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Published Columns: September 2007
Curbing Your Consumption:
Economics of Portable Power

By John Ford

Our lives are filled with battery powered devices these days. Many come with a space for one-use batteries or can be used with rechargeables. Others have custom battery cells with either old type Nickel-Cadmium (NiCad), the newer Nickel Metal Hydrid (NiMH), or the even newer Lithium Ion.

Whether to use batteries or a wall transformer to power these devices is one decision – which battery type to use where you have a choice is another. Which you choose could depend on performance or cost.

A wall transformer can use the same amount of power as the device itself, doubling your consumption. Since the power use is low, that isn't a big cost consideration for most small electronics.

Choosing a battery type can be complicated.

Traditional alkaline one-use batteries are a good option where the device is low-power, or needs to sit for a while, like an emergency device. If you use them in a digital camera – a high power device – they become really inefficient when trying to deliver a large amount of current and become drained very quickly. Most of the power is consumed inside the battery as heat. Rechargeable batteries of most types are good at delivering higher amounts of current – much better for high-drain devices.

NiCads were notorious for their 'memory effect' which dramatically reduced the power they could deliver from a charge. NiMH are much better, but still need to be fully discharged once in a while. It is also recommended to fully charge and deplete them through at least three cycles at the beginning to allow the cells to reach full potential. Lithium Ion actually prefer to be charged after each use – and they hold a charge much longer on the shelf.

NiMH AA cells are becoming available in capacities which are now greater than alkaline cells, which means you get just as much energy from one charge. Since they're about four times as expensive, and can be recharged hundreds of times, they can be much cheaper. A good quality charger that watches and controls the charge of each cell is a must to get the best life. (about $25 to $50 for four cells)

In order to recharge a cell, you need to put about 40% more energy in than they will deliver, and the recharger is often only 50% efficient.

Lithium Ion are still more expensive than NiMH, and their chemistry can only be made to deliver double the voltage of a standard AA, C, or D cell – so cannot be used as direct replacements, although there are some available apparently.

Both NiCad and NiMH are slightly lower voltage compared to an alkaline, but many devices have been engineered to take this into account. You can also buy adaptors to use AA cells in C and D sizes.

John Ford is a technology consultant, owner of a small energy conservation business,and the Nominated Candidate in Ottawa South for the Green Party of Ontario.