Trigger Diff Voltage is a key parameter for understanding how Active Balancing will function and the Smart Heltec BMS you have appears to be very different than the non-Smart Heltec BMS I am considering.The Heltec Smart BMS with active balance will balance any time the difference between cells exceeds the "Trigger Diff Volt" parameter setting.
From the manual -
e) Trigger Diff Volt - Triggered balance difference Voltage is the only parameter that controls balance. When the balance switch is on, when the maximum difference voltage of the battery pack exceeds this value, the balance starts, and the balance ends when the difference voltage is lower than this value. For example, the balance trigger voltage difference is set to 0.01V. When the battery pack voltage difference is greater than 0.01V, balance is started, and when the battery pack voltage difference is lower than 0.01V, the balance is ended. (It is recommended that the balance trigger difference voltage of the battery above 50AH is 0.005V, and the balance trigger difference voltage of the battery below 50AH is 0.01V)
This is from the Alibaba Heltec
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That non-smart BMS appears to have a Trigger Diff Voltage of 100mV, meaning it will only have any impact above the knee.
A Trigger Diff Voltage of 10mV or 5mV would mean Active Balancing would likely be effective through practically the full charge/discharge cycle which may not cause a problem but pretty much guarantees that charge will be added to the weak cell during discharge which will then need to be pulled back out near the top of the charge cycle.
Ideally, I’d like all my cells to reach 3.575V (for my 28.6V Charger) at the same time, or if not, for the weak cell to lag and get boosted up to the others with Active Balancing.
The situation I want to avoid is to have the weak cell reach 100% SOC before the rest of the cells, continue charging up to high-voltage Shut-Off (3.75V) and have the rest of the cells slowly inch their way closer to 100% SOC as the weak cell relaxes below high voltage reconnect.
If that ends up happening, I would rather have no additional charge going into the weak cell during discharge.