Jun. 20, 2008 - Will Lithium-Ion batteries power the new millennium?(2)

Charging the Li-ion battery

The Li-ion charger is a voltage-limiting device and is similar to the lead acid 92P1075 battery charger. The main differences of the Li-ion charger are higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge. 92P1071,92P1060,92P1011,92P1087

Whereas the VRLA offers some flexibility in terms of voltage cut off, the manufacturers of Li-ion cells are very strict about the voltage choice. When first introduced, the charge voltage limit of the graphite system was 4.10 volts per cell. Although higher voltages deliver increased energy density, cell oxidation severely limited the service life in the early graphite cells if charged above the 4.10V/cell threshold. This effect has now been solved with chemical additives and most new Li-ion cells are now set to 4.20V. The tolerance on all Li-ion 92P1075 batteries is a tight +/- 0.05 volts per cell.

The charge time of all Li-ion 92P1071 batteries is about 3 hours at a 1C initial charge current. The battery remains cool during charge. Full charge is attained after the voltage reaches the upper voltage threshold and the current drops and levels off at about 3% of its nominal rating, or about 0.03C. 08K8214

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. Figure 3 shows the voltage and current signature of a charger as the Li-ion cell 08K8195 passes through stage one and two.

Figure 3:  Charge stages of a Li-ion 08K8193 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.

Claims of fast charging a Li-ion 08K8192 battery in one hour or less usually results in lower charge levels. Such a charger simply eliminates stage two and goes directly into ?I>ready?/I> once the voltage threshold is reached at the end of stage one. The charge level at this point is about 70%. The topping charge typically takes twice as long as the initial charge.

No trickle charge is applied because the Li-ion 92P1101 is unable to absorb overcharge. Trickle charge could cause plating of metallic lithium, a condition that makes the cell unstable. Instead, a brief topping charge is applied to compensate for the small amount of self-discharge the FRU 92P1069 battery and its protective circuit consume.

Depending on the charger and the self-discharge of the 08K8196 battery, a topping charge may be implemented once every 500 hours or 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05 volts per cell and turns off when it reaches 4.20V/cell.

Protection circuit

Commercial Li-ion 92P1102 battery packs contain redundant protection devices to assure safety under all circumstances. Typically, an FET opens if the charge voltage of any cell reaches 4.30V, and a fuse activates if the cell temperature approaches 90°C (194°F). In addition, a pressure switch in each cell permanently interrupts the charge current if a safe pressure threshold is exceeded, and internal voltage control circuits cut off the battery at low and high voltage points. Exceptions are made to prismatic and cylindrical spinel packs containing one or two cells only.

The Li-ion is typically discharged to 3 volts per cell. The lowest ‘low-voltage?power cut-off is 2.5V/cell. During prolonged storage, however, a discharge below this voltage level is possible. Manufacturers recommend a ‘trickle?charge to raise such a battery gradually back up into the ‘acceptable?voltage window. Not all chargers are designed to apply a charge once a Li-ion 92P1077 battery has dipped below 2.5V/cell. 

Some batteries feature an ultra-low voltage cutoff that permanently disconnects the pack if a cell dips below 1.5 volts. This precaution is done to prohibit recharge if a battery has dwelled in an illegal voltage state. A deep discharge causes copper plating, which can lead to short circuit in the cell.

Most manufactures do not sell the Li-ion cells by themselves but make them available in a 92P1061 battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits. 92P1073

A major concern arises if static electricity or a faulty charger has managed to destroy the battery’s protection circuit. Such damage often causes the solid-state switches to fuse to a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but does not provide the required safely. If charged beyond safe voltage limits with a poorly designed accessory charger, the battery may heat up, then bulge and in some cases vent with flame. Shorting such a battery can also be hazardous. 08K8201

Analyzers for the Lithium Ion 08K8199 batteries

In the past, battery analyzers were used to restore batteries affected by ‘memory? With today’s nickel-free batteries, memory is no longer a problem and the emphasis of an analyzer is shifting to battery performance verification, quality control and quick-test.

Conventional wisdom says that a new 08K8198 battery always performs flawlessly. Yet many users have learned that a battery fresh from the shrink-wrap does not always meet the manufacturer's specifications. With a battery analyzer, all incoming batteries can be checked as part of a quality control procedure. In addition, warranty claims can be made if the capacity drops below the specified level at the end of the warranty period.

A typical life of a Li-ion is 300-500 discharge/charge cycles or two years from time of manufacturing. The loss of battery capacity occurs gradually and often without the knowledge of the user. Although fully charged, the battery eventually regresses to a point where it may hold less than half of its original capacity. The function of the battery analyzer is to identify these weak batteries and “weed?them out.08K8197

A battery analyzer can also be used to troubleshoot the cause of short runtimes. The charger may not provide a full charge or the portable device may draw more current than expected. Many of today’s battery analyzers can simulate the load signature of a digital device and verify the runtime based on the available battery capacity.

An important feature of modern FRU 08K8193 battery analyzers is its ability to read the internal battery resistance, a test that only takes a few seconds to complet. As part of natural aging, the internal resistance of a Li-ion gradually increases due to cell oxidation. The higher the resistance, the less energy the battery can deliver.

To utilize the OhmTest as a battery validation, it is essential to obtain a reference reading of a good battery with known performance. Because each battery type may be different, a reference reading will be required for each model.

A more reliable method of measuring the state-of-health of a battery is through quick-test methods. Cadex has developed a system that uses an inference algorithm to test ThinkPad R50e battery capacities. The Quicktest algorithm is made battery specific by using a trend-learning algorithm that resembles the thinking process of the human brain.

The Cadex Quicktest can be performed with a charge level of between 20 and 90 percent. If the battery is insufficiently charged, or has too high a charge, the analyzer automatically applies the appropriate charge or discharge to bring the battery within testing range. Within this range, different charge levels do not affect the readings. The test lasts about two minutes and supports Li-ion/Polymer, NiMH and NiCd ThinkPad R50 batteries.

The Cadex quick test is added to the Cadex 7200 and 7400 battery analyzers. These platforms feature interchangeable battery adapters that contain the battery configuration codes and matrix for the quick test.

Summary

The Li-ion receives good grades in performance and reliability. Supply shortages have eased and prices have become competitive with nickel-based equivalents. As a result, more portable equipment is being fitted with the Li-ion ThinkPad T43 battery.

The Li-ion has found a strong market niche with portable devices demanding small form factor. The most popular uses are mobile phones and laptop computers. Because of the aging aspect, the Li-ion is most suitable for applications with a hectic user pattern. Where the Li-ion falls short is on high current applications, such as power tools, heart defibrillators and two-way radios for public safety.

Another field where the Li-ion has proven less favorable is in applications that require only occasional battery use. On a laptop that is mostly powered by AC, for example, the Li-ion ThinkPad T40 battery ages over time and the full benefit of the battery cannot be realized. High heat levels inside most laptops also cause the Li-ion to age prematurely. Field tests have revealed, however, that Li-ion is less affected by heat than NiMH.

The Lithium Ion Polymer systems are struggling to meet and surpass the performance of the Li-ion battery. High energy cost ratio limits the Li-ion Polymer to small portable devices, such as mobile phones. It is expected that once mass-produced, the Li-ion Polymer will be lower priced than the Li-ion because of simpler packaging.