Background

As it is impossible to purchase single 18650 cells from their manufactures, we must go through a distributor instead. There are a large amount of fake or counterfeit cells on the market, so working with a legitimate supplier is paramount to ensure a safe and matched pack. In an ideal world, we would test each cell that goes into our pack for multiple charge and discharge cycles to characterize them, and build our modules based on cells with similar characteristics. However, this is not feasible in our situation, so we could either measure a random selection of the cells to get an idea of the variation, or just rely on the supplier to provide cells within tolerances.

Cell Supplier and Choice of Cells

Through research, we are currently talking to Liion Wholesale for sourcing our battery cells. As of March 2017, we have inquired about the following cells and the prices received along with any notes are listed below. In order to obtain these prices, we must agree to provide a link from our website to theirs, and social media or sticker on the car is optional according to them, but unlikely to affect the price too much. As per their FAQ on the website, large quantities are usually directly shipped from Asia so we will have to pay duties on top of shipping charges.

All prices are assuming a 1500 cell order quantity

Cell	Price (Per Cell)	Notes
Panasonic NCR18650B	$3.75 USD inc Shipping	Prices as of Feb28 are high on the market, it may go up or down depending on when we order
LG MJ1	$4 USD	Unclear if shipping is included

The NCR18650B was used in MSXI. It is also widely used by the E-Bike community for many years, so reliability should not be a huge concern. The downside of the NCR18650B is that it does not perform as well under large current draws. This however, is not a huge concern as 1C discharge amounts to ~120A for our pack. Source: e-cigarette-forum.com

The LG MJ1 is a relatively new cell to the market. It has one of the highest nominal capacity of currently avaliable 18650 batteries (3500mAh). It also has a max discharge current of 10A per cell, but only a cycle life of 400 cycles. Source: Batterybro

With a small price difference between them, it made sense to purchase a few to charge and discharge in order to get a sense of how well they live up to their claimed specifications.

Test Procedure

Test and discharge procedures are completed with an iCharger 306B charger and logged to a computer via Logview. Parameters logged are specified below, and are all written to a csv file. It is planned to then use MATLAB to process and visualize the data, deviations and distributions can also be calculated.

For the NCR18650B and the MJ1, 5 cells each were purchased from Liion Wholesale and labelled from 1 to 5 for testing.

Parameters logged:

Time
Input voltage
Battery voltage
Charge/Discharge current
mAh charged/discharged
External temperature (measured via magnetic temperature probe, accuracy unknown)
Internal temperature (internal to the charger, rises from cold start)

Test Parameters

Parameter	Value	Comments
Charge Voltage	1s Li-ion	Selected Li-ion program on the charger, modified end voltage to be 4.2V as per datasheet.
Charge Current	1.6A	Approximately 0.5C (1C = 3.3A and 3.5A)
Discharge End Voltage	3.0V	Selected based on presumed safe discharge voltage of the pack
Discharge Current	3.4A	Approximately 1C for both cells

Test Results

Battery Test Log - Log of all charges and discharges performed on the cells.

Interpolated discharge curve

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