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/
