recommended giving 7-9 days autonomy in this case. In a real application there will be a close connection between the service you can get and the battery size and installed PV module power, but also the user interaction can have a large influence. With a modern controller, the user can read the battery state of charge and the load can be adjusted to the available energy stored in the batteries. This means that more energy can be used during sunny weather and less during very cloudy or rainy periods. For continuous year-round use at latitudes higher than 50-60, larger battery banks (larger autonomy) often with seasonal storage will be needed. This requires professional system sizing and is out of the scope of this document.

Parrallel Connections

Build higher battery capacity with parallel connection:~

In case you need increased battery capacity or the possibility to connect heavy loads you can connect batteries in parallel. This means that you connect the positive terminal of one battery to the positive terminal of another battery. The voltage of this connection will still be the same as of one single battery but the capacity will be increased and the ability to handle heavy loads will increase. In the same way, it is possible to connect several batteries in parallel, all the positive terminals connected to each other and the negative to each other depending on how much capacity you need.

In the case of non-professional batteries and installations not more than 3-4 batteries in parallel can be recommended. One bad cell may otherwise destroy the whole battery bank. All batteries should also be of the same age and size to give the best life. When connecting batteries in parallel it is important that each battery will have the same cable resistance between the terminals and the other system components as a controller, loads or PV modules. The figure above shows a good way to achieve this.

Note: battery capacity will increase slightly at the beginning of operation:~

A new battery will not reach its full capacity during the first discharge cycle. In the standards, it is described that up to 10 charges/discharge conditioning cycles are allowed before performing the first capacity verification test. Normally, a battery shall be fully charged and discharged a few times to activate the electrode materials. This is often difficult to achieve when PV panels are used for charging, as their charging capability is limited. This might delay the activation. The full capacity might not be reached during the first 50 cycles. This effect should not be overestimated. It will be in the range of 10%-20% of the nominal capacity.

Temperature effect on battery capacity:~

The nominal capacity is normally measured at 20C battery temperature, with a constant current discharge, down to a certain fixed cut-off voltage for the battery. In cold climates, the usable capacity may be significantly reduced, as low temperatures will slow down the chemical reactions in the battery. This will result in a useable capacity at for example minus 10 C battery temperature of only 60% of the nominal one at 20 C. The capacity is still there if the battery is heated to 20 C but at a low temperature, one cannot utilize the full amount.

When possible the battery should be placed indoors or otherwise sheltered from low temperatures by insulation or perhaps even placed on the ground if any other heat sources are not available. Seasonal storage containers with phase change materials with water as the main storage component has shown to work well. In warm climates, the opposite effect on capacity does not occur at battery temperatures above 20 C. In this case, the battery should be placed in a way to avoid high temperatures. Already 10 C temperature increase above 20 C will double the corrosion velocity of the electrodes and reduce the battery life significantly.

Charging procedure will fill your energy storage:~

A lead-acid battery can generally be charged at any rate that does not produce excessive gassing, overcharging or high temperatures. In the laboratory, constant current charging is often used. Constant voltage charging is preferred in many stationary installations. This is especially true for sealed or valve regulated (VR) batteries.

However, for deep cycling applications, the constant voltage charging is not recommended due to the fact that the charging time is much longer than acceptable, several days or weeks. If the battery is charged at too high a voltage, which will shorten charging time, the corrosion is enhanced and the battery lifetime will suffer. In a PV system, the energy source is not regular and special charging considerations have to be made.

Different steps in the battery charging procedure:~

The charging steps mentioned below are all taken care of by a good PV charge controller and are mainly mentioned here for information and motivation to look for better controllers. The steps used in charging of an open or vented lead-acid battery are named:

>_ the main charge, used for charging the battery up to a voltage level when gassing starts and the voltage rises. The voltage limit is 2.39 V at 25 C and 2.33 V at 40 C.
>_ top-up charge, to reach the 100 % state of charge from a level of 90 - 95 %. Retain the voltage limit by decreasing the current.
>_ equalization charge, used for equalizing the capacity of the individual cells, in a multi-cell battery. This is an important issue for improving life but requires a special controller mode to create this in a system charged by PV panels. (Increase the voltage to 2.5 - 2.6 V/cell for a short time, 0.5 - 1 h, at regular intervals, once a week).
>_ maintenance charge, used for maintaining the full capacity of a battery that is already fully charged but not frequently used for some period. Approx. 2.20 - 2.25 V/cell or a current value equal to the capacity value divided by 100 (C/100).

The battery is not very sensitive to the abuse created during the main charge phase, except the situations implicating a rise in temperature. It is preferable not to start charging a very warm battery (>50°C) if there is a possibility to cool the battery first. When controlling the charging process according to the voltage of the whole battery, it is understood that the individual cells of the battery have the same voltage. If not, some cells may not be fully charged (undercharged). It is, therefore, important to check the voltage of each unit regularly.

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Visitor Comments
  1. Comment #1 (Posted by Stephen McLachlin )
    I greatly appreciate the science, work and thought put into this site and knowledge base. I would appreciate more knowledge/entries on: 1) 'correct' 12v battery bank cabling (meaning resulting in most even charging and discharging). by bank I mean 2, 4, 6, 8, 10 batteries in banks of 1, 2, 3 and so on batteries in each bank. 2) Validity of 'cross-tying' battery banks with advantages and disadvantages (each battery 12v). 3) Diagrams (or rules) of cross-tying 12v battery banks My interest is my RV batteries (4X8D 12V); Prosine 2.5; Onan generator; AIMS ATCS30W Again, thank you for all the effort and resulting great information on this site. regards, Stephen
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