Chargers for NiMH batteries are similar to NiCd systems but require more complex electronics. To begin with, the NiMH produces a very small voltage drop at full charge and the NDV is almost non-existent at charge rates below 0.5C and elevated temperatures. Aging and degenerating cell match diminish the already minute voltage delta further (SONY VAIO VGN-FZ Battery).
A NiMH charger must respond to a voltage drop of 8 to 16mV. Making the charger too sensitive may terminate the fast charge halfway through the charge because voltage fluctuations and noise induced by the battery (SONY VGP-BPS8 Battery) and charger can fool the NDV detection circuit. Most of today's NiMH fast chargers use a combination of NDV, rate-of-temperature-increase (dT/dt), temperature sensing and timeout timers. The charger utilizes whatever comes first to terminate the fast-charge.
NiMH batteries that are allowed a brief overcharge deliver higher capacities than those charged by less aggressive methods. The gain is approximately 6 percent on a good battery (SONY VGP-BPS11 Battery). The negative is shorter cycle life. Rather than 350 to 400 service cycles, this pack may be exhausted after 300.
NiMH batteries should be rapid rather than slow charged. Because NiMH does not absorb overcharge well, the trickle charge must be lower than that of NiCd and is set to around 0.05C. This explains why the original NiCd charger cannot be used to charge NiMH batteries(SONY VGP-BPL9 Battery).
It is difficult, if not impossible, to slow-charge a NiMH battery. At a C?rate of 0.1C and 0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must rely on a timer. Harmful overcharge can occur if a partially or fully charged battery (SONY VGP-BPS10 Battery) is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100 percent charge. Overcharge could occur even though the NiMH battery feels cool to the touch.
Lower-priced chargers may not apply a fully saturated charge. The full-charge detection may occur immediately after a given voltage peak is reached or a temperature threshold is detected (IBM Thinkpad T400 Battery). These chargers are commonly promoted on the merit of short charge time and moderate price. Some ultra-fast chargers also fail to deliver full charge.
Charging Lithium Ion
Whereas charges for nickel-based batteries are current limiting devices, Li-ion chargers are voltage limiting. There is only one way to charge lithium-based batteries. The so-called 'miracle chargers', which claim to restore and prolong batteries (HP PAVILION DV2000 Battery), do not exist for lithium chemistries. Neither does a super-fast charging solution apply. Manufacturers of Li-ion cells dictate very strict guidelines in charge procedures.
The early graphite system demanded a voltage limit of 4.10V/cell. Although higher voltages deliver more capacity, cell oxidation shortened the service life if charged above the 4.10V/cell threshold. This problem has been solved with chemical additives (HP PAVILION DV2 Battery). Today, most Li-ion cells are charged to 4.20V with a tolerance of +/?0.05V/cell.
The charge time of most chargers is about 3 hours. The battery remains cool during charge. Full charge is attained after the voltage has reached the voltage threshold and the current has dropped low and leveled off (Dell INSPIRON 1525 Battery).
Increasing the charge current does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer. Figure 2 shows the voltage and current signature of a charger as the Li-ion cell passes through stage one and two (ACER Travelmate 2300 Battery).
Figure 2: Charge stages of a Li-ion battery. Increasing the charge current on a Li-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer.
Some chargers claim to fast-charge a Li-ion battery (Dell Latitude D620 Battery) in one hour or less. Such a charger eliminates stage 2 and goes directly to 'ready' once the voltage threshold is reached at the end of stage 1. The charge level at this point is about 70 percent. The topping charge typically takes twice as long as the initial charge.
No trickle charge is applied because Li-ion is unable to absorb overcharge. Trickle charge could cause plating of metallic lithium, a condition that renders the cell unstable. Instead, a brief topping charge is applied to compensate for the small self-discharge the battery and its protective circuit consume. Depending on the battery (Dell Inspiron 6000 battery ), a topping charge may be repeated once every 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off at 4.20V/cell.
What if a battery is inadvertently overcharged? Li-ion batteries are designed to operate safely within their normal operating voltage but become increasingly unstable if charged to higher voltages. When charging above 4.30V, the cell causes lithium metal plating on the anode; the cathode material becomes an oxidizing agent, loses stability and releases oxygen. Overcharging causes the cell to heat up (Dell INSPIRON 6400 Battery).
Much attention has been placed on the safety of Li-ion to prevent over-charge and over-discharge. Commercial Li-ion battery packs contain a protection circuit that prevents the cell voltage from going too high while charging. The upper voltage threshold is typically set to 4.30V/cell. Temperature sensing disconnects the charge if the cell temperature approaches 90°C (194°F); and a mechanical pressure switch on many cells permanently interrupt the current path if a safe pressure threshold is exceeded. Exceptions are made on some spinel (manganese) packs containing one or two small cells (HP Pavilion DV6000 Battery). The charge process of a Li-ion Polymer is similar to Li-ion. These batteries use a gelled electrolyte to improve conductivity.
Charging at High and Low Temperatures
Rechargeable batteries work under a reasonably wide temperature range. This, however, does not automatically permit charging at these extreme conditions. While hot or cold temperatures cannot always be avoided, recharging a battery is at the control of the user. Efforts should be made to charge at room temperatures. No commercial battery (Dell Inspiron E1505 Battery)should be charged below freezing.
Nickel-based batteries should only be fast-charged between 10°C to 30°C (50°F to 86°F). Below 5°C (41°F), the ability to recombine oxygen and hydrogen is greatly reduced and the resulting pressure build up may cause the cells to vent.
The charge acceptance of nickel-based batteries(ASUS Eee PC 900 Battery) at higher temperatures is drastically reduced. A battery that provides a capacity of 100 percent if charged at moderate room temperature only accepts 70 percent if charged at 45°C (113°F). This explains the poor performance of vehicular chargers in the summer.
The Li-ion batteries offer reasonably good charge performance throughout the temperature range. Below 5°C (41°F), the charge should be with less than 1C. Charging at freezing temperatures must be avoided because plating of lithium metal could occur (ASUS A3000 Battery).
The Cadex Universal Conditioning Chargers (UCC) feature a built-in temperature sensor that applies a trickle charge if the battery is too cold. No charge is applied if too hot. These advanced chargers use reverse load charge and detect the full charge by NDV, dT/dt sensing and timers. Intelligent battery (ACER Aspire 5020 Battery) adapters configure the charger to the correct charge algorithm. The UCC chargers are available in single, dual and six bay (Figure 3) configurations. The two-bay unit is designed for vehicle mounting.
Commercial fast-chargers are often not designed in the best interests of the battery. The two common battery (FUJITSU LIFEBOOK S6120 Battery).killers are high temperature during charge and incorrect trickle charge after charge.
Choosing a quality charger makes common sense. This is especially true when considering the high cost of battery replacements and the frustration poorly performing batteries create. In most cases, the extra money invested in a more advanced charger is returned in longer lasting and better performing batteries.