DIY Powerwall: Cell Types
Difficulty level: Intro – Easy – Intermediate – Expert
Cells come in varied form factors. Today we will go over some of the terminology used to describe different cells.
The first main classification is cylindrical vs prismatic. The obvious distinction here is that one set are the shape of cylinders… Prismatic cells are often subdivided into prismatic / pouch cells, where the difference is the whether there is a hard or soft casing to the prismatic cell.
Prismatic cells have the advantage of being any shape you need. However, all types of prismatic cells require compression when in use to avoid delamination of the interior layers (swelling). Cylindrical cells on the other hand don’t need external pressure, making their use a bit simpler.
Prismatic cells are defined by their dimensions. There are no standard prismatic cell sizes, so just look for the dimensions listed by the manufacturer, and you will know what kind of volume you have to deal with. Cylindrical cells, on the other hand, have industry standard sizes and names associated to them.
The most common sizes are the following (but really, manufacturers can make any size they want):
14500 (AA) – 14mm wide, 50mm high. Lithium cells smaller than 18650 are normally primary cells (non rechargeable), or fraudulent listings. Although rechargeable lithium cells smaller than 18650 do exist, due to their rarity and the amount of fake products sold in that segment, we recommend being especially careful.
18650 – 18mm diameter, 65mm high; this is the standard “laptop battery” size cell.
26650 – 26mm diameter, 65mm high
2170 – also known as 21700, introduced by Telsa, it also brought on a change from the standard 5 number size to a 4 number. 21mm wide, 70mm high
4680 – 46mm diameter, 80mm high. These cells are (so far) only made by Tesla, and are “tabless” which is a new manufacturing process that allows for much higher current ratings for a given cell.
38120 – 38mm diameter, 120mm high. These cells usually have screw terminals, making them very popular to those who don’t want to spot weld. They are almost exclusively made in the LiFePO4 chemistry.
40152 – 40mm diameter, 152mm high. Like the 38120, these cells usually have screw terminals, making them very popular to those who don’t want to spot weld. They are almost exclusively made in the LiFePO4 chemistry.
40172 – 40mm diameter, 172mm high. These normally come without the screw terminals and are often used in street lamps and other low power appliances that can be run off a small solar pannel. They are almost exclusively made in the LiFePO4 chemistry.