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Because many hobby and commercial electronic products receive their operational power from batteries, I thought it suitable to include this section. Before getting into details, you must begin by learning some basics.
The term battery actually refers to an electrochemical DC power source containing multiple cells. Nominal voltage levels for individual cells can vary, depending on the type, from 1.25 to 2 volts per cell. The 1.5-volt electrical devices most people refer to as "flashlight batteries" are, technically speaking, cells.
Primary batteries, or "dry cells," are typically thought of as non rechargeable, although this description is not always true. The electrolyte used in primary batteries is not always "dry," either.
Secondary batteries are rechargeable, and usually do contain a liquid or paste-type electrolyte.
The ampere-hour (amp-hour, Ah) rating is a term used to define the amount of power that a battery, or cell, can deliver.
All batteries use a chemical reaction to produce electrical current. Battery types are usually defined by the types of chemicals or materials used in their construction.
There are seven main types of commercially available batteries: zinc, alkaline, rechargeable alkaline, nickel-cadmium, lead acid, "gelled" electrolyte, and lithium.
Zinc batteries are the most common type of "flashlight" battery. They are available in regular and heavy-duty types, but neither is very exemplary in regard to performance levels. They are not recommended for the majority of electronic projects.
Regular alkaline batteries last 300 to 800% longer than zinc batteries, depending on their application.
Nickel-cadmium batteries, or "nicads," are very popular secondary batteries. These are the type used in the majority of commercially available rechargeable products. A newer type of high-capacity nicad is also being used extensively today. It boasts a much longer service life, and a faster recharge rate (typically 5 to 6 hours). A bothersome peculiarity of nicads is their tendency to develop a "memory." Because of this characteristic, it is recommended that they be fully discharged before recharging.
Lead acid batteries are most commonly used as automobile batteries.
They are also available in smaller sizes that are sealed (except for a blow hole to allow gases to escape). "Motorcycle"-type lead-acid batteries are available in a wide range of amp-hour ratings, and are a good choice for heavy-duty projects.
Gelled electrolyte batteries fall into the same category as lead-acid types.
They are most often used in uninterruptable power supplies (UPS; power supplies intended to supply 120-volt AC power in the event of a power failure), burglar alarms, and emergency lights.
Lithium batteries are designed to supply a small amount of power for a long period of time. Lithium batteries are most often used to power memory backup systems in computers, and in wristwatches. They are very expensive.
The most recent entry into the common marketplace is called the rechargeable alkaline battery. It was developed and patented by Rayovac Corp. under the trade name Renewal. These batteries offer 2 to 3 times the energy storage capacity of nicad batteries, higher terminal voltage (1.5 vs. 1.2 volts), retention of full charge for up to 5 years in storage, and lower operating temperatures, and they are "environmentally friendly." Rechargeable alkaline batteries can be used through well over 25 full charge/discharge cycles with very little degradation in performance, so that can add up to quite a cost savings over buying "throwaway" batteries.
As stated previously, the power delivering ability of a battery is rated in amp-hours. However, this does not mean that a 5-Ah battery will deliver a full 5 amps for one hour, and then suddenly quit. The battery manufacturer will calculate this rating over a longer period of time and then back-calculate to find the rating. For example, a 5-Ah battery should be capable of providing 500 milliamps for 10 hours. This calculates out to 5 amps for 1 hour, 1 amp for 5 hours, or 500 milliamps for 10 hours.
Trying to operate a battery at its maximum current rating for 1 hour is destructive to the battery. A 5-Ah battery would be a good choice for a project requiring one amp of current, possibly even two amps. Primary battery life and secondary service life (the total number of charge/discharge cycles it can withstand before failure) can be extended (by factors ranging from 100 to 400%) by avoiding excessive current drain.
In addition to excessive-duty operation, the most destructive variable to batteries is heat. Batteries should always be stored in a cool place, even a refrigerator (not a freezer!). Batteries being used in vehicles that are left outside in the sunlight should be removed and brought indoors, if possible.
Secondary batteries should be maintained in a charged state. Lead acid and gelled electrolyte batteries need to be recharged about once every 3 to 6 months if not used. Nicads, on the other hand, have a self discharge rate of about 1% per day! For optimum performance, they need to be left on a trickle charge continuously when not in use.
A Few Words of Caution
Some types of batteries contain very potent acids, caustic substances, or highly poisonous materials. These materials include mercury, lead, hydrochloric acid, and other substances so toxic that the EPA (U.S. Environmental Protection Agency) classifies them as toxic waste. If you want to experiment with chemistry, buy a Gilbert Chemistry Set, but don't attempt to mess around with this stuff.
Don't try to recharge a battery that is not designed for recharging. It could explode. And if the explosion doesn't do enough damage in itself, toxic waste can be sprayed in the eyes and mouth. Even rechargeable batteries can explode if recharged too fast, or if overcharged.
Lead acid batteries can produce enormous current flows. An accidental direct short, with a lead acid battery as the power source, can literally blow up in your face; this could result in eye damage, or fires. Large batteries should be fuse-protected right at the battery terminals.
The recharging of secondary batteries is not a complex process, but there are a few rules to follow. Nicad batteries should be completely discharged before trying to recharge them. They should not be discharged too quickly, however. One good way to discharge them properly is to connect them to a small incandescent lamp rated for the same voltage as the battery. When the lamp goes out, they're discharged. Of course, other types of resistive loads will perform this function as well as a lightbulb, but they won't provide a visual indication of when the discharge has been completed. As stated previously, it is destructive to discharge any battery too quickly. Nicads, however, are the only secondary battery type in which mandatory discharge becomes a concern. Other types of batteries can be recharged without having to be fully discharged.
Recharging procedures are generally the same for all types of secondary batteries. The primary rule to keep in mind, in regard to charging rates, is to not try to recharge them too fast. In some cases, trying to recharge a secondary battery too fast may cause it to explode. A good rule of thumb to follow is to not allow the charging current to exceed one-tenth the value of the amp-hour rating. For example, a 5-Ah battery should not receive a charging current any higher than 500 milliamps (1 /10th of 5 amps).
In many cases, the recharge voltage applied to a secondary battery is higher than the rated battery voltage. For example, a typical automobile battery is rated at 12 volts (six cells at 2 volts each). The charging voltage applied to an auto battery is 13.8 volts. There are practical reasons for doing this with a car battery, but it is seldom advisable to follow this practice with batteries in other applications. In the majority of situations, the use of higher voltages ends up being a waste of power, and a potential risk toward overheating the battery.
Building a General-Purpose Battery Charger
By now, you may have come to the conclusion that all you need to properly recharge a battery is a variable DC power supply and the properly sized series resistor to limit the current. You're right! For example, if you wanted to recharge a 5-Ah, 12-volt battery, you will want to limit the charging current to 500 milliamps.
When the "dead" battery is first connected to the power supply, assume it to look like a short (it will come close to that if it is totally discharged). That means you need a resistor to drop 12 volts at 500 milliamps. Using Ohm's law, that comes out to 24 ohms. Because 24 ohms is not a standard resistor value, a 27-ohm resistor will do nicely. In the beginning of the charge cycle, this resistor will be required to dissipate almost 6 watts of power, so use one with a 10-watt rating. Alternatively, you could use two 56-ohm, 5-watt resistors in parallel; or, you could use three 8-ohm, 2-watt (or higher, 5-watt is preferable) resistors in series. Set the power supply to 12 volts, put the 27-ohm resistor in series with the battery, and the battery should recharge properly.
There is a small problem with this simple resistor power supply method. As the battery begins to charge, the voltage across it will increase. This causes a subsequent decrease of voltage across the resistor, and the charging current decreases. When the battery gets close to being totally recharged, the charging current drops to a very low value. The result is that the recharging process takes more time. This might or might not be a problem, depending on your needs.
If a more rapid recharge is desired, the circuit illustrated in Fig. 1 will provide a constant charge current, regardless of the battery voltage.
This charge current will be maintained until the battery is fully charged, then it will automatically drop to a safe level. This circuit can be used with any type of variable power supply (as long as the power supply can be adjusted to a voltage slightly higher than the battery voltage), and it can charge any type of battery up to about a 15-Ah rating.
Referring to Fig. 1, the variable power supply is connected to the positive and negative input terminals of the charging circuit, and adjusted to be about 1 volt higher than the battery voltage. (The "lab quality power supply" project, discussed in earlier sections, will work well with this circuit.) The rotary switch (RS1) is set to the desired charging current position:
Position 1 _ 15-Ah batteries (or larger)
Position 2 _ 7.5-Ah batteries
Position 3 _ 2-Ah batteries
Position 4 _ 1-Ah batteries
Position 5 _ 0.2-Ah batteries (or smaller)
RS1 is set to the position rated at, or below, the actual amp-hour rating of the battery. For example, a battery rated at 5 Ah should be recharged in position 3, not in position 2! FIG. 1 is a simple current-limiting circuit. It is identical in function to the current-limit circuit used in the "lab power supply" project discussed earlier in this guide. However, some component values have been changed to provide different current-limit values. D1 is designed to protect the circuit in the event the battery terminals are connected in the wrong polarity.
If you would like to modify the circuit in Fig. 1 to provide different charging currents for special needs, this can be easily accomplished.
Simply divide 600 mV by the desired charging current in milliamps. The answer, in ohms, will be the total resistance needed between Q2's base emitter leads to provide the desired current.
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