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/
