Battery energy -- What battery provides more?

Battery energy does not always remain quietly stored in the battery. Sometimes you must get that energy out in a hurry. Short bursts of power are required, for instance, when starting an automobile on a cold morning. Cold cranking requires high current from the battery. When you turn on the viewing screen of a digital camera, you also demand high current output from the camera's battery.

The power used comes from stored energy

During every interval in which power is used, a quantity of energy is drained from the battery. That quantity of energy is equal to the amount of power, multiplied by the time the power flows. Energy has units of power and time, such as kilowatt-hours or watt-seconds. As the stored battery energy is used up, the available voltage and current drops lower and lower until finally the battery is exhausted. Then it is time to recharge or replace the battery. A good battery must supply two requirements. First, it must be able to meet the power demand by supplying adequate voltage and current when needed. Otherwise it is useless from the start. Secondly, there must be sufficient energy to last a long time, or else the battery must be economical, readily replaced, or easy to recharge.

Joules are units of energy or work

The Joule is the International Standard unit of energy defined as one watt-second. One watt-second of mechanical work is the work done by a force of one Newton (or 0.2247 pound) pushing through a one-meter distance. 3600 Joules are contained in one watt-hour, since an hour contains 3600 seconds,. Batteries are often rated in milliampere-hours instead of watt-hours. This battery rating can be converted to energy if the average voltage of the battery during discharge is known. For instance, a 3.6-volt Lithium-ion battery rated at 850 mAh will maintain a voltage of 3.6 volts with little variation during discharge. Multiply the voltage of 3.6 volts times 850 mAh to yield 3060 mA-volt-hours, or 3060 milliwatt-hours. 3.06 watt-hours equal 11016 watt-seconds or Joules. Compare this value to those found in the tables below.

Joules may be converted to other familiar units using the numerical factors given below. Divide the number of Joules by 3.6 million to obtain kilowatt-hours. Divide the number of Joules by 1.356 to obtain the number of foot-pounds, a popular unit of work in the English system. Divide by 1055 to obtain the equivalent number of BTU (British Thermal Units). Divide by 4184 to obtain the number of food Calories! Yes, food Calories are energy, of course. This comparison does not put batteries in a good light compared to peanut butter. Two tablespoons of smooth peanut butter contain 191 Calories, or almost 800,000 Joules! It takes a huge battery to contain this much energy.

What battery type gives the most energy for the price?

In the table below we present the cost per Watt-hour, Specific Energy, that is Watt-hours per kilogram, Joules per kg, and the Energy Density, Watt-hours/liter for various types of batteries. It is not surprising that the well-known Lead-acid storage batteries head the list. Fine for use in our cars, but a little inconvenient in a laptop. And why are Alkaline long-life and Carbon-zinc batteries in the list? Aren't they non-rechargeable? This was thought to be the case, previously, but now they can have their lifetimes extended by recharging. Ordinary alkaline cells may be recharged literally dozens of times using the new technology built into the Battery XtenderTM. Recharging alkaline, nickel-cadmium and nickel-metal hydride cells side-by-side in one automatic charger opens up new possibilities for battery selection economy.

Battery
Type
Cost
$ per Wh
Wh/kgJoules/kgWh/liter
Lead-acid$0.1741146,000100
Alkaline long-life$0.19110400,000320
Carbon-zinc$0.3136130,00092
NiMH$0.9995340,000300
NiCad$1.5039140,000140
Lithium-ion$0.47128460,000230


Costs of lithium-ion batteries are falling rapidly in the race to develop new electric vehicles. The $0.47 price per watt-hour above is for the Nissan Leaf automobile, and they predict a target cost of $0.37 per watt-hour. Tesla Automobiles uses a smaller battery pack, and they are optimistic about reaching a price of $0.20 per watt-hour in the near future.


There is another type of battery that does not appear in the table above, since it is limited in the relative amount of current it can deliver. However, it has even higher energy storage per kilogram, and its temperature range is extreme, from -55 to +150°C. That type is Lithium Thionyl Chloride. It is used in extremely hazardous or critical applications such as space flight and deep sea diving.

The specifications for Lithium Thionyl Chloride are $1.16 per watt-hour, 700 watts/kg, 2,000,000 Joules/kg, and 1100 watt-hours per liter. For more information of Lithium Thionyl Chloride please contact Tadiran Batteries.

For more tables on energy storage, including the available storage in standard AAA, AA C, and D batteries, please follow the link to Battery Energy Tables. and for details on charging Alkaline batteries, please see Alkaline charging.