I want a simple way to keep a 12 volt gel cell charged between a range of voltages (approx 10.5 volts and 13.8 volts), and to disconnect the load when it falls below 10.5 volts, and reconnect at 13.8 volts. I am using an 18 volt solar cell as the power source, and this has an amplified zener to regulate its voltage to a fairly stable 13.8 volts, but it can fall below this when dark!
I have looked at a window comparator circuit, but it needs a stable supply voltage. Any ideas? Thanks

 

Regulate the supply voltage? duh very inefficeint. How about a LM339?

 

Put an isolating diode in series with your 12v cell and feed that with a regulated 14.4v from the solar cell. You can leave it permanently connected because this will 'float-charge' the cell at 13.8v.

 

like pebe said, just isolate the solar panel fro the battery with a suitable sized diode. many solar panels have this either built into them or have connection points provided. It is needed to prevent "back-feed" from the battery to the solar panel when there is no light, to prevent heating and damaging the solar panel. the solar panels I use are setup this way when multiple panels are strung together, and to prevent the batteries damaging the panels.

 

Thanks for the ideas. I already have a diode between battery and solar panel, but the battery is then used to power a 130mA 12 V cooling fan (switches on at 70F). During the day, the fan takes more current than is being replaced by the 18 V solar panel (100mA in sunlight) and eventually the battery gets to 9 V - not good for gel cells! I want to stop the battery discharging below a set point, say 11 volts, and switch off the fan until the battery has a healthy charge, say 14 volts. I need a simple switching circuit to control this voltage range.

 

Hi Jules, That LM339 Circuit I sent you will do what you want.

Gary