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Monday, May 11, 2009

Batteries Part 2 Nickel-Cadmium

These rechargeable cells have been around for a long time, and were the only rechargeable batteries available in the standard dry cell sizes for many years. They run at a lower voltage than alkaline cells do, having 1.25v fully charged and 1.0v when drained, so voltage-sensitive electronics (like digital cameras) do not work well or at all with them. Ni-Cads also have a lower capacity than alkaline batteries do, they usually only have 5-600mAh, or equivalent to the carbon-zinc cells. Cadmium is a toxic metal similar to lead, so these batteries need proper disposal.
They have advantages as well. Ni-Cads will discharge at very high rates without harm to the cells. A Ni-Cad AA cell can discharge at ten or more times the rate that an alkaline battery can handle. Cordless drills and RC cars almost always use Ni-Cad battery packs for this reason. They can handle as many as a thousand cycles before failing, and their self-discharge rate of 10-20% per month is the lowest of the rechargeable cells. Their efficiency (power in/power out) of 70-80% is very good as well.
To get the best use out of your Ni-Cad cells, it is essential to purchase a ‘smart’ Ni-Cad charger, many of the problems with these batteries are due to cheap constant-voltage chargers. Overcharging Ni-Cads is one of the fastest ways to destroy them, on a constant-voltage charger a fully charged Ni-Cad cell will actually draw more current as it heats up, which makes more heat, which draws more current… Ni-Cads suffer from ‘memory’, where if they are only partially used and then recharged, eventually they only have that much capacity to give. A ‘smart’ Ni-Cad charger has the capability to do a ‘deep-discharge’, which fully drains the Ni-Cad cells, wiping the memory and restoring capacity.
Nickel-cadmium batteries are the ‘workhorses’ of the rechargeable designs, and if treated well, will serve well.