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Rechargeable battery Protection CircuitIt is important for rechargeable batteries to neve go below a set voltage as the internal structure of the battery can be damaged if this occurs. Before this design complex circuits that were prone to failure and that also drew current from the battery were used. This design completely shuts down the loading of the battery and draws no current itself when active - this is because the gate current is virtually zero when the MOSTEF is active (unlike a transistor). a very clever design that is simplicity itself. Summary of the Rechargeable Battery Protection CircuitA Field-effect transistor is oupled to conduct current from a rechargeable battery to a load so long as the voltage potential across the rechargeable battery is sufficient to turn on the field-effect transistor to prevent the rechargeable battery discharging below a predetermined value, thereby protecting the rechargeable battery from permanent damage from overdischarge.According to the design, a field-effect transistor (FET) is coupled with its gate and source across the terminals of the rechargeable battery and its drain and gate forming the connections to the load. So long as the source-to-gate voltage exceeds a certain threshold, depending on the characteristics of the FET, the battery is effectively coupled to the load. When the voltage supplied by the rechargeable battery falls below a certain value, i.e., the voltage required to turn on the FET, the FET ceases to conduct, effectively disconnecting the battery from the load, preventing any further discharge of the battery. FETs having different characteristics are easily available so there is usually an FET type for any application. The characteristics of importance are the gate-to-source voltage needed to turn on the FET and the power rating. Since MOSFETs have low leakage currents, they are an effective switch for this application. The simplicity of the circuit for protecting rechargeable batteries makes it universally useful for circuits with rechargeable batteries from laptop computers to flashlights. Detailed Description of the Rechargeable Battery Protection CircuitThe design is the use of a three-terminal device having the characteristics of conducting electrical current from a first terminal to a second terminal so long as the voltage between the first and third terminals exceed a given threshold connected between a rechargeable battery and a load so that, when the rechargeable battery discharges to the point that its voltage is less that the threshold value, it disconnects the battery from the load to prevent any further discharge.The characteristics of field-effect transistors have cut-off voltages between the gate and source that make such transistors useful for making the design. The characteristics of a metal-oxide semiconductor field-effect transistor (MOSFET) are especially suitable for the design because, in addition to the cut-off characteristics, the gate draws negligible current and therefore does not bleed additional current after cut-off as would be the case with a bipolar transistor. Figure 1 : Mosfet Rechargeable Battery Protection Circuit
 In the circuit of FIG. 1, an MOSFET 17 is coupled so that current is passed from a rechargeable battery 11 to a load 15 so long as the MOSFET 17 is forward biased by virtue of its gate terminal being connected to the negative terminal of the rechargeable battery 11 and its source to the positive terminal. When the rechargeable battery 11 discharges to the point at which its voltage is at or below the value of the MOSFET 17 gate-to-source cut-off voltage, the MOSFET 17 effectively disconnects the battery 11 from the load 15, preventing further discharge of the rechargeable battery. Because of the extremely low gate current of the MOSFET 17, the rechargeable battery 11 does not discharge further. A typical commercially available MOSFET suitable for the described purpose is a Motorola MTP3N40 which has a cut-off voltage of approximately four volts. Figure 2 : Mosfet Rechargeable Battery Protection Circuit (Alternate design)
 If necessary, the cut-off voltage can be set by a voltage divider as illustrated in the embodiment shown in FIG. 2. The rechargeable battery 11, the load 15, and the MOSFET 17 operate as previously described except that the gate terminal of the MOSFET 17 is connected to a voltage divider formed by resistors 21 and 23. By choosing resistor values in a manner well known in the art, the cut-off voltage of the MOSFET 17 can be set to a predetermined value. In the case of the MTP3N40, the MOSFET 17 can be biased to cut-off when the voltage supplied by the rechargeable battery 11 falls below six volts. Jump from Rechargeable Battery Protection Circuit to Best Microcontroller Projects Home Page.
517-9337 |
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