The fast-charger offers several advantages over the other chargers; the obvious one is shorter charge times. Because of the larger power supply and the more expensive control circuits needed, the fast-charger costs more than slower chargers, but the investment is returned in providing good performing batteries that live longer.
The charge time is based on the charge rate, the battery’s SoC, its rating and the chemistry. At a 1C charge rate, an empty NiCd typically charges in a little more than an hour. When a battery is fully charged, some chargers switch to a topping charge mode governed by a timer that completes the charge cycle at a reduced charge current. Once fully charged, the charger switches to trickle charge. This maintenance charge compensates for the self-discharge of the battery.
Modern fast-chargers commonly accommodate both NiCd and NiMH batteries. Because of the fast-charger’s higher charge current and the need to monitor the battery during charge, it is important to charge only batteries specified by the manufacturer. Some battery manufacturers encode the batteries electrically to identify their chemistry and rating. The charger then sets the correct charge current and algorithm for the battery intended. Lead Acid and Li-ion chemistries are charged with different algorithms and are not compatible with the charge methods used for nickel-based batteries.
It is best to fast charge nickel-based batteries. A slow charge is known to build up a crystalline formation on nickel-based batteries, a phenomenon that lowers battery performance and shortens service life. The battery temperature during charge should be moderate and the temperature peak kept as short as possible.
It is not recommended to leave a nickel-based battery in the charger for more than a few days, even with a correctly set trickle charge current. If a battery must remain in a charger for operational readiness, an exercise cycle should be applied once every month.
Hitachi Ltd and Hitachi Vehicle Energy Ltd developed a lithium-ion battery cell that has a high energy density and a high output density for plug-in hybrid electric vehicles (PHEVs).
The energy density and output density of the new battery cell are 120Wh/kg and 2,400W/kg (SOC: 50%), respectively. While the output density is equivalent to that of an existing battery cell that the companies are producing in volume for hybrid electric vehicles (HEVs), the energy density is about twice as high.
The new battery cell measures 146 x 110 x 30mm and weighs 0.75kg. Its average voltage is 3.6V. Though the companies did not disclose the details, the positive and negative electrodes contain oxide including a manganese substance and a carbon material, respectively. They have already made a test production line and plan to ship samples to automakers in the spring of 2010.
Hitachi Vehicle Energy has been developing battery cells mainly for HEVs. The company defines the cylindrical cell that it is now mass-producing as the second-generation product.
Also, it plans to produce 300,000 units of the third-generation product per month for General Motors Corp’s mild hybrid vehicles and other vehicles in 2010. The third-generation product is a cylindrical cell that is 40mm in diameter and 92mm in length. Its current capacity, average voltage and output density are 4.4Ah, 3.6V and 3,000W/kg, respectively.
Furthermore, Hitachi Vehicle Energy has already developed the fourth-generation product, whose output density (4,500W/kg) is 1.5 times higher than that of the third-generation product and started shipping samples in the autumn of 2009. The fourth-generation product is an angular cell that has a high cooling performance. And its positive and negative electrodes are made by using a manganese substance and amorphous carbon.
Also known as ‘overnight charger‘ or ‘normal charger’, the slow-charger applies a fixed charge rate of about 0.1C (one tenth of the rated capacity) for as long as the battery is connected. Typical charge time is 14 to 16 hours. In most cases, no full-charge detection occurs to switch the battery to a lower charge rate at the end of the charge cycle. The slow-charger is inexpensive and can be used for NiCd batteries only. With the need to service both NiCd and NiMH, these chargers are being replaced with more advanced units.
If the charge current is set correctly, a battery in a slow-charger remains lukewarm to the touch when fully charged. In this case, the battery does not need to be removed immediately when ready but should not stay in the charger for more than a day. The sooner the battery can be removed after being fully charged, the better it is.
A problem arises if a smaller battery (lower mAh) is charged with a charger designed to service larger packs. Although the charger will perform well in the initial charge phase, the battery starts to heat up past the 70 percent charge level. Because there is no provision to lower the charge current or to terminate the charge, heat-damaging over-charge will occur in the second phase of the charge cycle. If an alternative charger is not available, the user is advised to observe the temperature of the battery being charged and disconnect the battery when it is warm to the touch.
The opposite may also occur when a larger battery is charged on a charger designed for a smaller battery. In such a case, a full charge will never be reached. The battery remains cold during charge and will not perform as expected. A nickel-based battery that is continuously undercharged will eventually loose its ability to accept a full charge due to memory.
Toyota Motor Corp has set up a division to accelerate developing next-generation batteries, a company executive told reporters at the North American Auto Show on Tuesday.
The world’s biggest carmaker and a leader of gasoline-electric vehicles has adopted nickel-metal-hydride batteries for the current Prius hybrid and decided to use more energy-efficient lithium-ion batteries for the plug-in hybrid car which will be launched in 2012.
“We believe a key to the electrical mobile technology lies in innovation of battery technology,” said Koei Saga, Toyota’s managing officer who is in charge of developing batteries. “Lithium-ion batteries will already be a step forward, but we need batteries that offer far superior performance,” he added.
The new division was established in January and about 50 engineers are studying production processes for the next-generation batteries. Saga did not elaborate on what the new batteries will be like or when they will come out.
Saga also said Toyota has eyed the possibility of procuring some batteries from outside the company although the carmaker will mainly keep on using its own batteries. Toyota has developed its batteries together with Japan’s consumer electronics maker Panasonic.
Cell design made a profound advance in 1995 when the pouch cell concept was developed. Rather than using an expensive metallic cylinder and glass-to-metal electrical feed-through to insulate the opposite polarity, the positive and negative plates are enclosed in flexible, heat-sealable foils. The electrical contacts consist of conductive foil tabs that are welded to the electrode and sealed to the pouch material.
The pouch cell concept allows tailoring to exact cell dimensions. It makes the most efficient use of available space and achieves a packaging efficiency of 90 to 95 percent, the highest among battery packs. Because of the absence of a metal can, the pouch pack has a lower weight. The main applications are mobile phones and military devices. No standardized pouch cells exist, but rather, each manufacturer builds to a special application.
The pouch cell is exclusively used for Li-ion and Li-ion polymer chemistries. At the present time, it costs more to produce this cell architecture and its reliability has not been fully proven. In addition, the energy density and load current are slightly lower than that of conventional cell designs. The cycle life in everyday applications is not well documented but is, at present, less than that of the Li-ion system with conventional cell design.
The pouch cell is highly sensitive to twisting. Point pressure must also be avoided. The protective housing must be designed to protect the cell from mechanical stress.
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