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 [1]
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+, tsenseor1-, tsensor2+, tsensor2-, 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 and only one temperature sensor is supported.
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 [2] or you can use Dave Fiddes "oic" tool for the command line[3]. It is important to know that the main module has node-id 3 and the subsequent ones 3+i (i index of submodule, so 1st submodule has node-id 4). The base node id can be configured to start at something else than 3.
All info given here is preliminary and will likely change!
Apart from that the modules output periodic CAN messages depending on their role.
All modules output on ID 500 (0x1F4) + 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 300 (0x12C)
| 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 3 dumpall #dumps all values including cell voltages from 1st module
oic -n 4 dumpall #same for second module
oic -n 4 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
0x603#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
0x603#0x40 0x07 0x21 0xD6 0 0 0 0
And you will receive the reply
0x583#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 0x604.
Configuration Parameters
There is currently a limited set of parameters which will be extended as the software grows.
| 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 | |
| balmode | 0 | 3 | 0 | Balancing mode
0=No balancing 1=Only bump up low cells 2=Only dissipate high cells 3=Do both | |
| 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 Limits | |||||
| ucellmin | mV | 1000 | 4500 | 3300 | Minimum loaded cell voltage. Discharge limit is dropped to stay above this value |
| ucellmax | mV | 1000 | 4500 | 4200 | Maximum loaded cell voltage. Charge limit is dropped to stay below this value |
| chargemax | A | 1 | 2047 | 200 | Maximum charge current |
| dischargemax | A | 1 | 2047 | 200 | Maximum discharge current |
| chargeXsoc | % | 0 | 100 | Percentage of charge current at soc X. In between two points charge current is interpolated | |
| Battery Characteristics | |||||
| nomcap | Ah | 0 | 1000 | 100 | Nominal capacity |
| ucellXsoc | mV | 2000 | 4500 | Unloaded cell voltage at SoC X. Voltages in between to points are interpolated. Used for SoC estimation. | |
| 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 | -1 | 3 | -1 | Temperature sensor -1=Off, 0=JCurve, 1=KTY81, 2=PT1000, 3=LeafBMS | |
| Communication | |||||
| pdobase | 0 | 2047 | 500 | base COB Id for cyclic messages. Main module uses pdobase, 1st submodule uses pdobase+1 and so on
Must always be set to the same value on all modules! | |
| sdobase | 0 | 63 | 3 | base node ID for SDO queries. Main module uses sdobase, 1st submodule sdobase+1 and so on
Must always be set to the same value on all modules! | |
Operation
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
Charging the battery happens via a pre-determined charge curve. Since we have no battery knowledge ourself we need to mimic what we see the OEM BMSes do. The charge curve usually starts at its maximum charge current when the battery is empty and then around 30-50% cuts back. 10 points can be configured of the charge curve at 0, 10, ..., 90%
In contrary when discharging there is no pre-determined curve, we only limit the minimum per-cell voltage by limiting the discharge current
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
These are not enforced yet. High temperature will cut back both charge and discharge current whereas low temperature will only cut back charge current.
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%.