5 things I learned from a $25 Battery Management Device

battery technology hardware

I teach an online course on battery management and was looking for an inexpensive off-the-shelf product to see what could be learned from it. Here's one that I found on Ebay that claims to be a BMS for up to four series cells.

Why should you spend the next 5 minutes reading this article? You'll takeaway two main things

  1. Save $$ by making better choices on which battery electronics to buy
  2. Develop an intuition of how battery management devices work

The way I like to examine a product is from the outside-in. Starting from the hook-up pads provided around the border of the PCB, I learned the following.

B+ and B- are the points for connecting the battery positive and negative

P+ and P- are the points for connecting an external power supply which can be used to charge the battery

B0 is the same as B-, both of which are the battery negative points

B1B2, B3 …. are the points for connecting the each of the cells in the battery. For example, if the battery is a 4S configuration (consisting of 4 series cells) - B1 will represent positive terminal of cell 1, B2 will represent positive terminal of cell 2, and so on.

  • Knowing this gives me a sense of what external devices need to be connected to the BMS for it to function properly.

The next thing I noticed, was that the pad B4+ has a larger surface area than B1+, B2+ and B3+. More copper area means more capability of current flow, which would mean that B4+ and B- have majority of the current.

But why is there not as much current through B1, 2 and 3 ?

This is because they are voltage sense terminals, so the current flow will be in the order of milli amps at most. They can however, also be used as balancing inputs for the cells, in which case the current would be a little higher, but still not as much as what would flow through B4+ and B-.

  • Surface area of pads and traces can give an idea of current flow
  • Voltage sensing pads will have much less current flow
  • A string of cells that constitute a battery can be actively balanced (charge is transferred from one cell to another) or passively balanced (excess charge in a cell is dissipated as heat through resistors)

The next component that caught my attention was R010, on the bottom left. It is a current sense resistor which measures the current flow between B- and P-. This essentially measures the charging current for the cell. The sense resistor used has a larger footprint as compared to the other resistors, that keep the power loss across it low.

  • We simply want to sense current, not dissipate it!

The two chips on the top of the board MDS2658 are 30V 16A FETs which seem to be obsolete parts now. These FETs are most likely switching P+ to B4+ to complete the connection for charging.

  • Check the supply chain of parts before you integrate a product in your design, or even when you are designing your own product.
  • Try to find a part that has a commonly available footprint. In this case, the FETs have a common SOP8 footprint, for which it might be possible to find an interchangeable part from another manufacturer.

Battery management systems should be able to communicate with other subsystems in the vehicle. On the left you see SDA and SCL pads which are for the I2C connections. They can be used to connect external devices, however I2C protocol stands for inter integrated circuit and is designed to work for on-board communication.

In this scenario, since the I2C pads are given for off-board communication, an increased capacitance in the I2C bus can cause communication issues. Using a logic analyser it can be seen what the rise time of the SDA and SCL pulses are. If the rise time is slow, that means the capacitance is high. A way to work around the higher capacitance on the I2C lines would be to lower the value of the 10kohm pull up resistors that are provided on the board.

  • I2C is meant for on-board communication and is sensitive to capacitance on its lines. If you are having issues with I2C capacitance, try reducing thevalue of the pull up resistors on the lines.

Since you've made it this far in the article, here are three more things I learned

  • NTC temperature sense provided as external hookup, but no indication as to resistance of NTC to be used and the temperature cutoff points programmed into the microcontroller.
  • Unpopulated components on the top side of the board likely allow for upgrading charging current capabilities. It is a technique used to keep a common PCB layout for supporting multiple versions of a product.
  • Missing silkscreen markings about what battery and charging voltages are supported. The product should clearly say the supported voltage, current and temperature ranges.

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