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Batteries, - a simple guide to practical usage (and a bit of supporting theory).
As a designer and manufacturer of battery belts and other associated power equipment, I have been asked to write an article about the benefits and disadvantages of the various battery technologies, with particular reference to the sort of applications used by readers of this journal. My notes here have been necessarily generalised, and are not necessarily related to any manufacturer's specific product, although I have used a few examples from manufacturer's product lists for comparison purposes. In particular the major manufacturers are continuously improving their products, so any cost, capacity or weight comparisons I make may not apply to the particular devices you are familiar with.
First of all, before we start, when dealing with batteries, please observe the following safety notes:
Do not dismantle any battery, most of the chemicals are poisonous and/or corrosive.
Do not dispose of batteries by trying to burn them, or otherwise get them too hot, since the chemicals will be forced out of the case, often violently.
Dispose of batteries properly. Ni-Cads in particular are far from biodegradable or environmentally friendly, so you shouldn't just put them in the dustbin, and in fact the use of Ni-Cads are prohibited in some countries.
In my experience, most confusion with electrical matters stems from folk not understanding the basic concepts and terminology. If you appreciate the difference between an Amp (I don't mean an amplifier!) and a Volt, between series connections and parallel connections, own and know how to use a simple multimeter, then you will be able to understand the following notes and be able to avoid getting into all sorts of trouble with basic battery usage and maintenance, so read on....
I will start with a few basic definitions, so that we can establish a common terminology so that we all know exactly what is being discussed. A battery consists of a number of cells. A cell may be either a primary cell or a secondary cell. Most common cells operate by converting a chemical reaction into electrical energy. A primary cell is a single use cell, i.e. it can not normally be recharged, it produces electrical energy until its active materials are consumed. A secondary cell can be recharged a number of times, to regenerate the active materials. In most cases, a battery consists of a number of identical cells connected in series. The nominal output voltage of a battery consisting of cells connected in series will be the sum of the individual cell voltages. e.g. if you want 6 Volts from Ni-Cads (which have a cell voltage of 1.2V), then you will need 5 cells, if you want to use lead-acid cells, which have a 2V cell voltage, then you use 3 cells.
Batteries and cells are normally rated as, say, x V at a capacity of y mAh (mili-Ampere-hours). Sometimes the manufacturer will quote mWh (mili-Watt-hours) instead of mAh. This results in a larger number being quoted, and the naive customer will think he is getting a more powerful battery. All manufacturer's figures will be nominal, since for virtually every type of battery, the voltage will vary over time, even if no load is connected, and the voltage also depends on the exact state of charge of the battery, ambient temperature and other factors. However, to convert from mAh to mWh, then simply multiply the mAh rating by the battery Voltage (e.g. a lithium-ion battery with a nominal voltage 7.2V, quoted as 1500mAh could also be rated at 10800mWh or 10.8 Wh.)
There is another definition that needs to be understood, which is mainly used in discussions concerned with charging secondary cells - C.
1C is a number which indicates the amount of current when the rated capacity of a cell is discharged in one hour. In general, charging and discharging currents are expressed in multiples of C. C's unit of measurement is in Amps (Do not confuse this with the standard electronic abbreviation for a capacitor...., with units measured in Farads). For example, a safe charge current for many secondary cells is 0.1C, so for a battery with a capacity of 2500mAh, a safe charge current would be 2500 x 0.1 = 0.25A . (It would be usual to charge at this rate for a period of 12 to 16 hours.)
To summarise the above - all batteries are not created equal. Reputable manufacturers will print the capacity on the battery case. They will also provide you with full specifications of their products. For comparison purposes convert capacity to mAh if given as mWh. The voltage normally stated will be a nominal voltage, the actual measured voltage will be different.
As an example, we can consider a 6V battery with a capacity of, say, 1500mAh. Does this mean we can extract 1500mA over a period of one hour, or can we extract, say, 150A over a period of 36 seconds and still have a battery voltage of 6V at the end of the period? In fact it depends on the manufacturer's specification. Many manufacturers quote a five hour rate, which in the case of our six volt battery means it will give 300mA over a period of five hours. However, another factor to bear in mind, is that the final voltage will not be 6V, but will be 'the discharge end voltage' - defined as the limiting voltage when the battery is considered to have no residual capacity. The standard end voltage for most Ni-Cad batteries is 1.0V per cell, and it is this value on which the manufacturers base the capacity calculation, but the actual end voltage will be dependant on your particular application. If we substantially increase the load current, then the active materials are used less effectively, and the effective capacity will be reduced. For most batteries, a low ambient temperature causes the internal resistance of the battery to be higher, so the output voltage will be lower, also the active materials will be less reactive, so the capacity will be reduced at lower temperatures.
These are not normally used as a main power source for audio or video equipment, but are often used in auxiliary devices, e.g. remote wireless or infrared controllers, microphones, etc. There are many different chemical systems employed in these cells, some using very expensive components. The requirement for, say, a clock battery, which will be expected to supply a comparatively low amount of energy for a year or so, is different than that required for a battery powering a digital camera with a flash lamp, which has to supply pulses of relatively high power. Within many electronic devices, for example real time clock circuitry and memory maintenance circuitry, there is a requirement for primary cells which will supply a low amount of power for ten or so years.
A few advantages of primary cells are that they are easily available, at least in the standard consumer sizes, have a long shelf life and a high power density.
As an emergency backup, then it can be useful to have a battery pack which will take standard primary cells, but chose an easily available cell size, AA, C or D
Originally, (well, in my childhood days) most primary cells were of zinc-carbon construction. A later development was zinc chloride which has a greater capacity, whereas now most primary cells are of alkaline construction. However, in some situations the high capacity of the alkaline cells can cause a hazard due to sparking on installation. For example, a standard alkaline Duracell D cell has a capacity of 18000mAh, and therefore Zinc based cells are still used in hazardous situations.
Later cell technologies include zinc air, which provide energy only when a protective seal is removed and they thus have a very defined operational life. Other chemistries include silver oxide, mercury, and a whole range of lithium based cells, such as lithium iron sulphide, lithium manganese dioxide and lithium thionyl chloride. These cells are usually highly matched to a particular application, and give a saving in size and weight (but not usually cost) when compared with an alkaline cell of the same capacity
These are the batteries that most of this article will be concerned with, and is the area that causes most energy supply problems, since we not only take energy from the cell, but have to replace it also, so the problems will be at least doubled.
I will only consider four types of secondary cell; lead acid gel, Nickel Cadmium (Ni-Cad or Ni-Cd), Nickel-Metal Hydride (Ni-MH), and lithium-ion (Li-ion).
Lead Acid Gel BatteriesThese are my preferred batteries, for the following reasons: Easily recharged with constant current or constant voltage system. The same cell can be used for fast cycling or long term float applications. Low internal impedance allows very high discharge currents. Excellent mechanical and vibrational strength. Absolutely no 'memory' effects, so can be recharged at any state of discharge. No damage due to cell reversal. 2V per cell, meaning fewer cells for lower battery cost and higher reliability. Cells can be paralleled for additional capacity. Construction allows air transportation without restrictions, and they are allowed in every country. They have an easily monitored capacity, since they have a gently sloping discharge voltage/time curve. Their operational voltage range -65 deg C to +65 degC is excellent, and they are readily available in rectangular format in 6, 12 or 24V packs. They give a very reliable and predictable performance at a low cost. They also have a long charge cycle life - up to 2000 cycles or eight years on float charge. Always store them in a fully charged condition, and check every month or so and top up their charge. They will self discharge at about 3% per month.
You won't like them because they are heavy (see example below to see how wrong you are!!). You will also forget to check their charge condition if not using them for a period of time.
Let us look at the Yuasa NP7-12 battery in more detail. This 12V battery has a capacity of 7.0Ah - that is at a 20 hour rate of 350mA to 10.5V. At a one hour rate of 4200mA then it has a capacity of 4.2Ah to an end voltage of 9.6V. The maximum discharge current with the standard spade terminals is 40A, and it has a charge retention of 85% over 6 months. It is in a robust plastic case about 151mm by 65mm by 98mm high, although it may be used in any position. It weighs 2.65Kg. Retails at about £15.00. A suitable constant current mains charger is readily available for £20.00, but it is feasible to use an even cheaper wall block type device at 0.1C for overnight charging.
Now, if we try and make up a similar capacity Ni-Cad pack, we need 10 cells, since each is 1.2V. We will require a case and we will also need to do some wiring to connect the cells in series. A suitable 7Ah Ni-Cad cell is 33mm diam x 91mm high, and weighs 230gms at a retail price of about £9.50 each (total of £95.00). If we mount them in a block, as two rows of five, then we end up with a pack 91mm high by 66mm wide by 165mm long. This will be larger than the lead acid battery with the same rating when we put a rigid case around it. The total weight will be at least 2.3Kg, since we must be add wiring and the weight of the case. A decent charger/discharger may well cost £100 or so, the assembled battery pack in a case is likely to cost £150.00.
Nickel Cadmium BatteriesBearing in mind my list of advantages for lead acid batteries, why use Ni-Cads? The lead based storage battery was invented in 1859 (about ten years before the first dry cell), and it was about 100 years later that the sealed gel type lead acid battery was perfected. The Nickel Cadmium storage cell was invented in 1899, and it was not until the early 1960's that the sealed Ni-Cad cell started to be used, although the first sealed unit was developed in 1947. Well, historically the sealed devices seemed to appear at about the same time, so we can not say that one is more popular than the other because of tradition, but lead acid batteries have existed for a long time in their liquid form, and they were, and still are used for heavy power requirements, and I believe that is why lead acid gel batteries are not more popular for AV users - it is not being marketed into the AV area. Most Ni-Cad cells are available as size for size replacements for the common consumer and industrial sized primary cells, and the cell voltage at 1.2V and a pretty level 1.2V at that, is comparable with the 1.5V of most primary cells. Lead acid, at 2V, just does not have this market, so they are not made in the smaller sizes, and if we were using four lead acid cells to replace four primary cells, then we would have a serious over voltage problem. That, I believe is the commercial reason, but what about the technical reason for Ni-Cad's popularity?.
The major advantage of Ni-Cads is that the discharge voltage is constant, until it reaches a knee point at the end of the discharge cycle, when it rapidly falls. For many electrical/electronic devices in the 1960's, then if you had a consistent 6V supply, say, from 5 Ni-Cad cells, then an amplifier would perform just as well after one hour's use as at the beginning. (Never mind that half an hour later you may be getting rapidly into total silence). Electronics were relatively expensive, and it was unnecessary to have voltage regulators if you used Ni-Cad batteries. With lead acid gel batteries, then the amplifier performance may not be as good after one hour, but it would never suddenly fail. Where a constant voltage was required, then regulators were necessary if using lead acid batteries, adding more cost to the device. The early regulators were also not very efficient, the excess power being mainly dumped as heat..
One problem with Ni-Cads, is the so called 'memory effect'. Many manufacturers have improved their batteries and now better understand this phenomenon.
Memory EffectIf a Ni-Cad battery is repeatedly charged and discharged with only a shallow discharge, (to perhaps a discharge end voltage of 1.13V/cell) and then subjected to a deep discharge, (to1.0V/cell) then the operating voltage will decline, and the capacity decrease. This phenomenon is known as the memory effect. Despite a common misconception, this is also a feature of Nickel Metal Hydride Batteries. This effect will disappear after one or two deep discharges to a discharge end voltage of 1.0V/cell.
Charging Ni-CadsBecause of the memory effect, before charging Ni-Cads, it is wise to ensure that they are fully discharged to an end voltage of 1.0V/cell. This is why many chargers have a built in discharger. It would be as well for you to actually measure the appropriate voltages of your charger and battery combination. It is obvious, that since this discharge end voltage is so critical to the capacity of the Ni-Cad battery, that you should not just leave the battery connected to a load overnight, since it may go well below the desired 1.0V/cell. If the cell is completely discharged, then polarity reversal may occur, and electrolyte creapage takes place. Gas pressure increases inside the cell, and the cell will be permanently damaged. For the same reason, it is essential you match the charger to the battery, in particular if you are using a fast charger. Fast chargers employ circuitry that detects the voltage decrease at the end of the charge cycle (negative delta V - the voltage peaks, and then falls slightly when the Ni-Cad battery is nearly fully charged) and switches to a lower current value. If this detection of negative delta V fails, then the second line of defence depends of detection of the increase in temperature of the battery pack, and if that fails it should finally time out if it has been giving a fast charge for too long a period of time. Before charging they measure the ambient temperature and the state of charge of the battery to calculate the charge rate and time required. These devices cost far more than a simple 0.1C charger.
If you have a number of different voltage/capacity Ni-Cad battery packs, then your charging situation may have to be more 'hands on', unless you have the associated expensive fast charger for each pack. The following notes should help.
The speed of charging is related to the speed of gas recombination at the negative plate and the rate of gas generation. For most cases that I am aware of a charge current of 0.1C for a period of 14 to 16 hours from complete discharge will bring the battery to full capacity, but check with your battery manufacturer to be safe. So, the first requirement is a means of discharging the battery to 1.0 V/cell.
A constant current charger is ideal, but not easy to achieve at low cost for high currents, so a quasi constant current charger is more usual. For the normal domestic UK electricity supply, this is simply a transformer/rectifier with a series resistor between the DC side of the rectifier and the battery . The value of the resistance is adjusted so that the charge current at the end of charging does not exceed the specified current value. This resistor control system is also easily employed if you wish to recharge your batteries from your car battery (assuming the Ni-Cads are less than 10V, say).
Some particular considerations with Ni-Cads: - do not mix old and new cells, or mix different manufacturers or different sized cells within a battery. Internal resistance and other features will be different and can lead to high current flows within the battery pack, and damage to individual cells. Do not overcharge or over discharge, or charge with poles inverted, which may damage the safety vent. For storage longer than about three months, discharge the battery and ensure it is disconnected from any load. You will have to go through two or three charge/discharge cycles before reuse, to get the battery back to maximum capacity. The normal rate of self discharge is about 15% per month, increasing in warm temperatures. With proper attention to their use, it is quite feasible to have over 1000 charge/discharge cycles from Ni-Cad batteries.
Nickel Metal Hydride BatteriesThese became popular in the early 1990's, particularly in the smaller sizes. Compared to Ni-Cads, these will have a higher energy density for the same weight of battery, and possibly double the energy density for the same volume. For example, I have some Ni-Cad AA batteries with a capacity of 750mAh (made by Varta), but the equivalent sized Ni-MH battery has a capacity of 1600mAh. (Admittedly, the Nicad is a consumer marketed battery, the other is industrial.) Ni-MH have different charging and discharging characteristics than Ni-Cads, and many Ni-Cad chargers are unsuitable for Ni-MH batteries, unless they specifically state they are also for Ni-MH cells. I previously mentioned that the Ni-Cad fast chargers were switched off on detection of negative delta V when the battery became charged. This rate of voltage decrease is less for Ni-MH batteries, and is not normally used to determine end of fast charge. For Ni-MH batteries it is more usual to monitor the temperature of the battery, which increases rapidly when it is fully charged. Many Ni-MH battery packs include a temperature sensor which connects with the charging circuit, for example mobile phone batteries, etc. When the battery gets warm, the charger will switch to a lower charge rate. Ni-MH battery chargers usually include temperature sensing circuitry, which is not present in the low cost Ni-Cad chargers.
Although less significant, there is still a memory effect with Ni-MH batteries, which is overcome in the same way as for Ni-Cads previously described. Storage requirements are more or less the same as for Ni-Cads, but they tend to self discharge more quickly, so will need recharging before use, if kept charged longer than a week or so. Treat as for Ni-Cads if not using for a period of a few months - i.e. store at the discharge voltage level in a cool place. A major advantage over Ni-Cads is that they contain no toxic heavy metals, and can be used anywhere in the world. The internal resistance of Ni-MH batteries is higher than for Ni-Cads, and they are not usually used for heavy load requirements.
Lithium-ion BatteriesThese are of a comparatively modern chemistry, originally developed by the UK Atomic Energy Authority, and I believe this technology is licensed to various manufacturers. They do not contain Lithium metal, but they operate on the insertion and extraction of lithium ions into and out of their electrodes. They are becoming more popular, and are being sold on their only advantage of a very high energy density, but in reality, it is only marginally better than that obtained with Ni-MH. There are a number of features with Li-ion, that you need to be aware of, some of which I've listed below.
The nominal cell voltage is 3.6V. However, in practice the cell voltage will vary from 4.2V when fully charged to the usual end point voltage of 2.6V. How can this be tolerable? Today, electronic development has progressed rapidly from the 1960's, and high efficiency switching regulators are available at low cost. For example Micrel make a small unit that is 97% efficient, needs about 3 low cost external components, costs less than £1.00 in quantities of 1000 and is rated at two amps. It contains all the protection that a power supply needs, without the necessity of fuses, etc. This and other similar devices allows the electronic designer to use, say, 3 Li-ion cells in series (Voltage range 12.6V to 7.8V) and the regulator to achieve a constant 5V with very little energy loss. If a linear regulator were used, say typical of the circuitry of fifteen years ago, then the losses would account for over half the energy stored in the battery. The battery voltage can be monitored to give a fairly accurate estimate of the remaining operational time (assuming a constant load of course). Li-ion cells are normally used in a similar situation to that which I have just described by most manufacturers. They are used almost exclusively in the small pro-sumer video cameras, portable mini disc and CD players, but are generally specific to a particular device, and are kept that way for reasons of the manufacturer's profit requirements. For charging and other purposes they often have a microprocessor built into the battery, and manufacturers such as Sony are protective in the protocol that the battery uses when communicating to the charger and the camera, so it becomes very difficult to substitute another power source.
Li-ion batteries begin to fail as soon as they are made. When they are a year old, whether or not you have used them, they will have a reduced performance. They are very sensitive devices, - if you drop them on the floor, they may not work again. If you overcharge them slightly, or subject them to a slight over voltage when charging, they will never work again, and that is one reason a microprocessor is often incorporated within the battery case, to closely monitor the voltage. Another reason for the microprocessor is to lock the consumer into the manufacturer. Some Sony cameras will only work if you use a Sony battery for example, and I believe that some models require an expensive repair shop visit just to unlock the camera again. It is not recommended that Li-ion cells are used in equipment not designed for their use, for reasons which by now should be pretty obvious. Li-ion have no memory effect problems and may be stored in any state of charge, but storing at nominal voltage is recommended.
Finally, for comparison purposes, we can compare a Sony Info-Lithium NP-F550 7.2V 10.8Wh (1500mAh) battery (costing about £50.00) with 6 of the Ni-MH cells mentioned earlier to give 7.2V at 1600mAh. (total cost £20.00). The Sony battery weighs approximately 100gms, six of the Varta cells weigh a total of 160gms + wiring + case. The volume, but not the shape would be more or less equal. My personal view is that there is little benefit to the end user by specifying Li-ion batteries, other than a small saving in weight. There is a large financial benefit to the manufacturer of the equipment, however.
Well, I don't have much more I want to say, right now, but if you have access to the Internet, then I have a web site at http://www.rwc.yertiz.com which has some other information, and if you search for it, you can find my e-mail address. I also have a software related web site at http://raywest.com. I will normally respond to e-mails within a couple of days.
If you would wish for a more detailed explanation of any power supply system, then if you contact the editor, then I may be requested to write another article - unfortunately this was put together at the last minute, due to a complete failure of my filing systems - a sort of compost heap on the desk/floor.
copyright - Ray West MSc - August 2000