Diagnosing sudden jumps in State of Charge (SOC) | Orion Li-Ion Battery Management System

The Orion BMS primarily calculates state-of-charge (SOC) by coulomb counting, which is a way of monitoring the amount of current going into or out of the battery pack. However, the BMS also has an algorithm for correcting the calculated state-of-charge which can result in rapid increases or decreases in the reported state of charge. State-of-charge drift may be occurring because of a damaged or weak cell, imbalanced cells, incorrect “drift point” settings in the battery profile, skin effect from poorly filtered inverters, a current sensor that is backwards, current sensor failure or lack of a current sensor.

If the Orion BMS is used without a current sensor or if a current sensor error code is present, the reported state-of charge is based only on voltage and will not be correct! For best results, use an approved current sensor with the BMS.

Resolving the issue:

Step 1. Determine if a current sensor is connected to the BMS.

If a current sensor is not connected, the state of charge is not calculated by the BMS and will be incorrect. If a current sensor is connected, using the Orion BMS utility, check for and ensure no current sensor fault codes are present. If current sensor fault codes are present, address the fault code since state of charge cannot be accurately calculated without a functional current sensor. Also ensure the correct polarity of the current sensor (amperage entering the battery pack should appear as negative, and amperage leaving the pack should appear as positive). If the current sensor polarity is incorrect, either physically flip the current sensor or “invert” the polarity in the BMS profile.

Step 2. Review the pack for weak cells or cells which are obviously out of balance.

Using the Orion BMS utility, open the live cell data tab. Look at cell voltages when not under a load and look for variance between cells. If cells are at substantially different voltages at rest (such as 3.35v and 3.0v for iron phosphate), there is likely either a balance issue of a weak cell. If cell voltages are still significantly different after a full charge, the issue is likely that the cells are not balanced. If the pack is not balanced, simply balancing should resolve the issue. If a cell is weak, the cell will need to be replaced. An unbalanced pack or a pack with a cell which has a low capacity will correctly result in state-of-charge drift. In these cases, the BMS is working correctly since the battery pack has a reduced usable capacity. To resolve this, either the pack must be balanced (if out of balance), or the weak cell must be replaced (if caused by a weak cell).

Step 3. Review the “drift points” set in the battery profile.

These points are the open cell voltages that are known to correspond to a specific state of charge. For iron phosphate cells, points in the flat part of the curve should be avoided.