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BMS, or Battery Management System
#1
What is a BMS and what is good for?

A BMS, or Battery Management System, is an acronym of a device that in some degree or another actively do some work with your batteries or protect them. A BMS can consist of several parts but some can also be left out. 

The attributes that are common in a BMS are (All described down below)
  • Fault protection
  • Balancing
  • Monitoring

Its important to understand that not all BMS boards or system do consist of all 3 attributes and some of them only have parts of above.
The main function of a BMS is to protect the batteries from being damaged, and possibly causing a fire or explosion, resulting in property damage, or loss of life. Balancing is a way to ensure that you over longer period of time will stay in shape and not end up with the fault protection kicking in.
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#2
Common Fault Conditions:
  • Under Voltage per cell and total voltage
  • Over Voltage per cell and total voltage
  • Over Discharge / SOC (State of Charge)
  • Over Current
  • Short Circuit
  • Temperature

Under Voltage
When:  A condition where the battery voltage drops below a set parameter. Most BMS's will be default set around 3.0V/cell (for standard lithium cobalt cells). 
Action: Most BMS system disconnect the load. Some BMS system have both a load and charge port and on those they only disconnect the load port to let the system charge back up.

Over Voltage
When: A condition where the battery voltage goes above a set parameter. Most BMS's will be default set around 4.2V/cell (for standard lithium cobalt cells)
Action: Same as under voltage action but if charge port exist that port is only disconnected

Over Discharge
When: A condition similar to Under Voltage, however some batteries will bounce back enough to allow the BMS to turn the power back on. If Over Discharge is a function of the BMS, then it will monitor the number of times of bounce back, and keep track of what's happening. If it records the voltage is getting progressively lower, it will completely disconnect the battery and only when applying a charger will it reconnect
Action: Generally only exist on more expensive BMS systems where you generally keep track of the SOC by counting energy going in and out of the system. 

Over Current
When: A condition where the battery current goes above a set parameter. Depending on the BMS rating, this can be 20A, 30A, 50A, 100A. This rating is usually included in the advertisement title of the BMS  "3S/4S 100A 12V BMS Li-ion Lithium Protection Module". Its meassured by a shunt.
Action:  Disconnects either the load or charge port or both

Short Circuit: 
When: A condition kind of similar to Over Current, but is much more specialized. If a BMS does not have SC protection, it is possible to take the Pos/Neg outputs of the BMS and connect them together, and destroy the BMS. With SC protection, the BMS detects a direct short and disconnects the power before anything has a chance to be fried. This is a feature that is not common, especially on cheap BMS units. This is rare to have since most BMS rely on the normal Current protection. 

Temperature
When: Both Ebike BMS systems and BMS systems made for larger installs tend to have temperature sensors. its important to have them installed on the battery pack. The main function is to sense how the battery pack is stressed or when it is in bad heath. A battery tend to heat up when to large current is drawn or sent to a cell and this will in conjunction with its internal IR result in heat being dissipated. A cell or battery that gets hot can result in a non stop able battery fire. 
Action: It can disable charge or discharge ports to let the battery calm down.
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#3
Types of BMS

There are 2 major types of BMS. There are active and passive. These two types work very much like active/passive balancers, which you can find out more info here: Active Balancing vs Passive Balancing

For BMS units, this goes a little further. The BMS will actually trigger the balancing at set points. On cheap units, it's programmed via hardware (usually resistors). On the more expensive types they can be set in software.
An example of a more expensive BMS that does balancing, is the Batrium. It has boards that are called Longmons that have rows of resistors to burn off excess heat. This is a passive balancer, but it is actively controlled by the BMS. The Batrium can be set when to turn on/off the balancing. However, if the Watchmon (the main unit) is not on, or working, the Longmons can work independently and keep the voltages in check; this would be passive balancing and passive bms.
Cheaper BMS units sold on eBay an AliExpress are normally passive BMS units. They are triggered by a preset value and you have to work within those confines, regardless of what voltage range you would like to build.
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