Preserving Lithium-Ion Battery Life
Discharging a li-ion battery below its minimum recommended cell voltage significantly shortens its lifespan. This circuit offers a solution by disconnecting the load from the battery once the cell voltage reaches a predetermined level, thus preventing excessive discharge and ensuring the battery’s longevity.
The voltage at junction A may be set to 3 V, for example, by selecting the correct ratio of R1 and R2. When the battery voltage drops below the minimum value, the voltage at junction A will be smaller than that at junction B. The latter voltage is equal to:
VB = 1.25 V + I R4 = 1.37 V
where: I = (Vmin. – 1.25 V) / (R3 + R4) = 800 nA
(Vmin. = minimum value)
Disconnecting Load to Preserve Battery
In this circuit, when the output of opamp LT1495 goes high. It triggers the P-channel logic level MOSFET, SW1, to block and sever the connection between the battery and the load. To prevent oscillation around the switching point, hysteresis is introduced using resistor R5. This added hysteresis ensures stability; the circuit will only reconnect the load to the battery when the voltage rises to 3.092 V, thanks to the 92 mV hysteresis provided by R5. Adjusting hysteresis can be achieved by altering R5’s value, considering the internal impedance of the battery and the load current.
Critical Setting of Switching Point
The switching point defined by R1 and R2 values is crucial in this circuit. If the switching point is too high, the battery’s capacity isn’t fully utilized. Conversely, setting it too low risks over-discharging the battery, leading to potential damage. With the specified values and parts’ tolerances, the switching point falls between 2.988 V and 3.012 V. For precise adjustment, it’s practical to use slightly lower values for R1 or R2 and incorporate a multi-turn trimpot in series. This approach enables accurate switching point adjustment and allows the use of ordinary 1%-tolerance resistors for R1 and R2.
Thorough Testing Before Implementation
Before deploying the protection circuit with an actual battery, it’s advisable to test it by connecting it to a power supply. Carefully verify all its features to ensure proper operation and avoid potential issues when used with a real battery.