Learn and understand the design of the BMS sampling board on BYD e5

The weather has turned cooler, and it is more comfortable to sleep at night; the CBA league has started, and you can watch football games early in the evening after get off work, hoping that the level of the league will get better and better.

This time let’s learn about the design of the BMS acquisition board on BYD’s e5.

Currently, the e5 has been discontinued because it is an older model. On the e5, there seem to be two types of battery packs, old and new, which also correspond to the old and new versions of the collector. The picture below shows the old version (lithium iron phosphate/75Ah). Battery pack and collector.

Further, the collector image corresponding to this version is as shown below.

The other version (ternary/100Ah) battery pack is as shown in the picture below. The layout of the collector and the way of outlet are very different.

The collector corresponding to this version of the battery pack is shown in the figure below: it is divided into single core, two core and three core. They are used to identify the number of internal AFE chips. Today we will learn the design of this sampling board together.

Specifically, the front and back of the E5 collector are as shown in the figure below: the upper and lower shells are made of plastic material, and the overall size is approximately 170mm*94mm*19mm; the upper and lower shells are fixed with four screws, and there is a label on the upper shell. BIC-B231A00 - Dual-core collector. BYD's collector is called BIC. Dual-core means that there are two AFE chips inside.

Remove the PCBA from the casing. The T side is as shown in the figure below: PCB thickness is 1.6mm, 4-layer board, surface is gold-plated, via holes are plugged with green oil, and solder mask is not opened; the minimum package of the components on the board is 0603, and the total number of external components is 0603. The four connectors are all plug-in packages; the T-surface area is basically coated with conformal paint, except for the mounting holes at the four corners.

Side B is shown in the picture below: Most of the area is coated with conformal anti-paint, and many devices are also arranged on side B. It can be clearly seen that the mounting holes are all non-metallized holes, and the PCB is slotted.

Basically, the core chip on all collectors is AFE (analog front end), and all other functional circuits are built around the AFE; the relevant functional circuits are mainly divided into single voltage sampling modules, equalization modules, and temperature acquisition modules. , communication module and AFE module are five major parts.

The division of functional modules on the B side basically corresponds to the T side; this collector only has two AFE chips welded, and there is actually one that is not welded, so that it can collect up to 36S cell voltage.

The AFE used is MAXIM's MAX17823B, which is a relatively mature model. The functional block diagram inside the chip is as follows: In summary, it can collect 12 channels of single voltage signals, each channel supports passive equalization, external communication supports daisy chain mode, and temperature sampling channel There are only two channels (AUXIN1\AUXIN2).

The functional architecture of this sampling board is shown in the figure below: The entire collector design goal is to use three AFEs, but this collector only has two AFEs welded, AFE3 is not welded, and the circuit for communication at the port is not welded, so Communication cannot be cascaded to the next AFE, so the two-AFE single board is located at the end of the entire battery pack, and the collectors at other positions should be three AFEs; in addition, the temperature sampling position of the entire collector is located at AFE1 On the board, communication between AFE boards is implemented using capacitors, and only transformers are used externally.

The battery cell voltage sampling filter and equalization circuit is shown in the figure below: Each sampling line is connected with a fuse in series. The sampling line uses a 1K+100Ω resistor in series, and a common mode filter capacitor and a differential mode filter capacitor are added; a balancing resistor Use two 82Ω/1812 in parallel, the equivalent resistance is 41Ω; use external equalization, use NMOS; use differential mode filter capacitors between adjacent SW pins, also use two capacitors in series.

Summarize:

The sampling board circuits look similar to everyone, but it is not easy to clearly explain the selection basis of each device. I have been busier at work recently, so I regret that I cannot reply to many students' messages one by one. All the above are for reference only.