[moved] Li-ion battery Protection Circuit

[btahir90]

Hello!

I am planing to make a high voltage battery from 3.6V Lithium ion batteries.
The problem is i want to protect individual cell from over voltage protection. Thinking to use Zener diode but its two lossy and will create short circuit effect.
I want some circuit that allows battery only that current which keeps it in voltage limits, I mean circuit detects voltage and limit current flowing towards battery.

 

[Audioguru]

Re: Li-ion battery Protection Circuit

You need to buy a balanced lithium battery charger or make its circuit with a balanced lithium battery charger IC. Then each cell has its voltage monitored by the circuit and has its charging adjusted for it.
The battery charger circuit also must monitor the total voltage and disconnect the load when it drops below about 3.2V per cell.

 

[btahir90]

Re: Li-ion battery Protection Circuit

Thanks for reply but i am looking for a customized solution and basic scheme of required design is in below image

 

[Audioguru]

Your circuit is not a lithium battery charger. Instead it is a simple regulated voltage power supply that will explode a lithium battery.

The Battery University.com explains that a Lithium battery charger needs:
1) The battery voltage is measured then if it is lower than about 3.2V per cell then it tries charging at a very low current. If the voltage does not rise then it stops charging and raises an alarm.
2) Measure battery temperature and stop charging if the battery is getting hot.
3) Constant current.
4) Each cell must have its voltage limited to 4.20V.
5) When the charging current drops to 3% of the capacity rating of the cells the charger must be disconnected.

A battery charger IC designed to charge a Lithium battery has these things.

 

[btahir90]

Can you share an IC model no which is most common one? I am planing to connect several cells in series may be up to 120 cells will these IC work there? these ICs monitor individual cells or series of cells?

 

[BradtheRad]

several cells in series may be up to 120 cells

This may work for a time, as long as all the cells are new. However someday one of the cells will start to decline. It will discharge more quickly than the other cells, or else it will not take a charge. It will be driven into reverse polarity by the stronger cells. No one can predict what that cell will do then.

Li-ion batteries require extreme care, or else you risk them starting a fire/ explosion. News reports tell of this happening, for instance, in commercially manufactured laptop computers.

 

[Audioguru]

Each Lithium cell must be monitored separately because it becomes chemically unstable (a fire and explosion hazard) if its charge exceeds 4.20V.
120 cells in series will be fully charged to 504DCV! Charger ICs are made by almost every large semiconductor manufacturer to charge only a few cells in series.

I have never built a charger for a Lithium battery because all my products using a Lithium battery came with a charger.

 

[btahir90]

To be in safer limit i will charge each cell up to 4V and discharge at 3V. It will reduce danger and enhance life. All i need is protection circuit for each string which is reliable. Charger will cutoff at 480V and load will cutoff at 360V but for individual string i need some circuits.

 

[KlausST]

Hi,

With series connection of cells, the weakest cell will determine overall lifetime.
And the weakest cell will have the most stressing charge / discharge cycles, because it has the largest voltage range.

To avoid this you urgently need a balancing circuit.

You may also review your idea. Maybe it is more robust and practical to lower the count of cells in series, but connect them in parallel.
Then you will need a step up / boost circuit.

Klaus

 

[Audioguru]

To be in safer limit i will charge each cell up to 4V and discharge at 3V. It will reduce danger and enhance life. All i need is protection circuit for each string which is reliable. Charger will cutoff at 480V and load will cutoff at 360V but for individual string i need some circuits.

A lithium cell charged to only 4.0V is nowhere near fully charged. It is about half of a full charge.
You do not want a "protection circuit" that some Lithium batteries have. Instead you want a proper Lithium charger circuit.
You cannot simply charge an entire string of battery cells in series. Instead you must balance the charging for each cell separately.
3V is too low for a Lithium cell discharge voltage because then the cell will be damaged and the weakest cell in the string will be less.

Look at Lithium battery charger IC circuits on the datasheets from manufacturers.

 

[btahir90]

Thanks to all but using an ic will not resolve my problem as i am connecting high no of cells in series and parallel.
here is another try please give your feed back about this.

For over voltage i am bypassing the cell as my current is limited via current control PWM circuit so no short circuit will occur and cells which are going full will be eliminated from string one by one. Micro controller will measure voltage and currents to implement CC/CV charging function. I forgot to insert temperature measurement but it will be implemented. For individual cell discharging i am not sure how to control.
Please advice.

 

[KlausST]

Hi,

Your circuit now maybe is a protection against over charging....But it doesn't protect against over discharging.

Klaus

 

[Audioguru]

Your circuit also does not measure the battery voltage before charging, and if it is too low then your circuit fails to do the things to prevent a Lithium fire or explosion that a battery charger IC does, and I said before. I guess you did not go to The Battery University.com and read about it and I guess you did not look at the datasheet of a Lithium battery charger IC.

 

[btahir90]

ok i will read all detail at battery university and will also look at lithium ion charger iC. Thanks for your advice.