A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area
monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.

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A battery pack built together with a battery management system with an external communication data bus is a smart battery pack. A smart battery pack must be charged by a smart battery charger.

BMS circuit is used in most of rechargable electronics including smartphone, electric cars, Electric cycle, & etc

in this instructable i made a 16.5Volts battery pack with BMS to use it for my DIY ringlight

checkout in the instructables which i have made it

https://www.instructables.com/id/Ringlight-V1/

Supplies

Parts Required

1x 4s 30A BMS battery circuit https://amzn.to/2OjBX1E

16x Lithium ion battery. salvaged from old laptop

Mains wire

1x Lterminal block

1x Buck converter

20Volts 3amps laptop power supply

A gangbox

Step 1: Grouping the Cells

Since in this project i have used old laptop batteries so not all the batteries work in the optimum level

In this project i have used 16 batteries.

4 in series and 4 in parallel

if you want more knowledge of the battery

check this link where they have explained it

https://batteryuniversity.com/learn/article/serial...

Batteries achieve the desired operating voltage by connecting several
cells in series; each cell adds its voltage potential to derive at the total terminal voltage. Parallel connection attains higher capacity by adding up the total ampere-hour (Ah).

Some packs may consist of a combination of series and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve a nominal voltage 14.4V and two in parallel to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4s2p, meaning four cells in series and two in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short.

Most battery chemistries lend themselves to series and parallel connection. It is important to use the same battery type with equal voltage and capacity (Ah) and never to mix different makes and sizes. A weaker cell would cause an imbalance. This is especially critical in a series configuration because a battery is only as strong as the weakest link in the chain. An analogy is a chain in which the links represent the cells of a battery connected in series