DIY E-Bike Battery

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. Their specifications are as follows:

• 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, 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, it was given 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

K2 26650 Li-Ion Cells

Serial Number	Label	Out-Of-The-Box Voltage	Self-Discharge Voltage	Remarks
PH190223-132431	000	3.29	3.33
PH190223-132415	001	3.29	3.33
PH190223-132362	002	3.29	3.33
PH190223-132351	003	3.29	3.33
PH190223-132270	004	3.29	3.33
PH190223-132286	005	3.29	3.33
PH190223-132393	006	3.29	3.33
PH190223-132397	007	3.29	3.33
PH190223-132328	008	3.29	3.33
PH190223-132317	009	3.29	3.33
PH190223-132458	010	3.29	3.33
PH190223-132442	011	3.29	3.33
PH190223-132426	012	3.29	3.33
PH190223-132378	013	3.29	3.33
PH190223-132265	014	3.29	3.33
PH190223-132281	015	3.29	3.33
PH190223-132457	016	3.29	3.33
PH190223-132424	017	3.29	3.33
PH190223-132392	018	3.29	3.33
PH190223-132344	019	3.29	3.33
PH190223-132367	020	0.00 😭	3.21
PH190223-132350	021	3.29	3.34
PH190223-132334	022	3.29	3.34
PH190223-132233	023	3.29	3.34
PH190223-132269	024	3.29	3.34
PH190223-132462	025	3.29	3.33
PH190223-132398	026	3.19 🙁	3.32
PH190223-132414	027	3.29	3.33
PH190223-132413	028	3.29	3.33
PH190223-132333	029	3.29	3.33
PH190223-132394	030	3.29	3.33
PH190223-132361	031	3.29	3.33
PH190223-132345	032	3.29	3.33
PH190223-132329	033	3.29	3.33
PH190223-132296	034	3.29	3.33
PH190223-132280	035	3.29	3.34
PH190223-132409	036	3.29	3.33
PH190223-132425	037	3.29	3.33
PH190223-132440	038	3.29	3.33
PH190223-132360	039	3.29	3.32
PH190223-134729	040	3.29	3.33
PH190223-134648	041	3.29	3.33
PH190223-134519	042	3.29	3.33
PH120223-134644	043	3.29	3.33
PH190223-134515	044	3.29	3.33
PH190223-134691	045	3.29	3.33
PH190223-134713	046	3.29	3.33
PH190223-134628	047	3.29	3.33
PH190223-134632	048	3.29	3.33
PH190223-134760	049	3.29	3.33
PH190223-134552	050	3.29	3.33
PH190223-134712	051	3.29	3.33
PH190223-134631	052	3.29	3.33
PH190223-134708	053	3.29	3.33
PH190223-134595	054	3.29	3.33
PH190223-134755	055	3.29	3.33
PH190223-134536	056	3.29	3.33
PH190223-134692	057	3.29	3.33
PH190223-134696	058	3.29	3.33
PH190223-134599	059	3.29	3.33
PH190223-134756	060	3.29	3.38
PH190223-134675	061	3.29	3.33
PH190223-134697	062	3.29	3.33
PH190223-134584	063	3.29	3.33
PH190223-134744	064	3.29	3.33
PH190223-134596	065	3.29	3.33
PH190223-134740	066	3.29	3.33
PH190223-134643	067	3.29	3.33
PH190223-134681	068	3.29	3.33
PH190223-134568	069	3.29	3.33
PH190223-134579	070	3.29	3.38
PH190223-134739	071	3.29	3.33
PH190223-134761	072	3.29	3.33
PH190223-134680	073	3.29	3.33
PH190223-134551	074	3.29	3.33
PH190223-134676	075	3.29	3.33
PH190223-134563	076	0.65 😢	3.33
PH190223-134723	077	3.29	3.33
PH190223-134745	078	3.29	3.33
PH190223-134664	079	3.29	3.33
PH190223-134728	080	3.29	3.33
PH190223-134564	081	3.29	3.33
PH190223-134724	082	3.29	3.33
PH190223-134627	083	3.29	3.33
PH190223-132318	084	3.29	3.33
PH190223-132446	085	3.29	3.33
PH190223-132365	086	3.29	3.33
PH190223-134709	087	3.29	3.33
PH190223-134693	088	3.29	3.33
PH190223-132399	089	3.29	3.33
PH190223-134535	090	3.29	3.33
PH190223-134660	091	3.29	3.33
PH190223-134531	092	3.29	3.33
PH190223-134707	093	3.29	3.33
PH190223-132382	094	3.29	3.33
PH190223-132301	095	3.29	3.33
PH190223-132445	096	3.29	3.33
PH190223-134516	097	3.29	3.33
PH190223134757	098	3.29	3.32
PH190223-132223	099	3.29	3.40
PH190223-132441	100	3.29	3.33
PH190223-132376	101	3.29	3.32
PH190223-132349	102	3.24 🙁	3.32
PH190223-132430	103	3.29	3.36
PH190223-132297	104	3.29	3.32
PH190223-132302	105	3.29	3.33
PH190223-134521	106	3.29	3.33
PH190223-134762	107	3.29	3.33
PH190223-132410	108	3.29	3.33
PH190223-132383	109	3.29	3.33
PH190223-132312	110	3.28 😟	3.33
PH190223-132456	111	3.29	3.34
PH190223-132429	112	3.29	3.34
PH190223-132285	113	3.24 🙁	3.32
PH190223-132377	114	3.29	3.33
PH190223-132366	115	3.29	3.32
PH190223-134633	116	3.29	3.33
PH190223-134585	117	3.29	3.33
PH190223-132218	118	3.29	3.33	Slightly torn wrapper 😟
PH190223-132447	119	3.29	3.32

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 3.65V, 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 3.70V.

Self-Discharge

This unintentional test was the result of effectively shelving the project for 16 months after the cells were charged. Cells with greater self-discharge (bad) will show lower voltages; cells with voltages closer to 3.65V have low self-discharge. Voltages were measures the same way out-of-box voltage was measured.

The results of this test were very encouraging. The cells held a much more consistent voltage than they had had out of the box, even the ones that had arrived severely discharged. This bodes well for their long-term performance in a battery.