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3.7V lithium ion battery charger

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I am trying to design a lithium-ion battery charger for a single battery 3.7v, 2500mAh. I will be charging the battery using CC/CV method with a constant charge current of 0.5C or 1250mA and CV of 4.2V. I will be using an LM317 and the charging configuration from the datasheet. An Arduino will monitor the battery voltage and switch between the modes. According to my simulation, my CV mode is working, but my CC mode isn't. I am using the configuration as per CC mode 1 and my input voltage to the LM317 is 12V whereas, the regulated output voltage is 7V with a constant current of 1250mA. The output voltage of 7V is beyond the 4.2V required for the lithium battery, and this will damage it. I am thinking about using CC Mode 2 as per the attachment with a constant current of 1250mA and voltage of 3V which will increase to 4.2V as the battery charges. Would this work? I will also like to use a high pass transistor to pass the majority of the current around the LM317 because my LM317 will need a large heatsink. How can I create a circuit using a pass transistor to pass 1000mA and send 250mA through the LM317?


High Pass Transistor.JPGCC Mode 2.JPGCC Mode 1.JPG
 

Hi,

I will also like to use a high pass transistor to pass the majority of the current around the LM317 because my LM317 will need a large heatsink. How can I create a circuit using a pass transistor to pass 1000mA and send 250mA through the LM317?
Please keep in mind: The generated heat will always be the same. LM317 with / wihtou pass transistor, or any resistor solution or any other linear solution.
--> You will always need the same size of heatsink.

Abot the other: Why don´nt yo simly use a dedicated LI-Ion charger IC? It does all you need in one IC and it will work reliably.
If you use a switch mode charger then you don´t generate that much heat. Maybe you then need no heatsink at all.

--> go to a IC distributor or a charger IC manufacturer internet site to find the suitable IC for you.

Klaus
 

Hi,



Abot the other: Why don´nt yo simly use a dedicated LI-Ion charger IC? It does all you need in one IC and it will work reliably.


Klaus

It's for a school project and the circuit have to be designed without a battery charger IC.
 

Caution is called for. Li-ion are the type in news reports for causing fires and explosions.



You ought to find out what is a safe maximum voltage and safe maximum charging current for your battery.

It appears you want to have simultaneous current and voltage control. You may need to choose which one takes precedence over the other.
For a battery under 4.2v you can charge it by applying 4.2v (and wise to put a safety resistor inline). Now imagine the battery has reached 4.2V. How many Amperes can 4.2v supply push into the battery? None. A constant current control cannot come into play then. And we aren't sure if the battery is fully charged at that time.

As for charging ordinary nicads and NIMH type (nominal 1.25v), a voltage reading can go higher than 1.25v as you charge the cell (maybe as high as 2v for an aging cell). Then you stop after a certain amount of time. Eventually its voltage settles to 1.25-1.45v.

Try experimenting with nicads or nimh first, since they can tolerate abuse better than Li-on type. Your home-brew charge circuit should be designed with an aim to duplicate 'smart' chargers according to Li-ion safety practices.
 

Your specs
CC=1.25A
CV=4.2


Missing specs suggested:

- Vinput = 5V 6W min.
- CV cutoff = 10% of CC or 125mA threshold
- CV Tolerance = +1% max ( ageing accelerates rapidly for charge flow duration times spent at 4.2V and below 3V with risks increase with Dendrites developing and short circuits.) Life span can be doubled at 15% loss in capacity using CV=4V
- 85% efficiency min With 5W charging, and 5V input and 3V out, 2W dissipation or 40% loss.

(optional)
- under-overtemp sensing
- charge timer limit cutoff and fault indicator
- short circuit detection cutoff
- reverse battery detection shutdown
 

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