From the electronics viewpoint, the battery chargers can be divided into linear- and switching-regulator chargers.
For some devices, for example digital cameras, preferred battery charging is done by using a cradle charger in which the device, or just the battery, is placed. With this method, the heat dissipated by the charger has much less importance with respect to that of an embedded charger. In these cases, a linearregulator charger is used, which has relatively large dimensions and an efficiency hardly exceeding 60%.
In some larger devices, for example notebooks, the battery charger is part of the system and, therefore, heat generation has to be limited. A switching charger is remarkably smaller than a linear one and can be embedded in the device. This is made possible by the use of a switching type regulator (or switchmode power supply (SMPS)). The main drawback of switching regulators is the noise level generated by their switching action (10 000 times more than in linear regulators). However, there are means to cope with this drawback, for example connecting special capacitors from the AC input terminal to ground.
The electronic industry produces a number of ICs for the correct charging of batteries according to their chemistry. Original equipment manufacturers make their choice as a function of the IC performance and price. Therefore, they might want just a single chemistry charger (Ni–Cd/Ni-MH or Li-ion or Pb-acid), or a multi-chemistry, smart-battery-compliant charger/controller.
Multi-chemistry charging is now available in several ICs. The latter charger is based on bq2000, an IC by Texas Instruments, whose details are given here as a way to illustrate how similar ICs work. Bq2000 is a programmable IC that can charge Ni–Cd, Ni-MH or Li-ion batteries. The battery chemistry is detected by monitoring the voltage profile in the early charge stages. Afterwards, the charge is brought to completion with an appropriate algorithm.
A programmable timer is also available to allow a further charge control. Fast charge is not allowed until voltage and battery temperature fit in a given range. If V is low, the IC provides a trickle charge to raise it to an acceptable level. If T is too high (typically above 45_C), charge is only started after cooling down; if T is low (typically below 10_C), a trickle charge is applied. The power efficiency is more than 90% and power dissipation is low. Other features of chargers/controllers of this type include: sleep mode for low power consumption, battery removal and insertion detection, continuous temperature monitoring and fault detection, user-programmable charge current and voltage.
In multi-chemistry chargers, Ni–Cd and Ni-MH batteries are charged at constant current and charge termination is based on voltage, temperature or time values. In the first case, the peak voltage typical of the end of charge of these batteries is used. If a top-up charge is requested, this is made at a lower current and termination is time-dependent. Finally, a trickle charge, usually at a rate between C/20 and C/100, is applied to the battery to compensate for self-discharge. For Li-ion batteries, smart chargers normally apply the constant current (at the 1C rate), constant voltage (within 1%) method. Charge is typically stopped when the current in the constant-voltage step falls below C/30 or when the timer tells that time is out. Li-ion batteries have low self-discharge rates, so trickle charge is not applied. Depending upon the power requirements of the application, battery packs can consist of up to four Li-ion cells in a variety of configurations. These batteries can be charged with different power-source types: AC adapter, USB port, or car adapter.
Chargers may be linear or switching type. With reference to Li-ion batteries, more details are given below.
A linear charger is preferred when the input voltage of the charger is only moderately higher than the OCV of fully charged cells and the 1C fast-charging current is not much higher than 1 A. For example, there is typically one Li-ion battery cell in an MP3 player, with capacity ranging from 700 to 1500mAh and an OCV of 4.2 V. Because the power source for the MP3 player is either an AC/DC adapter or a USB port with a well-regulated 5-V output, a battery charger with a linear topology offers the simplest and most cost-effective solution. Because of the high power dissipation of linear chargers, they are only suitable for small-capacity devices (<_1300mAh).
The buck, or step-down, switching charger is a better choice when the 1C charging current is higher than 1A or when the input voltage is much higher than the open-circuit voltage of the fully charged cells. For example, in a hard-drive-based PMP, a one-cell Li-ion battery with OCV of 4.2V and a capacity of 1200–2400mAh is typically used. PMPs are now frequently charged by car kits, which output a voltage between 9 and 16 V. The high voltage differential (at least 4.8 V) between the input voltage and the battery voltage makes the linear topology very inefficient for this application. This inefficiency, coupled with the high fast-charging current (1.2–2.4 A, i.e. 1 C) can create heat-dissipation problems. To avoid this, the buck technique needs to be adopted.
In certain applications where three or even four Li-ion/Li-polymer cells are connected in series, the input voltage to the charger might not always be higher than the battery voltage. For instance, a laptop PC uses a three-cell Li-ion battery pack, with a 12.6-V (4.2 V_3) OCV at full charge and 1800–3600mAh capacity. The input power source can be either an AC/DC adapter with an output voltage of 16V or a car kit with an output voltage between 9 and 16V. As such, the input voltage can be either lower or higher than the battery voltage. Obviously, neither a linear nor a buck charger is capable of charging a battery pack under this circumstance. This gives way to a switching charger with the same structure of a buck-boost regulator.
A recently introduced method for charging Li-ion batteries, that is constant- current pulse charging, combines the benefits of a linear charger and a switch-mode charger. It limits the charging current by employing a currentlimited AC wall adapter. The current from the converter is switched to the battery for constant-current charging. As the battery voltage rises to the voltage limit, the current source is switched on and off, thereby supplying the required average current to the battery without exceeding the voltage limit. Power dissipation is low, because the switch is either on or off, as for a switch-mode charger. Yet the circuit is simple, as in a linear charger, because no output filter is required. It can dissipate more power while in the current-limit mode (depending on the AC wall adapter used), but that has little effect on the battery or its load if the maximum safe temperature is not exceeded. The circuit of this charger is smaller and less complex than a switching charger: a complete charger can be made with only two capacitors and one resistor in addition to the IC (e.g. 1679 by Maxim) and external MOSFET.
According to Sanyo, a producer of this type of charger, full charge can be reached in 90 min vs 2–3 h for a conventional CC-CV charger. Pulse width modulation may also be used: as the battery reaches full charge, the period between pulses increases. The objective of using a pulse current is to increase the rate of electrode reactions. When a sufficiently high charging current flows continuously through a cell, the electrodes becomes polarized. Consequently, the cell voltage increases and tends prematurely to the upper limit. If the current is stopped for a while, some relaxation may occur, that is the electrodes can have some degree of depolarization and the ionic flux at the electrode/electrolyte interface can be faster.
Reconsidering the power sources, the most flexible source is certainly the AC wall adapter. However, the USB port of a notebook, desktop computer, or computer peripheral offers an interesting alternative. Indeed, charging a cell phone, MP3 player, or digital camera from a notebook computer is highly convenient and can be performed anywhere; moreover, it allows users to perform two functions at the same time, such as downloading music or updating files, while recharging their battery.
A typical notebook’s USB port provides a 5-V source with up to 500mA of current. The ability of a USB port to supply a charge for another portable device is highly dependent on how much power the portable system draws to run other functions, while the battery is being charged. When the system requests power to run subsystems or applications, the current available to charge the battery will be reduced because the USB port can only provide a finite amount of current, and this will extend the charging time.
Our website is not responsible for the information contained by this article. Webworldarticles.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
This article was sent to us by:
Giani Rimeollo at
06282010
1. A PRACTICAL GUIDE TO GET THE MOST OUT OF YOUR LEAD ACID BATTERIES
All articles in this directory are property of their respective authors. Additionally, read our Privacy Policy
© 2010 WebWorldarticles.com - All Rights Reserved. Partners: Gunblade Saga