Nickel cadmium batteries, aka Ni-Cd or Nicad, are popular rechargeable batteries.
The main uses are in cordless tools, and as rechargeable replacements for 1.5v and 9v consumer batteries. They're also found in emergency Lighting, some early laptops, and various rechargeable small consumer appliances.
Voltage & Current
Terminal voltage is 1.2v. Since this doesn't match the standard 1.5v per cell, a minority of battery appliances won't work if 1.2v nicads are fitted in place of 1.5v dry cells. Most appliances are designed to accept nicads, but there are exceptions. Its not a problem with 8.4v nicads, which contain more cells than 9v dry batteries and are a closer match.
Terminal voltage stays steady through the life of the battery until the end.
Nicads have very high current delivery ability, with for example high performance D cells able to deliver 60A in use. This makes them ideal for cordless tools. The downside is that an accidental short can cause temperatures high enough to boil the battery, with venting of steam. Charged batteries should not be stored or transported carelessly.
Typical nicad capacity is smaller than zinc carbons, and only a fraction of that of alkaline cells. Nicads are available in differing capacities with prices to match, but capacity never compares to a good dry battery. NiMH are generally much higher capacity than NiCd.
Parallelling nicads to increase capacity is not good practice.
NiCds typically manage around 1000 recharge cycles if treated as designed. Exact life varies, with good quality cells lasting much better than cheapos.
Nicads can be left indefinitely, charged or flat, without affecting their life expectancy.
The main thing that shortens nicad life is fully discharging under load. Exact capacity and leakage varies from cell to cell, so the cells don't discharge at exactly the same time. There is thus a short time at the end where a nicad pack will deliver power but at a reduced voltage. If this power is used, some flat cells get reverse charged at high current, and this reduces their life significantly. Once a nicad pack begins to wane it should stop being used, don't run it totally flat.
NiCds don't normally leak, but they can at end of life.
Nicads gradually discharge themselves when stored. Rate of discharge is one of the measures of battery quality, but times of 3 to 6 months from full to flat are typical. Batteries in poor condition (near end of life) discharge much faster.
This makes nicads poorly suited to appliances only used very occasionally, such as torches. When used like this they must be left on continuous float charge to keep them ready for use when needed. Such treatment results in a lower number of cycles of battery life.
Some companies have advocated fully discharging nicads before each charge, and sell a battery holder plus resistor to do so. The practical result of such dischargers is to shortern the useful life of the nicads, and lighten the wallet.
The memory effect is widely blamed for loss of capacity over time with nicads. Memory effect does exist, but is much misdescribed. Its an unusal phenomenon rarely seen, and not one the end user need worry about in ordinary circumstances. Apparent loss of capacity is more often due to the user noticing that capacity doesn't compare well with dry batteries, or due to increasing self discharge near end of life.
Chargers & Charging
Nicads are available with charging times of anything from 16hrs to 15 minutes. Faster batteries cost more, and require chargers to match. Both cells and charger must be capable of the charge speed used, and its the charger that determines the charge speed. Don't charge slow cells in a fast charger, but fast cells can safely be charged slowly.
There are several charger types, from simple to complex. Note that the charging time (and hence current) of the charger and battery need to roughly match, if there's a big difference all won't be well.
Current limited These are the simplest of all charger types. They provide a current limited supply with no other control. These are ok for slow charging. Charging should be stopped manually once the 16 hrs is up, leaving them on much too long reduces battery life.
Float chargers These are very low current chargers designed to be left charging batteries all the time. Almost all are of the current limited type. Battery life is reduced by this treatment, and recharge times are prolonged.
Timed Current limited chargers with built in timer are suitable for medium speed charging. These are used with low to medium cost cordless tools.
Smart charger These monitor battery behaviour in detail, and stop charging when the battery needs no more charge. Smart chargers are the only option for fast charge speeds. They maximise battery life and minimise energy consumption.
Many chargers charge cells in pairs. Keeping the cells in the same pairs every time helps maximise performance and life by minimising overcharging.
Cordless tool batteries are nearly always sub-c size cells in a plug-in plastic container. Typical capacities are 1.3Ah, 1.7Ah and 2Ah. Quality varies widely, with much poorer internal resistance, self discharge, life expectancy and reliability seen in low cost cells.
Replacing all the cells with new ones is the one way to ensure good long term performance. But its often uneconomic.
Putting high current through a cell for a few seconds burns out the dendrites that cause self discharge & shorting. Although this brings a lot of dying cells back to life, they don't last long, as by the time dendrites are shorting a cell, other dendrites are very close to doing so too. A repair option this isn't much use.
Higher voltage battery packs, such as 24v, experience only a smallish performance drop if 1 or 2 cells are removed. Hence this is a practical option where 1 or 2 cells are bad, and complete replacement isn't worthwhile.
Nicads contain toxic cadmium, and aren't meant to be disposed of in household rubbish.