The ever-increasing demand for high capacity batteries promoted wide research since the last five decades. It was felt that the various portable appliances, which were being marketed since that time, needed a more powerful electrical source, of greater capacity source and therefore longer lasting operation in portability. The need to have environmentally friendly devices and the possibility of re-charging the cells instead of disposing of them was an ever-increasing demand. Battery disposal, in fact is a problem in itself, and most countries have formulated legislation in recent years to this effect in order to avoid environmental hazards.

The ideal re-chargeable battery must have a short charging time, high capacity, has to be able to supply any current on demand and has to have a long lifetime. Current technology does not provide all the solutions for having an ideal situation, but significant progress has been made in battery technologies over the recent years, which have somehow approached the ideal situation.

Prior to the invention of the nickel metal hydride technology (NiMH), the other types of re-chargeable batteries available on the market were the lead acid accumulators and nickel cadmium (NiCd) cells. NiCd cells are still much in use today, being rather cheaper than NiMH cells. However, the so called memory effect of these cells presented a drawback which constrained the user to be very careful on how to re-charge his cells in order to obtain the specified lifetime. Very often, these batteries die a pre-mature death and rarely last their lifetime. On the other hand, NiMH technology is constantly improving. More capacity is being installed into the standard sizes, and better, more stable materials are constantly being introduced. NiMH technology is well on its way to substitute the environmentally hazardous NiCd technology.

The newer NiMH cells, besides having roughly 30% more capacity over the NiCd cells for the same equivalent size, are much less prone to the memory effect. Moreover, as already stated, their constituents are not as hazardous as the nickel cadmium cells and may be considered as the most environmentally friendly of all technologies. However there are other considerations.

Nickel metal hydride cells are very sensitive to charging current. Their re-charge has to be carefully controlled in order to obtain the best performance and attain maximum charge out of the cells. They also have a tendency to overheat during charge. A temperature-sensing device is normally installed in the cells to enable them to control the charge temperature and also detect full charge. Nevertheless, even using sophisticated chargers, the charging time can be as much as double that of normal NiCd cells. Chargers for NiMH also obviously more expensive than their counterparts for NiCd. NiMH cells are not the ideal choice when high discharge currents (ex. Power tools) are required. This was initially a draw back for mobile phone usage, which required high peak currents during conversation exchange. The newer phones have overcome this problem, and together with their very low standby current drain, now ensure a decent time between charges. For both technologies, the self-discharge rate is rather high. Self-discharge can be explained as the amount of charge lost when the battery is actually idle. The NiCd cells can lose up to 10% of their full charge overnight, after which the discharge rate reaches equilibrium of about 0.3% daily. Early NiMH cells had as much as twice this amount. However newer improved batteries have much less. Another parameter is the exercise requirement. This can be defined as the frequency, a battery needs to be exercised in order to achieve its maximum service life. NiCd cells tend to be more demanding on this. Typically once every 30 days, a NiCd battery has to exercise. By comparison, a NiMH cell requires exercising every 90 days. This is a clear advantage when regular exercising is impractical and when users are less careful.

One last consideration is the number of cycles. Provided a NiCd cell is charged and discharged properly, it can give up to 1000 cycles. Otherwise, due to the memory effect, its lifetime can be much shorter. A NiMH cell can only give half as many cycles, but the fact that it possesses minimal memory effect, often makes it longer lasting than a NiCd cell.

NiMH batteries are ideal in applications where cost is secondary, and higher capacities for the same cell volume are required. Nowadays, NiMH has practically fully replaced NiCd in mobile phone applications.

In practical terms, self discharge causes a free standing battery to lose all or most of its capacity by itself over a period of say one month or so without being utilised by an application (like a mobile phone) during this time.

A typically NiMH cell can be thought of a positive plate and a negative plate, whose surfaces are both dosed with nickel hydroxide and then separated by an insulating material moistened in electrolyte A fully charged battery is an object under tension, which needs to release this force on first opportunity. The amount that the battery will remain in this condition depends on the materials used in its construction. However, this is rarely the case. All batteries lose energy with time. Elevated temperature also accelerates this phenomenon.

The chemistry of the battery releases oxygen at the anode in a non-linear manner, but normally depending on its state of charge. Manufactures have tried to use super conductive electrolyte to improve capacity, but this also increases self-discharge. Much depends also on the insulating materials used to isolate the negative from the positive electrodes.

This insulator or separator can be physically damaged after several cycles or by improper charging, causing the battery to self-discharge more rapidly. On the other hand, swelling of the +ve and -ve plates causes pressure between these plates and the dividing insulator and therefore increase the risk of self-discharge. A compromise is to make the electrolytes less active, but in this case, capacity will also be drastically reduced. This is the challenge designers are facing. Maintaining capacity at high levels without increasing self-discharge properties.

Some thirty years ago, Lithium batteries started to appear on the commercial market. It was thought that Lithium, having the highest electrochemical activity, would possess the greatest energy density. Lithium is the lightest metal known to man, just 3 times heavier than hydrogen, with an extraordinarily active chemistry. Due to its chemical activity, and instability, it is not found in nature as an element. Having such a chemical inside a battery is not without risks. An unexpected short circuit inside the battery causes the temperature to rise uncontrolled and induces the risk of explosions or other undesirable effects.

Further research promoted the use of a non-metallic battery containing Lithium ions instead of Lithium. A compound called Lithium-cobalt dioxide (LiCoO2) was introduced, which although inferior in energy density terms, was much safer to use. Nevertheless, precautions had still to be taken during charging and discharging of these new Li-ion cells.
The Li-ion cell has twice the capacity of Ni-Cd for the same volume. The discharge capabilities of the cell are better than that of Ni-MH, permitting higher current peaks. The self-discharging effect is also much less. The voltage of the cell is typically 3.6V.
The Lithium ion battery is not without its disadvantages however. The charging and discharging of the cells must be controlled. There is still the risk of overheating during charging and the cell voltage must not exceed a certain limit. During discharge, the cell voltage has to be monitored and the battery disconnected, when the cell voltage drops to a certain amount. This is because of the chemistry of these cells. Neglect of these precautions could result in the possibility of formation of metallic Lithium on the battery plates with the risks mentioned before for Lithium batteries.

To overcome these problems, miniature control circuits were developed and these are used in conjunction with the batteries. These tiny PCBs contain circuits and components that control the peak (Vcc) and low voltage threshold (Vss) limits together with the charging parameters. The device reduces the charging current to 0mA whist maintaining the charge voltage (typically 4.2V) applied to the terminals. Excess charging currents are also avoided by means of the control circuitry.

Due to use of relatively precious materials (Li and Co), these Li-ion cells are more costly than their rivals. In future, newer materials will be used, with the effect that both performance and costs will be improved. At the moment, substitutes like LiNiO2 and LiMnO4 for the positive terminal are already available (see construction details)

A typical Li-ion battery is charged using a 4.1V or 4.2V charger (depending on battery supplier). When the peak voltage is reached, the charging current drops to 0C keeping the voltage steady at 4.1V or 4.2V.
The charge is to range between 0.1C and 1.0C (typically 0.5C). Charge termination can be set by timer or by current (0.05C would be quite suitable)

Typically, a Li-ion battery is discharged at 0.5C during its operation. Higher current peaks are possible however. The cell voltage has to stay within a certain window as prescribed by the manufacturer. The lower threshold voltage is normally around 3V, although some suppliers have gone lower than this, thus permitting more useful capacity between cycles.

As already explained, self-discharge is particularly low for Li-ion batteries, with the batteries retaining over 80% of their capacity after 1 month.