Why does a multi-cell lithium battery need a battery management system, while a multi-cell lead-acid battery uses just a simple charge controller?

Explanation:A battery management system is essentially the “brain” of a battery pack; it measures and reports crucial information for the operation of the battery and also also protects the battery from damage in a wide range of operating conditions.  

Let's take a look at a battery management system to familiarize ourselves with its components.  

The single most important function that a battery management system performs is cell protection.  

Lithium ion battery cells have two critical design issues; if you overcharge them you can damage them and cause overheating and even explosion or flame so it's important to have a battery management system to provide overvoltage protection .

Lithium ion cells can also be damaged if they're discharged below a certain threshold, approximately 5 percent of total capacity. If the cells are discharged below this threshold their capacity can become permanently reduced.  

To ensure a battery's charge doesn't go above or below its limits, a battery management system has a safeguard device called a dedicated Lithium-ion protector  

Every battery protection circuit has two electronic switches called "MOSFETs." MOSFETs are semiconductors used to switch electronic signals on or off in a circuit.