16-cell BMS

16-cell BMS

The goal of this BMS is to use mostly decade old components to create a 16 channel cell voltage meter. It uses a very accurate sigma/delta ADC for superb resolution. It can drain about 100mA from a cell or charge it with 50 mA via the DC/DC converter. The ability to both charge and discharge cells speeds up balancing despite the low balancing current.

When turned off it drains no current from the connected cells at all.

On the low voltage side it features two inputs for temperature sensors and one input for a 5V current sensor, differential or single ended.

It can be cascaded to be used with more than 16 cells and has an automatic addressing scheme to forgo the use of jumpers or DIP switches. All communication is done via CAN. Supply voltage is 12V with about 20 mA draw when not balancing.

The software is not yet finished, see forum for progress

The boards can be bought in the shop.

Pinout

The pinout is also written to the bottom silkscreen.

On the left side you see 17 inputs. The first one is connected to the negative pole (GND) of your (sub)pack, the subsequent ones to the positive poles of up to 16 cells. At least the lower two cell inputs must be connected. When cascading modules the positive-most input of module n is also the GND of module n+1, so must be spliced to connect to both.

On the top right is the vehicle interfacing or low voltage connector. From top to bottom it is low voltage GND, 12V, enable_in, enable_out, CANH, CANL. GND, 12V, CANH, CANL are connected to all modules in parallel whereas enable_out is chained to enable_in of the next module. The first enable_in must be connected to 12V whenever the BMS should become active, e.g. when charging or driving. The module with 12V on the enable_in auto-detects as "main" module, collects data from the subsequent modules and sends more high level CAN messages

On the bottom right is the temperature sensor and current sensor input. Top to bottom it is tsensor1+, tsensor1- (not silkscreened), tsensor2+ , tsensor2-(not silkscreened), 5V, CUR+, CUR-, GND. A single ended current sensor is just connected to CUR+.

Right now the current sensor is only supported on the main module.

Connector part numbers

Connector	Manufacturer	Header	Counterpart	Pins
LV connector (supply, CAN, enable)	Phoenix Contact	1803468	1803617
Temperature and current sensors	Molex	640454-8	1375820-8	1445336-1
Cell taps	Molex	1-640456-7	1-1375820-7	1445336-1

Power supply

The pins GND and 12V must be connected to a permanent 12V source in order to ensure correct operation. In other words, don't connect 12V for example to your cars ignition 12V that is interrupted whenever you stop the car.

The enable input of the first (main) module is connected to an interruptable 12V source. This input starts up the BMS. Enable can stay high but even a 1s pulse is enough to start the BMS, as it will then keep itself running as long as needed.

The BMS will then keep running as long as current flows through the battery. 2 hours after it has measured the last current flow it will shut down. It will use these 2 hours to let the cells settle to their open circuit voltage and will do balancing and SoH calculation. Afterwards it will shut itself down and consume only a few uA from your 12V source.

Communication

All configuration, queries and firmware updates are done via the CAN bus. There is no independent web interface like known from e.g. the inverter. However there is a solution to connect an ESP32 via CAN and then have the known web interface or you can use Dave Fiddes "oic" tool for the command line. It is important to know that the main module has node-id 10 and the subsequent ones 10+i (i index of submodule, so 1st submodule has node-id 11). The base node id can be configured to start at something else than 10.

All info given here is preliminary and will likely change!

Apart from that the modules output periodic CAN messages depending on their role. The base address for cyclic messages defaults to 0x1F4 and can be changed by modifying pdobase on the main module. pdobase is ignored on the sub modules.

All modules output on ID 501 (0x1F5) + i

start	length	item	description
0	13	umin	minimum cell voltage of module i
16	13	umax	maximum cell voltage of module i
30	2	counter	counts from 0-3 (anti-freeze)
32	13	uavg	average cell voltage of module i
48	8	tempmin	Minimum temp +40 on module i
56	8	tempmax	Maximum temp +40 on module i

The main module does not output its local accumulated values but accumulated values over all modules.

In addition the main module outputs on id 500 (0x1F4)

start	length	item	description
0	11	chargelim	maximum charge current in A
11	11	dischargelim	maximum discharge current in A
22	10	soc	Pack soc in 0.1%
32	16	idcavg	Averaged current reading of last 1s in 0.1A - signed!
48	10	utotal	Total pack voltage in V
62	2	counter	counts from 0-3 (anti-freeze)

Reading individual cell voltages

Cell voltages are only available via CAN SDO from the module that measures them. So voltages 1-16 must be queried from the first (main) module, 17-32 from the 2nd (1st submodule) and so on. With Dave Fiddes tool this translates to:

oic -n 10 dumpall #dumps all values including cell voltages from 1st module
oic -n 11 dumpall #same for second module
oic -n 11 read u0 #Read first cell voltage of second module

For querying voltages say in your arduino sketch you'll need to acquire every voltage separately by sending and receiving SDO messages

0x60A#0x40 idhigh 0x21 idlow 0 0 0 0

idhigh and low are the bytes of the internal ID. The id of the first cell voltage is 2006 (0x7D6) so for querying the 1st cell voltage of the first module you send

0x60A#0x40 0x07 0x21 0xD6 0 0 0 0

And you will receive the reply

0x58A#0x43 0x07 0x21 0xD6 0xD0 0xED 0x01 0

0x0001EDD0 is the voltage value which decodes to mV x 32. So 0x1EDD0/32 = 3950.5 mV.

The next cell would be on 0x7D7 and the last on 0x7E5. The next 16 voltages are then queried from node id 0x60B.

Configuration Parameters

There is currently a limited set of parameters which will be extended as the software grows. This table always reflects the latest git commit. Check the page history for previous versions.

These are the parameters of v0.20.B

Name	Unit	Min	Max	Default	Description
BMS
gain	uV/dig	1	1000	586	Scaling factor from ADC digits to mV
correction0	ppm	-10000	10000	-1250	The first two and the last channel have slightly different topology and need their own correction factor
correction1	ppm	-10000	10000	1500
correction15	ppm	-10000	10000	1000
numchan		1	16	16	Number of cells connected to the module, power cycle required following saving parameters to update.
balmode		0	3	0	Balancing mode 0=No balancing 1=Only bump up low cells 2=Only dissipate high cells 3=Do both By default balancing starts 60s after power up or in fact 60s after idcavg settled below 1A. You will see opmode changing to RunBalance
ubalance	mV	0	4500	4500	Voltage above which top balancing is started
idlewait	s	0	100000	60	Number of seconds to wait after stop of current flow to measure unloaded cell voltage und start balancing
Battery Characteristics
dischargemax	A	1	2047	200	Maximum discharge current
nomcap	Ah	0	1000	100	Nominal capacity
icc1	A	1	2000	50	Maximum charge current of empty battery
icc2	A	1	2000	30	Maximum charge current of roughly half-charged battery
icc3	A	1	2000	20	Maximum charge current of rather full battery
ucv1	mV	3000	4500	3900	First constant voltage set point. Transition to next one when dropping below icc2
ucv2	mV	3000	4500	4000	Second constant voltage set point. Transition to next one when dropping below icc3
ucellmax	mV	3000	4500	4200	Third constant voltage set point and maximum cell voltage
ucellmin	mV	1000	4500	3300	Minimum loaded cell voltage. Discharge limit is dropped to stay above this value
chargeXsoc	%	0	100		Percentage of charge current at soc X. In between two points charge current is interpolated
sohpreset	%	10	100	100	This is the starting value for the state of health (SoH) calculation. The value is continuously updated by the BMS as SoH is estimated.
Sensor Setup
idcgain	dig/A	-1000	1000	10	Gain (or actually division factor) of current sensor
idcofs	dig	-4095	4095	0	0A offset
idcmode		0	3	0	Current sensor mode 0=Off, 1=Single Ended ADC, 2=Differential ADC, 3=ISA current shunt
tempsns		0	3	0	Temperature sensor 0=None, 1=Channel1, 2=Channel3, 3=Both (set tempsns 0 in dashboard if dropdown not availble and value set as -1)
tempres	Ohm	10	500000	10000	Nominal sensor resistance at 25°C
tempbeta		1	100000	3900	beta factor of Steinhart equation (see data sheet)
Communication
pdobase		0	2047	500	base COB Id for cyclic messages. Main module uses pdobase and pdobase+1, 1st submodule uses pdobase+2 and so on
sdobase		0	63	10	base node ID for SDO queries. Main module uses sdobase, 1st submodule sdobase+1 and so on

Spot values

All values that the BMS either measures or calculates are called spot values. While all modules show all spot values, some only make sense on the main module. The column "scope" denotes that. "M" means the value is only relevant on the main module, "S" means it's relevant on main and sub modules.

Name	Unit	Scope	Description
version		S	Firmware version
opmode		S	State of operation BOOT - System starting GETADDR - Waiting for address assignment SETADDR - Setting address of next node REQINFO - Requesting info from sub modules RECVINFO - Receiving info INIT - Initialize BMS hardware SELFTEST - Running hardware self tests RUN - normal operation when current flows through battery IDLE - battery is idle, i.e. no current through it for a given time ERROR - selftests detected a critical error, operation halted
lasterr		S	Last detected error, see Errors
errchan		S	Cell tap channel causing the error, if applicable
modaddr		S	Address of module
modnum		S	Total number of modules
totalcells		M	Total number of cells
counter		S	Revolving counter 0-15 100 ms clock rate
uptime	s	S	Time in s since power was applied to 12V pin
chargein	As	M	Charge into the battery
chargeout	As	M	Charge from the battery to load
soc	%	M	State of charge, i.e. available charge in percent of fully charged battery
soh	%	M	State of health, i.e. available total charge in percent of data sheet value when new
chargelim	A	M	Current limit for charging the battery, see Charge Curve
dischargelim	A	M	Current limit for discharging, see Calculating current limits
idc	A	M	Present current through battery, negative for current out of the battery, positive for current into the battery
idcavg	A	M	Average current of the last second
power	W	M	Average battery power of the last second
tempmin	°C	M	Minimum temperature of all modules
tempmax	°C	M	Maximum temperature of all modules
uavg	mV	M	average cell voltage over entire pack
umin	mV	M	Minimum cell voltage of entire pack
umax	mV	M	Maximum cell voltage of entire pack
udelta	mV	M	Difference between minimum and maximum cell voltage of entire pack
utotal	mV	M	Sum of all cell voltages, i.e. pack voltage
uX	mV	S	Present voltage of cell X of current module, see also Reading individual cell voltages
uavgX, uminX, umaxX, tempminX, tempmaxX	mV, °C	M	Average/Minimum/Maximum cell voltage and temperature of sub module X
uXcmd		S	State of balancer
cpuload	%	S	Time the CPU spends actually doing something

Operation

State of Charge calculation

The State of Charge (SoC) lets you know how many percent of charge you battery still holds, in percent of a fully charged battery.

Determining SoC is split into two methods: determining the SoC from cell voltage and then tracking it as current flows in or out of the pack.

To estimate the SoC a 10-point lookup table is implemented that maps from cell voltage to SoC. The accuracy of this table is directly related to the accuracy of the estimation. You can edit the lookup table by changing the chargeXsoc parameters. The voltages you enter are open circuit voltages. These can only be obtained when there was no current flow through the battery for a certain amount of time, typically 10-30 minutes. This time is configured via the idlewait parameter.

So whenever the BMS has the chance to estimate the SoC it will do so. You can adjust idlewait to a low value to start estimating quickly. It means the first estimates will be off but it will become more accurate if the pack remains unloaded for longer time.

The subsequent tracking of SoC requires the BMS to know the current through the battery. Right now it supports analog current sensors (e.g. hall effect) with an operating voltage of 5V. Within this class it supports differential and single ended sensors, configurable via idcmode. The parameters idcofs and idcgain are used for calibration. When no current is flowing adjust idcofs until idcavg shows 0A. Then put a known current through the sensor and adjust idcgain until idcavg shows the correct current.

Lastly the BMS needs to know the energy or charge content of your battery in Ah. This is configured with the nomcap parameter and can be found in the data sheet.

State of Health calculation

The State of Health (SoH) lets you know how the remaining fully charged capacity, in percent of the data sheet value, i.e. when the battery was new. Batteries slowly degrade over time and usage cycles.

Whenever we do a SoC estimation as described above, we also update the SoH. If the estimated SoC has changed by more than 20% we compare the energy that we have taken out of the battery to the difference in SoC. If the two match, the SoH is 100%. If we have taken more energy from the battery then the difference in SoC suggests, SoH is above 100% (and you likely configured a wrong nomcap). Otherwise SoH is < 100%.

As this method comes with certain inaccuracies we only change the long term SoH slowly, i.e. every estimate is slowly averaged onto the long term SoH. This should balance out inaccuracies.

The spot value soh shows the current long term SoH. The value is also reflected onto sohpreset, a saveable parameter. Whenever you need to interrupt the BMSes power supply you should go should save parameters so that next time it starts up with the correct long term SoH.

Calculating current limits

Limiting charge and discharge current are the only means of this BMS to protect the battery. It has no means to turn off the current flow by itself but instead relies on the connected components to obey the calculated current limits that are sent out via CAN. The current limits are calculated in a number of steps.

Charge Curve

The initial approach was to assign a certain charge current to points in the SoC curve. This method can be skewed if the SoC calculation is off for some reason. The new method deploys 3 CC/CV regulators. This was copied from oberving VWs charge curve of the MEB cars. The first regulator has the highest charge current and aims for a voltage typically around 3.9V for many NMC chemistries. When current drops below the CC value of the second regulator then that one is used, aiming for a higher voltage of typically 4V. And finally the 3rd regulator takes over aiming for the charge end voltage of typically 4.2V. When setting all 3 to the same values a simple CC/CV curve is generated.

Voltage limits

Voltage limits are enforced via a P-regulator. I.e. as we approach the upper or lower voltage limit current is cut back to stay away from the limits until it reaches 0A. The upper and lower limit is configurable

Temperature limits

High temperature will cut back both charge and discharge current whereas low temperature will only cut back charge current. These limits only work when you've set up your temperature sensors correctly. Please check the readings against a reference before relying on it.

Charge current has a proportional, multi-step temperature derating, i.e. the charge curve defined by iccX and ucvX is scaled with a derating factor the lower the temperature or when the maximum temperature is approached

• Below -20°C no charging is possible
• Between -20°C and 0°C charging with 0-30% of the charge curve is allowed
• Between 0°C and 25°C charging with 30% to 100% of the charge curve is allowed
• Between 43°C and 50°C charging with 100% to 0% of the charge curve is allowed, i.e. above 50°C charging is no longer allowed.

Discharging is not limited at low temperatures as high discharge currents at low temperatures are not known to cause damage. As the battery heats up discharge current is limited, starting at 46°C until discharging is no longer allowed when reaching 53°C.

Energy limits

Not enforced yet. Some OEM BMSes cut back charge current after a certain amount of energy has been put into the battery. E.g. When starting charge at 50% charge rate will end up higher at 70% than if we start charging at 10%.

Calibration

As every instrument the voltage measurement needs to be calibrated to cancel out tolerances in the involved components. There are 4 parameters for calibration.

• gain: The translation ration between the ADC digits and mV. This applies to all channels
• correction0, 1 and 15: This allows special tuning of said 3 channels because they use a slightly different current path and thus need a small gain correction

Calibration procedure

First we determine the general gain. Pick any of the voltages between u2 and u14 which share a common gain factor. Use a good multimeter to measure the physical voltage of your reference channel. Now tune the gain parameter until your multimeter and the BMS voltage read the same value +-0.5mV. Now all channels from 2-14 should read correctly.

Then for channels 0, 1 and 15 also measure the physical voltage input and tune the respective correctionX parameter until multimeter and BMS report the same voltage. The correction factors can be negative if a channels reads too high.

Save your parameters to flash to make the calibration permanent.

Flashing firmware

If you have sourced the BMS boards from an alternative source then you will need to flash the bootloader and firmware onto the BMS. This is a relatively straightforward process once understood but took me a while to get it to work.

Firstly you will need an STLink dongle, I used this one but I believe any will do.

I used STM32CubeProgrammer on Linux to flash my board and also (prior to flashing) upgrade the STLinkdongle to the latest firmware (might not be necessary but felt it was wise to do so).

Once you've got STM32Cubeprogrammer installed you're probably best erasing the STM32 Chip to ensure you're starting from a clean board. Navigate to the "Erasing & Programming" tab on the left and connect to your STlink dongle (if you havent already). Once connected, (you can also update the firmware on the dongle at this point on the "Memory & File Editing" tab), connect the dongle to the BMS board via the VDCG pins...

(V = Voltage 3.3V, D = SWDIO, C = SWCLK, G = Gnd)

and perform a "full chip erase".

Once thats complete head over to Github to download the Bootloader and download the hex file (you may need to be logged in to download the files).

Once downloaded, in STM32CubeProgrammer, navigate to the "Memory & File Editing" tab and open the hex file you just downloaded, once opened you can use the "Download" button to flash the bootloader to your BMS board.

You can now repeat the above process of opening the hex file and "downloading" to the BMS board for the main BMS firmware which is available from the BMS Github repository .

Once all that is done, you can move onto the Web interface. You'll need the "Can Backend" version of the ESP32 Web Interface available here (click the green code button and download zip)

Once downloaded download Arduino IDE 1.8.19 (you'll need this version, nothing beyond version 2 as this does not support SPIFFS flashing as far as I'm aware) and set it up for flashing to ESP32 boards and also flashing ESP32 Spiffs .

To setup the Web interface Arduino sketch, you'll want to unzip and (IMPORTANT) RENAME the folder to "esp32-web-interface" to match the ino file within the folder, if you don't do this you'll have problems further down the line. Once unzipped and renamed, open the esp32-web-interface.ino sketch file and upload it to your ESP32 dev board. when this completed you'll also need to upload the SPIFFS image, you can do this by going to Tools and "ESP32 Sketch Data Upload" and choosing SPIFFS. once this is complete, you can connect your ESP32 to your BMS board via CAN and power them and they shuld work (note you'll need the 12V enable in line connected also).

Once on the Web interface, select Node ID 10 and a speed of 500kbps and you'll have all your wonderful BMS spot values and params ready to go.

Operating Limits

• Max voltage for the board is 70v
• Max number of cells is 16