Lithium Ion Cell Charging With 5 volts DC

Thread Starter

bkhan10000

Joined Jan 18, 2017
31
Hey guys,

I am trying to charge a single cell lithium ion battery with a five volts phone charge power supply. The problem is 5 volts is too high to charge the battery and I am trying to make a simple switched mode circuit to reduce to voltage to get a desired amperage (in this case 200mA). I am using an arduino to PWM to a logic level mosfet to control current through the cell with the 5 volts. Is this something that is workable and safe? My battery seems to be charging fine and is not seeing any degradation. Using voltage measurement and a current sense resistor, the microcontroller PWM is varied until the battery reaches 4.2 volts (the maximum voltage of a lithium-ion cell) while maintaining the desired 200mA. Am I missing something to make this a safe circuit? I know every time the mosfet is switched on, 5 volts is technically going through the cell but the average voltage and amperage are at safe values. Is this okay/safe?

-Khan
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

That's probably not a good idea. They make chips that run at 5v that charge 4.2v Li-ion cells.

At least put a series resistor in there to limit peak current unless your sense resistor is of adequate size. However, the ripple voltage peak should not go above 4.25v or you risk damage to the cell.

Another idea would be to use a resistor capacitor filter followed by another resistor. That would smooth out the peaks and also limit peak current to the cell.

The peak current will be:
I=(5-Vbatt)/Rs

where Rs is the current sense resistor value in Ohms.
 

AlbertHall

Joined Jun 4, 2014
12,343
That's probably not a good idea. They make chips that run at 5v that charge 4.2v Li-ion cells.

At least put a series resistor in there to limit peak current unless your sense resistor is of adequate size. However, the ripple voltage peak should not go above 4.25v or you risk damage to the cell.

Another idea would be to use a resistor capacitor filter followed by another resistor. That would smooth out the peaks and also limit peak current to the cell.
Agree.
 

MrAl

Joined Jun 17, 2014
11,389
Hi again,

I have a couple of those cheap $2.00 chargers and they work but they dont put out much current even though they are said to put out 400ma or so. It takes a good 10 hours to charge a cell.

For my good charger i use a buck switching regulator circuit with current limit. That naturally puts out a fairly clean DC for charging. The heart is one of the Simple Switcher IC's so is fairly simple to build but you have to add your own current limit circuit.
 

ian field

Joined Oct 27, 2012
6,536
Hey guys,

I am trying to charge a single cell lithium ion battery with a five volts phone charge power supply. The problem is 5 volts is too high to charge the battery and I am trying to make a simple switched mode circuit to reduce to voltage to get a desired amperage (in this case 200mA). I am using an arduino to PWM to a logic level mosfet to control current through the cell with the 5 volts. Is this something that is workable and safe? My battery seems to be charging fine and is not seeing any degradation. Using voltage measurement and a current sense resistor, the microcontroller PWM is varied until the battery reaches 4.2 volts (the maximum voltage of a lithium-ion cell) while maintaining the desired 200mA. Am I missing something to make this a safe circuit? I know every time the mosfet is switched on, 5 volts is technically going through the cell but the average voltage and amperage are at safe values. Is this okay/safe?

A trick I've used in the past; borrow the charge control circuit from an E-cigarette. The small ones done up to look just like a cigarette are the cheaper ones and usually not too expensive to sacrifice.

-Khan
 

ian field

Joined Oct 27, 2012
6,536
You need that bit as well - its best to use the right charging adaptor for the E-cig you borrowed the charge control circuit out of.

The charging adaptor is basically just a current limiting resistor and nothing to stop the cell overcharging.

They usually also have a LED indicator - a transistor senses current draw and connects the red junction of a bi-colour LED - that has a lower Vf and cuts off the green junction.
 
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