Difference between revisions of "DIY E-Bike Battery"

Revision as of 14:37, 16 November 2021

A project by Jpt4 and AVH to homebuild batteries for their personal ebikes.

Cells

120 (total) LiFePO₄ 26650 cells purchased from Battery Hookup for the $228. Specifications:

• Make/Model: K2 LFP26650E
• Size: 26650
• Voltage: 3.2
• Capacity: 3.8 amp-hours (12.16 watt-hours)
• Maximum Voltage: 3.65 (safe) 4.1 (maximum)
• Minimum Voltage: 2.5 (safe) 2.0 (minimum)
• Continuous Discharge Current: 12 amps
• Maximum Discharge Current: 28 amps for 30 seconds
• Recommended Charging Current: 3.8 amps

Packaging

The cells arrived in a tightly-fitting two-piece vacuum-formed plastic package that holds all the cells vertically in a 10x12 grid. The top of half of the package had some splits and cracks near one corner. The package, along with a desiccant pack, was in a thin plastic bag, zip-tied shut, in an exactly-fitting cardboard box.

Labels and Markings

Each cell is marked from the factory:

  Li-Ion IFpR27/65
+ K2 Energy LP26650E-3800-21 -
  3.2V 3.8AH(12.16Wh)

Also, each cell has a serial number in a QR code near the negative terminal, (usually, but not always) on the side opposite the label. The dot-matrix printer that made the codes wasn't perfectly aligned, so many cells have codes that are slightly distorted and difficult to scan. Using the Barcode Scanner app, I found the codes could be scanned more reliably sideways, or at not-quite-90-degree angles.

To make it easier for us to keep track of the cells, the first time each cell was removed from the packaging, we gave them a three-digit number corresponding to the row and column they were from in the box. 000 is from the top-left, 009 is from the top-right, 110 is from the bottom-left, and 119 is from the bottom-right. The labels are written in permanent marker on the ample blank, white space on the cell wrappers, and underlined so that there is no ambiguity which way the number is supposed to be read.

Individual Cell Measurements

Cell Testing

Out-Of-Box Voltage

When the package arrived, each cell's voltage was measured with the bench multimeter. (Also, its QR code was scanned, and its label written on.)

First Charge

A custom 5-cell holder was made from wood and brass strips (26650 cell holders are wiedly expensive , given how cheap 18650 cell holders are) and plugged in to a ToolkitRC M7, which was plugged in to the benchtop power supply. This setup was used to charge 5 cells at a time (24 times) to charge the cells to 3.60V, the maximum setting the M7 can charge LiFePO₄ cells to. Then, the cells were charged 2 at a time on a NiteCore Digicharger D2 to 3.65V.