Battery Low-Voltage Cutoff

Don't kill your rechargeables.

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I'm knocking together a gadget to run 12V devices off of 18V Ryobi One+ battery packs (of which I have a few). I found a sufficiently beefy buck converter a while back that knocks 18V (nominal) down to 12.6V, but there's nothing in it to keep the battery from being discharged too much. This circuit will cut off its output at around 14.28V...2.86V per cell for Li-ion, 952 mV for NiCd (since I still have some of those).

It's a fairly simple circuit.  The input voltage feeds both a voltage divider and a 5.1V reference, which are compared by an op-amp.  If the input voltage exceeds 14.28V (set by the 18k and 10k resistors in the voltage divider), the op-amp turns on the MOSFET, which turns on the load.  Since a freshly-charged Li-ion battery pack can exceed the MOSFET's 20V gate-to-source rating, the gate is driven through another voltage divider.

(The idea is largely cribbed from, but with the bipolar transistor and relay replaced with a MOSFET.)

  • Boards arrived today

    Scott Alfter08/12/2020 at 04:25 0 comments

    15 days to get here from JLCPCB, and that was with the cheaper shipping option.  When I got home, I pulled the parts off the breadboard, stuffed them into one of the PCBs (panelized 3-up, so I received 15 total), and hooked it inline between a Ryobi NiCd battery pack and a buck converter...and it worked!

    Now I need to work on the packaging to turn all of this into a durable adapter that I can bring on the road with some battery packs...might need to start another project for that.

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John L. wrote 11/16/2021 at 16:12 point

Does this circuit turn itself completely off to prevent parasitic drain on the battery?

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Scott Alfter wrote 12/02/2021 at 22:50 point

I didn't provide for that.  I suspect the drain of the opamp is fairly low, but the way I've used mine so far, I usually just plug in a battery pack long enough to run a device (like an inverter to power the inflator on an air mattress) and then unplug it, so I haven't tried measuring no-load drain.

  Are you sure? yes | no

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