Build Your Own Batteries Copyright © 2008 www.DIYPowerSystem.com
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Table of Contents Table of Contents......................................................................................... 3 Storing Electricity ........................................................................................ 4 Types of Batteries ........................................................................................ 7 Battery Parameters ..................................................................................... 13 Safety and Maintenance ............................................................................. 19 Building a Battery ...................................................................................... 20
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Storing Electricity The simplest definition of energy is the one according to which energy is a particular property of objects and systems that is described as conservable scalar physical quantity. That means that energy is a simple physical property of objects, which can be quantified. Energy is widely found in nature in a variety of forms. Thus, according to several criteria, we deal with potential and kinetic energy, thermal and magnetic energy, nuclear and chemical energy, magnetic and mass energy and, most importantly for people as consumers of energy, we also talk about electric energy. This list of forms of energy is far from being complete. However, the point is that electricity is a form of energy and, as any other form of energy, it can be converted into a different form, and it can be obtained, as a result of transformation, from a different form of energy, virtually without loss of energy, according to the law of conservation of energy. However, in order to transform one form of energy into another, certain devices are necessary. For instance, in order to produce energy from water, a dam is needed. Dams turn gravitational potential energy of moving water into kinetic energy, and by means of an electric generator, this kinetic energy is turned into electric energy. A similar process is implied by the generation of electricity from wind power, except that in this case water is replaced by wind, and instead of dams we deal with wind turbines. Batteries, at their turn, are able to turn chemical energy into electricity. But unlike dams or wind turbines, batteries have the advantage of being designed also to store energy in the form of electrochemical energy. Storing electricity is, indeed, an issue, particularly with respect to renewable power systems that rely on somewhat elusive sources of energy, such as the sun or the wind. These intermittent sources of energy are subject to weather conditions and, 4
as a consequence, people, as consumers of electricity, are also subject to such an environmental aspect. The thing is that when it comes to renewable power systems, we can benefit from peak periods, when the amount of electricity produced is higher than the amount we actually need in order to be able to use the appliances commonly found in any home. The rest of the energy generated by such systems is to be lost, unless devices for storing that excess of electricity are available. Batteries are excellent for storing additional electricity that can not be consumed as it is produced. This is why batteries should be comprised in renewable power systems, in order to increase the load availability. If we decide to employ batteries in order to store energy within a large range of applications, we may either choose a primary battery or a rechargeable battery, also referred to as secondary battery. The difference between these types of battery is that with primary batteries, which are able to turn chemical energy into electricity by means of an electrochemical reaction, this electrochemical reaction is not reversible, meaning that after the battery is discharged, it can no longer be used. With rechargeable batteries, the electrochemical reaction by which chemical energy is converted into electricity is reversible, meaning that at its turn, electrical energy – direct current – from an exterior source can be converted into chemical energy within the battery. At this moment, the battery is in the charge mode.
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Storage of electrochemical energy within the battery
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Types of Batteries According to the electrochemical process inside the battery, we distinguish between lead-acid batteries, nickel-cadmium and nickel-metal hydride batteries, lithium-ion and lithium-polymer batteries, and zinc-air batteries. Each type of battery has its own advantages. For instance, lithium-ion batteries have the highest level of cell voltage – 3.4 – but despite this level of efficiency, they are not the most widely used. On the contrary, lead-acid batteries are the most popular due to a particular feature, that is, they are less expensive than any other type, but for that price they prove a high performance all the same. The main drawback – lead-acid batteries have the lowest level of energy density as compared to weight and volume. While the battery releases electric energy – that is, when it is in the discharge mode – the water produced reacts with the sulfuric acid electrolyte, generating its dilution. As a consequence of this reaction, a decrease of the specific gravity of the electrolyte is triggered along with a decrease of the state of charge. While charging the lead-acid battery, the reaction is completely reversed.
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Specific energy and energy density of lead-acid batteries Vertical axis: energy density Wh/liter Horizontal axis: specific energy Wh/kg Among lead-acid batteries, deep-cycle batteries are the most recommended for applications that require full discharge and charge cycles that must be repeated. Yet, motor vehicles, for instance, can run perfectly on shallowcycle batteries, since they only need a small and rapid amount of energy in order to start to work. But should we be interested in deep-cycle batteries with a longer life span, or in batteries that tolerate better certain temperature conditions, nickelcadmium batteries might be what we are looking for. However, the main drawback of nickel-cadmium batteries is that they have what is generally referred to as memory effect. This memory effect describes the propensity of the battery to remember and to repeat its “behaviors” in the past, meaning that if the battery has been charged and discharged at a certain level of its capacity for a longer period
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of time and repeatedly, than the battery will only charge and discharge until that level is attained, even if the respective level does not overlap the full capacity of the battery.
Specific energy and energy density of nickel-cadmium batteries Vertical axis: energy density Wh/liter Horizontal axis: specific energy Wh/kg The long term effect is that the nickel-cadmium battery will lose its capacity after being subjected to incomplete charging and discharging processes. Nickel-cadmium batteries are the only ones that experience the memory effect. But besides this matter, another aspect is that nickel-cadmium batteries are debated with respect to their impact on environment, the main reason for which other types of electrochemistry are internationally recommended for use. In nickel-metal hydrate batteries, the concerns linked to the impact of cadmium on the environment are removed along with the removal of cadmium from the anode. In this new electrochemistry, besides such concerns, the memory effect is also eliminated. But despite these two 9
advantages nickel-metal hydrate has on nickel-cadmium, its downsides must be mentioned. For instance, constantly overcharging a nickel-metal hydrate battery can lead in time to damage. More over, the discharge rate of selfdischarge of a nickel-metal hydrate battery is fairly high.
Specific energy and energy density of nickel-metal hydrate batteries Vertical axis: energy density Wh/liter Horizontal axis: specific energy Wh/kg Batteries relied on lithium-based technologies can provide an energy density which is three times higher than the one proved by lead-acid batteries, due to the particular features lithium has, that is, due to an atomic weight of 6.9, as compared to 207, as it is the case with lead. Another feature that makes lithium-ion electrochemistry much more efficient is the cell voltage, which, a 3.5, is higher than the cell voltage level of lead-acid. Lithium-based technologies refer to lithium-ion and lithiumpolymer.
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Specific energy and energy density of lithium-ion batteries Vertical axis: energy density Wh/liter Horizontal axis: specific energy Wh/kg
Specific energy and energy density of lithium-polymer batteries Vertical axis: energy density Wh/liter Horizontal axis: specific energy Wh/kg
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Another type of battery is the one based on zinc-air. This battery charges and discharges due to the fact that the positive electrode, made of carbon, is exposed to the air. While charging, the zinc electrode is oxidized, since the oxygen in the air is reduced at the cathode, whereas during charging, the reaction is reversed.
Specific energy and energy density of zinc-air batteries Vertical axis: energy density Wh/liter Horizontal axis: specific energy Wh/kg
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Battery Parameters While choosing or building a battery for a certain application, in order to make sure the application will run smoothly on the battery we choose or build, some features must be taken into consideration. Since it is the efficiency of the entire system that we are looking for, we should pay a special attention to features like: charge and discharge rate, charge and discharge duration, voltage and current, temperatures released while the battery charges and discharges, and, the number of cycles of charge and discharge expected during the life span of the battery. The pictures below illustrate the performance of a battery in different circumstances.
Charge rate and discharge rates affecting cell voltage Vertical axis: battery cell voltage
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Horizontal axis: cycle time – in minutes Left side: discharge rate Right side: charge rate
The internal resistance of a 25 Ah nickel-cadmium cell affected by temperature Vertical axis: milliohms Horizontal axis: temperature – in º C Curves (percent): First upper curve (the continuous curve) – 40 Second upper curve – 60 Third upper curve – 80 Fourth curve – 100 However, other parameters must also be considered in order to make sure the battery matches perfectly the requirements of the entire system in which it is comprised because, indeed, different systems impose constraints on batteries. Thus, for a renewable energy system, we should mind a large 14
range of basic parameters, other than the ones presented above. For instance, the type of battery is essential for a proper functionality of the system. Deep cycle batteries are definitely much more recommendable and must be chosen of shallow cycle batteries. Moreover, the electrochemistry on which the battery relies is yet another parameter one should take into consideration. A particular case refers to the above mentioned memory effect in nickel-cadmium batteries. The picture below shows how this impact of this effect on the discharge voltage.
Vertical axis: voltage – volts per cell Horizontal axis: depth of discharge – percent Upper curve: complete discharge before the memory effect Lower curve: depth of discharge lower with 25% after the memory effect Also, various systems have various requirements with respect to voltage. The load conditions are also to be born in mind, and this is why one should always be aware of the Ah discharge. If the Ah discharge is determined, then
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we can easily calculate the Ah capacity that we obtain by dividing the Ah discharge by the maxim acceptable depth of discharge. In order to meet the total Ah capacity, one should calculate the number of battery packs necessary to attain that capacity. Finally, a due attention must be paid to thermal and charge and discharge rate controls, since we don’t want to shorten the life span of the battery by neglecting these aspects, knowing that all batteries have a particular tolerance to such aspects. Temperature is a very important aspect, since it can seriously affect the particular electrochemistry on which the battery relies. For instance, leadacid batteries are functional between -10º C and 50º C. On the other hand, nickel-cadmium seems to have a wider range of operating temperature, being able to work properly between -20º C and 50º C. Nickel-metal hydrate batteries have the same operating temperature range as lead-acid batteries, and lithium-based electrochemistry seem to have a rather narrow scale of temperatures. Thus, lithium-ion batteries work best between 10º C and 45º C, whereas lithium-polymer batteries are functional between 50º C and 70º C.
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Operating temperature range according to electrochemistry Vertical axis: range of temperatures – in º C If we look for a battery able to work properly within the largest range of temperatures, we should definitely opt for nickel-cadmium electrochemistry. However, we should not rush into making a decision, since there are other parameters we must consider. If we think about overcharge tolerance, we should know that nickelcadmium has only a medium tolerance, whereas lead-acid batteries have the highest tolerance. Nickel-metal hydrate and lithium-based technologies have either a low or a very low overcharge tolerance. The point is there are many features we have to consider before deciding what battery is best suited for our renewable power systems, and even if a certain type o battery excels from a certain point of view, it might just as well have serious drawbacks with respect to other parameters. But this is
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precisely why we must accurately acknowledge the requirements the system we intend to build impose to the battery. And since the life span of batteries was mentioned above, it must be said that it depends on the charge and discharge cycles and on how properly these cycles evolve. Thus, lead-acid batteries are functional between 500 and 1000 cycles, and so do lithium-ion and lithium-polymer batteries, whereas we can only rely on a life span of 200 to 300 cycles when it comes to zinc-air. The best batteries from this particular point of view are the ones working on nickel. Both nickel-cadmium and nickel-metal hydrate can undergo between 1000 and 2000 cycles before wearing out.
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Safety and Maintenance If we have already decided what battery we are to employ within the homebuilt renewable power system, some other issues are to be handled with. For instance, we always have to supervise the charging process, due to the fact that overcharging determines loss of water – at it is the case with leadacid batteries – on short term, and the shortening of the life span, on long term. Since it is extremely inefficient and almost impossible to supervise the battery in person, charge regulators or controllers must be employed. Charge regulators don not only prevent the shortening of the life span, but they also represent a guarantee that the battery performance will not be affected. However, it’s not just the charge and discharge cycles that must be monitored. Other performance parameters should be observed all the same. Modern control devices supervise, for instance, depth of charge, or rate and sate of charge and discharge. Voltage and current, as well as Ah released or consumed by the battery must also be monitored. As a safety issue, preventing overcharge remains, however, the most important, mostly if wee talk about employing a battery within a solar power system, and particularly if no charge controller is used and if the battery is supplied directly from the PV module. Due to the fact that overcharging causes overheat, we deal with the risk of explosion.
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Building a Battery As an experiment or, at least as a curiosity, on should try to build a battery at home. Even if this battery will not be powerful enough to sustain a wind or solar power system, the building process itself will prove that virtually all important elements of such systems can be made at home and o a low budget, since almost every component of the battery can be improvised or, at least, obtained for free or for a low price. The basic elements of a homemade battery are: a container, some aluminum and copper pipes and bars, water, bleacher, some silicon and tape for insulation, and a connector that ensures the link between the two electrodes. In addition, a D.C. voltmeter may prove to be of help if we want to see how efficient our homemade batteries are. Water and bleacher are used to create the electrolyte solution that facilitates the chemical reaction between the two electrodes, that is, between the positive electrode represented in this case by the copper pipe, and the negative electrode for which the aluminum bar stands. By if we want to simply the battery and the building process even more, we may just as well use an aluminum can as container, and at the same time as negative electrode. In order prevent the copper pipe from touching the aluminum can, silicon is used to seal these components from each other. With respect to electrolyte, that is, the solution that facilitates the chemical reaction between the two electrodes, we may be tempted to use more bleacher in order to obtain a higher level of amperage from the cell. Because, indeed, the more bleacher we use, the higher the amperage will be. However, we have to consider the fact that bleacher has a corrosive effect
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both on aluminum and copper, and this is why we have to mind how concentrate the electrolyte solution will be.
Aluminum cells connected in series in order to prevent corrosion and to deliver a satisfactory amperage level In order to prevent corrosion and to have a satisfactory amperage level, we can build more cells and connect them in series, as it is shown in the picture above. This way, even if we use more aluminum cans and more copper pipes – which would lead to higher expenses, but not as high as to exceed a fairly low budget – we get both a satisfactory result with respect to amperage and a longer life span for the cells. Along with a proper maintenance – regular cleaning – the copper and aluminum components are expected to last between 4 and 5 years. As rudimentary as they may be, such batteries prove to be surprisingly efficient for emergency cases when power supplies shut down for relatively short periods of time, or when, given some particular circumstances, we need lighting, for instance, but no grid or other supplies are at hand.
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This experiment should be interpreted rather like an attempt to demonstrate that our reliance on fossil fuel can be overcome, and that premises for an enhanced autonomy are already set.
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