Circuit to shut off inverter at low battery voltage set point = 50%

[DSII]

Charge controlers have an adjustable high voltage set point, so the batteries are not over charged. If you place a relay circuit that disconnects the batteries from the inverter so the voltage does not fall below the 12.06 50% level it would reduce damage to the batteries. The problem is that the circuit will need to not enguage the inverter until the voltage is some what higher like 12.30 volts. If needed the relay could open the 115 volt side and the inverter would only draw a small amount of current, until you checked the battery voltage level.

Thanks for any help in this matter.
Dale La Roy Smith II
Cocoa, Florida

[Mike90250]

Some inverters have a "Low Battery Disconnect" in them, but they are set too low usually.
I think you could use a 12V relay, and a trim pot in series with it, to control the 115V output, adjust the trimpot so the relay just drops out at your desired voltage, and have a "override" push button switch to short out the trimpot, to reset the relay.
Has to be home brew, i've see nothing in stores.

[DSII]

Relay will bounce unless close voltage is set higher.

Here is what happens:
1. Voltage goes down to low set point = 12.06 ~ 50%
2. Relay opens, no load on batteries, now voltage goes above 12.06
3. Relay closes, which places load on batteries, now voltage goes below 12.06
4. Go back to step 2. repeat process ( relay now is in circular cycle )

Do you think the OUT BACK inverts have Low Battery Disconnect that is adjustable?

Thanks for your help.

[Sunking]

Here is the problem, State Of Charge Voltages is meaningless when the battery has a load on it or being charged. The only time SOC has any meaning is when the battery is disconnected from any load or charge currents and rested for several hours.

The only true measure of the state of charge is by using a temperature corrected hydrometer reading, then record the loaded voltages down as each data point is reached.

Secondly you are getting the bounce effect because you need 2 set points:

1. LVD or low voltage disconnect, say 11.9 volts
2. LVR or low voltage reconnect which has to be higher than LVD, like 12.3

If the two are equal or too close, you will just sit there and bounce.

[DSII]

Theory great actual circuit ?

Dear SunKing,

In theroy I understand but not sure how to connect circuit. Could you steer me in a direction to make a circuit that would do this?

Thanks.

[Mike90250]

My suggestion - use a manual reset, or a 4PDT 120VAC relay, and route the relay coil wire thru the NO cntact. Then you HAVE to manually reset it, the battery recovering won't trip it back on

[opanin17]

Low battery protection is good for all solar inverters as well as every battery powered device. But using just a relay and a trimpot wouldnt solve the problem fully because when the battery voltage is close to the switching point, the relay will be bouncing. In some cases, the bounce can be stopped by connecting 100uf capacitor accross the coil pins of the relay.
In other cases as the one we have here, bouncing can be stopped by using a comparator with hysterises as in this link circuit

http://helpersalone.blogspot.com/2016/07/low-battery-off-and-alarm.html

The hysterises function sets two operating points for the relay turn on and turn off. In that case bouncing is overcomed and the circuit is made automatic.

[Mike90250]

This is an ALERT function, and requires a human to intervene and protect the batteries (start the generator or shut off the inverter)

[Sunking]

Low battery protection is good for all solar inverters as well as every battery powered device. But using just a relay and a trimpot wouldnt solve the problem fully because when the battery voltage is close to the switching point, the relay will be bouncing. In some cases, the bounce can be stopped by connecting 100uf capacitor accross the coil pins of the relay.
In other cases as the one we have here, bouncing can be stopped by using a comparator with hysterises as in this link circuit

http://freecircuitworks.blogspot.com/2016/03/low-battery-protection-and-alarm31.html

The hysterises function sets two operating points for the relay turn on and turn off. In that case bouncing is overcomed and the circuit is made automatic.

Really? Please inform us. I mean really this is a 6-year old post we have been waiting 6-years to be answered. Please enlighten us with your superb knowledgge.

[opanin17]

Mike, this is not an alert function only, the circuit in the link has an alert for low battery and a disconnect function using either a relay or P mosfet. You only adjust the variable resistor when your battery is close your stop voltage to start your beeping notification.
When the battery level reaches your desired turn off, you adjust the other variable resistor to switch your relay off. In this case because of the positive feedback, the relay will turn on after the battery voltage has been made to rise a little through charging.

[Mike90250]

I don't think a relay or mosfet is going to control a serious inverter. Maybe a small 5-20A load. after 20A DC relays get expensive, and 4Kw mosfets even more.

[opanin17]

No mike. You are not going to switch or control the main DC . You are only going to use the circuit to control either the switch cable or even connect it directly to your inverter IC shutdown pin. Check this post and see how its done.

http://helpersalone.blogspot.com/2016/07/low-battery-off-and-alarm.html

Every inverter uses either of this.

[bcroe]

I don't think a relay or mosfet is going to control a serious inverter. Maybe a small 5-20A load. after 20A DC relays get expensive, and 4Kw mosfets even more.

The good approach would be an electronic control if available to shut off loads, instead of just breaking the main
feed. Actually low voltage MOSFETS have gotten cheaper these days. Find cheap ones, and just parallel a bunch
of them; no issue with sharing in saturated mode. And use a simple Op Amp + Voltage Ref set up as your detector,
with either a flip flop or a lot of hysterises to drive the MOSFETS. Did this at work at the 48V 180A level. Bruce Roe

[clinecustoms]

Has anyone yet found a reasonably priced in line adjustable low voltage cutoff, like the battery savers on trucks? for 24v 60 or 80 amp max draw. looking for something independent to the inverter or CC cutoffs to really just protect the battery from going below say 22.5v at any given time like either under load or resting.

[Mike90250]

Most inverters have a built in LVD. Better ones have programmable ones. Your manual will state what the disconnect voltage is

[clinecustoms]

Most inverters have a built in LVD. Better ones have programmable ones. Your manual will state what the disconnect voltage is

looking for something independent to the inverter or CC cutoffs. on occasion there are things connected to the battery not through an inverter or CC, in my case I have one system that is for a dvr and cameras, which I run direct to the battery as I do not want the controller shutting them off unless really needed, now that particular one is on 12v and not more than 7a however I am using a cheaper 24v which I would like a secondary backup shutoff if things go bad. this would be a total safety for the batteries and shut everything off until manually recharged above a set voltage.