Circuit protects battery from overdischarge

Martin Wuzik, Implex AG Hearing Technology, Ismaning, Germany -- EDN, 4/12/2001

All electronic circuits powered by a battery discharge the battery. In some applications, it is undesirable to overdischarge the battery, because it could irreversibly reduce the battery's capacity and the number of discharge/charge cycles. The circuit in Figure 1 protects a single NiMH (nickel-metal-hydride) cell by disconnecting the load from the battery. Figure 2 shows the output voltage, V_SYS,versus the input voltage, V_BAT. For this NiMH battery, the switching points are 1.1 and 1.3V. If the battery discharges and V_BAT drops below 1.1V, Q₁ switches off ,and the node Main Circuit disconnects from the battery. In that case, the battery's only load is the pair of voltage detectors IC₁ and IC₂ from Ricoh (http://www.ricoh-usa.com). The load current of one detector is typically 800 nA, so the battery drain is 1.6 µA. The user must now charge the battery. Once the battery charges and the voltage reaches 1.3V, the load reconnects to the battery and remains connected as long as V_BAT stays above 1.1V.

IC₁ is a voltage detector with a 1.3V setpoint and a push-pull output. IC₂ has a 1.1V setpoint. An important difference between the two detectors is that IC₂ has an open-drain output. If the battery voltage drops but remains within the 1.1 to 1.3V range, IC₁'s output is low, and Q₂ switches off. Q₃ switches on because IC₂'s output is still in the high-impedance state. If V_BAT drops below 1.1V, IC₂'s output switches low, Q₃ turns off, and, as a result, Q₁ also switches off. As soon as V_BAT drops below 1.1V, the load disconnects from the battery. The load reconnects to the battery only when the battery charges to a voltage higher than 1.3V. At voltages of 1.1 to 1.3V, IC₂ cannot switch on Q₃ because the IC's output is an open-drain type and V_SYS is low. IC₁'s output must assume a high state to switch on Q₂ and to finally switch on Q₁ on. The transistors are low-threshold MOSFETs from Supertex (http://www.supertex.com). The circuit uses no trimming resistors. You can select IC₁ and IC₂ off the shelf with 100-mV steps and 2% switching-point accuracy. You can adapt the circuit for the higher voltages of Li-ion batteries by selecting the voltage detectors.