Building the Goodrum-Fechter “Zephyr” BMS – Part 3
The completed Goodrum-Fechter ‘Zephyr’ BMS unit (board available here)
Assembling the case, PCB and end-plate
If you’re building a 16s BMS, then in your parts you get one long case to house the PCB, but for the 24s version I was doing, I had two smaller cases that I had to put end-on-end. The end-plate that comes with the PCB is meant to replace the blank end-plate that comes with the case. Mine had to be sanded down a bit at the corners to make it fit in the plastic coupler that secures it to the case.
This done, it was time slide it together to see if it was a good fit, and that it allowed the PCB to slide in such that the resistors made thermal contact with the case. This was a bit tricky, as it was somewhat stiff and needed some gentle force to make it go in. In the end I took some sandpaper to the surface of the resistors smooth down some uneven parts, however I realised that this caused a few bits of bare metal on the resistors to show and I quickly stopped this. If any of the bare metal were to make contact with the case, then it could potentially cause a catastrophic short.
The PCB is slid into place in the case, with the status LED engaged with its hole in the end-plate
When I finally had it slid together, I took resistance measurements between the exposed metal of the case-end and the resistors, and no shorts seemed to be in evidence. The case itself is insulated by the black paint it’s covered in, but just as a precaution I got hold of some Kapton thermal insulating tape and ran a layer along where the resistors would be touching the case.
One of the two cases used to house the BMS board
Wiring it all up
The instructions don’t offer specific guidance on what grade of wiring to use for the battery main power coupling, charger power input or cell-tap wires, but the holes in the board give you a rough idea of what the designers have in mind.
The size of the wires you need depends on what kinds of power supply current you intend to use with the board. The designers of the Zephyr say that you can safely use it with up 20 amps (though you will need the Q3 FET in addition to Q2 installed on the PCB), but the guage wiring depends on how much power you are actually going to support.
Obviously the best policy is to ‘future-proof’ it if you want to leave open the option of upgrading to a more powerful charger, and that’s what I did. Although I’m only using an EMC-900 charger, which is capable of supplying 9 amps, I still wanted to be able to support the maximum rated current. Having the thickest wires possible also makes the whole thing more economical and efficient. There’s less energy lost on the way to the pack through wires heating up, and it all runs cooler.
Eventually I settled on the 6mm² wires (around 10 AWG) that the holes on the board were designed to accommodate. Apparently 12 AWG is enough to carry 20 amps, so these would more than enough. I used this grade of wire for both the power supply wires, and for the wires serving the main terminals of the battery bank.
The holes in the end-plate were not big enough to accommodate them, so I had to drill them out a little. Here you can see the wires threaded through their designated outlet holes.
For the cell-tap wires, I once again opted for the largest wires that would fit into the cell-tap holes, using 1mm² wire (about 17 AWG). This however caused difficulty for my choice of connector.
The parts list includes three 8-pin connectors intended for soldering directly onto the tap holes on the board. These would then be connected – via another connector head – to groups of eight cell-tap wires running from circuit board, out of the case and directly to the battery bank. I assumed that the idea of this was so that you would be able to unplug the cell-tap wires, separating the BMS from the battery bank. However you wouldn’t be able to detach the cables completely from the BMS unit doing it this way: The tap-wires would be detachable from the circuit board, but not from the case itself because the connectors would still be on the inside of the end-plate. This approach also involved complications of the board-mounted connectors clashing with the positions of some of the shunts and other components, necessitating moving some of them to the underside of the board.
Though this clearly must have served a useful purpose for some people, for me I couldn’t see any benefit of doing it this way. Instead, I opted to solder the wires directly onto board, and have them run to a connector external to the case. I was originally planning to use a 25-pin serial port connector for this purpose. Though these are usually used with computer peripherals, they’re perfect for the tap-wires many people are using in their BMS arrangments. These connectors, though, aren’t big enough to house the thicker, 1mm² wire I’d chosen, so I had to find something a bit chunkier. After much hunting around I settled on another PC-based connector – a 24-pin ATX “Molex” connector. These were ideal: affordable and similar in design to motorcycle mini connectors, except with the many pins I’d need for my 24s Lithium pack.
I started my wiring for the cell-tap connections from the connector, then worked back to the circuit board, threading the wires through their respective holes in the end-plate.
The end-plate has three rows of nine holes, one row for each 8 cell-circuit bank on the board. For reasons I’ll explain, I arrange things so that I only need to use only 8 wires per bank so that it ties in nicely with my 24-pin connector. I didn’t like the colour of the end-plate, so I sprayed it black to match the case.
You can see the final form of the unit starting to emerge, now. I decided I wanted about 12 inches free-play for the cell-tap wires outside the case, like I had allowed for the charger cables and master battery bank wires.
Once I had the wires all laid out close to their final arrangement, I measured and cut off extraneous lengths of the wires to minimise kinks and bunching of wires inside the unit. Then they were soldered carefully to the cell-tap points along the length of the board.
Once all the wiring was complete, I used cable ties to tidy it up and keep it out of the way of the orange LEDs, which I wanted to be visible so I could see what all the cell-circuits were doing.
The ninth-wire issue
One consequence of having lots of cells in series is that the wires in between the cells act as both the positives for one cell and the negative of the next cell along. The diagram from the Zephyr assembly manual shows how a Lithium Bank (in this case a 16s bank) is wired up to the cell circuits on the board. For reasons of architectural practicality, the cell-circuit sections of the board are split up into 8-cell sections, within which cell-circuit poles are shared with the cell-taps to either side (cell tap 2 is the positive of cell 1, and the negative of cell 2, and so on…).
External connection diagram from the Zephyr assembly manual
This means that each bank serving 8 cells of the pack, actually ends up with nine wires, one last one to seal of the unbounded cell in a series. The problem with this arrangement is that for each bank you end up with one extraneous wire – unnecessary as it goes to exactly the same cell pole as one other wire on the previous bank. From the point of view of using an external connector, I needed to keep a ‘rational’ number of cell-tap wires: While 24-pin connectors aren’t hard to come by, 27-pin ones are practically impossible to find.
To avoid having an extra wire per 8 cells to deal with, what many people do is simply put jumpers between the individual banks, connecting cell-tap connection 9 of one bank to the first cell-tap of the following bank. This is exactly what I did. I left the first cell-tap of each bank empty, adding jump-wires to connect it to the last cell-tap of the previous bank. To accomplish this, I just drilled a 2mm hole in the first and last track of each bank, connecting them with a short jump-wire on the back of the board.
Below you can see the back of the board where my jump-wires connect cell-tap 9 of bank 1 to cell-tap 1 of bank 2, and cell-tap 9 of bank 2 to cell-tap 1 of bank 3.
But what about the first cell-tap? Since this is the master negative pole of the entire bank, I decided it would make sense to just cross-connect it with the master negative cable which I connected earlier. The picture below shows the board before the master negative was connected. You can see the hole I’ve drilled for the jump-wire which will join them both here.
And here it is as it is today (below). You can see the black master negative cable on the far left, and the soldered jump-wire that connects these two on the back of the board. The first cell-tap hole therefore doesn’t need to be used, just like those in the other two banks.
The negative track of cell-circuit one is jumpered to the site of the pack negative (far left)
The end result is that I get to confine myself to a nice, ’round’ number of wires, and need only 24 cell-tap wires, rather than the awkward 27 that the end-plate allows for.
Once assembled, the final article is pictured below. The cases that house it (two of them, joined end to end) come with metal covers that can slot into the uppermost groove of the case, sealing it shut. But instead of using these, I ordered a single piece of 96mm X 320mm perspex to go in their place. That way I could see the LEDs while I was testing and fine-tuning the unit. It was also a nice way of putting my handywork on display so it could be better appreciated…
The EMC-900 came with a pair of red/black Anderson connectors to be used with whatever the charger was to be connected to, and these were duly attached to the charger input wires. For the main battery bank terminals, I used a 50 amp Anderson connector to connect to the one I have on the battery pack.
This is the BMSBattery EMC-900 charger, which I have nothing but praise for. Its voltage and current can be fine-tuned, and it has a digital display telling you exactly what it’s doing. The charging cycle can be read by the amount of current that it’s drawing, which starts out at 9.o amps, and slowly reduces during the final few minutes of a charge cycle.
The BMSBattery EMC-900 charger
Once I put it it into commission, there was a certain amount of fine-tuning with the BMS end-of-charge (EOC) cut-off point and the charger voltage. I’ll cover this in a separate section shortly…