In a previous post I have shown how to physically mount six Nissan Leaf battery modules in two series groups of 3 parallel modules to build a 180 Ah by 16 volt Lithium (LiNMC) battery.
The batteries are covered by these very cool laser cut acrylic protective covers (which obscure the BMS wiring).
Anthony Felix asked for more information about the BMS units I’m using on my batteries, so here it is! (Jump down to the last picture if you just want to see where the BMS units are attached….all of the text between here and there is an explanation of WHY they are attached there…)
I purchased mini-BMS units from Clean Power Auto to use on my batteries.
The Mini-BMS boards serve two functions:
First, they monitor the cell and “sound the alarm” if the voltage goes too high (overcharging) or too low (overly discharged).
Second, when the battery pack is charging, or any time a cell voltage is “too high” they attempt to actively “balance” or manage the cells to make sure that they all reach the “fully charged” level without any cells going to the “overcharged” level. If the voltage reaches the “100% charged” level (4.1 volts in the case of LiNMC chemistries such as in the Nissan Leaf) the mini-BMS board will shunt a resistor across the cell, “burning off” about 0.5 amps of power in an attempt to bring the voltage down to just below 4.1 volts, or at least keep it from raising any higher.
How they “sound the alarm”
The mini-BMS cells are designed to be linked to each other and form a large “loop”. When all is well, they conduct through the entire loop from one end to the other. If any mini-BMS board detects a problem (overly high voltage, or overly low voltage) they will disconnect (or “open”) the loop, meaning that the other end will drop to zero volts, indicating that a problem exists.
When does the loop open?
If the voltage raises to the maximum allowed (by the battery manufacturers specifications) of 4.2 volts, the mini-BMS board will “open the loop”. Also, if you are discharging the cell too much (such as driving your car longer than you should) and the voltage drops too low, the mini-BMS will also “open the loop”.
Who “listens for” the alarm?
You need a head unit, or intelligent charger, or EV charge controller that connects to a loop based BMS system. It will apply 5 volts to one end of the loop, and check to see if most of it (4.8 volts or so) comes out the other end. If you are currently charging the pack, it means that the charger needs to shut off, to protect the cell that reached 4.2 volts from getting overcharged. If you are currently drawing from the pack (e.g. driving) it should sound an alarm or put the vehicle into turtle mode to reduce the draw and tell the driver that they need to pull over and charge the pack!
I’m using an EVCC unit from ThunderStruck motors to control my chargers and sound the alarm when driving, and it is functioning as my head unit for the BMS loop, but you can also buy a head unit from Clean Power Auto to go with their distributed mini-BMS boards.
When I purchased the BMS units, I didn’t know exactly how I was going to arrange the modules into my batteries, so I bought individual cell BMS units, designed to be mounted across each cell. If I were to do this project again, I would definately purchase one of the “4S cell board” ($55) for the entire 16 volt battery, as opposed to using four individual “cell modules” ($12.75 each, or $51). The extra $4 is well worth having them all connected together when you purchase them, as I spent about 30 minutes per battery soldering together the 4 BMS units.
But, since I already had the individual cell units, I soldered together four individual units myself to make the equivalent of a 4S cell board for each 16 volt battery, and mounted them onto a piece of wood.
This assembly is attached to the 16 volt battery by five wires. (It also will have two wires that form the left and right side of the “loop”.)
Leaf Modules have two cells!
To understand the attachment points, you need to understand how a Nissan Leaf module is constructed internally. It actually has four cells inside it, 2 series sets of 2 parallel cells, each cell rated at 30 Ah. When you connect cells in parallel, their voltage will always be the same, so you can essentially treat them as a single larger cell. This means that for all practical purposes, the Nissan Leaf modules is a series arrangement of two 60 Ah cells. Each cell is nominally 4 volts when charged, so a leaf module (with its 2 series arrangement) is around 8 volts. (8.4 volts absolute maximum.) BUT, if you measure from the negative terminal to the “sense” terminal in the middle, you get 4 volts, or one cell. If you then measure from the “sense” terminal in the middle to the positive terminal, you get another 4 volts from the other cell.
This is why I am connecting the sense terminals together for my parallel set of 3 Nissan Leaf modules…to REALLY keep all six of the cell in the lower half (and all six of the cells in the upper half) of the 3 leaf modules in parallel. The photo below shows one of my “sense terminal” buss-bars. Hopefully, no real current will flow through this buss-bar, it’s just there to keep the parallel cells at the exact same voltage. I could have used much smaller copper stock for this connection, but I found it was easier to use the same copper for all of the connections.
Because the mini-BMS boards are cell level boards, you need TWO (2) BMS boards for a single Nissan Leaf module. My 16 volt battery has six leaf modules, BUT, I have two sets of 3 in parallel. This basically means that I have a total of 4 cells in series (except my 4 cells are 180 Ah each, because they are made up of 3 Nissan leaf modules, at 60 Ah, each of which is actually two 30Ah cells inside the module (just to further confuse you).
The picture below shows my “series” buss-bar, which connects the three positive battery terminals (on the right) to the three negative battery terminals (on the left), putting the two parallel groups of 3 modules “in series”.
Below you can see that I have rotated the battery and installed the two (Positive and Negative) “terminal” buss-bars. By connecting to the two terminal buss-bars, you get a 16 volt potential for my entire “battery” of 6 Nissan Leaf modules.
Back to the BMS connections
In my 16 volt battery, I have six Nissan Leaf modules total, arranged in two (in series) “groups” of three (in parallel) modules . The lower one which is the most negative side, and the “upper” one which is the most positive side of the 16 volt battery.
Individually, the four BMS units each have two wires (+ and -) and need to be attached in the following places so that they span each of the 4 cells:
1. To the most negative terminal, and the sense terminal of the lower group.
2. To the sense terminal, and the positive terminal of the lower group.
3. To the most negative terminal in the upper group, and the sense terminal of the upper group.
4. To the sense terminal of the upper group and the most positive terminal of the upper group.
Note that BMS unit 2 can share it’s negative connection with the positive connection on BMS unit 1…. and BMS unit 3 can share it’s negative connection with the positive connection of unit 2, and unit 3 can ALSO share it’s positive connection with the negative connection of unit 4, etc…
So in this way, it is possible to have five wires connect all four BMS units. (the end most wires are individuall, but the 3 middle wires are shared by the BMS unit “above and below” that connect.)
The picture below will probably serve to explain things a lot better than all of the previous words. Click the picture to make it bigger. Note that the small white wires connecting the BMS boards are part of the larger “loop”, but that they currently do not have the connections to the batteries to the left or right of this one.