The lead/acid batteries of my system wear out much faster than expected. I really want to get rid of them now and switch to a more efficient technology. I have decided to build a Lithium/Ion battery pack made of 1000 or more 18650 cells. This type of cells is used in Tesla cars and Tesla's Powerwall.

These cells have a 3.7 Volts nominal voltage and have a typical capacity of 2000 mAH. One cell weighs about 43 grams.

The maximum capacity for these cells is near to 3000 mAH (high end Panasonic). By the way, Cells announced for much more than 3500 mAH (current maximal available capacity) are FAKES.

They cost between 0.5 € (used cells bought en Ebay) to several € for high end Panasonic cells.

A 1000 cell pack would have following characteristics:

• Energy: 3.7 * 2 * 1000 = 7400 WH = 7.4 KWH
• Weight: 50 * 1000 = 50 000 g = 50 Kg
• Cost (2 € unit cost): 1000 * 2 = 2000 €
• Life expectancy: 5 years

For comparison my current lead/acid pack has following data:

• Energy: 6 KWH (max)
• Weight: 60 * 8 = 480 Kg
• Cost 8 * 200 = 1600 €
• Life expectancy: 2 years - :-(

If I can get healthy cells for a unit cost of 2 € or less, it is really worth the effort.

My plan is to evaluate first the technology with a "mockup" made of about 100 used cells.

For this I build a test rig in order to check rapidly the health of each battery. I will also design a monitoring board capable of measuring the voltage and temperature of each group of cells. The BMS boards will send these data over isolated CAN buses or BlueTooth to the global system.

See my 110 kWh Powerbucket project.