Voltage monitor help !

[yusuf]

Hello Guys !

I want to design "voltage monitor circuit" to monitor Li-on 3.7V battery.

The specification of Li-ion 3.7V battery is :

Fully charged voltage is : 4.2 V
And low voltage is : 3.7V

So, As the battery goes low , the voltage monitor circuit should energize the relay.

Same , As the battery goes high(Full Charge) , the voltage monitor circuit should de-energize relay.

I have attached the required schematics..
But guys , what are the modification require in this circuit....

Thanks in advance....

 

[simonbramble]

The circuit you have attached will trip below a certain point and untrip that same certain point. It sounds like you need a gap between the trip and untrip point. In which case you need a window comparator. Have a look at this circuit:
https://cds.linear.com/docs/Datasheet/1042fa.pdf

Simon

 

[audioguru]

You should learn about Lithium Battery Charging at www.batteryuniversity.com .
When the voltage drops to 3.7V then there is plenty of charge remaining. Maybe 3.2V or 3.3V would be low.

Also when the charging battery voltage reaches 4.2V then the battery is only 60% to 70% charged. The charger should continue charging at 4.2V until the current drops to about 2% of the battery capacity which takes time but will fully charge the battery.

 

[yusuf]

You should learn about Lithium Battery Charging at www.batteryuniversity.com .
When the voltage drops to 3.7V then there is plenty of charge remaining. Maybe 3.2V or 3.3V would be low.

Also when the charging battery voltage reaches 4.2V then the battery is only 60% to 70% charged. The charger should continue charging at 4.2V until the current drops to about 2% of the battery capacity which takes time but will fully charge the battery.

OK.. so what modification I will have to do in the above circuit...
or is there any circuit of battery monitor...

 

[ronsimpson]

The circuit posted relies on a Zener diode. They are not the best reference. Use a TL431. It is more stable.

 

[chemelec]

According to the Li-on Batteries I have, It states: Full charge is 4.2 and Empty is 2.75 Volts.

For a Window Comparator, you need a Dual comparator, such as an LM393 is good.
Window comparator circuits are easly found on Google.

 

[audioguru]

According to the Li-on Batteries I have, It states: Full charge is 4.2 and Empty is 2.75 Volts.

But when the charging voltage reaches 4.20V then the cell is only 60% to 70% fully charged. Extra time must be spent at 4.20V until the charging current drops to 3% of the cell's capacity then it will be fully charged.

2.75V is too low for a modern (cobalt type) rechargeable Lithium cell. Then the cell might ignite when it is attempted to be charged. Most Lithium cell chargers will (should) refuse to charge a cell with a voltage below 3.0V.