Inverter LBCO vs Battery Low Voltage Limit

Post by raysun » Thu Apr 29, 3:03 pm

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It is imperative that batteries avoid reaching their low voltage cutoff (LBCO) level. For lead-acid batteries, reaching this point significantly diminishes their lifespan. In the case of lithium iron phosphate batteries, there is a significant risk of one or more cells being harmed to the point of failure.

In practical terms, a battery rated at 600AH (or 700AH) should not approach the LBCO unless it is improperly managed — this can lead to costly repercussions.

The Radian inverter is configured to trigger LBCO at 44V, which corresponds to an almost fully discharged lead-acid battery. This setting primarily protects the inverter rather than the battery itself. Many users choose to adjust this setting higher in hopes that it serves as a "failsafe" to maintain a Depth of Discharge (DoD) typically around 50%. However, this strategy is fraught with potential inaccuracies owing to voltage fluctuations under varying loads.

Incorporating lithium batteries prompts additional considerations. Their flatter voltage/discharge profiles complicate voltage monitoring compared to lead-acid batteries. Although the discharge curve of lithium batteries appears flat, it varies depending on the load. Therefore, all concerns surrounding LBCO in lead-acid batteries also pertain to lithium batteries, especially given that the voltage differential between a fully charged cell and an entirely depleted one can be as narrow as 0.4V.

It is critical to understand that a "48 Volt" lithium battery essentially has a nominal voltage of 52V. At 50V, the cell voltage may fall to around 3.1V, indicating a charge state between 1% and 4% — a state indicating near-total depletion.

Under active loads, it is possible for the battery voltage to dip to 50V without the cells being fully discharged, contingent on the current being drawn.

Ultimately, becoming acquainted with typical system loads and how the battery reacts to them is essential. The LBCO on the inverter must be calibrated to ensure that standard loads do not deplete the battery beyond a manageable DoD.

If the Battery Management System (BMS) resorts to protective measures that shut down the battery, it signifies that the battery has been drained excessively.

Does the system incorporate a battery monitoring mechanism?