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electrical:12v:lvd

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Low Voltage Disconnect (LVD)

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 system load is heavier than the LOAD output can provide, one may use:

  • a stand-alone LVD with higher current rating (more expensive)
  • or run loads off a relay triggered by the LVD (less expensive)

except the inverter, due to the heavy current draw. The inverter will have its own internal LVD.

common LVD setpoints

Lead-acid deep cycle batteries last the longest with best capacity if they are never cycled more than 50% Depth of Discharge1). That point is somewhere around 12.20v when rested or very lightly loaded.

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.

LiFePO4 nominal 12v banks can be safely drained to 80-90% Depth of Discharge. That point is something like 12.0v.4)

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 load5) moderate load6) heavy load7)
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.

electrical/12v/lvd.1529787007.txt.gz · Last modified: 2018/06/23 13:50 by frater_secessus