A Simple 24V Li-Ion Battery Low Voltage Disconnect W/ Hysteresis Using Six Parts
I was at Lowes the other day and I saw their new 24V Kobalt tools. With a great surprise they sell a low capacity 24V Li-ion 1.5A/Hr battery for $10.00.
Wow! I was hooked I bought a the starter set with the drill and 1/4 inch impact driver with charger and a battery for $119.00 and 3 of the 1.5A/hr batteries.
I am excited about having these low cost Kobalt 24V Li-ion batteries since they will be more useful as power sources for other projects. I quickly designed an adapter on the laser cutter out of acrylic that I could plug into the battery.
First project to use the battery pack on: A dual USB power outlet and an 800 lumen LED light
I installed a DC-DC boost board to produce the 28V needed for the series LED string and a DC-DC buck to produce the 5V needed for the USB ports.
Great it works, lets test it.
Wow runs for about 3.5 hours with the LEDs running.
Doh it runs the batteries below their critical cutoff point of 15V. Yikes this will destroy the batteries in no time. (apparently these batteries do not have a low voltage cutoff built into them)
Hmm time for a low voltage cutoff circuit. So here is the simplistic circuit that I came up with: It cuts the battery off at ~18.2V and then with a little hysteresis it will turn back on again at 19.8V I figured this was a good trade off. This is simulated, I am ordering the parts to see how the circuit will perform. I plan on using a TLV431 since it will use less current to function and the PMOS will also be a different part. Edit: I just looked at the datasheet for the TLV431 and it is a low voltage part so a nogo there.
TLV431 2.5V reference
R1 and R2 form a divider for 2.5V (the switching point of the TL431)
R3 Bias for TL431 and PMOS Gate
R5 creates the hysteresis ( this is so that we do not oscillate when switching the battery off )
R4 is just a simulation load ( this is where your battery powered circuit will be )
M1 is a P-CH MOS FET sized accordingly for the load. ( I like a low rdson value and at least 2x the voltage for the rating so 48V or greater PMOS)
V1 is a simulation voltage source that is programmed to run from 24V @0.0S to 18V@0.5S then back to 24V@1.0S
Red Trace is simulation load current, Green trace is simulation Battery Voltage