Battery Pack - 3300mAh 18650 Lithium Ion Battery Cells

Construct a battery pack, charge, test, discharge, and charge for storage (optional).

Supplies

1. 2 - 3300mAh 18650 Lithium ion battery cells
2. 1 - 2S battery management system (BMS) board
3. 1 - pair (male/female) XT-30 battery connectors
4. 1 - piece pure nickel battery tab
5. 2 - 14 gauge battery lead wire (black/red)
6. 1 - piece of shrink wrap tube 

Tools

1. 1 - battery charger (I used the SkyRC BD250 Battery Discharger/Analyzer)
2. 1 - multimeter
3. 1 - spot welder
4. 1 - soldering iron
5. 1 - heat gun (if using shrink wrap)

1. Step 1: Check your batteries with a multimeter to make sure the batteries have the same voltage.
2. Step 2: Place the two batteries side by side (in series) with the positive and negative opposite of each other.

Image 1: Batteries taped in series

1. Step 3: Tape or glue them side by side to secure them.
2. Step 4: Solder nickel tabs to the BMS board and the red and black 14 gauge wires to the BMS P+ and P- terminals (red + and black -) and connect to the XT-30 connector.

Figure 1: BMS manufacturer's connection schematic

1. Step 5: Using a spot welder, attach the connectors. In my case, I used the 2S BMS for the protection benefits. Ensure that when welding the connections on one end of the BMS, the positive end of the battery connects to the part that says B+ and the negative end connects to B-.

Image 2: Batteries in series next to BMS

1. Step 6: Perform a visual inspection to make sure all connections are correct and secure. 
2. B+ of BMS connects to positive end of the battery
3. B- of BMS connects to negative end of the battery
4. Red wire connects to P+ terminal
5. Black wire connects to P- terminal
6. Red and black wires connect to XT-30
7. P side of BMS connects to other end of the batteries
8. Step 7: Use a multimeter to check the voltage again by placing the ends in the two connection points of the XT-30. Voltage should be double the nominal voltage value of a singular cell.

Image 3: Battery pack complete

1. Step 1: Connect the battery pack to a charger and charge to full. In my case, I used the iMax B6 charger.
2. Step 2: Use the SkyRC BD250 discharger to safely pack. Log discharge time and voltage drop data to be analyzed.

Image 4: Battery discharge information on SkyRC

1. Step 1: Upload data from discharge to Matlab.
2. Step 2: Use Matlab to calculate average voltage, average current, and total capacity.

Results during discharge:

Current: 6.6A

Voltage: 8.4V - 6.6V

1. The starting to ending voltage.

Capacity: 1235mAh

1. The amount of charge the battery delivered during discharging.

Power: 43.92W

1. Starts at 43.92W but decreases slightly over time due to voltage drop.

Discharge time: 11 minutes 17 seconds

From Matlab code:

Average Voltage: 6.96 V

Average Current: 6.57 A

Total Capacity: 61880.87 mAh = 61.88 Ah

Average Time Interval: 0.5s

VoltageInitial = 8.1010 V

VoltageFinal = 6.6260 V

CurrentInitial = 0.0300 A = 30mA

CurrentFinal = 6.6130 A = 6613mA

If choosing to store the battery, discharge the battery completely and charge to a storage-safe voltage (3.7-3.8V).

1. When storing the battery, ensure it is stored in a cool, dry place, with low humidity. 
2. It is also important to check the battery voltage about every few months if storing it for more than 6 months. Ensure it is still within about a 3.7-3.8V range, if below 3.5V, recharge the battery.
3. When analyzing the battery, if the battery capacity you get during discharge is less than the value the manufacturer provided, then it may mean your battery had degraded or it wasn’t fully charged before the test.

Other Information:

1. Lithium-Ion batteries - The numbers 18650 for the lithium ion battery cells stand for the dimensions. The first two numbers (18) represent the diameter in millimeters. The last three numbers (650) represent the length of the cell in millimeters. For the storage-safe voltage, it is important to place it at 3.7-3.8VDC per cell so that while in storage there is no stress on the battery chemistry or degradation overtime. If it is fully charged or near the minimum, it can degrade and experience chemical stress.
2. Max potential: 4.25-4.35VDC per cell
3. Min potential: 2.3-3VDC per cell
4. Storage-safe voltage: 3.7-3.8VDC per cell
5. Max continuous current: 8A
6. Max burst current: 16A
7. Battery Management System (BMS) - BMS is a management system that helps keep your battery functioning properly and within its limits.