You can get those cell protection boards for as low as 35 cents on Aliexpress. There's no reason to do it any other way with some kind of homebrew circuit, and definitely would be a bad idea to ever consider charging any size Lithium battery with no charge HV cutoff. Even small ternary cells (much smaller than an 18650) can hit over 500°C when they go into thermal runaway, and this can easily burn a house down. The cheap Chinese "vapes" do this often enough that every medium size fire department has had at least one call by now!
People don't respect these cells until they see what kind of energy can be released all at once, which will happen in mere seconds if they go into thermal runaway! An easy way to force one into runaway is to overcharge it! Those of us with battery experience/knowledge should set good examples and not encourage people to do dangerous things and take chances. On Mon, Feb 23, 2026 at 6:37 PM DOOLEY PHILIP G JR via EV <[email protected]> wrote: > The relay ckt is also bad because its sense voltage will change with > temperature and position and vibration. I think cheap flashlights rely on > the charger to limit the max voltage and the dimming of the LED to limit > the min voltage.The white LEDs I tested were very dim at 2.5V, but at -30C > that would be 2.4V. It is far safer to have a BMS PCB to preserve battery > life and prevent fires. Cells are available with this ckt built in. > Phil > > On Monday, February 23, 2026 at 05:45:01 PM EST, (-Phil-) via EV < > [email protected]> wrote: > > That's not going to stop overcharge though! You need protection on BOTH > ends! > > These Chinese protection boards also only draw in the uA class, and when > they hit LVCO, they usually stop drawing current. The biggest problem with > your relay solution is it will run the cell into the ground after LVCO! > > On Mon, Feb 23, 2026 at 1:55 PM Lee Hart via EV <[email protected]> wrote: > > > There are some very simple ways to do it. If you don't mind loading the > > cell at a few mA, use a small DIP reed relay with SPDT contacts and a 5v > > coil. A typical relay turns on when the coil voltage rises to about 80% > of > > rated voltage (4v for a 5v coil), and turns off when it falls to about > 20% > > (1v for a 5v coil). > > > > Wire the coil and a resistor in series across the lithium cell. Adjust > the > > resistor so the relay turns off at the desired voltage (say, 2.5v). > > > > Now connect the contact's common to the lithium cell, the normally-open > > contact to your load, and the normally-closed contact to the junction > > between the resistor and coil. > > > > How it works: When the relay is off, the coil sees the full cell voltage. > > When it rises over is over 4v (i.e. when you charge it), the relay turns > > on, and connects your load. It also puts the resistor in series with the > > coil. When the cell voltage falls to about 2.5v (1.5v across the > resistor, > > and the other 1v across the coil), the relay drops out, to disconnect > your > > load. > > > > Small 5v reed relays have a coil resistance of around 500 ohms, so this > > circuit only draws about 6 mA from a 3v cell. > > -- > > Excellence does not require perfection. -- Henry James > > But it *does* require attention to detail! -- Lee Hart > > -- > > Lee A. Hart https://www.sunrise-ev.com > > > > _______________________________________________ > > Address messages to [email protected] > > No other addresses in TO and CC fields > > HELP: http://www.evdl.org/help/ > > > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20260223/b547fd1a/attachment.htm > > > _______________________________________________ > Address messages to [email protected] > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20260224/e4147155/attachment.htm > > > _______________________________________________ > Address messages to [email protected] > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20260223/775e07d5/attachment.htm> _______________________________________________ Address messages to [email protected] No other addresses in TO and CC fields HELP: http://www.evdl.org/help/
