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USB Charging Circuit for 3x 3.1V Lithium Batteries Answered

I am trying to make reliable charging circuit using 5V(USB) as source. With it I aim to charge 3 Li-Ion batteries - small coin batteries(MS621FE, datasheet). How can I make such a circuit with:

 - light indication (LED)  while charging
 - light indication (LED) for fully charged that stops the first LED
 - protection against overcharge

There are a lot topics covering similar circuits but all of them are at least for 3.7V batteries which is dangerous for such a small battery (suggested charging voltage between 2.8V and 3.3V). Does anyone have any idea how to make such a circuit?



3 years ago

Overcharge protection could be done by using a comparator with a voltage reference on one input, and the voltage divider at the other terminal so that way when the full charge voltage is reached, a resistor divider will apply a proportionally smaller voltage to one input of the comparator, and if that smaller voltage is greater than the voltage reference, the comparator turns on or off from its previous state depending which input is connected to the Vref and divider. That can drive a small LED and also an "enable" or "disable" pin for a charging circuit.

If I am not mistaken, lithium ion batteries charge using CC (constant current) mode, at less than 1C. So take the mAH, and charge it at that same rating dividing away the H. (a 10mAH battery will need to be charged at 10mA for approx. 1H.*) Then when a the voltage nears the nominal maximum, (4.2v for standard LiPo's and lithium ions.) the charger needs to go into constant-voltage mode, and stay that way for quite a while until the current drops off to I believe 1/10C of the rating (so again, 0.1C of 10mAH is 1mA) then the charger stops feeding the battery and the battery should be done charging.

***NOTE Make sure to charge at considerably less than 1C if the LiPo voltage is less than 2.6V as internal resistance becomes really high as the cell voltage falls below 3V.

Dave from the EEVblog has some great videos discussing lithium charging, I am merely repeating what I remember him saying.

NO. These batteries are explicitly charged constant voltage, with current limiting. They are so small they would be extremely hard to charge CC. The manufacturer specifically says do not CC charge.

What types of batteries are they? Why with constant voltage, is the internal resistance so high that CV is more practical? Lithium ion batteries are charged with CC for the first half I am pretty sure, but I am at a loss as to what the batteries in the datasheet are. LiFePO
4 maybe? I do not know the way to charge those, just ions and LiPos.

Yes, the internal resistance is huge - look at the maximum discharge currents employed.

REAL "power" LiPo batteries get a CC charge, and then a CV charge, but then you need to have temperature, voltage and current monitoring.

I just noticed the battery is NOT Li-Ion but rather Manganese Silicone Lithium

With constant voltage charging set properly there is no overcharging.
The charging indication as mentioned by Steveastrouk is simple enough, like in this example:


Just adopt for your voltages.
Add a diode to indicate the USB power and all is good - second LED on means charging, second LED off means full battery.

Thank you for the fast answer. Similar suggestion is posted on the seiko site as well: http://www.sii.co.jp/en/me/battery/support/charging-circuit1/ . But I'm trying to make it with automatic overcharge protection and with the light indicators described above. This is something I can't find anywhere.

Done by constant voltage charging, it HAS automatic protection.

Put a small amplifier across the limiting resistor, when the voltage across the resistor is high, the battery is charging, when it isn't, its not. Amplify the voltage to drive your LED

Charging one small battery is pretty simple, you need a constant voltage of around 3.1V, and a current limiting resistor, so that in the worst case, you won't exceed the limit of the battery, which is only 100uA, so I'd use a 33K resistor.