Charging two 3.7v Li-Ion Cells

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.

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

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.

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...

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.

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'

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.

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 to batteries in this circuit

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