Problems with the circuit

[Unmesh Sawant]

Hi friends,
Iwas just working on battery charging and power supply projects which gave me an idea of building a Mobile/ cellphone battery charger with auto cut off. this charger will auto matically cut off the power supply when the battery is full/100%. I have seen circuits on google but main problem is the are for greater voltages e.g 12v , 24v . I was thinking of making a 3.7v charger whose max voltage or cut off voltage will 4.3 v. Please suggest me about how to go on this project . Should i need a microcontroller or an analoge cicruit will work. wating for your kind replies most intersting topics ( which i seem to be).

Thanks in advance

Unmesh

 

[ArticCynda]

Hi Unmesh,

It would depend on what kind of charging circuitry you're using. In case of a dedicated Li-Ion charger IC, this kind of auto cut-off is generally already included.

If not, I'd recommend using an opamp as comparator with positive feedback (hysteresis) to detect that the cell voltage is 4.3V and then use this output to turn off the charger circuitry.

Good luck!

 

[D.A.(Tony)Stewart]

There are 3 stages

1. CC depending on rating battery from C/10 to 10C or more which is the current rate for 20h use.
2. CV such as 4.3V/cell
3. Shutdown.. when the current drops below x mA such as 20mA

How you implement it is up to you. USing a smart charger IC is easiest.

 

[Audioguru]

A rechargeable Lithium battery might catch on fire or explode if you over-charge it to 4.3V. Its maximum voltage is 4.20V.
When the Lithium battery voltage reaches 4.20V then it is only about 70% fully charged. Continue charging at 4.20V until the charging current drops to a low amount then the cell is fully charged.
Go to www.batteryuniversity.com and read all about it.

Of course this will not charge a phone because the charger circuit is built inside the phone and uses a 5V input.

 

[ArticCynda]

Go to www.batteryuniversity.com and read all about it.

Please be aware that www.batteryuniversity.com is not a reliable source: its articles are written by only a few people, it does not cite its sources, and is not peer reviewed.

 

[Unmesh Sawant]

I respect all your views about it i thank you all for replies, but is it not possible to built any circuit. It would be great as it will reduce our work and will benifit all of us. Can you please send any shematic about it. And i would like to remind you that its a lithium- ion battery rated at 3.8v and its max charging is 4.2 and not 4.3 sorry for that mistake. Also i dindt understand how their is an existing cutoff circuit inside the charger by(artic cynda).

Thanks in advance
Unmesh

 

[ArticCynda]

Also i dindt understand how their is an existing cutoff circuit inside the charger by(artic cynda).

Like Audioguru mentioned, Li Ion and Li Polymer batteries have the annoying habit to burst into flames when they're continuously overcharged. To avoid this, integrated battery chargers (IC's) will always feature a cut-off circuit that turns off the charge current after the charging process has been completed.

 

[Unmesh Sawant]

Hi friends sorry i was out,was not able to look at your sugesstions. Thank you cynda but do you know any circuit related to that , i mean i am a newbie so if you know any please post and not onh cynda but others too audioguru plz show me some directions how about to go for it.

Thanks in advance
Unmesh

 

[ArticCynda]

(...) do you know any circuit related to that , i mean i am a newbie so if you know any please post (...)

The topology of the circuit would depend on whether you take the integrated approach (with a dedicated charger IC) or if you want to implement it discretely (with constant current source and comparator to detect full charge state). Do you have any microcontroller or other configurable device in your design that can be used as control logic?