Understanding Battery Cell Imbalance
Battery banks configured in parallel are common in solar energy storage
systems. While parallel configurations increase amp-hour (Ah) capacity and
enable higher energy reserves at a constant voltage, they also introduce a
critical challenge: State of Charge (SOC) imbalance.
If not addressed, these
imbalances can degrade battery health, reduce system efficiency, and increase
the risk of system failure over time.
What Causes
Battery SOC Imbalance?
SOC
imbalance arises when batteries connected in parallel show different charge
levels. Although the inverter reports the average SOC across the battery bank,
each individual battery may be at a different charge level—often due to
non-uniform wiring resistance or asymmetric current paths.
Inverter
Misreading Due to Wiring Design
When one
pair of cables connects from the inverter to just the topmost battery in the
stack, most of the charge and discharge current flows through that unit. This
battery becomes overused, leading to:
- Overcharging of the topmost battery (due to constant higher current flow),
- Undercharging of the bottom-most battery,
- Uneven wear and capacity degradation over time.
The result: Battery 1 might show 100%
SOC while Battery 4 lags at 70%, even though the inverter averages them at
around 85%.
How Stacked
Batteries Are Wired
As shown in
the diagram, stacked batteries are typically connected in parallel:
- All positive terminals are connected together, often leading to a common bus or directly into the inverter’s DC input.
- All negative terminals are likewise connected, completing the parallel circuit.
However, where and how these wires connect impacts current
flow distribution. A single-point connection (e.g., only at the top or bottom)
increases resistance in the furthest batteries, reducing their current flow.
For other
useful links, please see:
Related Articles
How to Fix Battery Cell Imbalance (BSL Battery)
A battery cell imbalance may present through unusual SOC (State of Charge) behavior, red LED indicators, or system instability. If one battery in the stack is out of balance, it can cause inconsistent charging and affect overall performance. The goal ...Understanding AC-Coupled Batteries
AC vs DC Coupled Battery Systems DC-Coupled System In a DC-coupled setup: Solar panels generate DC electricity → this DC power directly charges the battery → then it is converted to AC by a hybrid inverter for use in the home or export to the grid. ...BSL Battery – Basic Troubleshooting Guide
Use this guide when setting up a BSL battery stack, or if your BSL batteries are connected but not working. Quick Check Verify CAN cable connection The cable from the inverter must be connected to the CAN port of the master battery. Inspect the RS485 ...Pylontech Battery – Basic Troubleshooting Guide
This guide outlines the correct wiring, startup, and shutdown procedures for integrating Pylontech US5000 lithium batteries with a Victron inverter system via CAN-bus communication. It is compatible with Victron and Deye hybrid inverters. Quick Check ...Pylontech Battery - Basic Troubleshooting Guide (Batteries not discharging - Battery Monitor [512] Internal Failure Alarm)
Once this is reported by the customer, we must first request for photos of the whole setup to check any wiring/connection anomalies. Then confirm the number of affected batteries by following the steps below 1: On the VRM portal, Go to Remote Console ...