Current Capacity

Batteries are rated according to voltage and current capacity (ampere-hours). Each battery is composed of one or more cells. In our traffic analogy, these cells are representative of parking garages for our cards, or electrons. When the battery is hooked into a circuit, the cars have a path on which to travel. They then begin to flow and the circuit has current. When all of the cars have left the garage and run out of gas, the battery is dead and the current flow ceases. Current capacity is the approximate gas mileage of our cars. The voltage of a battery is determined by the chemical composition of the material within the cells. Just as diesel fuel and gasoline both have the capacity to do work within our cars, the different chemical compositions have capabilities of doing work similar to one another.
 

mA x hours = mAH
(current capacity)

The current capacity measures how quickly a battery will discharge under certain circumstances. This figure is determined by multiplying the flow of the current from the battery into the circuit by the amount of time the battery is able to provide that current.
 

So,
Time = mAH/mA

If you are given the current capacity of a battery (in mAH), dividing this number by the current requirement of the circuit will give you the time of the battery will last under a constant load.

Let’s take, for example, a 9.6 volt battery rated at 1500 mAH (milliampere-hours) at 250 milliamps. By dividing 1500 mAh by 250 mA, we find the battery will discharge in about six hours.

1500 mAh ÷ 250 mA = 6 hours

In contrast, the battery might be rated at 1250 mAh if used in a circuit requiring 500 mA. In this case, the battery would discharge in about 2.5 hours.

1250 mAh ÷ 500 mA = 2.5 hours

It is this concept that makes predicting the usable time of camcorder batteries difficult. Camcorders require a great deal of current, especially when using certain features, such as auto-focusing, fast forward, or rewind. Each of these features requires a different amount of current. This factor alone makes the usable time difficult to predict. However, other factors must be taken into consideration, such as temperature. Therefore, a 2000 mAh battery will not necessarily last twice as long as a 1000 mAh battery -- although it will typically be close.

Alkaline cells, regardless of their size, produce 1.5 volts. So, the parking garages may only hold a certain number of cars (remember, the TOTAL number of cars is voltage). Therefore, a “D” battery is composed of one alkaline cell. Conversely, a 9-volt battery is composed of six alkaline cells wire together in series. To explain this, we will deviate slightly from our traffic scene to a nearby railroad. Hooking batteries or cells in series is just like using multiple locomotives to pull cargo on a railroad -- the train is able to utilize the cumulative power of all of the engines to pull the railroad cars.

Voltage Discharge Curve

The voltage discharge curve is a vital tool to help you determine the appropriate type of battery to sell. This factor is determined by the chemical properties of the battery. Certain batteries exhibit what is known as a sloped curve. In this type of battery, the voltage gradually degrades as the battery gets older. Batteries exhibiting flat voltage curves maintain their voltage at a somewhat constant level until the end of their life, where the voltage suddenly drops off. In the graph at left, the top curve represents a flat discharge curve, the bottom curve represents a sloped discharge curve.

Alkaline Batteries

The most popular battery composition is alkaline. Alkaline batteries have a sloped discharge curve. Their voltage gradually drops off over the life of the battery until eventually, the battery is no longer usable. Although the discharge curve is sloped, it is still flatter than zinc-chloride and carbon batteries. Alkaline batteries can deliver up to 80% of their original capacity after being stored for four years. This makes alkaline batteries ideal for emergency devices such as flashlights, radios, and TVs.

Alkaline batteries are 8 to 10 times more powerful than the zinc-chloride or zinc-carbon batteries. They’re very good for use in high-current applications, such as motorized toys and portable TVs. Since they are able to emit large amounts of current for short periods of time without significant voltage loss, alkaline batteries are also ideal for photo flash units.

Lithium Batteries

Lithium batteries are great for long-term use. They last 2-3 times longer than alkaline batteries. Lithium batteries also perform well n extreme temperatures and are an excellent choice for devices such as smoke detectors and wireless alarm systems. These batteries are able to ahndle excessive current applications, like CD players and portable computer equipment. The discharge curve of a lithium battery is similar to that of an alkaline battery; however, some lithium batteries have shelf lives of up to 10 years!

Nickel-cadimum

Nickel-cadmium (Ni-Cd) batteries introduce a different spectrum of battery use. Once these batteries have reached the end of their effective lifespan, they may be recharged over and over again, up to 300 times. Ni-Cd batteries feature a flat discharge curve, which means they will deliver their full voltage for a length of time, then suddenly discharge. This works well in motor-driven devices, such as RC cars and CD players.

Hi-capacity Ni-Cd

Hi-capacity Ni-CD batteries are a great step-up from standard Ni-Cd batteries because of greater current capacity. The voltage rating on hi-capacity Ni-Cd batteries are the same as standard Ni-Cd batteires. One major disadvantage of Ni0Cd batteries is the cell capacity.  The cells in Ni-Cd batteries produce only 1.25 volts each. Batteries with multiple cells are usually rated in multiples of 1.2 volts. For example, the 9 volt equivalent Ni-Cd battery only provides 7.2 volts (6 cells x 1.2 volts). As a result, Ni-Cd batteries are not recommended for voltage-dependent devices such as clocks.

Battery Memory:
Ni-Cd batteries have a unique property known as memory. After frequent partial discharges and charges, a Ni-Cd battery will no longer deliver its full capacity. The battery “remembers” how long it is used. Fully discharging and recharging the battery several times will usually lessen the effects of memory and restore nearly full capacity. In some cases, discharging and recharging the battery will restore full capacity and ERASE the memory!

Nickel-metal-hydride (Ni-MH)

Ni-MH batteries have all the advantages of Ni-Cd batteries and more, and lack their primary drawback. Ni-MH batteries:

• Do not have the memory feature that Ni-Cd batteries do
• Are longer lasting than Ni-Cd
• Can be recharged more times than Ni-Cd (500 or more times, compared to approximately 300 for Ni-Cd)
• Are more expensive than either type of Ni-Cd battery.

Rechargeable Alkaline

Rechargeable Alkaline batteries are yet another option. They have the advantage of having the exact same voltage as standard alkalines, so they are interchangeable in every situation. Unlike any of the other rechargeables, they come charged and ready to use (because they do not lose their charge as quickly as the others). Rechargeable alkaline batteries can not be recharged as many times as other rechargeable batteries, however. An absolute maximum is about 100 times, but the maximum voltage is reduced as the number of cycles increases.