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Forum Charging two 3.7v Li-Ion Cells
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Forum Thread Details
  • Replies 14 replies
  • Subscribers 291 subscribers
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  • series
  • lithium
  • 7.4
  • 3.7
  • charger
Related

Charging two 3.7v Li-Ion Cells

Former Member
Former Member over 10 years ago

So for a portable project, I will be using two lithium ion 2200mah 3.7v batteries in series to give me 7.4v. I have a USB powered 3.7v Lithium Ion charger found here https://www.adafruit.com/products/1905 . How could I charge both the batteries in parallel and then use them in series for 7.4v?

 

All help is greatly appreciated.

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  • michaelkellett
    michaelkellett over 10 years ago in reply to dougw +2
    I see some problems with this idea - if one battery is at a significantly different state of charge and thus voltage form the other (and this happens as the batteries age) then a huge current will flow…
  • Kilohercas
    Kilohercas over 10 years ago in reply to derkarhu +1
    You can also make very expensive circuit for battery charging, with CCCV push/pull 3A converters, just treat them as capacitors http://cds.linear.com/docs/en/datasheet/8052ff.pdf Super-capacitors are equivalent…
  • dougw
    dougw over 10 years ago

    You can use a double pole, double throw switch to switch from parallel to series, or you could use 4 transistors if you need to do it under digital control.

    image

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  • michaelkellett
    michaelkellett over 10 years ago in reply to dougw

    I see some problems with this idea - if one battery is at a significantly different state of charge and thus voltage form the other (and this happens as the batteries age) then a huge current will flow when the switch is set to parallel operation for charging. If the batteries have internal overcurrent protection this will be triggered - if not then some bad things may happen.

     

    There is no easy way to do the switching from series to parallel connection- how about using the batteries in parallel all the time and using a boost convertor to get the battery voltage up to the system working voltage ?

     

    MK

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  • Former Member
    Former Member over 10 years ago

    Thank you both for responding, I'm thinking I will go with Michael's idea, to charge them at 3.7v and step up the voltage within my project. Since I don't have any overcurrent protection in my batteries, It should be safer. A 7.4v switching regulator is what I plan on using.

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  • derkarhu
    derkarhu over 10 years ago in reply to Former Member

    you could use a simple charge pump voltage doubler, driving the 'top' cell... this pretty much guarantees that both cells get exactly the same end-point voltage...

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  • dougw
    dougw over 10 years ago in reply to michaelkellett

    Some good points articulated by Michael, and you definitely always need to be careful when dealing with high energy density devices like lithium batteries. I have seen some of them puff up like balloons - I'm not sure if they were charged in reverse or were over charged since I didn't connect them, but it was a most un-nerving sight to come across.

    However, if you want to push a bit beyond the traditional safety rules, I have found it hard to fast-charge 3.7V Li-Po batteries because you can apply a full 3.7 volts to a depleted battery and it won't draw more than 1 amp and even then the current rapidly drops to a half amp. So my batteries wouldn't be damaged by parallel connection, but you need to do your homework on your batteries with a current-limited supply and understand their performance before designing them into a system. For example, shorting a battery is probably much worse than connecting two in parallel and an appropriate fuse might be prudent whether you connect in parallel or not.

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  • derkarhu
    derkarhu over 10 years ago

    Normal charge voltage for a 3.7 V Li-ion cell is 4.2 V _during_ the main charge, BUT only if the cell is within a normal charge temperature, above 0°C and <+40 (not sure about the maximum, please check that), and a 'float voltage of 3.8 to 3.9 V.

     

    If you simply apply 3.7 V per cell, you will not charge the battery most efficiently, and will not get the cell(s) to their full charge state...the fully-charged state is quite a bit above the nominal 3.7 V cell voltage, which is actually the rated voltage witha load applied. Fully-charged, with no load, the Li-ion cell will be > 3.9 V 'open circuit'

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  • Former Member
    Former Member over 10 years ago

    Please note that I am using the Adafruit Li Ion 3.7v charger, which safely charges the cells. I have decided that it is safer to just charge the 3.7v cells in parallel, which I have already tested and confirmed, and then boost the voltage to what I need for my project, using a 7.4v switching regulator.

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  • derkarhu
    derkarhu over 10 years ago in reply to Former Member

    Whatever works for you. ...

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  • Kilohercas
    Kilohercas over 10 years ago in reply to derkarhu

    You can also make very expensive circuit for battery charging, with CCCV push/pull 3A converters, just treat them as capacitors image

    http://cds.linear.com/docs/en/datasheet/8052ff.pdf

     

    image

    Super-capacitors are equivalent to batteries in this circuit

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  • D_Hersey
    D_Hersey over 10 years ago

    I'm just here to underline MK.  He may be cousins with Groundskeeper Willie, but he is right here, as usual.

     

    Image result for groundskeeper willie

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