# 12V Battery Shutdown

#### anotheruser1

Joined Dec 6, 2011
30
I have a bank of batteries that i want to shut off at around 12 volts. The normal running voltage with generator running will be approx 13.8. I have looked at anything i could find here that was close, but nothing matches what i need. Anyone have any diagrams of such would be appreciated

#### praondevou

Joined Jul 9, 2011
2,942
I have a bank of batteries that i want to shut off at around 12 volts. The normal running voltage with generator running will be approx 13.8. I have looked at anything i could find here that was close, but nothing matches what i need. Anyone have any diagrams of such would be appreciated
How much money can you spend?

How much current does your load draw at most?

What hysteresis do you want? Off at 12V and On at ...?

#### anotheruser1

Joined Dec 6, 2011
30
How much money can you spend?

How much current does your load draw at most?

What hysteresis do you want? Off at 12V and On at ...?
Hi, I have a ton of different ic's and things I got from a plant shutdown, so hopefully I have everything already. It is not a super critical thing, but I would like it to cut off at 12 and cut back on basically anything above 12, so for numbers ill just say cut on at 12.8. As for current it will be controlled by separate relays because it is a 400 amp load. Thanks for helping

#### praondevou

Joined Jul 9, 2011
2,942
Something like this would work:

It turns on the output voltage (relay) when the input voltage makes a transition from <12.9V to >12.9V. It turns off the output voltage at about 12.2V. (high to low transition)
The p-channel MOSFET could also be substituted by an PNP transistor with some changes to resistors made.
The ON transition is determined by the resistor divider 41.2k and 10k. The hysteresis (OFF transition) by the 41.2k in parallel with the 750k resistor and the previously mentioned 10k resistor.

The small relay would have to switch your 400A eek contactor.

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#### iONic

Joined Nov 16, 2007
1,662
I have a bank of batteries that i want to shut off at around 12 volts. The normal running voltage with generator running will be approx 13.8. I have looked at anything i could find here that was close, but nothing matches what i need. Anyone have any diagrams of such would be appreciated
Normal operating voltages of 13.8V From a 12V battery is not practical as this is the float charge voltage. As soon as this voltage is removed the battery will drop quickly, especially under a load down to in the ballpark of 12.7V.

Since Lead Acid Batteries begin to build sulfate on their terminals at about 12.4V I would not recommend having the lower set point much lower than that.

#### anotheruser1

Joined Dec 6, 2011
30
Something like this would work:

It turns on the output voltage (relay) when the input voltage makes a transition from <12.9V to >12.9V. It turns off the output voltage at about 12.2V. (high to low transition)
The p-channel MOSFET could also be substituted by an PNP transistor with some changes to resistors made.
The ON transition is determined by the resistor divider 41.2k and 10k. The hysteresis (OFF transition) by the 41.2k in parallel with the 750k resistor and the previously mentioned 10k resistor.

The small relay would have to switch your 400A eek contactor.
Thank you I will give it a try and see what happens.

#### anotheruser1

Joined Dec 6, 2011
30
Normal operating voltages of 13.8V From a 12V battery is not practical as this is the float charge voltage. As soon as this voltage is removed the battery will drop quickly, especially under a load down to in the ballpark of 12.7V.

Since Lead Acid Batteries begin to build sulfate on their terminals at about 12.4V I would not recommend having the lower set point much lower than that.
The battery bank is for a emergency back up system, when its in use there is a generator running. If the generator fails, I want it to shut off. So about 12 volts would be good for this purpose. Thanks for your input

#### iONic

Joined Nov 16, 2007
1,662
The battery bank is for a emergency back up system, when its in use there is a generator running. If the generator fails, I want it to shut off. So about 12 volts would be good for this purpose. Thanks for your input
Yes, you are right, it's worth a little sulfation for emergency power!

#### anotheruser1

Joined Dec 6, 2011
30
i have one question, the diode with the arrow pointing to it (see attachment) where does that go? i have never seen a connection in the middle. thanks

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#### praondevou

Joined Jul 9, 2011
2,942
i have one question, the diode with the arrow pointing to it (see attachment) where does that go? i have never seen a connection in the middle. thanks
That's not a simple diode, it's a voltage reference, a very common one.

Here goes the datasheet: http://www.fairchildsemi.com/ds/TL/TL431A.pdf

When the voltage at the middle pin goes higher than 2.5V with respect to the anode then it conducts (cathode anode).

It can be done with other parts, that's the easiest I could think of.

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#### anotheruser1

Joined Dec 6, 2011
30
That's not a simple diode, it's a voltage reference, a very common one.

Here goes the datasheet: http://www.fairchildsemi.com/ds/TL/TL431A.pdf

When the voltage at the middle pin goes higher than 2.5V with respect to the anode then it conducts (cathode anode).

It can be done with other parts, that's the easiest I could think of.
Learn something new everyday, thank you for your time