How to make an 11v shutoff circuit?

[BatteryCharger]

As some of you read in my previous thread I plan on making a giant 13.2v battery from a ton of AA NiMh cells because I can get them cheap. The main power output will go out through a relay. I want the battery to stop at 11v, or 1v per cell to avoid damage. Is there a relatively simple circuit I can build that will detect 11v, and then flip my main relay off? Something to do with a zener diode maybe?

 

[JimmyM]

As some of you read in my previous thread I plan on making a giant 13.2v battery from a ton of AA NiMh cells because I can get them cheap. The main power output will go out through a relay. I want the battery to stop at 11v, or 1v per cell to avoid damage. Is there a relatively simple circuit I can build that will detect 11v, and then flip my main relay off? Something to do with a zener diode maybe?

A resistor and an 5-7V Zener diode as a voltage reference, a pot to adjust voltage and a comparator to "compare" the reference to the pot. When the battery connected to the pot produces a voltage less than the reference voltage, the comparator output will go low, thereby shutting of the FET that operates the Relay. Or just use a Big FET instead of the relay. Don't forget hysteresis to prevent chattering when the voltage drops and recovers. There are about a billion electronics circuit schematics out there on the internet. Search for "voltage comparator", "low battery shutdown", etc.

 

[VidPro]

lazy way:
put a pot on the power going to the drive coil of the relay, raise the resistance on the pot till the relay drops out like between 11 and 10.5
use a center off switch, when you start up the thing, you hit the relay coil with full power, then retract back to center, to turn it off, you switch it down disconnecting the relay coil.

all the relay coils will HOLD with very little current and voltage once triggered (with higher power), so you adjust the pot till it lets loose again at that voltage (triggering the relay initally without the pot).

once they let loose from thier magnetic bond, they fly back pretty fast, and they have the springy contacts stuff in there, so it still does a fast clean disconnect, even if its a limp hold that finnaly becomes undone.

Lasy ways have caveats only good for solid instalations, if you bump the relay with it holding at the low end of the magnetic hold , it will come undone earlier than your calibrated voltage.

i use that method as a mega size cutoff for huge li-ion bank , once the li-ion battery is to far depleated , the coil will not hold, and it cuts off the battery, if i am desperate i can manually trigger it for a short burst still. (of course you could also manually trigger a digital electronic curcuit too). also i adjust the relay itself as needed, bending the contacts to bounce away different, and changing the spring pressure, easy to do with big ones, still possible to do with smaller ones. and at lower DC voltages you dont really have Arcing issues so the throw range doesnt need to be large, like for 120v ac would.

for examples: a 12V relay might kick in at as little as ~8-9volts on the coil, and not kick back out till it gets below ~6-7v
a 6 volt relay will still kick in at ~3.5v but not kick back out till about 3V.
huge voltage ranges between triggering on and turning off, make it a problem, but its still usefull with something triggering the inital magnetic pull, and the low voltage dropping it back out.

so you have a button/switch to kick it in Hard, then the loose hold through the calibrated resistance kicks back out when the voltage drops to low.
2 parts, recalibratable in seconds, easy to hands on visualise, wont work if its going to get bumped.