This circuit protects the battery against deep discharge (discharge below the minimum voltage) and load against undervoltage.
After discharging battery to the minimum supply voltage circuit disconnects the load from the battery and does not draw any current.
Suitable for protection of batteries such as lead-acid (Pb), NiCd, NiMH, Li-Ion and Li-Pol accumulatores.
In Fig. 1 you can see the schematic of protection that disconnects the battery when the voltage reaches the minimum value. The threshold voltage is determined by the sum of the voltage of zener diode ZD1, E-B drop of transistor T1 and some low voltage on R1. To connect the load you must press TL1. As long as the voltage is sufficiently large, the T1 and T2 maintain each other in on state. When the voltage decreases, the current stops flowing through the Zener diode, which closes T1 and subsequent T2. With positive feedback is always close immediately, thus there is no slow gradual closing of transistors. To achieve low losses, MOSFET is used as a power switch device T2.
In Fig. 2 you can see a modified diagram, where TL1 button allows you to power on and off load. The device thus serves not only as protection but also as a power switch. The principle of operation is similar to Single button ON-OFF toggle.
Maximum input voltage depends on the maximum voltage Ugs of the transistor T2. The minimum input voltage depends on the voltage at which T2 still opens reliably. For conventional MOSFETs work from about 5V, the low voltage (logic) MOSFETs can work with lower voltages. This allows applications such Li-Ion/Li-Pol one cell which has min. voltage of approximately 3.4 V. For small voltage the zener ZD1 can be replaced by a combination of series connected diodes. I tested the circuit with the IRF3205 and IPB06N03LA in function of T2. Note: it is advisable to connect the fuse in series with the battery, otherwise there is a risk of fire in case of failure.