====== Low Voltage Disconnect (LVD) ======

A low voltage disconnect is used to stop loads from dragging a battery bank below a given voltage (Vlvd).


===== examples =====

[[https://amzn.to/3Ri94RY|{{ https://m.media-amazon.com/images/I/51ODnDvz-rL._AC_UL400_.jpg?125}}]]

  * [[https://amzn.to/3Ri94RY|Victron Battery Protect]]
  * [[https://amzn.to/47Xzz5n|Blue Sea 7635 m-LVD]]
  * [[https://www.amazon.com/s?k=low+voltage+disconnect&link_code=qs&tag=themousepad|various amazon listings]]

One can also **DIY a low voltage disconnect**.  

In theory the simplest way to implement an LVD would be to wire all loads go through the [[electrical:solar:charge_controller#using_load_output|LOAD output]] of the charge controller, and set the controller's [[electrical:solar:charge_controller_setpoints|Vlvd]].  In practice this is rarely done for reasons given in the link above.  It is more common to run a [[electrical:12v:relay|relay]] on the LOAD output, which is triggered ON/OFF by the LVD setting:

controller LOAD output -> relay -> loads((the LOAD triggers the relay, but actual power comes from the battery bank))  

If the controller has no LVD (or not no LOAD output at all) you can use a   [[https://amzn.to/3m2OTp3|stand-alone LVD]] powered by the battery. 





===== common LVD setpoints =====

Lead-acid deep cycle batteries are typically discharged 50% [[electrical:depth_of_discharge|Depth of Discharge]] or less ((and fully charged afterwards)).  That point is somewhere around 12.20v when rested or very lightly loaded.  

This following chart((http://www.trojanbattery.com/pdf/GEL_Trojan_ProductLineSheet.pdf)) shows DoD vs expected duty cycles.  In an offgrid scenario we will assume the bank is cycled every night.  If the bank is drained to the usual 50% DoD the bank((under lab conditions)) is predicted to last 1000 cycles, or about 3 years.  If the bank cycle is shallower to 20% DoD the bank is expected to last 2500 cycles, or about 7.5 years.  If the bank is deeply cycled to 80% DoD the bank is expected to last 500 cycles, or about 1.5 years.

{{ :electrical:12v:ladutycycles.png?direct |}}

The sweet spot appears to be 40% DoD based on the graph.




[[electrical:12v:deep_cycle_battery#lithium_chemistries[electrical:12v:lifepo4_batteries_thread|LiFePO4]] nominal 12v banks can be safely drained to 80-90% Depth of Discharge.  That point is something like 12.0v.((http://rvwiki.mousetrap.net/doku.php?id=electrical:12v:lifepo4_batteries_thread#notable_voltage_reference_points))

===== challenges =====

LVD rely on measured voltage to know when to disconnect, but heavy loads can cause distortions (//voltage sag//) in that reading.

|  **State of Charge**|  **Voltage, light or no load((1.0 per Trojan))**|  **moderate load((interpolated as 0.9))**|  **heavy load((0.8 per Trojan))**|
|  100|  12.70|  11.43|  10.16|
|  90|  12.60|  11.34|  10.08|
|  80|  12.40|  11.16|  9.92|
|  70|  12.30|  11.07|  9.84|
|  60|  12.20|  10.98|  9.76|
|  50|  12.10|  10.89|  9.68|
|  40|  12.00|  10.80|  9.60|
|  30|  11.80|  10.62|  9.44|
|  20|  11.65|  10.49|  9.32|


Observe and adjust based on your experience with your own system.  

===== further reading =====

  * [[https://www.youtube.com/watch?v=cFUcXOznENQ|quick demonstration]] of a generic LVD