How does a BMS work

hsors

10 mW
Joined
Mar 14, 2014
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29
Where can I find reliable and precise information on how a BMS like this one is working: https://bmsbattery.com/bmspcm/323-16s-lifepo4-bms-battery-management-system-bms-pcm.html (ideally with circuit diagram)
- What does it do during charge (it is recharging fist at CC then at CV and if yes how does it gauge when it's time to move from CC to CV; does it do charge rebalancing when a cell is giving a lower voltage than another one...)
- what does it do during discharge (why is there is need to move the battery current through a complex high power electronic circuitry ? If it's only a question of switching off the battery pack when voltage is too low, there must be simpler ways of doing it
Thanks !
 
All the BMS does is balance the cells, protect against too high/low cell voltages and too high charge/discharge current. The balancing circuits cut off the charging current and bleed down any cells which are too high when the battery is being charged. This is called top balancing. It doesn't balance when there's a certain difference in cell voltage, it only makes sure they're charged to the same voltage.

If you use a BMS that doesn't have the balancing function, it only protects against high/low voltages and high current. It will just have some mosfet switches that turn off when a limit has been exceeded. What could be simpler than that?

There is nothing that actually switches from CC to CV mode. The charger has both a current and voltage limit at the same time. At first, the battery draws the maximum current (CC phase), then when the battery hits the maximum voltage it's in the CV phase of the charge and the current gradually drops to 0. This is just based on Ohm's law, not any electronics enforcing a certain charge curve or profile.

Have a look at this charge simulator: http://www.ebikes.ca/tools/charge-simulator.html The bulk current is the current limit. The voltage limit is set based on the battery you choose (4.2V * number of series cells).
 
can i just use a boost converter from 12v to 50.4? i already have a 12v charger and the boost converter cost like 5 bucks.

btw just to make sure im right i need to supply 50.4v to the bms?
 
BShady said:
can i just use a boost converter from 12v to 50.4? i already have a 12v charger and the boost converter cost like 5 bucks.

btw just to make sure im right i need to supply 50.4v to the bms?


There are two limits you have to keep in mind.
Voltage.. your 50.4 (Or 49.8 would be a safer goal)
Current 1C or less.

What is C? If your battery is 5 amp-hours... 1C is 5 amps. half C (.5C) is 2.5amps.. and 2C is 10amps

so based off the size of your battery, you have to limit your current...

That is why we use a CC/CV power supply... that stands for Constant Current / Constant Voltage.


The way it works is simple. Your battery is low, so if you tried to raise it up to your 50.4 volts instantly, it would take a HUGE amount of current and fry something. If you have a smart BMS it will shut you off... so you cant charge.
So we limit the current to what is safe for the battery with the charger.

So we run our current limit, say that 5 amps... until the battery hits 50.4... but we're not done. if we shut off the charger the battery would drop to a lower voltage. the constant current part is done.
so we start running less current into it... keeping the voltage constant at 50.4... the current goes lower and lower until the charger is shut off.

a CC/CV power supply can do this automatically if the limits are set properly.
 
So if I charge the lipos thru a BMS with a 12v computer power supply boosted to 50.4v with a boost converter I should be fine?

On the boost converter I can limit the max current. So I can limit it to 3 amps for my 5Ah lipo, which is .6C.
 
999zip999 said:
Just get a 50.4v cc-cv charger 3a. It will shut off when full.

I have a 12s BMS. All I need is something to power it. I have a ton of old computer power supplies capable of out putting 12v 20A. Therefore I would rather spend $6 on a boost converter and use the old power supplies then spend $25 and buy a whole new power supply!
 
Im agree with Bshady

You are right.... use what you have.... it is easy for people to say "go and buy a charger capable of doing bla bla bla" .... sometimes is imposible to find such thing around town or cant wait to order or simply it is stupid to buy a power source of poor quality compared to whatever pc stuff you already have..... if can limit current to 3A and the BMS will regulate the current as other guy explained.... why then is required to go and buy another charging box??? It is a matter of making the "market /trade gears" work?
Why is that every advise "is go and buy" like if one would wash their hands in case of an accident or something happening due to my advise
 
Anyway bshady can you post link or explain what kind of inverter 12 to 50.4v are you using? If you opted for such option? And what is the max Amps it would support?
 
if you have tons

12vdc power supplies

wire in series

for desired output voltage
 
isolate grounds duh
 
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