A low voltage disconnect is used to stop loads from dragging a battery bank below a given voltage (Vlvd).
The simplest way to implement an LVD is to wire all loads go through the LOAD output of the charge controller, and set the controller's Vlvd. If the load is heavier than the LOAD output can provide, you can run loads off a relay triggered by the LVD.
If the controller has no LVD (ie, no LOAD output) you can use a stand-alone LVD powered by the battery.
except the inverter, due to the heavy current draw. The inverter will have its own internal LVD.
This following chart2) 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 bank3) 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.
The sweet spot appears to be 40% DoD based on the graph.
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 load5)||moderate load6)||heavy load7)|
Observe and adjust based on your experience with your own system.