CC/CV charging and BMS - How to reconcile?

Hardware diyBMS
1

Hi,

I am hoping that someone can help me reconcile what I see as a contradiction when implementing a multi-pack charger and a Battery Management System (BMS). I’m sure that the problem is at my end, so a correction to my understand will be most welcome.

  1. As I understand it, when charging Lithium-Ion batteries a CC/CV strategy must be used.
    charging_graph
    Source: Cadex/Battery University

The relevant point for my question is at Stage 3. That is, the charger MUST switch off the charger when the current is <3% (30mA in the example graph) of the starting current.

  1. If one is charging a multi-pack Lithium-Ion battery, a BMS should be used to ensure that each cell in the pack remains within limits. From my enquiries, it seems that the “bypass” technique is very common design.
    Balance_circuit
    Balance_circuit2998×2248 446 KB

    Source: TI.COM

In this design, once a cell reaches its maximum charge voltage (4.2v), a controller shunts current around the cell to keep the voltage within limits.

My assumptions from the arguments above are: Once the three cells in the battery pack (shown above) have reached their maximum charge voltage, all three bypass circuits will be conducting. The current flowing through each cell will eventually drop to the <3% value, but the bypass current will be adding to this value. Thus, it is 30mA (through the cells) plus the 50mA through the bypass circuits that is seen by the charger.

Question. How does the CC/CV charger know when to turn off?

Regards, M

2

As the Total voltage approaches the set voltage of the charger, The charger will automatically reduce the current and switch to constant voltage mode. The current will keep reducing while the total voltage stays constant.

3

Hi Jim,
Thank you for taking the time to read and post a solution to my question. And yes… I understand the point you are making, but that still leaves me puzzled!

As I point out in my question, if the charger is “looking” for the Switch-Off point (<3% of C, or 30mA in the example) and 50mA is passing through the the bypass circuits, the charger has no way of knowing this. Consequently, the charger will continue supplying 80mA (30mA to cells and 50mA to bypass circuits) and will never switch off.

Regards, M.