NiMH battery discharger

[Andrew Eckhardt]

Are you sure what to expect from deep cycling NiMH batteries? Most sources say it doesn't help anything, and not doing it ensures the dreaded polarity reversal never happens. You'd likely improve cycle life without deep cycling.

[nattawa]

Originally Posted by Bradford336

Does C9 polarity not matter?

C9 is a 0.1uF ceramic or film cap, has no polarity.

Originally Posted by Bradford336

What would the V and A specifics be for D2?

D2 can be any generic LED of any color of your choice, forward voltage drop is usually in the range of 1.7V - 2.5V. When working with 5.5V +B, the voltage across R6 is about 3.5V, the current through the LED would then be about 3.5mA.

Originally Posted by Bradford336

...how the Q1-A is actually put into the schematic...

Q1-A is a power MOSFET, its source goes to ground, drain is at R4, and its gate is driven by the U1-A output.

Since the comparator LM139 is powered by the battery, its output goes between the ground level and the battery voltage. That works just fine with a 5-6V battery. However, if you use this circuit on different voltage batteries you need to take caution about a few things. The mosfet requires a minimum 4.5V across gate-source to turn on, the circuit may not work on batteries of a voltage lower than that. The mosfet can be damaged if 20V or higher voltage is applied across gate-source, you want to put in some protection when working with higher voltage batteries to limit the gate drive under, say 12V or 15V. The feedback resistor R3, together with R1, R2, and R7, give the comparator a hysteresis behavior. The hysteresis level is affected by the battery voltage as well. I would suggest a separate power supply, 9V or 12V, for the comparator and the voltage reference D1, if you want the circuit to suit various voltage batteries.