We used PCB as bus bar to get rid of spot welding but it provided other benefits.
Removing the need for wires
Traditional batteries need to have wires going from the BMS to the cells (to measure voltage, to ballance the cells and to measure temperature. It is not uncommon to have more than 20 wires to spot weld. Besides this wires get pintched and are a massive cause of failures and hazards.
In our design, all these wires are integrated inside the PCB, resulting in something very neat but more importantly error proof:
Temperature measurement
The contact plates can optionally be equipped with SMT thermistors in order to measure the temperature of the cells:
An SMT thermistor on the contact plate.
When comparing this solution to traditional wire thermistor glued to cells, the SMT thermistor reduces complexity and cost, due to the lack of glueing, cabling, and connectors.
However, it does require a pick-and-place manufacturing step, and may have a lower accuracy, due to the absence of direct contact of the thermistor with the cells.
Gouach has tested and used this solution extensively, and found it to be an interesting trade-off, especially when the pack is enclosed with good temperature homogeneity inside the case and reasonable temperature rise rates, such as micro-mobility batteries.
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
SMT thermistors characterisation
Test Procedure
Select the 2 cells nearest to the SMT thermistors
Use SE027 probes (back probe in the slot 1 and the front probe in the slot 2 of the TC-08)
Tape the thermocouples on the middle of the cell
Put kapton tape around the thermocouple to isolate it
Tape the thermocouple to the cell with some thermal paste
Actions
For all measurements, wait for the temperature to be stable (<2°C in 5 minutes)
Do not put any current.
Measure the temperatures of the 2 thermistors, and the 2 thermocouples.
Measure the thermistors & thermocouples at the following currents:
10A, 20A, 30A
Measurements should be at 1Hz.
Stop measure when any of the followings:
70°C measured by thermocouples.
Temperatures are stable (<2°C in 5 minutes)
The BMS cuts due to overtemp
Thermocouples shall be measured using the PICO
Thermistors shall be measured through UART on the BMS.
Picture of thermocouple’s postions according to the thermistance’s positions.
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 10A
20A
Test01
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Test02
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Test04
Graphic of T cells back thermistor and T cells back thermocouple according to the duration during test 20A
Test05
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Graphic of ΔT cells back thermistor and ΔT cells back thermocouple according to the duration during test 20A
30A
Test01
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 30A
Graphic of ΔT cells back thermistor and ΔT cells back thermocouple according to the duration during test 30A
Test02
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 30A
Graphic of T cells back thermistor and T cells back thermocouple according to the duration during test 30A
Offset
20A
Test01
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Graphic of T cells back thermistor and T cells back thermocouple according to the duration during test 20A
Test05
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Conclusions
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Graphic of T cells back thermistor and T cells back thermocouple according to the duration during test 20A
This test shows that, when discharging at 5A / cell, the thermistors overestimate the temperature, by up to 5°C.
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
Graphic of T cells front thermistor and T cells front thermocouple according to the duration during test 20A
This test shows that there is no major delay (<30s) in the response of the thermistor to temperature changes, as it is well synchronised with the thermocouple in direct contact with the cell.
However, in these tests, the thermistor ends up slightly under-estimating the temperature by up to 4°C, when drawing 6A / cell.
Note that the thermistor temperatures are obtained from BMS reading, which has not been calibrated. Calibration of the BMS temperature readings may be able to improve accuracy at high temperature.
💡
Further tests are planned, to provide a more definitive answer on the characterisation of the SMT thermistors. This document will be updated accordingly.
Cell-level fusing
Gouach contact plates integrate a PCB fuse (in the form of a small copper trace) on each pole of each cell.
These do not replace the use of a BMS, as they are not as precise as proper ceramic fuses, but improve the safety of the battery, in the following cases:
Internal short-circuit of a cell, to reduce propagation of thermal runaways to parallel-connected cells
External short-circuit of a cell, bypassing the BMS protection
Insertion of a cell in the wrong polarity
Fuse protection before burning
Fuse protection after burning
Fuses have been characterised on currents from 60 to 100A, on multiple PCBs, and different manufacturing batches.
The results are the following:
Up to 30A/fuse, all tests were stopped due to the cell rising in temperature, showing that the fuse is not a limiting factor.
When the current was lower or equal to 70A/fuse, the tin ball melted before the fuse could take effect. Moreover, a large variation in trigger time was measured.
At 80A/fuse, the fuse consistently blows in less than 1s.
At 100A/fuse, the fuse consistently blows in approximately 100ms.
This indicates that this fuse can be used to protect against currents 80A and above, which are easily reached in cases of reverse assembly, or internal short-circuit.
Repeated tests were carried out on multiple cell models, where a cell was mounted deliberately in reversed position during assembly.
This produced instantaneous currents in the order of 120 to 200A.
The cell-level fuses triggered reliably, thus protecting the cells against dangerously high currents and polarity inversion.
Individual test results. A large variation is shown below 70A, while results above 80A are stable enough for operation.
Cell fuse characterisation
Test setup
8 x 18650 power cells (equivalent to Sony/Murata VTC6) are used to pass current through the fuses of a contact plate (2.5mm footprint). An electrical load is also used to control the amount of current to put in the fuse.
Context
The tests were realised with an oscilloscope and CC-65 current clamp.
Ambient temperature was ~25°C.
All tests are realised with 2.5mm diameter tin balls.
We use 8 18650 liion cells high current (DEMEG) to pass current through the fuses and we use a electrical load to control the amount of current we want to put in the fuse. We also use thermal camera to control the temperature of the fuse during the test.
Goals
The goal of this tests is to determine the maximum current of the fuse one the cells footprint. in case of overcurrent, shortcircuit or polarity reverse of the cells to protect them. also to see if the fuses are all the same depending on the contact plate used.
The goal of the test is to draw the curve of the fuse and obtain the i2t of the fuse.
Test-01: Discharge @ 10 A, open case
This test is done with a 10s1p
Current
Duration (s)
Fuse T (°C)
10A
0
24.2
10A
30
25.4
10A
60
27.5
10A
90
27.6
10A
120
27.6
10A
150
28.1
10A
180
28.5
10A
210
29.5
10A
240
29.6
10A
270
29.6
10A
300
31.3
Fuse T(°C) vs time
Test-02: Discharge @ 15 A, open case
This test is done with a 10s1p
Current
Duration (s)
Fuse T (°C)
15A
0
24.4
15A
30
25.5
15A
60
27.5
15A
90
28.5
15A
120
28.6
15A
150
30.8
15A
180
31
15A
210
33
15A
240
33.7
15A
270
34.4
15A
300
35
Fuse T(°C) vs Duration
Test-03: Discharge @ 20 A, open case
This test is done with a 10s1p
Current
Duration (s)
Fuse T (°C)
20A
0
25.5
20A
30
29
20A
60
32
20A
90
35
20A
120
40
20A
150
43
20A
180
46
20A
210
52
20A
240
54
20A
270
55
20A
300
55
Fuse T(°C) vs time
Test-04: Discharge @ 25 A, open case
This test is done with a 10s1p
Current
Duration (s)
Fuse T (°C)
25A
0
25.5
25A
30
34.8
25A
60
35
25A
90
40.5
25A
120
42
Fuse T(°C) vs time
Test-05: Discharge @ 30 A, open case
This test is done with a 10s1p
Current
Duration (s)
Fuse T (°C)
30A
0
26
30A
30
32.4
30A
60
41.1
30A
90
42.7
30A
120
44.6
Fuse T(°C) vs time
Test-06: Discharge @ 35 A, open case
This test is done with a 10s1p
Current
Duration (s)
Fuse T (°C)
35A
0
26.9
35A
1
26.7
35A
2
27.2
35A
3
27.7
35A
4
29.1
35A
5
31.3
35A
6
33.1
35A
7
37.8
Fuse T(°C) vs time
Test-07: Discharge @ 60-65 A, open case
This test is done with a 10s1p
at 60Amp the solder point dissolder at 11s
Picture of the wiring of 60 A discharge set-up
Picture of the global of 60 A discharge set-up
at 65Amp the solder point dissolder at 11s
Test-08: Discharge @ 70-75 A, open case
This test is done with a 10s1p
At 70 amp the fuse do not burn and the temperature increase too high and desolder the wires connections to the fuse.
The solder point desolder whith a temperature higher than 200°C at about 7 second.
At 75 amp the fuse do not burn and the temperature increase too high and desolder the wires connections to the fuse.
The solder point desolder whith a temperature higher than 200°C at about 7 second.
