Intro - Primary cells - Secondary cells - Comparison tables
Primary cells (non rechargeable)
Primary cells can deliver a huge capacity for their weight but suffer from the disadvantage that once they are
run down that's it, throw them away and buy another set. If you are prepared to take this extra cost you can
achieve surprising results using a non rechargeable battery system.
Alkaline MnO2
These are the 1.5v "Alkaline" cells from a variety of manufacturers sold almost everywhere for general purpose
use in consumer electronic devices. Your personal stereo almost certainly runs on a set of Duracell or similar
alkaline AA cells. These cells have the advantage of being readily available and fairly cheap for a very reliable
high capacity. An alkaline AA cell has a capacity somewhere around 1500mAH while an AAA cell's capacity is about
1000mAH. Unfortunately they suffer from the disadvantage of weight, an alkaline AA cell weighs about 25g while an
AAA cell weighs about 12g.
Most robot builders who use these batteries have 4 AAA cells in series to give a 1000mAH 6 volt battery pack weighing just under 50g. As an alternative, there have also been robots powered from 9v PP3 style alkaline batteries. These have the advantage of a convenient connector for changing batteries and have a typical capacity of 550mAH for a weight of about 45g. However as with any 9v system you will need a 5v regulator or BEC unit to protect your radio gear if you use one of these batteries.
The most interesting alkaline cell from the point of view of an antweight builder is the AAAA cell. This weighs 6g and has a capacity of around 600mAH. In the UK these cells are only available by mail order but they can also be found as the internal cells for some 9v alkaline PP3 batteries. If you can find a supply of these cells then a 24g 600mAH 6 volt battery pack becomes a very intriguing possibility.
Lithium MnO2
"Lithium" cells are sold for a variety of electronic devices from cameras and digital watches through to
medical and scientific instruments. Most lithium cells are of little interest to antweight robot builders because
of their size. For instance a digital watch battery simply does not have the capacity to power an antweight. The
larger lithium cells however provide a very attractive combination of lower weight and higher capacity than their
alkaline MnO2 equivalents while retaining the same 1.5v cell voltage, which certainly makes them worth a second look.
Obvious lithium cells for use in antweight robots are the larger 6 volt photographic batteries, for example
type numbers 223 and 2CR5. These cells or their equivalents are produced by all the major manufacturers and are
available from retail photographic stores as well as mail order suppliers. The 223 weighs around 38g for a typical
1400mAH capacity while a 2CR5 is a bit heavier at 39g for its 1500mAH capacity. Lithium cells are also available
in more familiar sizes: a typical lithium AA cell weighs just under 15g with an astounding capacity of 2900mAH
while 9v lithium PP3 cells have a 1200mAH capacity and weigh only 34g.
The disadvantage of a lithium cell is the price. For an item that gives at best an hour or two of use from an
antweight robot, the cost, equivalent to that of a pair of radio control crystals or a less expensive servo, is
too great. Lithium cells may have a place in high end competition where money is no object but they are too
expensive to use in an ordinary everyday robot.
Zinc Air
There is one further battery technology that is worth mentioning. Zinc air cells are lightweight high power
cells that are activated when their internal chemistry is exposed to air. They are normally supplied with a paper
tab, which you pull to uncover an air hole and activate the battery. Most zinc air cells are small coin cells or
watch batteries but both Energizer and Duracell list an 8.4v PP3 with a weight of around 33g and a capacity of
about 1100mAH. These cells are more expensive even than lithium cells and as yet no known antweight robot has
used one but they have interesting enough technical features to make them worth a mention.
Other primary cell systems.
The original dry cell was the zinc carbon cell, followed by a refined version, the zinc chloride cell. The far
superior alkaline cells have superceded both these batteries and though they are both cheap and readily available
the extra performance of the alkaline cells is well worth their extra cost.
Finally, I have to mention another primary cell, as a warning to avoid it. Lithium Thionyl Chloride cells have a very impressive specification on paper and are readily available from electronics suppliers as replacement motherboard batteries for older Apple Macintosh computers. In the past this combination has led some antweight builders to experiment with these cells. These cells are designed to give tiny currents to retain memory settings for a period of years, not to give large currents to power motors for a period of minutes. They are hermetically sealed (i.e. they have no vent) because the electrolyte, SOCl2, is highly toxic and corrosive. They are to be avoided at all costs because when they are used heavily they can overheat and explode, spraying their dangerous contents everywhere. You have been warned!
Intro - Primary cells - Secondary cells - Comparison tables
© copyright John W. List 1998 - 2007