Originally Published MEM Fall 2003
Understanding different charging methods is key to extending the life of batteries destined for medical electronics.
To a large extent, the reliability and longevity of a battery hinges on the quality of the charger. In a price-competitive market, chargers are often given low priority, especially for consumer products. This article assesses the charger as the quintessential provider and guardian of the battery. It also examines recommended charge methods to increase the performance of nickel-cadmium (NiCd), nickelmetal hydride (NiMH), and lithium ion (Li-ion) packs. Well-performing batteries and chargers are especially important for instruments where battery failure is not an option.
A battery should always remain cool during charging because high temperatures shorten battery life. However, some temperature rise cannot be avoided when charging nickel-based batteries. The temperature peaks when the battery approaches full charge, then moderates after the battery switches to trickle charge. The battery should eventually cool to room temperature.
If the temperature remains above room temperature after a few hours in ready mode, the charger is performing incorrectly. Remove the battery when ready because any prolonged trickle charging will damage the battery. The caution applies especially to NiMH because this chemistry cannot absorb overcharge well. A lithium-based battery should never get warm during charge. If this happens, either the battery or the charger is faulty. Discontinue its use.
Nickel-based chargers are grouped into three categories: slow, quick, and fast. The slow charger, also known as overnight charger, applies a fixed charge of about 0.1 C (one-tenth of the rated capacity) for as long as the battery is connected. The charge time is 1416 hours. The C rate is a unit by which charge and discharge currents are scaled. A charge current of 1000 mAh (1 C) will charge a 1000-mAh battery in slightly more than one hour.
Slow chargers are found in cordless phones, portable CD players, and similar consumer goods. The quick charger, or rapid charger, serves in the middle of the range, in terms of both charging time and price. Charging takes from 3 to 6 hours, and the battery switches to trickle charge when ready. Quick chargers accommodate nickel-based or lithium-based batteries and service commercial products such as cell phones, laptops, and camcorders. The third type, fast chargers, offer several advantages; the obvious one is shorter charge times. At a 1-C charge rate, an empty NiCd or NiMH battery typically charges in a little more than an hour. Accurate full-charge detection is important. Once fully charged, the charger switches to topping and then to trickle charge. Medical batteries are normally charged on fast chargers. This method enables empty packs to be put back into mission-ready status as quickly as possible. Fast chargers are also used for industrial equipment such as two-way radios, video cameras, and power tools.
It is important to remember some simple guidelines when selecting the appropriate charger. A charger for NiMH can also accommodate NiCd, but not the other way around. A charger especially designed for NiCd would overcharge the NiMH battery. Nickel-based batteries prefer fast charge because it reduces crystalline formation (memory).
Nickel- and lithium-based batteries require different charge algorithms. Normally, it is not possible to interchange the two chemistries in the same charger. In addition, if a battery is not used regularly, remove it from the charger and then just apply a topping-charge before use. Do not leave the battery in the charger for standby.
Battery manufacturers recommend slow-charging a new NiCd for 24 hours before use. This process brings the cells within a battery pack to an equal charge level because each cell self-discharges at a different rate. The initial trickle charge also redistributes the electrolyte to remedy dry spots on the separator brought on by gravitation of the electrolyte during long storage.
Some battery manufacturers do not fully form the cells before shipment. Full performance is reached after the battery has been primed through several charge/discharge cycles, either with a battery analyzer or through normal use. In some cases, 50100 charge/discharge cycles are needed to fully form a nickel-based battery. Quality cells, such as those made by Sanyo and Panasonic, perform to specification after five to seven cycles. The initial readings may be inconsistent, but the capacity becomes steady once fully primed. A slight capacity peak is observed between 100 and 300 cycles.
Most rechargeable cells are equipped with a safety vent to release excess pressure if they become overcharged. The safety vent on a NiCd cell opens between 150 and 200 psi. (The pressure of a car tire is about 35 psi.) With a resealable vent, no damage occurs on venting but some electrolyte is lost and the seal may leak afterward. A white powder accumulating at the vent opening indicates venting activities.
Commercial chargers are often not designed in the best interests of the battery. This is especially true with chargers that measure the battery's charge state solely through temperature sensing. Although simple and inexpensive, charge termination by absolute temperature is not accurate.
More-advanced NiCd chargers sense the rate of temperature increase. Defined as ΔT/Δt (delta temperature/delta time), this charge-detection system is kinder on the batteries than a fixed-temperature cutoff, but the cells still need to generate some heat to trigger detection.
More-precise full-charge detection can be achieved with the use of a microcontroller that monitors the battery voltage and terminates the charge when a certain voltage signature occurs. A drop in voltage signifies full state-of-charge. Known as negative delta V (NDV), this phenomenon is most pronounced on NiCd charging at 0.5 C and higher. Chargers based on NDV must also observe battery temperature because aging cells and cell mismatch reduce the voltage delta.
Fast charging improves charge efficiency. At 1 C, the efficiency is 1.1, or 91%, and the charge time of an empty pack is slightly longer than one hour. On a 0.1-C charge, the efficiency drops to 1.4, or 71%, and the charge time is about 14 hours. On a partially charged battery or one that cannot hold full capacity, the charge time is shorter accordingly.
In the initial 70% of charge, the charge acceptance of a NiCd battery is close to 100%. Almost all energy is absorbed, and the battery remains cool. Currents of several times the C rating can be applied without causing heat buildup. Ultrafast chargers use this phenomenon to charge a battery to the 70% level within minutes. The charge continues at a lower rate until fully charged.
|Figure 1. Charge characteristics of a NiCd cell. The cell voltage, pressure, and temperature characteristics are similar in a NiMH cell.
(click to enlarge)
Past 70%, a battery gradually loses the ability to accept charge. The pressure rises and the temperature increases. In an attempt to gain a few extra capacity points, some chargers allow a short period of overcharge. Figure 1 illustrates the relationship of cell voltage, pressure, and temperature as a NiCd is being charged.
Ultra-high-capacity NiCd batteries tend to heat up more than standard NiCd batteries if charged at 1 C or higher. This is partly due to increased internal cell resistance. To moderate temperature buildup and still maintain short charge times, advanced chargers apply a high current at the beginning and then lower the amount to harmonize with the charge acceptance.
Interspersing discharge pulses between charge pulses improves the charge acceptance of nickel-based batteries. Commonly referred to as burp or reverse-load charging, this method promotes high surface area on the electrodes to improve the recombination of the gases generated during charge. The results are better performance, reduced memory, and longer service life.
After the initial fast charge, some chargers apply a timed topping charge, followed by trickle charge. The recommended trickle charge for NiCd batteries is between 0.05 and 0.1 C. Because of memory concerns and compatibility issues with NiMH, modern chargers tend to use lower trickle-charge currents.
Charging NickelMetal Hydride
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 nonexistent at charge rates below 0.5 C and elevated temperatures. Aging and degenerating cell match diminish the already-minute voltage delta further.
A NiMH charger must respond to a voltage drop of 816 mV per cell. Making the charger too sensitive may terminate the fast charge halfway through the charge because voltage fluctuations and noise induced by the 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 (ΔT/Δt), temperature sensing, and timeout timers. The charger uses 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% on a good battery. The negative aspect is shorter cycle life. Rather than 350400 service cycles, this pack may be exhausted after 300.
NiMH batteries should be rapid charged rather than slow charged. Because NiMH does not absorb overcharge well, the trickle charge must be lower than that of NiCd and so is set to around 0.05 C. This explains why the original NiCd charger cannot be used to charge NiMH batteries.
It is difficult, if not impossible, to slow-charge an NiMH battery. At a C rate of 0.1 and 0.3 C, 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 is charged with a fixed timer. The same occurs if the battery has aged and can only hold a 50% instead of a 100% 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. These chargers are commonly promoted on the merits of short charge time and moderate price. Some ultrafast chargers also fail to deliver a 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, do not exist for lithium chemistries. A super-fast charging solution is also not applicable for lithium-based batteries. Manufacturers of Li-ion cells dictate very strict guidelines in charge procedures.
The early graphite system demanded a voltage limit of 4.10 V/cell. Although higher voltages deliver more capacity, cell oxidation shortened the service life if charged above the 4.10-V/cell threshold. This problem has been solved with chemical additives. Today, most Li-ion cells are charged to 4.20 V with a tolerance of ±0.05 V/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.
|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.
(click to enlarge)
Increasing the charge current does not shorten the charge time by much. Although the voltage peak is reached more quickly 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 stages one and two. Some chargers claim to fast-charge a Li-ion battery in one hour or less. Such a charger eliminates stage two and goes directly to ready 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 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, a topping charge may be repeated once every 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05 V/cell and turns off at 4.20 V/cell.
It is important to avoid inadvertently overcharging a Li-ion battery. 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 4.30 V, 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.
Much attention has been placed on the safety of Li-ion to prevent overcharge and overdischarge. 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 4.30 V/cell. Temperature sensing disconnects the charge if the cell temperature approaches 90°C (194°F); and a mechanical pressure switch found on many cells permanently interrupts the current path if a safe pressure threshold is exceeded. Exceptions are made on some spinel (manganese) packs containing one or two small cells. 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. Although 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 temperature. No commercial battery should be charged at below-freezing temperatures.
Nickel-based batteries should only be fast charged between 10° and 30°C (50° and 86°F). Below 5°C (41°F), the ability to recombine oxygen and hydrogen is greatly reduced and the resulting pressure buildup may cause the cells to vent.
The charge acceptance of nickel-based batteries at higher temperatures is drastically reduced. A battery that provides a capacity of 100% if charged at moderate room temperature only accepts 70% if charged at 45°C (113°F). This explains the poor performance of vehicular chargers in the summer.
|Figure 3: Universal conditioning chargers come in one-, two-, and six-bay configurations. Intelligent battery adapters accommodate different battery types on the same charger.|
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 1 C. Charging at freezing temperatures must be avoided because plating of lithium metal could occur. Some chargers feature a built-in temperature sensor that applies a trickle charge if the battery is too cold. No charge is applied if the battery is too hot. These advanced chargers use reverse-load charge and detect the full charge by NDV, ΔT/Δt sensing and timers. Intelligent battery adapters configure the charger to the correct charge algorithm. These chargers are available in single-, dual-, and six-bay configurations (see Figure 3). 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 killers are high temperature during charge and incorrect trickle charge after charge. Choosing a quality charger makes common sense. This is particularly important for medical equipment for which dependable service of the battery and equipment is essential. 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.
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. (Richmond, BC, Canada). He can be reached via e-mail at firstname.lastname@example.org. This article contains excerpts from the second edition of Batteries in a Portable World--A Handbook on Rechargeable Batteries for Non-Engineers. The 300-page book is available from Cadex Electronics Inc. (email@example.com) and http://www.amazon.com. For additional information on battery technology, visit http://www.buchmann.ca or http://www.batteryuniversity.com.
Copyright ©2003 Medical Electronics Manufacturing