# Low voltage disconnect question



## cowboy joe (Sep 14, 2003)

Finished the design & install of the low voltage disconnect (LVD) / grid backup. So far, so good. If the batteries drop too low, the circuit disconnects them from the loads, then turns on a 12VDC, grid powered DC supply as a back up. I did this so the lights don't go off and stay off if the batteries are low. This way, there is only a momentary drop out while the relays switch. 

The circuit has adjustable disconnect / reconnect settings. Right now, the LVD is set to disconnect the battery at 11.5V and reconnect only after the battery charges back to 12.75V. Do these seem like reasonable settings? I searched the internet and found information that says the disconnect could be anywhere from 10.5V (seems aweful low) to 11.75V and reconnect between 12.25V and 13V. Thanks in advance.


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## TheSoundGi (Feb 10, 2010)

cowboy joe said:


> Finished the design & install of the low voltage disconnect (LVD) / grid backup. So far, so good. If the batteries drop too low, the circuit disconnects them from the loads, then turns on a 12VDC, grid powered DC supply as a back up. I did this so the lights don't go off and stay off if the batteries are low. This way, there is only a momentary drop out while the relays switch.
> 
> The circuit has adjustable disconnect / reconnect settings. Right now, the LVD is set to disconnect the battery at 11.5V and reconnect only after the battery charges back to 12.75V. Do these seem like reasonable settings? I searched the internet and found information that says the disconnect could be anywhere from 10.5V (seems aweful low) to 11.75V and reconnect between 12.25V and 13V. Thanks in advance.


hi Joe, the low voltage disconnect is really determined by the percentage of discharge that the battery is experiencing. Also the voltage of course, across the battery will be relative to the load that it is on the battery. also the disconnect voltage is directly related not only to the chemistry of the battery, I assume you are using standard lead acid. there are several types or levels as you probably already know. the standard vehicle type battery will not tolerate long-term will voltages. Marine batteries are really just across between deep cycle and standard automobile type batteries. When I first started building up a system of my own home. Several years ago, I went through long nasty expensive stupid and foolish school to learn the best way of wasting the most money spent on the worst batteries. Just for some fast background. I have over home setup on the grid with time of use billing. I charge the batteries at night when the cost is about five cents per kilowatt and off grid during the day when the cost is about $.23 a kilowatt. I like you set up automatic connects and disconnects when I operate on a 24 vote system, it provides a higher efficiency ratio. now back to your situation, even good Trojan L. 16 each cycle are really only designed to tolerate a sustained discharge down to about 66%. and they should be charged as soon as possible to prevent sulfation within a lead acid battery. Just for right now the long and short of it is. I could recommend a voltage to you but I would need to know the exact type of battery if you are using. Also another question I would have is our using the pulsating or multistage charger to prevent damage to your batteries. I am sorry for such a lengthy response, but the use of batteries is actually quite complicated. an especially awfully darn expensive when you make mistakes with them. One of the places where I learned a lot from the web is a site called http://www.utterpower.com/ I registered within a few years ago and is a bunch of guys that are super smart on all of this kind of stuff. Let me know what kind of batteries using, and I will try to help. Catch you later, Michael


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## WisJim (Jan 14, 2004)

My inverter has an adjustable low voltage disconnect, and it can be set for different voltages and times, so that it would shut off quicker at a lower voltage. For example, it could be set so it shut off if the voltage was 10.5 volts for 5 seconds, but 12 volts for 10 minutes, etc., so it doesn't shut off when there is a momentary voltage slump when the pump motor starts, etc.


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## TheSoundGi (Feb 10, 2010)

what make, what model, and what type and size is your battery.


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## cowboy joe (Sep 14, 2003)

WisJim said:


> My inverter has an adjustable low voltage disconnect, and it can be set for different voltages and times, so that it would shut off quicker at a lower voltage. For example, it could be set so it shut off if the voltage was 10.5 volts for 5 seconds, but 12 volts for 10 minutes, etc., so it doesn't shut off when there is a momentary voltage slump when the pump motor starts, etc.


Finding that the delay becomes more important as the load types / size increase in comparison to the battery capacity. High inductive loads (motors, pumps) tend to cause momentary brown outs which will trip the low voltage detector on start up. Found that my original design lacked sufficient filtering to compensate for the spikes so there were occasional drop outs when the aquaponics pump cycled. Hoping to add battery capacity in the next few weeks to minimize the issue. 

Thanks for the feedback.


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