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Charging two 3.7v Li-Ion Cells

Former Member

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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  • 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.

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  • 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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  • 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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  • 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.