What Is Inside a Citycoco Battery?

We all know they are lithium ion batteries, but how are they put together?

When we open up the case we get a blue box with a couple of wires sticking out of it.

The blue casing is a form of heat shrink wrap use to encase the battery and the BMS (battery management system). The two wires sticking out are the positive (red) and neutral (black). Depending on the battery they may have an additional fuse (e.g. 60A) attached and will usually have an adapter to connect them to the connector type on the case.

When we remove the wrapping we find the battery and box encased in a yellow plastic box.

Inside the yellow box are the individual lithium cells. This battery has 16 strings of 7 cells. When you connect cells in series (i.e. one after another) you add the current of the cells together, whilst when you connect the cells in parallel you increase the voltage. Each cell has a nominal voltage of 3.6V so this battery pack has a voltage of 16 x 3.6V = 57.6V. In use the cells will vary from 4.2V (fully charged) to ~2.9V leading the battery pack as a whole to vary from 67.2V to ~46.4V (in practice the BMS will kick in to protect the battery).

The BMS sits at the end of the battery and is individually connected to each string. This allows it to monitor the voltage of each string and keep them in sync.