Building the Goodrum-Fechter “Zephyr” BMS – Part 1
The completed Goodrum-Fechter ‘Zephyr’ BMS circuit board (PCB now available here!)
Once I had my LiFePO4 Lithium Pack, the next stage was to find some way to charge it, and also to manage the LVC (low-voltage cutoff) at the cell level. LiFePO4 cells are lighter and have much better lifespan and performance than SLAs, but they can only survive if their voltages are kept in a certain range. If the cells are charged beyond 3.65V or are discharged to below 2.0V, they can be permanently damaged, and have their lifespan and performance significantly impaired. Once this happens, the cells of the pack need to be replaced.
It’s therefore essential to keep the pack properly balanced. A charging system needs to be in place where the individual cells are charged only up to a certain point (usally 3.6 or 3.65V), and while using the pack, the cells need to have their levels monitored so that an alarm or throttle interrupt is triggered once they fall below a certain point – usually 2.5V or 2.0V. Lithium cells quickly fall out of balance at the end of a discharge cycle, so just measuring the pack voltage is not good enough to warn the user about individual cells. A system needs to be in place that responds to this too.
The Goodrum Fechter Zephyr PCB
To do both of these jobs, you need a battery management system (BMS). It is possible to find off-the-shelf BMS systems, but usually – like much EV hardware – these items need to be imported from Chinese vendors, and documentation is notoriously poor. I’d heard stories of people coming unstuck and ending up with useless, non-functioning boxes, with no technical support to turn to for help. Users were often left high and dry with no option but to try and return the items for a refund.
Because I wanted a tried and tested product with a strong user base and good support, I opted for the latest incarnation of the Goodrum-Fechter board. At the time of writing, this comes as a kit that you have to solder together yourself, so some soldering skills are required. However the board comes with solid assembly and testing instructions, and a spreadsheet containing a parts list that can be drag-and-dropped into the on-line shopping cart of electronics vendor Mouser. Mouser is a U.S. based company, but has outlets worldwide, so I was able to get my own order in from their European division.
I wanted to build my own so that in the process I would understand more about how the board worked, but – more importantly – so I would have technical help if I ran into problems. The Endless Sphere forum has a ‘Zephyr’ thread dedicated to the ongoing development of this board, and troubleshooting of any issues that people might have getting it working. It is well-attended by the board’s designers Gary Goodrum and Richard Fechter, who have been immensely helpful and patient with all of my questions.
I ended up with version 4.4 of the board. It came with one 8-pin chip already in place and wired to fix a ‘bug’ with the tracks on this version. This has been rectified on newer versions of the board.
The Goodrum-Fechter ‘Zephyr’ PCB as purchased – now available in the shop
The PCB comes with two end-plates for use with the case (which is included in the parts list). There is an end-plate for 16s and 24s pack configurations, with holes for the respective numbers of tap-wires. The forum thread offers links to the spreadsheet with the parts list, and the assembly, testing and operating instructions:
Zephyr LiPo-LiFePO4 BMS BOM-v4.4a.doc
Zephyr BMS Assembly-Test & Operating Instructions-v4.4a.pdf
The board is clearly laid out and labelled, with component names and values stamped where the components need to go. Any polarised components – like diodes or capacitors – get a square pad (the mounting for the hole on the board) where the negative end needs to go.
Mounting the Surface Components
Once my parts had arrived from Mouser, it was simply a matter of soldering a lot of components onto the board. It’s best to start with the smaller components, and then move onto the larger ones. Below the cell tap holes (those holes along the middle of the PCB which link to each cell in the Lithium pack), each cell circuit has seven resistors, a diode, an LED and three transistor-type pieces to mount.
I started by mounting the resistors and small capacitors to the control circuit and the individual cell circuits, and the cell-circuit diodes. Next I added the larger, metal 47uF capacitors above the cell-tap holes. On my version of the board the negative terminal for these wasn’t indicated by a square pad on the PCB, causing some confusion. Not that these capacitors should go with the white stripe on their case (negative pole) pointing right. Note that the illustrations below show only two out of the three banks – 16 out of the 24 cell circuits on the full board.
Stage 1: Resistors, capacitors, and cell-circuit diodes mounted
Next I mounted the orange LEDs, the two ‘TO-220’ FETs (top-left), and the 8-pin U1 and U3 chips (far left). Each chip has a white dot on it which indicates the negative pin, and this goes in the square pad. I also added the diodes to the control circuit. The diodes have a stripe at their negative end, which likewise needs to be mounted on the square pad.
Stage 2: FETs, diodes, 8-pin control circuit chips and orange LEDs mounted
Next came the shunts, which are used to divert the ‘overflow’ voltage away from fully charged cells to the power transistors. These need to be installed with the nomenclature (the little writing on the components) face-down.
Stage 3: Shunts mounted
Finally, the ‘TO-92’ transistor type components needed to be added to each cell circuit. Once again care needs to be taken to put them in the right way round, but this is easy to tell by just making sure the flat faces of the components point in the right direction, as shown in the pictures. I also added the 1KΩ pot (blue-capped variable resistor), shunt resistor (that metal bar to the left) and the 24 8-pin chips at the bottom of the cell-circuits. Once again, the white dot on the chip tells you which leg goes on the square pad.
Stage 4: TO-92 parts, pot, shunt resistor and remaining 8-pin chips mounted
The main tri-colour LED also went in here, however I found I had to mount this on the underside of the board in order for it to meet up with the hole in the end-plate.
The Underside Components
In earlier incarnations of the board, the power resistors were mounted on the top of the board, so they sat on top of those empty rectangular pads you can see. But later someone came up with the bright idea of using the case as a heat-sink by mounting the resistors on the underside of the board in such a way that they would make contact with the side of the case once the board was slid into place.
Getting the spacing just right calls for some precision in mounting the components, but is aided by the suggestion of using a piece of old PCB as a spacer to hold the resistor the exact 2mm above the board it needs to for the resistors to touch the side of the case once the PCB is slid into place on the correct ‘rung’.
There were 48 of these things to solder on for my 24s board, so it was some job! The resistors can go either way round, but it’s nice to have them all the same way so that it looks pretty…
Testing the Board
At this point, the board should be ready to start testing. For this stage I slid the board into its case for support, and gave it a temporary power connector in the form of a couple of ring connectors bolted to the holes for the main charger input wires.
The next section (coming soon) will deal with testing and callibrating the board prior to assembling the final unit.
Part 2 – Testing and Callibrating the “Zephyr” Circuit Board