Intro - Primary cells - Secondary cells - Comparison tables
Secondary cells(rechargeable)
Rechargeable cells have the obvious advantage over primary cells that once they run down there is no need to
throw them away, just charge them up and use them again. This comes at a price of course, they don't have quite
the capacity that you'd find in a primary cell and they can be quite expensive but the ability to recharge makes
up for that.
NiCd
Nickel cadmium cells were the first rechargeable consumer cells to reach the market. They have a cell voltage
of only 1.2v so compared to alkaline or lithium cells a greater number of cells may be required to reach a
particular voltage. They have a useful capacity and a reasonable price, though they have been superceded in recent
years by nickel metal hydride (NiMh) cells. NiCd cells are available in all the usual sizes as well as a range of
specialist cells.
Typical NiCd cells are the Sanyo range. A standard AA cell weighs 21g and has a capacity of 600mAH while their high-performance AA cell has the same capacity with a longer life and weighs only 24g. Of slightly more interest to antweight builders are the AAA cells with a 250mAH capacity and a weight of 11g but the specialist cells are likely to be of most use. Specialist suppliers such as Overlander will sell you 1/3AA cell which weighs 7g with a capacity of 110mAH or a 1/3AAA cell which weighs only 4g but has a small capacity of 50 mAH, just to name two examples. You may also see NiCd cells designed to be mounted on printed circuit boards for computer memory backup applications, but in general these cells have been replaced by NiMh equivalents with higher performance.
NiMh
Nickel metal hydride cells are a recent arrival on the marketplace, they offer the same voltage and very similar
properties to those of NiCd cells with a significant enhancement in capacity and cell life. These cells give an
antweight robot a significant boost over the performance of NiCd cells for a slightly higher price and wherever
possible should be used in preference to NiCd cells. NiMh cells are available in the full range of consumer and
industrial cell sizes and provide many options for antweight robot builders.
A typical NiMh AA cell weighs about 27g and has a capacity of 1100mAH while a typical NiMh AAA cell has a 650mAH capacity and weighs about 13g. Of the many specialist NiMh cells on the market the ones most used by antweight builders have been the Varta PCB mount packs. These batteries are designed to provide backup power to low power electronic circuits and are available in a useful range of voltages with respectable weights and capacities. The two examples of most interest to antweight builders are 150mAH packs, the 4.8v 4 cell version weighing just over 25g and the 6 volt version weighing 32g. These packs are ready encapsulated in a very small size and have been used in a large number of antweight robots.
Li-Ion
Lithium ion batteries are a relatively new technology, most seen in the consumer marketplace fitted to high-end
tech gadgets. These cells have a very impressive capacity for their weight but suffer from several disadvantages
for an antweight robot builder. Their relatively high cell voltage of 3.6v is not enough on its own to power radio
control gear and other electronic systems on an antweight robot. This means that either a complex and expensive
electronic switching regulator like the one you will find in a mobile phone or else a stack of two batteries giving
7.2v with a 5 volt regulator or BEC unit is required to give a 5 volt output. In addition to these problems, lithium
ion cells require a very specialised charger, both this charger and the batteries themselves are very expensive.
A limited number of robot builders have experimented with lithium ion batteries but the large extra investment has
not delivered any competitive success.
Lead acid.
Most large rechargeable batteries used for higher power applications than can be satisfied by consumer cells will
use lead acid cells. These have, as their name suggests, lead electrodes and sulphuric acid as the electrolyte. Even
though they are available in smaller sizes and with safe gell electrolytes which do not leak they are too heavy for
use in antweight robots. Some older Sony Walkman personal stereos used rectangular 2.2v lead acid cells about the
same size as an AA cell but sadly these cells no longer seem to be in production and no data can be found for them.
Battery packs
A single NiCd or NiMh cell delivers 1.2v. For a competitive antweight robot a minimum number of 4 of these cells
is required to give 4.8v, just enough to run the electronics. If 4 bare cells are to be connected together some kind
of battery holder will be required which can add unnecessary weight to your robot. Luckily most rechargeable cells
are available fitted with solder tags to allow easy connection between cells so it can be a fairly easy process to
assemble your own battery packs. If self assembly seems like a little too much, why not try one of the many commercial
battery packs available from a good model shop? These packs are available in 4.8v, 6v and 7.2v variants in a wide
range of capacities and best of all they are supplied ready fitted with a power connector for your radio. As an
example I have a commercial 4.8v 4 cell 180mAH pack that can run a simple antweight robot for 20 minutes or so on a
charge and weighs only 30g.
Charging NiCd and NiMh cells
NiCd and NiMh cells require different charging methods due to their different chemistry. The simplest NiCd
charger is a straightforward constant current power supply delivering about a tenth the capacity of the cells, for
instance 15mA to charge a 150mAH cell. An NiMh charger by contrast is a higher current power supply with extra
circuitry to monitor the condition of the cell during the charge cycle. A NiCd charger will shorten the life of
NiMh cells and a NiMh charger can damage NiCd cells so it is important that the right charger is used for each cell
technology. It is possible to build a charger of either type yourself if you are an electronics enthusiast but it
is a lot more convenient to use a commercial charger if you can afford one.
The more expensive commercial chargers such as those available from model shops allow almost any combination or capacity of cells to be charged, but all these features come with a price. Depending on the cells you choose it may be possible to use a cheaper charger instead if you don't fancy breaking the bank. For example the Uniross consumer NiMh charger is designed to charge 8.6v PP3 batteries, the same charger will quite happily charge two 150mAH 4.8v Varta NiMh packs in series connected to the PP3 terminals.
Intro - Primary cells - Secondary cells - Comparison tables
© copyright John W. List 1998 - 2007