Objective:
The performance of a battery is determined by its worst-performing cell.
“Testing is the only way to ensure that we have a well-built, balanced, and reliable battery pack. An outlier cell inside a module will be detected at the module level. Without individual cell testing, an outlier cell could find its way into our pack and cause a sudden capacity drop during competition. ” [2]
Context:
put our packs/modules/cells through <100 cycles; “The major effects (sudden capacity loss) are most likely to appear after 100 cycles of the pack, however, minor effects will always be present” [2]
our pack will be assembled for 1-2 months
we can expect a normal distribution for every metric (IR, self-discharge, capacity)
Background Information:
OEM Specs:
It is important to inform our test procedure from the OEM specs for the cells we are planning to use:
Metrics:
Capacity
“A capacity drop in any one of our modules will affect the whole pack. If one cell in one module drops by 10% capacity due to an uncaught cell parameter outlier (IR or capacity), then we lose (3.5Ah * 10% * 3.7 * 36 = 47Wh) 47Wh of energy in our pack. That single cell will then also start to degrade quicker and the module will have to be replaced. Going by the efficiencies of other winning solar cars, 47Wh could add an additional 1-2 Km of range to the car (and avoid headaches from increases in variation after many cycles)”. [2]
DC IR / Impedance
“Internal resistance mismatch between cells can lead to sudden capacity losses and a decrease in overall cycle life of around 40%” [3] The 4-point test is virtually instantaneous but needs to be conducted 1 at a time. Test using Multimeter & Gamry Battery Holder. Additionally, observe temperature during charge and discharge. temperature ∝ internal resistance. Impedance testing is not worth it for us: The Difference Between Battery Resistance & Impedance Testing (eepowersolutions.com); Li-Ion BMS - White Paper - Resistance vs impedance (liionbms.com).
Self Discharge
“Over long periods of time (such as sitting idle in the bay for a month), this self-discharge current will cause an imbalance in SoC of the cells and lead to decreased pack capacity. Over the course of a week-long competition, Self-Discharge will not cause any significant difference in SoC.” [2]. Check OCV. Manufacturers tend to supply cells at a 30% SoC; OCV is a good proxy for checking self-discharge. Charge all the batteries and leave them idle for a couple of weeks (if we can afford the time) then check the voltage with a multimeter.
Thermals
The temperature variation (unless caused by a cell with initially high IR) will be very small and thus its effects are negligible. [2] IR strongly correlates, thus we don’t need to test this. Set up is time-consuming. If we really want we can set up thermistors or a heat gun on the charging/discharging batteries to detect any outliers
Procedure:
Prototyping (OCV & Impedance)
Pick up a cell from the packaging
Number it with Sharpie
Place in the Gamry cell holder with a Keysight 34410a
Measure the OCV of the cell
Collect the IR via the Keysight 34410a
Add data to the designated row: MSXV_celltracking
Place the cell in a box labelled ‘Tested’
Production (Capacity) [DRAFT]
Place cells into [parallel cell charger(s)]
Charge cells at 1.45A (well below the standard charge spec [5])
Observe the thermal performance of batteries via thermistors/heat gun [optional]
Once fully charged, allow the cells to rest for 10 mins (recommended per spec sheet [5])
Commence discharge on [parallel cell charger(s)] at 1.70A
Once fully discharged, record values in the following MSXV_celltracking
Allow the cells to rest for 20 mins
Charge cells at 1A (well below the standard charge spec [5])
Observe the thermal performance of batteries via thermistors/heat gun
Once fully charged, allow the cells to rest for 4+ hours for the temperature to normalize temperature
Place all batteries in a box until module/pack assembley - when placing them in your module, check ocv and make sure it hasn’t dropped to much (self-discharge check), if discrepancies: record in MSXV_celltracking
References:
[1] Sample Cell Testing Procedure - Mechanical - Confluence (atlassian.net)
[2] Individual Cell Testing Evaluation - Electrical - Confluence (atlassian.net)
[3] http://web.mit.edu/bazant/www/papers/pdf/Gogoana_2013_J_Power_Sources.pdf
[4] Separating GOOD 18650 cells from BAD ones - Battery talk #2 - YouTube