Part 1 – Battery type

Choosing a battery type

There are so many things to consider when choosing a battery system. How does it consume power? Does it require quick recharging? What is the ambient temperature during charging and discharging? The desired total longevity? Price? All of these factors are important.

Some batteries are used as back-up for critical systems like life support, while others simply keep a light on longer. However, it is always worth investigating the scope, size, and maintenance requirements – in order to make the right investment.

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First off, it’s important to remember that a battery is an energy converter that stores chemical energy. This energy is converted into electricity when it completes the circuit through an external circuit, load, etc., or conversely takes electrical energy from a charger and stores it as chemical energy.

A capacitor can also be recharged with electricity and emit energy, but in this case it is really an electric charge which is stored up. See below for capacitors and batteries.

Small and medium-sized sealed lead acid (VRLA, AGM / GEL) batteries are sometimes called, a bit erroneously, “maintenance-free” batteries. They manage to deliver large currents for a short time, but to get out the battery’s total capacity, the power consumption should be relatively low. The nominal capacity applies normally with 20 hours of discharge, with higher currents (less time) the total capacity becomes lower. Energy density around 30-50Wh / litre or 20-30Wh / kg, nominal cell voltage is 2V.

Lead acid batteries provide a cost-effective solution and are relatively easy to load and take care of. They have no so-called memory effect, can be recharged even when they are not entirely empty, but should never be left discharged, or they can be destroyed in a short time.

Ni-Cd batteries can also handle large transient currents, and can leave their nominal capacity even with relatively large power outlet. Available in many varieties, sizes and capacities including for high ambient temperatures, rapid charging etc. Can be assembled into “desired” shape and performance, is well proven. Energy density of 80-120 Wh / litre or 40-60 Wh / kg, nominal cell voltage 1.2V. Manufactured in cylindrical, prismatic (rectangular) and in the button form.

They are easy to maintenance charge, but require monitoring for quick charge.

Ni-Cd are at their best when working hard, and should always be fully discharged before recharging, otherwise the so-called memory effect occurs, in which the battery’s capacity will gradually degrade. This can usually be “repaired” with repeated full charge / discharge, or with equipment for reconditioning of rechargeable batteries. Due to the content of the heavy metal, cadmium is Ni-Cd covered by high environmental fees and is being phased out increasingly in both the EU and the rest of the world.

Ni-MH batteries are reminiscent of Ni-Cd for the most part and are more environmentally friendly as they do not contain any heavy metals. Have about 50% higher capacity than a corresponding physically similar Ni-Cd, energy density 280-340Wh / litre or 80-120 Wh / kg, nominal cell voltage 1.2V. Can not handle quite as high currents as Ni-Cd, self-discharge is also somewhat higher than with other systems which one should consider if the application means that there is a time lag between charging and use.

There is also a newer generation of Ni-MH cells that have very low self-discharge (Low Self Discharge or LSD cells). Primarily developed for home use and in smaller sizes (AAA and AA) up to about 2200mAh. The characteristics of charging Ni-MH generally follow the Ni-Cd with somewhat stricter control of voltage curve and temperature. The memory effect is less than for Ni-Cd. Manufactured in the same shape and structure as Ni-Cd, and often have, but not always, green coloured housing. The price is somewhat higher, but due to environmental fees for Ni-Cd, the overall cost is balanced down for Ni-MH.

Lithium-ion batteries have a very high capacity relative to volume / weight, 320-380Wh / litre or 150-200 Wh / kg and can be the right option when weight plays a major role. They are more expensive than the others, but the prices are falling gradually as more and more start using this system. At present the automotive industry is the strongest proponent of research on batteries based on lithium chemistry.

The cell voltage lies at 3.7 V often uses a single cell in for example mobile phones. Li-Ion always have a small add-on electronic circuit (PCM Protection Circuit Module) which provides for the safety, prevents overcharge / under-discharge etc. The charging technique is reminiscent of the lead battery’s, but the battery can be recharged in less time, down to 2-3 hours. During charging is monitored often the balance (voltage level) between the individual cells with a special electronic circuit (VBB) that can be a separate circuit board but which often is combined with the protection circuit. The balancing can be passive or active where the latter provides a faster and more accurate result.

The cells are built both cylindrical and prismatic (rectangular). Li-Ion batteries are becoming increasingly common in applications with low to medium power consumption.

Lithium-ion Manganese dioxide is a variant that is designed for high discharge currents. Lithium-ion has no memory effect, requires no reconditioning and the self-discharge rate is low.

There is a certain amount of consumption that must be taken into account as well due to electronic protection, etc.

Lithium-Polymer (Li-Pol) batteries have a similar chemical composition as the regular Li-ion cells. The construction is such that one, in principle can shape the cell in the desired way, a dream for the designer. There is no need basically for a “hard” housing, only plastic or aluminium foil and can be “squeezed” in an optimal way in for example the computer or the phone’s battery casing. Works like Li-Ion, best with low or medium flows, requires somewhat longer recharge time. Pricewise it can be advantageous due to the simpler manufacturing than Li-Ion, and is the most common Li-cell in portable devices today.

Also called Lithium Iron Phosphate or LiFe cells. This belongs to the same family but has a different chemical structure and slightly lower cell voltage, nominally 3.2 V, but usually with high current capacity and good of cycling -ability, up to 2000 cycles for certain types, they can even load faster than other lithium-ion cells. Energy density about 180Wh / litre or 90Wh / kg. They are more stable and less prone to thermal runaway compared with ordinary lithium-ion / Lithium-Polymer and in some cases require somewhat simpler monitoring circuits, like built-in balancing of the cell voltage between cells connected in series.

Several Li-ion based battery chemistries LiCoO2 Lithium-Koboltdioxid, LiMn2O4, Lithium-Manganese dioxide, LiNiO2 Lithium-Nickel oxide are some of the many battery chemistries with energy density between 200-300 Wh / litre or 100-130Wh / kg, several new chemical formulations can be expected in the future, mostly driven by the automotive industry in cooperation with battery manufacturers.

There is a less common variant of alkaline cells (primary cells) that are designed to be recharged. They have the usual non-rechargeable terminal voltage of 1.5 V, but can be recharged 10-50 times or more depending on how deep they are discharged. They can not handle especially high currents and require a special charger. One advantage is that they have the same terminal voltage as ordinary alkaline disposable cells.

There is a variety of lithium technologies, all characterised by very high energy density in relationship to weight and volume and long shelf life, up to 10-15 years. Cell voltage 3.0-3.6 V and manufactured in sizes and shapes as the usual consumer cells, as well as for PCB mounting and industrial use, for memory backup etc. Common types are lithium thionyl chloride (3.6 V) for small and medium-sized currents and Lithium-Sulfur dioxide and lithium-manganese dioxide (3.0 V) for large currents, The latter are common in for example some cameras and flashlights. Today, there are even several manufacturers of lithium cells with 1.5 V voltage (lithium iron disulphide, LiFeS2) that directly replace conventional alkaline consumer cells in AAA and AA-size and in 9V batteries. These have many good properties like good cold resistance, very long shelf life, good high current properties and so on. The price is higher than that of the alkaline cells but may be outweighed by the long running time, a classic example is the 9V battery for smoke alarms that can work up to 8-10 years before replacement must be done.

So-called smart batteries are something that can be seen with increasing frequency, also called Info batteries (in connection with cameras , for example) or intelligent batteries. This is not really tied to any particular chemical system has long been used in for example certain laptop and video batteries and in several other contexts. The so-called Smart circuit functions as time counter / power gauge on the battery.

A circuit board with microcontroller, etc. (BMS Battery Management System, SBS, the Smart Battery System) is built into the battery and accurately measures in- and discharged energy, temperature, number of cycles and others parameters.

The result is presented in the attached equipment display or monitor, and you can always see for example remaining capacity in percent or operating time in hours / minutes, in comparison with the earlier simple battery meter in the form of a few LEDs, bars in a small LCD display or the like, which pretty roughly showed the battery’s status. The information between the battery and external equipment can take different forms of a physical data bus, regular standard SM-bus, I2C bus, CAN bus and so on.

On larger battery systems, like for example electric vehicles or similar one often has many parts and modules, instruments, displays, motor control, etc. charging circuits , etc., that share data from the battery’s common communication bus. Components of batteries and cells can be split between master and slave modules, different manufacturers can use a little different names for the parts, BMS, BMU and so on.

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Power consumption is decisive

Ordinary alkaline cells are sometimes used in for example a portable two-way radio instead of rechargeable cells. These have on paper a higher capacity than a NiCd or NiMH cell of the same size. For example an AA / LR6 alkaline cell can have 2800 mAh while the corresponding Ni-Cd perhaps has around 900 mAh but the operating time is not always longer. This is because the internal resistance is higher, and the terminal voltage drops rapidly when the battery is charged. The battery is simply not optimised to work with large currents. Even the so-called manganese dioxide battery, which is the predecessor of alkaline batteries is sold as an alternative with a lower price, most suitable for applications with low consumption

Capacitors and batteries, “Supercaps”

In some contexts, the combination of a rechargeable battery and a capacitor with high capacitance can be useful. Primary Lithium (non-rechargeable) are connected in parallel too often with capacitors, in order to counteract the short voltage dips that would otherwise arise in the lithium battery when one applies a load after the battery has “rested” for a long time. In conjunction with major effects, in rechargeable systems, like with electric vehicles and in conjunction with energy storage on a large scale so-called Supercaps are often used, which are a special type of multilayer electrolytic capacitors.

Very high capacitance from a few F (Farads) to several thousand F may occur and the capacitor can briefly give many kW, at the same time as it can be recharged as quickly. Compared to a battery, however, the capacitor has significantly lower total energy content or energy density, around 2-3 Wh per litre or kg where for example a lithium battery can easily have 100 times higher or more. The effect that can be handled instantaneously, however, is higher for the capacitor; cycling lifespan of capacitors is significantly better than for batteries. The combination of Supercaps and batteries or other power sources often gives good results.

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Fuel cells

A system that directly and continuously converts chemical energy into electrical energy. Can be likened to a battery that can continuously be filled with a fuel. The most common fuel is hydrogen gas which then decomposes electrochemically and generally uses a so-called reformer that converts a hydrocarbon based fuel such as methanol into hydrogen and carbon dioxide.

The principle behind the fuel cell stretches back to the early 1800s, but not until the 1950s were more practical fuel cells produced with high effect. NASA and the space program was a driving factor and today you can buy practical, usable systems. The efficiency can theoretically reach more than 80% but in practice usually around 50-60%. Emissions from a continuously operating fuel cell are “clean”, often plain water if the cell for example is supplied with hydrogen.

The longevity and economy of batteries and battery systems

The lifespan of the various battery types depends on a variety of factors, ambient temperature, depth of discharge etc. A Ni-Cd battery can last 500-1000 cycles or more in favourable conditions, especially for large industrial systems, Ni-MH and Lithium-ion batteries last usually slightly fewer. Many newer Li-technologies, like Lithium Iron Phosphate can be optimised to last through many cycles, considerable resources are being applied and research is being done in this area, especially in relation to electric vehicles.

Even lead-acid batteries can be optimised for either cyclic or standby operation, and can in the latter case function for at least 10-15 years.

One factor that is important to, for example, lithium and lead technology, is the depth of discharge (DOD), which then is determinate for the overall life expectancy. If you for example utilize only 80% of the nominal battery capacity, you get significantly more cycles in total and a longer life, lower risk of battery failures and so on.

Rapid developments make it difficult to give too detailed data and special formulations exist within the different chemical systems. Always consult with Celltech for your current application.