The emergence of the Smart Battery System (SBS) greatly simplifies the design of stand-alone battery systems, so its application has surpassed the notebook computer field and appeared in a variety of other applications, such as backup power systems, high-reliability military and aerospace applications. Other key applications include automotive, security/surveillance/anti-counterfeiting systems, medical devices, blade servers, telecommunications and portable electronics.
Smart batteries use internal electronics to measure, calculate, and store battery data, making power usage more predictable. Moreover, smart batteries have an important advantage in that they prevent accidental system downtime.
A basic SBS system consists of the following: System Management Bus (SMBus), smart battery charger and smart battery.
The modular nature of SBS makes it easy to design a closed-loop battery charging system that allows the use of a battery-independent charger (smart charger) to minimize the cost of non-recurring engineering (NRE) of hardware and software. It also contributes to a rugged system that is especially important for high-reliability battery backup applications. The high-accuracy barometer integrated into the battery pack keeps the battery accurately monitored, even when the battery is not in the system. The barometer is calibrated to the actual capacity of the battery, thus eliminating deviations and ensuring accuracy.
The main function of the smart battery charger is to provide a voltage source and a current source for charging the smart battery. The smart battery communicates with the smart charger via the SMBus interface and optionally communicates with the host. To prevent overcharging due to loss of SMBus functionality, the watchdog timer continues to run to monitor the frequency of calls between the smart battery and the charger. If the battery has been inactive for more than 3 minutes, the charger will pause and wait for the battery to request charging again. In addition, the battery can be controlled by a forced shutdown feature that bypasses the SMBus to provide redundancy and let the charger know that the battery is indeed present.
The smart battery charger has the following advantages over a fixed stand-alone charger.
1 True plug and play, independent of battery chemistry and battery configuration. Any smart battery pack can be paired with any smart battery charger. Batteries with different chemical properties, configurations, and even different charging algorithms can be replaced with charger circuits without modification.
2 Built-in security features. The SBS standard provides a watchdog timer and a special "safety signal" interface between the battery and the charger.
3 Reliable battery detection system.
4 Automatic charging management, no need for host.
5 Closed loop charging system without host intervention. The host can collect power measurement information as needed.
LTC1760 Dual Smart Battery System Manager
The LTC1760 is a highly integrated three-level battery charger and selector for products that use dual smart batteries. It is a buck switch topology battery charger with a variety of features and other new features that meet the smart battery standard definitions, such as input current limit and safety limits, and more. Three SMBus interfaces enable the LTC1760 to implement servo functions such as tracking the internal voltage and current of two batteries and allowing an SMBus host to monitor the status of any battery. This servo technology enables the accuracy of the charger to be ±0.2% less than the internal voltage and current measurements of the battery.
Traditionally, dual battery systems are sequential discharge systems that allow the battery power to be consumed sequentially to simply extend the total battery operating time. The LTC1760 uses proprietary analog control technology that allows two batteries to be safely charged or discharged in parallel. This structure increases the charging speed by 50% and the battery operating time by 10%. In addition, parallel discharge not only enhances current capability, but also reduces I2R losses and improves voltage regulation under very high load conditions. Reducing I2R losses and improving voltage regulation all extend the total discharge time of the timing solution.
â— Main features of the LTC1760
1 Independent Level 3 charger polls the battery for charging requirements and monitors the actual current and voltage (with an error of ±0.2%) as determined by the internal battery charge measurement for fast, safe and thorough charging.
2 Fast charging mode can be used to further shorten the charging time.
3 Support battery inspection for barometer calibration.
4 3 power path FET diodes allow safe and low loss simultaneous discharge from DCIN and both cells.
5 Two FET diodes enable both batteries to discharge safely and with low loss.
6 Hardware programmable current and voltage safety limits and many other safety features to complement the battery's internal protection circuitry.
Although the LTC1760 is very precise, it is very easy to use. Only four key parameters need to be determined in any given design: input current limiting sense resistor RICL, current limiting resistor RILIM and matching charge current sense resistor RSENSE, voltage limiting resistor RVLIM, short circuit protection resistor RSC.
The LTC1760 adds some smart batteries and an AC adapter to form a simple system.
Figure 1 LTC1760 dual battery charger / selector system architecture
â— Input current limit detection resistor RCL
As shown in Figure 2, this circuit limits the charging current to prevent overloading of the AC adapter when system power is increased. To set the input current limit, the most important thing is to minimize the rated current of the wall adapter. The current limiting resistor can be calculated by the following two equations.
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