ElectroClassic EV
Classic Cars Reborn into the Electric Future

Battery Brains

The battery management system (BMS) is an apparatus designed to prevent cells from overcharging, and to ensure all cells in the pack are charged evenly. It also protects cells from over-discharging, which directly effects their long term health. One BMS setup that appealed to me as the simplest and most economical was the MiniBMS from CleanPowerAuto. In this scheme, every battery cell is assigned a module (above right), which signals the MiniBMS control board (above left) when its designated cell reaches the charge limit. The module then shunts current away from the cell, and when all modules are in shunt mode, the control board turns the charger off. In extreme cases, the BMS shuts the charger down if any cell is in danger of overcharging. The modules also monitor the cells during discharge when the car is driving. If any cell approaches an LVE (low voltage event), the module signals the control board and an alarm is sounded to alert the driver.

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The installation of the MiniBMS was pretty straightforward. This shot shows a pile of yellow wires that were cut, stripped and crimped to prepare for the install of the signal loop that lets the modules communicate with the control board. Two circuit modules above were arranged in their relative working position to figure the exact lengths needed for the signal wires.

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The individual modules are installed on the battery with the circuit board ring connector on the negative terminal, and the red wire on the positive side. The MiniBMS instructions require the loop connectors to sit atop the battery connection straps, but under a washer so the ring doesn’t twist when the terminal bolt is tightened. The NordLock lock washers performed nicely in this regard.

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The BMS modules were installed finger-tight until all were in place, and then the terminal bolts were tightened with a box wrench wrapped in heat shrink to prevent accidental shorts. In that dramatic event, the high current would probably melt the wrench before any batteries were damaged, putting injury from molten steel higher on the list than electrocution.

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These yellow signal loop wires create a daisy chain that allows all of the cells to communicate their status to the control board: too full, too empty, or just right. The signal loop is an analog circuit where all the modules broadcast simultaneously on the same wire like a party-line.

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The signal loop connections are non-polarized on the modules, and do not need to follow the same battery order as the power circuit. The only requirement is that the loop starts and ends at the control board, and connects through every module. So even though the battery power loop starts and ends at the motor controller in the motor bay, the signal loop starts and ends at the BMS control board in the former fuel bay.

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The MiniBMS main board lives in the control box ahead of the passenger-side dashboard. Using a short length of multi-strand sprinkler control wire from OSH, all connections were first made on the board, then the dangly ends were gathered into a 10-pin plug from Radio Shack. For the AC-50 system with Elcon charger, the connections to the board are as follows:

– On the lower left are the remote buzzer + and – terminals. As you may notice, there is a buzzer on the board itself, but installing a remote buzzer in the driver compartment will increase audibility in the event of a low voltage warning.

– At upper left are the red and green leads which connect to the DIN plug on the charger. They give the BMS jurisdiction to shut off the charger when all cells are full, or in case of an overcharge event.

– Next clockwise is an orange wire that supplies 12 VDC to the control board from the DC/DC converter and auxiliary battery. This insures constant operation of the BMS, especially when the key is off during charging.

– The next brown wire is ground.

– The blue wire connects to the ignition, which signals the BMS control board to shut down the charger if the key switch is turned on.

– At the top right are the cell module signal loop wires, colored white and yellow. These two pins are the beginning and end of the signal loop path, which allows the individual cells to broadcast their status to the control board.

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Shown here is the 10-pin Radio Shack connector that allows me to unplug the control board from the entire battery management system, making it convenient to service. It was much easier wiring the plug to the control board, rather than trying to attach wires from the car directly to the tiny terminals on the board.

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Placement of the auxiliary BMS buzzer is under the dash so it can be clearly heard by the driver. The buzzer will sound if any of the cells dip below a charge level that will endanger their health. Once startled into crisis mode, the buzzer has done its job. The driver and passengers don’t need the added stress of a continual loud annoying noise, so I epoxied a cutoff switch to the buzzer body. The shot above shows the buzzer and switch mounted under the dash near the odometer reset knob, easily within reach of the driver.

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Installation of BMS complete! Here you can see the BMS modules glowing green, meaning that the cells are maintaining a healthy charge state. During the charge cycle, the modules will glow red when cells reach their upper limit and begin shunting current away from the cells. They would also glow red during a low voltage event to flag the cell in peril, but hopefully that never happens.

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*** NOTE – The MiniBMS performed perfectly as advertised, but I wanted access to more pack data with a better display option, so at a later date I decided to pop the extra cash for the Orion BMS. That entry is here.

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5 Responses to “Battery Brains”

  1. I get so excited every time there is a new update on the project! You’re so meticulous and thorough.

  2. I plan on using the same BMS in my project. I’m thinking of setting my charger to turn off just before the BMS would terminate the charge. I’m thinking 3.55 Volts per cell for the charger. The BMS cuts the charger if any cell reaches 3.6 V. The reason is that if the cells are in balance, and healthy, then the charger is in control of the charging process. Then the BMS is a backup system that only acts if there is a problem. This seams more reliable to me, because the shunts will only be used if needed.

    • I totally get it. I was marveling at how they promote no BMS at all over at EVTV, and their arguments make complete sense. I expect the next wave of chargers will have everything built-in.

      • If the cells stay in balance and you don’t charge too close to the upper voltage or discharge close to the lower limit, then going without a BMS may make sense. I just think that having a BMS is worth it.

        Your project is coming along nicely. I’m looking forward to your next couple updates.


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