Tim Lash, June 20, 2011
There are many manufacturers of the ubiquitous AA size battery. A single manufacturer may have several different models of AA in their product line. How does one know how a particular make and model performs? How a AA battery behaves in a circuit is a complex question to answer. All most people really care about is how long does the battery last and how much does it cost. Manufacturers do state some performance metrics in their datasheets. However, the test conditions may vary between manufacturers. A similar discharge test applied to multiple makes and models can verify performance claims and allow a consumer to determine if that particular battery is worth the price. This article looks at how different makes and models handle a 100 mA constant current discharge test.
The following piece of test equipment was used:
There are two major chemical formulations of the 1.5 Volt, AA battery: Alkaline and Carbon-zinc. In the past few years, a third chemistry, Lithium Iron Disulfide, has appeared. How long a battery lasts depends on many factors. Some of the factors include:
A thorough discussion on the factors that affect battery discharge is beyond the scope of this document. This article attempts to see how long a battery lasts when undergoing a 100 milliamp discharge. The batteries tested are bought at various stores and tested as soon as possible after purchase. The metric of comparison is how many amp-hours is available from new batteries. Each battery was exposed to a 100 milliamp constant current discharge until the cell voltage reached 0.8 volts. Most people consider 0.8 volts the lower limit of useful voltage on a AA. That is why the tests were ended when the cells reached 0.8 Volts. There is still some energy left in the battery, but most circuits cannot make use of the lower voltages. The Joule-Thief is an example of a circuit made from discrete parts that can function down to lower voltages. New DC-DC boost converters such as the TPS61200 integrated circuit from Texas Instruments can function down to 0.3 Volts.
Ten different AA batteries were purchased. Some were carbon-zinc types and some where alkaline. Usually, when a AA battery is described as “Super Heavy Duty” it is a carbon-zinc type chemistry. The CBA II was used to pull a constant current discharge of 100 mA on each battery until it reached 0.8 Volts. The amp hours and a discharge graph were computed in Scilab.
Data files of the results are available in the aa_100_mA_discharge_data.zip file. The Scilab script used to do the plots is also included.
The figure below shows a graph of the discharge curves. The plot is the cell voltage versus accumulated amp hours at 100 mA constant current discharge:
The table below shows costs and amp hours:
Unlike the AA 2 Amp Discharge Test, the carbon-zinc batteries are useful at a 100 mA discharge. All batteries provide over 0.700 amp-hours of energy. Alkalines do last 2 to 3 times longer than the carbon-zinc.
It is interesting to note that the Duracell Ultra was the longest lasting battery in the 2 A discharge test. However, the Duracell Copper Top and Rayovac Alkaline are the best performing batteries in this test as far as an amp-hour basis is concerned. The Ultra is formulated to last longer during higher current draw but this construction does not necessarily provide the longest operation for low discharges.
As for value of the batteries, we must look at the metric of $ per amp-hour. A lower number is better. The Sunbeam Super Heavy Duty offers the best value. A pack of 8 is available for $1 at the Dollar Tree Stores. The Sunbeam AA is tested at 0.828 amp-hours. A single Duracell Copper Top is rated at 2.610 amp-hours. The Copper Top lasts about 3 times longer than the Sunbeam Super Heavy Duty. Essentially, one would have to use 3 Sunbeams to every 1 Copper Top. If one doesn't mind switching out batteries often, and your circuit can work down to 0.8 volts, a carbon-zinc AA may be cheaper to use than an alkaline.
Value is a complex question to answer so it will be explored in another article in the future.
AA 2 Amp Discharge Test – shows results of a 2 amp constant current discharge on AA batteries.