This design implements a charger for a lead-acid battery as a subfunction in a microcontroller whose main function can be any more complex task. Furthermore, the MCU gets its power from the same battery. The charging process is so slow and uses so little processor time that it doesn't jeopardize the MCU's primary task.
The goal of the circuit (Fig. 1) is to ensure an uninterruptible power supply. Depending on battery condition, there are three operating scenarios:
the battery is charged
the battery is discharged
the battery is disconnected
In the first case, the MCU must monitor battery voltage and temperature, as well as control the power switch to maintain the optimal battery condition. In the second case, the battery voltage is too low to maintain MCU operation. With the MCU inactive, the power switch must be on. In the third case, the power switch must also be on. In this condition, the voltage on the MCU input is significantly higher than in the other cases. As a result, the MCU's input voltage can be used to recognize this condition.
The charger's schematic (Fig. 2) can be broken down into three sections:
the voltage divider
the temperature sensor
the MOSFET driver and MOSFET
Also to be considered are MCU ports TEMP/AD0, VOLT/AD1, and STOP and the charging algorithm (implemented in the microcontroller).
The charging algorithm (see the code listing) provides two charging modes: full charging and standby. As the rectifier starts to deliver power, the algorithm goes into the full-charging mode.
In this mode, the circuit charges the battery to between 14.4 and 15 V, which is "full battery voltage" (FBV). The algorithm then goes to standby mode, which keeps the voltage between 13.5 and 13.8 V, or "standby voltage" (SBV). The digital oscilloscope printout in Figure 3 shows an example of this cycle.
The battery voltage through voltage-divider R5/R6 goes to the MCU's ADC input (VOLT/AD1). R5 and R6 should be temperature-stable devices.
Lead-acid battery manufacturers recommend including temperature correction in the charging process, since battery voltage changes with temperature at about 4 mV/°C/cell.1 Therefore, the charger incorporates a temperature sensor consisting of two diodes (D1 and D2) and one resistor (R4).
Voltage on these forward-biased diodes changes with temperature at about 2 mV/°C, so two diodes are used to double the voltage signal. This kind of temperature sensor can measure temperatures from about 20°C to 150°C.2 That's more than enough for this application. The sensor's output is connected to the MCU's ADC input (TEMP/AD0).
The MOSFET driver circuitry provides voltage gain and level shifting. When the battery is discharged, the driver turns on the MOSFET and starts battery charging. If the battery is disconnected, the MCU will detect a very high input voltage and keep the MOSFET on.
The charger circuit and control algorithm was implemented using an Atmel ATMega16 microcontroller and WinAvr GNU GCC compiler.3,4 With an 8-MHz clock rate, the algorithm's ChargerSubfunction takes approximately 2.1 ms with a repetition time of 5 to 10 seconds. Accordingly, the charger uses less than 0.00042% of the MCU's time.
Finally, we should mention that this charger also can be built as a stand-alone device.
References
D. Berndt, Maintenance-Free Batteries: Aqueous Electrolyte Lead-Acid, Nickel/Cadmium, Nickel/Metal Hydride, Research Studies Press; 3rd edition, Oct. 2003
Adel S. Sedra and Kenneth C. Smith, Microelectronic Circuits, Oxford University Press, USA; 5th edition, Nov. 27, 2003
OFFER FOR COOPERATION ON THE REALIZATION OF NEW TECHNICAL SOLUTIONS Dear Ladies and Gentlemen, I would like to ask you to consider the possibility to cooperate on the realization of following program and projects. Accept our business proposal according to your interests! A program under the short name “PARS” is referring to the production and marketing of devices for a modern, efficient and full treatment of all battery types (Pb, NiCd, NiMH, Lilon and LiPol). The charging period lasts from 15 minutes to 2 or 4 hours, depending on battery’s capacity. Today are constantly used over 3 billion batteries and around 1 billion battery chargers. And the numbers are still increasing.The Pars Program has two devices to offer: the PARS Set and the PARS Adapter, Those devices would satisfy all users of devices with battery power supply. According to market needs the annual production of PARS devices would be counted in millions. For all those, who are interested in this program a practical demonstration of the efficiency and the potentials of the PARS program devices could be organized so they could compare them to other devices existing on the market. Cooperation on the realization of the PARS program is possible with more than one subject from more than one country, because of the programs extensive assortment and production. A device from the PARS program consists of following parts: - A controlled battery discharger - A controlled battery charger - A controlled battery conditioner - A battery’s capacity measuring device - A device for analyzing battery’s condition and other parameters which are important in maintaining the well functioning of all modern battery types The efficiency of the devices that depend on a large number of batteries (commonly used by the military, police, and mine workers) can be highly increased with some of the devices from the PARS program. The PARS program is applicable in 7 projects 1. For military and police: - For the purposes of the Serbian army ??For the purposes of the US army ??For the purposes of the NATO army ??For the purposes of the Russian army ??For the mobile accu stations 2. For modernisation of accu stations used in mines (miner's lamps) 3. For GOLF vehicles and accu stations on golf terrains 4. For scooters and similar battery charged vehicles 5. For wheelchairs and shopping carts 6. For forklifts and other battery charged service vehicles 7. For electro cars and traffic infrastructure (charging stations) PARS Program is a very ambitious program because it offers modern devices, a whole new family of electrical devices, which should satisfy a world market of over 3 billion batteries and around 1 billion battery chargers. So, for the needs of the PARS program it would be necessary to activate over 30 production plants. Author, Dr. Slavko Radosavljevic, Eng. Tel. + 381 11 2154894 E-mail. r_slavko@yahoo.es
Slavko Radosavljevic -June 06, 2007
it is very useful
kulandaivelus -April 24, 2007
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
