General description
Here is how it works in details:
The Gouach design is based on the following principles:
- Cells are placed within a plastic “spacer”, which provides the following functions:
- Positions and maintains the cells at their intended position
- Protects them against mechanical damage (impact, deformation)
- Reduces the propagation in case of thermal runaways.
- Those cells are then compressed between “contact plates”, which are PCBs providing the following functions:
- Ensures electrical contact with the cells.
- Connects the cells together, acting as a bus-bar, replacing the nickel strips commonly found in traditional lithium-ion batteries.
- Provides cell-level fusing.
- Provides cell voltage measurement.
- Provides temperature measurement.
- Those contact plates are being maintained pressed against the cells through compression screws.
- Optionally, a plastic “pressure plate” may be added behind each contact plate, providing the following functions:
- Distributes equally the pressure applied by screws on the PCBs, allowing to reduce the number of screws.
- Protects the contact plates against electrical and mechanical damage.
How the contact is ensured
When the screws are tightened, each cell is compressed between the two contact plates, and makes contact with a tin ball. The mechanical properties of the PCB material (FR4) are used to provide a spring effect to ensure a reliable contact, even in case of vibrations and shocks.
This design has the benefit of being simple and cheap to produce, allowing its use even in cost-sensitive applications.
Moreover, the design has been validated through lab-tests, as well as real use in micro-mobility applications.
The safety of the design has been validated, as can be shown by the list of certifications Gouach batteries have passed.
This idea has been patented under WO2021048028A1.
Vibration & shocks
The goal of this test is to prove the robustness of the design under vibration and shocks.
Vibration and shock profiles from the UN38.3 certifications are used.
Acceptance criteria are:
- UN38.3 criteria (no leaks, no venting, no disintegration, no rupture and no fire, no voltage drop after the test).
- No damage to the pack, contact plates, or cells.
- Vibration: No disconnection of a cell.
- Shocks: No extended disconnection of a cell.
Test setup
The battery is tightened to the vibration pot. Tests are done in the 3 axes, as per UN38.3.
The battery has the correct number of cells, but all except 1P are isolated using kapton tape.
This allows to detect disconnection of a single cell, through the resulting opening of the circuit.
This setup is reproduced with different sets of P, so that each cell position has been tested.
The output of the battery is connected to:
- An oscilloscope, with a falling edge trigger meant to detect disconnections
- An electronic load, configured at 4A (hence 4A / parallel-cell, which is representative of a 16A discharge on a 4P battery).
Conclusions
Vibration tests showed no damage to any element, nor any cell disconnection. This indicates that the PCB springs correctly held their function, ensuring the absence of arcing, heating, or additional wear-and-tear to the electrical contact (tin ball).
Shock tests showed no damage to any element, and only a brief (<1ms) disconnection.
Shock tests were run at 150G, which is not a shock profile representative of a normal use, but instead destined for safety tests where the typical acceptance criteria are lack of damage / fire.
Such results conform to the defined acceptance criteria.
A shock test, with a brief disconnection. Click to view the video.
Gouach battery shock test
https://youtube.com/shorts/RQr2T5sLA48?feature=share
Measuring contact resistance
The goal of this test is to measure the electrical resistance of the contact between the cell poles and the contact plates.
Test setup
All tests are realized on a special 10s1p contact plate assembly with representative Tin Balls, using power cells (equivalent to Sony/Murata VTC6).
The 10s1p test contact plates are equipped with exposed copper pads on the outer faces, which allow to measure the impedance of each cell + tin ball contacts when the assembly is complete.
Conclusions
The test shows an average contact resistance of per pair of tin ball.
When extrapolating to a full battery pack, it corresponds to up to ~6.5W losses on a 13s3p at 25A (~0.5% of the total output power)
How much current can go through each contact
The goal of this test is to characterise the high current discharge behaviour of the battery / Gouach contact plates and determine the maximum current that can be drawn for different durations.
Test setup
Three thermocouples are installed on a 10s1p battery, using power cells (equivalent to Sony/Murata VTC6).
The thermocouples are installed on:
- a cell
- its associated tin ball
- its associated PCB fuse.
The test is stopped when a cell temperature is above 60°C or when BMS temperature is above 100 °C.
Conclusions
In these tests, the battery has always hit the defined cell safety limit (60°C) before the tin ball could hit a melting temperature (>230°C) or the cell fuse got triggered.
Thermal rises shown below indicate that tin balls and fuses are not limiting factors for high current discharging, up to at least 30A / cell.
Certifications
This table presents the tests that are mandatory for each battery certification:
Test | UN38.3 | IEC 62281 | UL 1642 | IEC 62133 | UL2271 |
Purpose | Transportation | Transportation | Cell integrity | Cell & pack integrity | Batteries in LEV |
Region | US | International | US | International | US |
Level | Cell & pack | Cell & pack | Cell | Cell & pack | Battery |
Self-declaration possible? | ✅ | ||||
External short circuit | ✅ | ✅ | ✅ | ✅ | ✅ |
Abnormal charging | ✅ | ✅ | ✅ | ✅ | |
Imbalance charging | ✅ | ||||
Abusive charging | ✅ | ||||
Overdischarge | ✅ | ||||
Forced discharge | ✅ | ✅ | ✅ | ✅ | |
Crush | ✅ | ✅ | ✅ | ||
Impact | ✅ | ✅ | ✅ | ||
Shock | ✅ | ✅ | ✅ | ✅ | |
Vibration | ✅ | ✅ | ✅ | ✅ | ✅ |
Heating | ✅ | ✅ | |||
Thermal cycling | ✅ | ✅ | ✅ | ✅ | ✅ |
Altitude simulation | ✅ | ✅ | ✅ | ||
Fire exposure | ✅ | ||||
Drop | ✅ | ✅ | ✅ | ||
Continuous low-rate charging | ✅ | ||||
Internal short circuit | ✅ | ✅ | |||
Immersion | ✅ | ||||
Water exposure | ✅ | ||||
Label permanence | ✅ | ||||
Dielectric voltage withstand | ✅ | ||||
Isolation resistance | ✅ | ||||
General temperature | ✅ | ||||
Continuity | ✅ | ||||
Failure of cooling/thermal stability system | ✅ | ||||
Roll over | ✅ | ||||
Strain relief (cords) | ✅ | ||||
Handle loading | ✅ | ||||
Rotation | ✅ |
Batteries using the Gouach design (and Gouach BMS) have received the following certifications. Reports related to one of such batteries are linked for reference.