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. In this article, we assess the charger as the quintessential provider and guardian of the battery (SONY VAIO VGN-FZ Battery). We look at recommended charge methods to increase the performance of NiCd, NiMH and Li-ion packs.
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 (SONY VGP-BPS8 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 (SONY VGP-BPS11 Battery). The caution applies especially to NiMH because this chemistry cannot absorb overcharge well.
A lithium-based battery (SONY VGP-BPS9 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 Charger - The slow charger, also known as 'overnight charger', applies a fixed charge of about 0.1C* (one tenth of the rated capacity) for as long as the battery (SONY VGP-BPS10 Battery) is connected. Charge time is 14 to 16 hours. Slow chargers are found in cord-less phones, portable CD players and similar consumer goods.
Quick Charger - The quick-charger, or rapid charger, serves in the middle of the range, both in terms of charging time and price. Charging takes 3 to 6 hours and the battery switches to trickle charge when ready. Quick-chargers accommodate nickel-based or lithium-based batteries (IBM Thinkpad T400 Battery) and service commercial products such as cell phones, laptops and camcorders.
Fast Charger - This charger offers several advantages; the obvious one is shorter charge times. At a 1C charge rate, an empty NiCd or NiMH 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 trickle charge. Fast chargers are used for industrial equipment such as two-way radios, medical devices and power tools (HP PAVILION DV2000 Battery).
Simple Guidelines:
- 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 (HP PAVILION DV2 Battery).
- Nickel-based batteries prefer fast charge because it reduces crystalline formation (memory).
- Nickel and lithium based batteries require different charge algorithms. The two chemistries can normally not be interchanged in the same charger.
- If not used regularly, remove the battery (Dell INSPIRON 1525 Battery) from the charger and apply a topping-charge before use. Do not leave the battery in the charger for standby.
* The C-rate is a unit by which charge and discharge currents are scaled. A charge current of 1000mAh (1C) will charge a 1000mAh battery in slightly more than one hour.
Charging Nickel Cadmium
Battery manufacturers recommend to slow charge a new NiCd for 24 hours before use. This process brings the cells within a battery(ACER Travelmate 2300 Battery) pack to 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 (Dell Latitude D620 Battery )analyzer or through normal use. In some cases, 50 to 100 discharge/charge cycles are needed to fully form a nickel-based battery. Quality cells, such as those made by Sanyo and Panasonic, perform to specification after 5 to 7 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 over-charged. 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 afterwards. A white powder accumulating at the vent opening indicates venting activities (Dell Inspiron 6000 battery ).
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 (Dell INSPIRON 6400 Battery).
More advanced NiCd chargers sense the rate of temperature increase. Defined as dT/dt (delta Temperature/delta time), this charge detection system is kinder on the batteries (HP Pavilion DV6000 Battery) than a fixed temperature cut-off, 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 0.5C and higher. Chargers based on NDV must also observe battery (Dell Inspiron E1505 Battery) temperature because aging cells and cell mismatch reduce the voltage delta.
Fast charging improves charge efficiency. At 1C, the efficiency is 1.1 or 91 percent and the charge time of an empty pack is slightly longer than one hour. On a 0.1C charge, the efficiency drops to 1.4 or 71 percent and the charge time is about 14 hours. On a partially charged battery(ASUS Eee PC 900 Battery) or one that cannot hold full capacity, the charge time is shorter accordingly.
In the initial 70 percent of charge, the charge acceptance of a NiCd battery is close to 100 percent. Almost all energy is absorbed and the battery remains cool. Currents of several times the C-rating can be applied without causing heat build-up. Ultra-fast chargers use this phenomenon to charge a battery(ASUS A3000 Battery) to the 70 percent level within minutes. The charge continues at a lower rate until fully charged.
Past 70 percent, the 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 while a NiCd is being charged (ACER Aspire 5020 Battery).
Figure 1: Charge characteristics of a NiCd cell. The cell voltage, pressure and temperature characteristics are similar in a NiMH cell.
Ultra-high capacity NiCd batteries tend to heat up more than standard NiCd if charged at 1C 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 (FUJITSU LIFEBOOK S6120 Battery).
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 (SONY VGP-BPS3 Battery).
After the initial fast charge, some chargers apply a timed topping charged, followed by trickle charge. The recommended trickle charge for NiCd is between 0.05C and 0.1C. Because of memory concerns and compatibility issues with NiMH, modern chargers tend to use lower trickle charge currents (ACER Aspire 3020 Battery).
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