Balance charging your Lithium battery pack is important!

Balance charging your battery pack will help it last longer and prevent damage to internal cells that may be getting under or over charged. But before getting into the details of how balance charging works, lets look at how a typical battery pack is configured.

For a battery pack to have a higher voltages requires that individual cells are wired in “series”. This is basically connecting cells end to end (positive to negative) in order to get the voltage you need from the battery pack. Shown in the diagram below is a battery pack with a nominal voltage of 10.8V made up of 3 cells wired in series ( a 3S battery pack).

Three 18650 cells connected in series to make a 10.8V or 3S battery pack

Here is a list of Battery pack voltages and their corresponding cell counts:

  • 1S = 1 cell = 3.6V nominal battery pack voltage
  • 2S = 2 cells = 7.2V nominal
  • 3S = 3 cells = 10.8V nominal
  • 4S = 4 cells = 14.4V nominal
  • 5S = 5 cells = 18V nominal
  • 6S = 6 cells = 21.6V nominal

Each individual cell in the battery pack will have a nominal voltage of 3.6V. When fully charged the cell will be 4.2V and hopefully is never discharged below 3.0V For a very detailed description of how a lithium cell is charged / discharged read this page on the Battery University website: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

Direct charging a battery pack:

A direct charging configuration is when a battery pack is connected to a charger with only 2 leads (positive and negative). In this example we will be talking about the simple case where the pack does not have a BMS (Battery Management System) board. We’ll look at a battery pack with a BMS later in this article.

Direct charging = Charging using only the main leads

Notice that only the thick Main Lead cables of the charger are being used. In this configuration the charger is not monitoring the voltage of the individual cells. This could result in the battery pack not being 100% charged because individual cell may be not fully charged or worse yet,  slightly over charged cells.    This  results in an “unbalanced”  battery pack.

Balance Charging a battery pack:

With balance charging the charger monitors the voltage of each cell, and can charge them individually while trying to keep them at the same voltage level across each cell. This is the safest and most recommended way of  charging a battery pack. This configuration requires that the battery pack have an additional cable connection to the charger, as shown in the photo below:

Battery pack connected to charger balance port

The wires connected to the charger’s balance port are called “balance leads” and connect to the individual cells in the pack. These balance leads allow the charger to monitor the voltage of each individual cell while charging. This ensures that each cell in the battery pack ends up with similar voltages. Here is a diagram showing the connections:

Balance lead connections from a battery pack

Battery packs with a BMS board

The last configuration that we’ll look at is a battery pack that has a BMS board included. The advantages of a BMS board is that it will:

  • protect from overcharging individual cells and the battery pack overall
  • protect from over discharging individual cells
  • provide short circuit protection and current limiting the entire battery pack
  • it may even include a thermistor to monitor the temperature of the battery pack to protect against thermal run away

Here is an example of a 3S BMS board:

3S BMS showing wiring diagram

Notice the separate connections to the battery pack and the output connections for the load or battery charger. The BMS board will typically be wired in front of your batteries, so that charging and discharging are controlled by the BMS. In our example this BMS has a working current of 15A (your max load) and can handle instantaneous peaks of 30A.

Here is a diagram showing our battery pack with a BMS wired in:

3S Battery pack with BMS

Notice that the connection to the battery charger now only requires 2 leads (positive and negative). This is because the balance charging is being done by the BMS board and it will ensure even voltages across the individual cells. This also means that your battery charger can be simpler. In our example here it can be as simple as a 12.6V power source capable of providing at least a minimum amount of current

In Conclusion:

Balance charging is the safest and most recommended way of  charging a battery pack. Including a BMS in your battery pack design is preferable since it will provide not only balance charging but also over voltage / over charge protection and short circuit / max current protection.

If you are interested in the battery charger that I use or the BMS boards shown in this article here are Amazon links:

SkyRC iMax B6 charger:  https://www.amazon.ca/ORIGINAL-Charger-Balancer-Lithium-Battery/dp/B00HED90RU

3S BMS board: https://www.amazon.ca/Electric-3Series-Lithium-Battery-Protection/dp/B075277M6Y

4S BMS board: https://www.amazon.ca/4Series-Current-Lithium-Battery-Protection/dp/B07524GFSL

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