cell-monitor

Battery cell monitor and balancer based on the AVR ATTiny85 MCU.

• reads cell voltage and temperature
• switches balancing FET
• communicates with a host controller via isolated serial
• low power consumption (<1mA when idle, ~2mA while active)

A cell monitor is intended to monitor a single cell as part of a larger series pack. The cell monitor is powered directly from the cell it monitors and works with any chemistry where the voltage remains within 1.8V-5.5V (i.e. lithium-ion, LiFePO4).

Build

Dependencies:

• pio
• make

make
make fuse
make flash

Wiring

Cell monitors are wired in a daisy chain with the TX from the host controller connected to the RX of cell monitor 1, the TX of cell monitor 1 connected to the RX of cell monitor 2, etc. The TX of cell monitor N is connected to the RX of the host controller.

+-----------+
| HOST   RX | <------------------------------------------+
|        TX | >---+    +--->---+    +--->  ...  >---+    |
+-----------+     |    |       |    |               |    |
                  |    |       |    |               |    |
                +--------+   +--------+           +--------+
                | RX  TX |   | RX  TX |    ...    | RX  TX |
                |    1   |   |    2   |    ...    |    N   |
                +--------+   +--------+    ...    +--------+

Cell monitors are addressed by their position in the daisy chain.

Serial protocol

The host controller initiates all communication with the network of cell monitors and does so at 9600 baud. The host controller sends a request packet which is forwarded through the daisy chain to an addressed cell monitor. The addressed cell monitor will reply with a similar response packet which will be forwarded through the remaining chain and eventually read by the host controller.

Packet structure

Communication consists of 5 byte packets with the following structure:

Byte	7	6	5	4	3	2	1	0
1	ADDR6	ADDR5	ADDR4	ADDR3	ADDR2	ADDR1	ADDR0	REQ
2	REG6	REG5	REG4	REG3	REG2	REG1	REG0	WRITE
3	VAL15	VAL14	VAL13	VAL12	VAL11	VAL10	VAL9	VAL8
4	VAL7	VAL6	VAL5	VAL4	VAL3	VAL2	VAL1	VAL0
5	CRC7	CRC6	CRC5	CRC4	CRC3	CRC2	CRC1	CRC0

• ADDR6-0 - cell monitor address
• REQ - 1 if packet is a request, 0 if packet is a response
• REG6-0 - register (described below)
• WRITE - 1 if packet is a write command, 0 if packet is a read command
• VAL15-0 - value
• CRC7-0 - CRC8 of prior 4 bytes

Registers

• 0x1 - cell address (1 byte, unsigned)
• 0x2 - reference voltage (2 bytes, unsigned)
• 0x3 - cell voltage (2 bytes, unsigned)
• 0x4 - celsius temperature (2 bytes, signed)
• 0x5 - balance (1 if balancing, 0 otherwise)

Example

Reading the voltage from cell 1

Request:

Byte	Value
1	00000011
2	00000110
3	00000000
4	00000000
5	11010110

• ADDR = 1
• REQ bit set to 1
• REG = 3
• WRITE bit set to 0
• VAL is not applicable for read request
• CRC = crc8(50724864)

The following response will be received:

Byte	Value
1	00000010
2	00000110
3	00001100
4	11100100
5	01110111

• ADDR = 1
• REQ bit set to 0
• REG = 3
• WRITE bit set to 0
• VAL is the returned voltage (3300mV in this example)
• CRC = crc8(33950948)

Address assignment

A cell monitor has an undefined address when it is initially powered on. The host controller needs to initiate an address assignment request (which has slightly different semantics than other value requests):

Byte	Value
1	00000001
2	00000011
3	00000000
4	00000001
5	00110011

• ADDR = 0 (broadcast to all cell monitors)
• REQ bit set to 1
• REG = 1
• WRITE bit set to 1
• VAL = first cell address
• CRC = crc8(16973825)

The first cell monitor will receive the request and set its address to the value in the VAL field. Instead of sending a response it will increment the VAL field and forward the request onward. Eventually the host controller will receive the same request packet (with an incremented VAL). The host controller should assert that VAL equals 1 more than the number of cell monitors in the network.