Lite BMS master description

Principles of operation

The typical Li-Ion EV traction system consists of:

• A traction pack composed of Li-Ion cells
• A charger
• A motor controller, with a throttle
• A motor
• A BMS to protect and manage the pack, by controlling the charger and the motor controller

The Lithiumate BMS using a Lite BMS master consists of:

• A BMS master
• The traction pack, divided into banks of cells: up to 8 banks, up to 20 cells per bank
• Cell boards mounted on the cells: one cell board per cell (or block of cells in parallel)
  • The cell board mounted on the most positive cell in a bank is a positive end cell board
  • The cell board mounted on the most negative cell in a bank is a negative end cell board
  • All other cell boards are mid-bank cell boards
• Communication cables between the BMS master and the end cell boards
• Connections to the charger, to power its AC input, and measure its DC output current to the battery
• Connections to the motor controller, and its throttle, to limit it or disable it, and to measure the battery current to it (and from it)
• Connections to any dashboard display

During set-up, the BMS master is connected to a Windows computer to configure and monitor the BMS.

The BMS master interfaces to the cells, divided into banks. It can handle up to 8 banks of cells.

The BMS master communicates with the banks through up to 4 shielded RJ45 cables; each cable handles 2 banks. At the far end of each cable, a breakout adapter goes from the single RJ45 cable, to 4 individual, small cables. Each small cable is a shielded pair, terminated with a 2-pin connector. Two of those 4 small cables are used for one bank (one to the most positive cell of the bank, one to the most negative one) and the other 2 cables are used with another bank.

The BMS sends a serial digital stream (with a proprietary format) through the RJ45 cable, through the breakout and through a small cable, to a 2-pin connector on the positive end cell board.

The positive end cell board (mounted on the most positive cell of the bank), provides electrical isolation between the incoming cable (which is referenced to ground) and its own electronics (which are referenced to the high voltage of the battery). The cell board detects and executes any commands in the serial stream, adds data to the serial data stream, and passes it to the next, more negative cell board. The communication between cell boards is through a single wire link.

The next cell board (a mid-bank board) receives data form the positive end cell board through the single wire link. Just like the positive end board, it executes commands, adds data to the stream, and passes it to the next cell board.

The following cell boards operate the same way, down to the last cell board in the bank.

The negative end cell board (mounted on the most negative cell of the bank), performs the same functions as the other cells. But, instead of passing the serial stream to another cell board, it isolates it, and it passes it to its 2-pin connector.

A small cable, connected to the 2-pin connector on the negative cell board, carries the return serial stream back to the breakout, through the RJ45 cable, and back to the BMS master.

Other than the isolation at the positive and negative end cell boards, all the cell boards are identical. There is no need to configure each cell board to give it a unique ID, to tell it where it is placed along the bank. Instead, the BMS master does that, on the fly, at power-up. Therefore, cell boards may be interchanged without a problem.

Through the serial stream, the BMS master is able to tell the cell boards to measure their cells' voltage and temperature, and to turn on their balancing loads.

Each cell boards adds its measurement results to the data stream and passes it to the next cell board, and then back to the BMS master.

The BMS master gathers the data from all the cell boards, and calculates various key parameters:

• Total pack voltage (by adding the voltages of each cell)
• Average, minimum and maximum cell voltage
• Average, minimum and maximum temperature

Then, the BMS master uses the minimum and maximum cell voltage and temperature to determine whether charging or discharging should be enabled, as described in the following sections.

The BMS is on and running whenever the EV is plugged in a wall outlet.

While the electric vehicle is plugged into the wall outlet, the AC power from the plug powers the BMS master through its "AC Power" connector.

Inside the BMS master, as 12 V power supply powers the BMS.

That voltage appears on all the "12 V" terminals on the control connector (they are powered whenever the EV is plugged in the wall or when the ignition switch is on).

This supply can only provide enough power for the BMS master, plus a bit of current for external loads, such as small relays and indicator light bulbs.

If any cell voltage is above the V-cell-max setting, or any temperature is outside the charging temperature range, the BMS turns off the charger, by switching off the AC voltage from the BMS master to the charger AC input.

As soon as all cell voltages are back down to below the V-cell-high setting, and all the temperatures are within the charging temperature range, the BMS turns the charger back on.

The EV MUST must be wired so that the BMS is able to shut off the charger

As the BMS turn on the charger switch, it also turns on the "Charge OK" LED and sets the "CHG EN" output to 12 V. Conversely, as the BMS turn off the charger switch, it also turns off the "Charge OK" LED and opens the "CHG EN" output.

The charger output current is routed through the "Charger Current Sensor" terminals before reaching the pack.

Knowing the charging current allows the BMS master to:

• Report the current through the Amp Meter output (10 mV / A), which normally feeds a 0 to 5 V meter with a 500 A full scale face plate.
• Integrate the current to calculate the SOC (State Of Charge) of the pack; as the pack is charged, the SOC increases towards 100 %

Charging with a BMS and a CCCV charge occurs in 3 stages:

1. Full charge
2. Balance
3. Top off

The Constant Voltage of the charger must be set to CV = number of cells in series x max cell voltage.

During this stage, the BMS keeps the AC switch turned on, to power the Charger. The charger operates in its CC (constant Current) mode, charging the pack at full current. The BMS increases the SOC.

The first time the pack is charged, the BMS has no idea what the pack's initial SOC is: the SOC is uncalibrated. It will remain uncalibrated until the pack is fully charged for the first time.

When powered up the first time, the BMS will assume an SOC of 50%. If the initial actual SOC had been less than 50%, at some point the calculated SOC will reach 100 %, even though the pack is still not full. On the other side, if the initial actual SOC had been more than 50%, the calculated SOC will be still be below 100 % by the time the pack becomes full.

Once the voltage of the most charged cell reaches V-cell-max, the BMS enters the 2nd stage: balancing.

During this stage, the BMS balances the pack.

The point of balancing is to maximize the pack capacity, by ensuring that the cell with least capacity is the limiting factor to charging and to discharging.

A pack is balanced if all of its cells are at exactly the same State Of Charge - at some point. In an EV, that point must be the top (fully charged).

A BMS top balances the pack by removing charge from the most charged cells, to bring them down to the level of the rest of the cells.

The Lithiumate BMS top balances the pack using the "final voltage" algorithm: it uses a cell's voltage near 100 % SOC to decides whether to remove some of its charge.

Each cell board has a resistor load that can be placed across its cell to bleed off part of its charge, for the purpose of balancing the pack.

In this stage, the BMS top balances the pack by:

• Turning the charger on and off, to keep the voltage of the most charged cell between V-cell-high and V-cell-max. The charger operates in its CC (constant Current) mode, at full current.
• Turning on the loads across the cells whose voltage is the highest to remove some charge, to make room for more charge. That way, when the charger is turned back on, it can add a bit more charge to all the cells, including the rest of the cells that are not quite as charged.

This results in the charger switching on and off every few minutes. The on-off duty cycle is such that the average charge current is equal to the balance current. For example, with a 10 A charging current, and a 200 mA balance current, the charger may be on for 12 seconds, and off for 10 minutes.

After each cycle of removing charge from the most charged cells (slowly, at about 200 mA) and adding charge to all the cells (fast, at on the order of 10 A), the cells' SOC levels are a bit closer to each other. After a number of cycles, all the cells are at the same SOC, all close to 100 %.

The first time, this process can take as little as a few hours, or, if the pack was built with cells at significantly different SOC, as long as a few weeks. After a pack is balanced the first time, the BMS can keep it in balance in just a few minutes after each charge.

Tip: to avoid having to wait a long time while the BMS does gross balancing the first time, balance the cells to each other before building the pack, by connecting them all in parallel for a few days.

Once all the cells are at the same SOC (their voltages are all the same), the BMS enters the 3rd stage: topping off.

In this stage, the charger operates in the CV (Constant Voltage) mode, maintaining its output at the voltage that it was set for: the voltage of a full pack. The BMS keeps the charger continuously on, but the charge current decreases exponentially to 0 as the pack voltage gets closer and closer to completely full. This reduction in current is not due to the BMS or to the charger: neither one is purposely decreasing the current. Instead, this effect is the result of the physics of the pack voltage reaching the charger's Constant Voltage setting: as they become closer, the voltage difference decreases, resulting in less and less current.

When the voltage of each and every cell is exactly equal to V-cell-max, the charger current is down to 0 (because the pack voltage is the same as the charger's CV voltage, so there is no longer a voltage difference to cause any current).

At that point:

• The BMS master sees that a cell reached V-cell-max (actually, they all did) and it turns off the charger, for the last time.
• The BMS master calibrates the SOC by forcing it to 100 %.

The BMS is on and running whenever the EV is on.

When the EV is on, the 12 V from the Ignition line powers the BMS through the "IGN" terminal in the control connector.

That voltage appears on all the "12 V" terminals on the control connector (they are powered whenever the EV is plugged in the wall or when the ignition switch is on).

The load current from the pack is routed through the cable-mounted Load Current Sensor before reaching the motor driver.

Knowing the load current allows the BMS master to:

• Report the current through the Amp Meter output
• Integrate the current to calculate the SOC (State Of Charge) of the pack; as the pack is discharged, the SOC decreases towards 0 %

Ideally, the SOC will reach 0 % just as the lowest charged cell becomes empty. However, if the BMS is not configured properly, it may introduce errors in the SOC calculation; for example:

• If the BMS setting for pack capacity doesn't match the actual pack capacity, the rate of decrease in the SOC will be wrong
• If the offset in the current sensor is not calibrated, the calculated SOC will drift with respect to the actual SOC
• If the V-cell-min setting is too high, the BMS will shut off the pack before it is completely empty

Even if the BMS is configured properly to calculate the correct SOC, if the EV is used aggressively, the cell voltages may sag considerably, and the BMS may shut off the pack before the SOC reaches 0% SOC.

You cannot rely just on the reported SOC: in some cases the BMS may report a significant SOC level and yet shut down the pack because a cell voltage is too low; in other case, the BMS may report that the SOC is 0 %, yet the pack may be able to generate more charge.

The only true indicator that the pack is nearing being empty is the voltage of the lowest charged cell. The BMS does not report that voltage directly to the driver. Instead, the BMS makes the user aware of that condition in ways described in the next section.

Whenever any cell voltage is below V-cell-low, the BMS warns the user that the pack is nearly empty. It can do so in 2 ways, each of which may be implemented in an EV:

• Driving the "Lo-BAT" line to 12 V, which can light a "Reserve" indicator on the dashboard, or put the motor driver is a low power ("valet" or "limp home") mode
• Gradually increasing the resistance in the "Throttle Reduce" terminals proportionally to the drop in cell voltage; that can be used to reduce the output from the throttle to the motor driver, to reduce its effective range

The "Reserve" indicator solution is least intrusive, but it relies on the driver reacting to the warning, which may not be wise.

The onset of the "Valet mode" can be disconcerting to the driver, and may be dangerous if full power is needed at the time.

The gradual reduction of throttle range is not sudden, which is nice, but may not be noticed when driving slowly.

If, after a period of rest, all the cell voltages relax back up to above V-cell-low, the BMS will end the warning by opening the "LO BAT" line and bringing the Throttle Reduce output back to its minimum resistance.

Whenever any cell voltage reaches V-cell-min, or any temperature is outside the range for discharging, the BMS tells the motor driver to shut off. It can do so by opening the "DCH EN" output (which normally would sit at 12 V). The EV can use that line in 3 ways:

• Power the 12 V supply to the motor driver
• Control the "Enable" input of the motor driver
• Power the main contactor, which switches off the high voltage to the motor driver

The EV MUST must be wired so that the BMS is able to shut off the motor driver in one of the above ways.

The BMS re-enables the motor driver once all cell voltages return to above V-cell-low, and all the temperatures are within the discharge range.

As the BMS turns on and off the "DCH EN" output, it also turns on and off the "Discharge OK" LED.

If the motor driver is capable of regenerative braking, it will extract energy from the car's motion and generate current to be sent back to the pack.

That current flows through the load current sensor, in the reverse direction. The BMS master integrates that negative current into the SOC, increasing its value. When driving down a long mountain, regen will recharge the pack noticeably, and the SOC will increase towards 100 %.

If the pack is full (a cell voltage is above V-cell-max), or a temperature is outside the range for charging, the pack cannot accept any more current.

If so, the BMS will prevent regen, by opening the "CHG EN" line (which normally sits at 12 V).

The EV can use that line in 2 ways:

• Control the regen enable input of the motor driver
• Disable the output of a regen pot on the brake pedal

The EV MUST must be wired so that the BMS is able to disable regen from the motor driver in one of the above ways.

When all the cell voltages go back down below V-cell-high, and all the temperatures return within the range for charging, the BMS will re-enable regen by connecting the "CHG EN" line back to 12 V.

The BMS master detects the following faults.

• Loss of communications to the banks
• Excessive charging current
• Excessive discharging current
• Excessive cell voltage
• Insufficent cell voltage
• Excessive cell temperature

Should any of them last for a while^[1], the BMS controller will go into a Fault state.

In this state:

• Neither charging nor discharging is possible
• The Fault outputs are activated (which may be used to light a fault lamp on the dashboard or to turn off a contactor)

To reset the fault state, correct the problem.

[1]

The delay is not a fixed number, but depends on the fault settings, on the severity of the problem and on other factors.

For example, for discharge current, all of the following delays apply, one after the other:

1. Current sampling, which is done every 1 s, so it can be 0 to 1 s, depending of the timing between a current pulse and the sampling time
2. Current integral delay, which depends on the value of the discharge current with respect to the Nominal DCL and the Peak DCL:
   • One second if the discharge current is at I = Nominal DCL setting + 10 * (Peak DCL setting - Nominal DCL setting)
   • Five seconds if the discharge current is at I = Nominal DCL setting + 2 * (Peak DCL setting - Nominal DCL setting)
   • Ten seconds if the discharge current is at the Peak DCL setting
   • Twenty seconds if the discharge current is at I = Nominal DCL setting + 0.5 * (Peak DCL setting - Nominal DCL setting)
   • One hundred seconds if the discharge current is at I = Nominal DCL setting + 0.1 * (Peak DCL setting - Nominal DCL setting)
   • Infinite if the discharge current is at the Nominal DCL setting
3. Fault delay, which by default is 10 s, though it can be set between 0 and 25 s

For another example, for low cell voltage, all of the following delays apply, one after the other:

1. Cell board scanning, which is done every 1 s, so it can be 0 to 1 s, depending of the timing between a voltage dip and the scanning time
2. Cell voltage time averaging delay, which depends on the value of the lowest cell voltage before the dip, its value during the dip, and the Cell voltage averaging time constant (whose value is 25 s nominal, though it can be set between 0 and 255 s):
   • Infinite if the lowest cell voltage is at the Min value and stays there
   • Immediate if the lowest cell voltage had been at the Min value and it drops below that valueit by any minute amount
   • Equal to the Cell voltage averaging time constant if the lowest cell voltage had been at Vnormal, and the dip is at Vdip, and (Vmin - Vdip) / (Vnormal - Vdip) = 1 / e
3. Fault delay, which by default is 10 s, though it can be set between 0 and 25 s