two Li-ion 3.7V series charger from USB

Hello,
I want to make a charger from USB 5V port, to charge two series 3.7V Li-Ion batteries. What would be the standard cheapest approach? Should I buy a 2cell li-ion charger ic (MCP73213 but the price is huge) and place a step up to 7.4V? I want to make small and cheap ic. So currently I am thinking about taking two 1 cell chargers (MCP73831) and put them for each battery.
Would this be possible? Or do I need a switch between two batteries, cuz when lower battery is charged, there might be not enough potential for charging higher. Or maybe this schematic is illogical?

Or maybe this schematic is illogical?

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Absolutely. The circuit doesn't work.

To charge a two-cell battery, you need a higher input voltage (e.g. when using MCP78213) or a boost converter. Or you need to disconnect the cells and charge it individually.

A USB port might give you 100-130mA into a homebrew circuit. Specs state that it doesn't provide 500mA until a protocol called enumeration.

This charging circuit handles two 3.7V batteries in series. Each battery gets current at a 50% duty cycle. It is not too different from charging the batteries separately.



The clock can come from a 555 IC pulse generator. In fact I tried to make do with the half-bridge inside a 555, although I got a meager charge rate. So my schematic has an external half-bridge.

Li-Ion batteries

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This requires extreme caution. It's wise to limit voltage and current levels, to avoid having something go wrong, like a fire or explosion.

Removable 18650 cells can benefit of not huge price for charging :

----> **broken link removed**

If there is a spare charged pair, takes no time to recharge.

BradtheRad said:

A USB port might give you 100-130mA into a homebrew circuit. Specs state that it doesn't provide 500mA until a protocol called enumeration.

This charging circuit handles two 3.7V batteries in series. Each battery gets current at a 50% duty cycle. It is not too different from charging the batteries separately.



The clock can come from a 555 IC pulse generator. In fact I tried to make do with the half-bridge inside a 555, although I got a meager charge rate. So my schematic has an external half-bridge.



This requires extreme caution. It's wise to limit voltage and current levels, to avoid having something go wrong, like a fire or explosion.

Click to expand...

Thank you for the schematic. This looks like it should work and I am going to try it in next days. Do you have any recommendations for clock frequency? should it be like 10Hz or can be 100kHz? Also what precautions should be taken for th Li-Ion as you mention voltage and current limitation. I will use batteries with internal protection circuits, so should this be enough or not?

Externet said:

Removable 18650 cells can benefit of not huge price for charging :

----> **broken link removed**

If there is a spare charged pair, takes no time to recharge.

Click to expand...

Thank you for sharing this option, but in this case the point is to charge from USB port without removing batteries from circuit.

Adiz said:

Also what precautions should be taken for th Li-Ion as you mention voltage and current limitation. I will use batteries with internal protection circuits, so should this be enough or not?

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Li-ion are the type in news reports regarding fires and explosions. Maybe a way exists to make sure that yours have the internal protection? So it won't be hurt by a homebrew charger (provided you can verify that your batteries are not counterfeit).

Do you have any recommendations for clock frequency? should it be like 10Hz or can be 100kHz?

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Any frequency is probably suitable. To get assurance it's working, consider a frequency of 2000-5000 Hz (that is, if you can hear any sound from it at all).

If you prefer not to tap a wire to the join between the batteries, then install capacitors where the batteries are located in my schematic. Connect the ends of the battery pack across the capacitor stack. You have a voltage doubler which goes just high enough to overcome 7.4V.

Ok so for the protection I will use NCP360, this will provide the necessary protection for Li-ion batteries. I experimented a bit with circuits without protection and ended up having inflated battery that now is worthless so this lesson learned.

For the charging schematic, I need something that will show if the batteries are charged, so I need something additional. I was reading NCP360 datasheet and it shows that undervoltage and overvoltage will raise a flag, so i guess this can be as indicator for a LED to show that it needs to be charged. Next I am thinking about the situation, if one of the batteries are empty and other is full, so will this charging circuit work? I think the NCP360 should block everything for battery that is full, ant other battery will be charging, or am I wrong?

Adiz said:

Ok so for the protection I will use NCP360, this will provide the necessary protection for Li-ion batteries. I experimented a bit with circuits without protection and ended up having inflated battery that now is worthless so this lesson learned.

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You are lucky to not have had a fire or explosion. Very lucky.

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Oh, yeah, I sure hope you don't just throw it away in the trash and dispose of it properly as hazardous waste, if anyone were to puncture the protective casing they would probably be injured when it catches fire or ejects it's contents under pressure and catches on fire.

Hi,

The pdf "Battery Charging" is worth looking at: "This chapter will present charging methods, end-of-charge-detection techniques, and charger circuits for use with Nickel-Cadmium (Ni-Cd), Nickel Metal-Hydride (Ni-MH), and Lithium-Ion (Li-Ion) batteries."

It shows three different Li-Ion charger circuits, maybe you could put one of these (or similar using other Li-Ion battery charging IC manufacturer's designs) after the USB protection IC you are using, the NCP360. Li-Ion have a specific charging procedure, and level, and temperature, and so on, it's probably a tiny bit more complicated than shoving current and voltage into it "until it's full."

Pages 12 onwards of the pdf are about Li-Ion. Enjoy your project!

ads-ee said:

You are lucky to not have had a fire or explosion. Very lucky.

- - - Updated - - -

Oh, yeah, I sure hope you don't just throw it away in the trash and dispose of it properly as hazardous waste, if anyone were to puncture the protective casing they would probably be injured when it catches fire or ejects it's contents under pressure and catches on fire.

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I am aware of explosions and fire, I know what can happen with li-ion batteries. I still have to see myself what happens and why, so no need to worry about me. I have an electronics disposal place. No need to get off topic here.

d123 said:

Hi,

The pdf "Battery Charging" is worth looking at: "This chapter will present charging methods, end-of-charge-detection techniques, and charger circuits for use with Nickel-Cadmium (Ni-Cd), Nickel Metal-Hydride (Ni-MH), and Lithium-Ion (Li-Ion) batteries."

It shows three different Li-Ion charger circuits, maybe you could put one of these (or similar using other Li-Ion battery charging IC manufacturer's designs) after the USB protection IC you are using, the NCP360. Li-Ion have a specific charging procedure, and level, and temperature, and so on, it's probably a tiny bit more complicated than shoving current and voltage into it "until it's full."

Pages 12 onwards of the pdf are about Li-Ion. Enjoy your project!

Click to expand...

Thx for the pdf! I looked through it and got the idea about charging graphs. Started to look for the charging IC, it will not be safe and efficient enough only with transistor schematic.

I would never have that aliexpress VERY cheap charger in my home.