battery charging - What Circuitry is needed to charge in parallel, but power in series?

^{simulate this circuit – Schematic created using CircuitLab}

I've read several other battery posts, and didn't find the answer I was looking for. My question is, can the Diagram I've provide do what I intend it to, or am I missing something fundamental.

I have 2 x 3.7v batteries in series to power an Arduino. I also have a 3.7v charging circuit that I would like to be able to recharge the batteries with. Can I simply use two diodes to block any back voltage/current and then simply switch between connecting the second battery to the charger, or to powering the Arduino.

I don't plan to ever have the unit charging while the Arduino is on, but based on my diagram it would be powered in series while one battery was being charged, if this did happen, could it damage anything?

Thanks, please feel free to throw links at me if there is a place out there that already answered this type of question.

Answer

Try this design, it should work for what you are trying to do:

Make sure your power supply has a regulated current output & it does not exceed 4.2V.

hth, best regards!

p.s.: This small design modification will automatically switch off power to the Arduino when SW1 is switched down:

Or better yet, this design does the following: 1. power from the charger can not be applied to the cells (as shown in the diagram) 2. when the ganged-switches are switched to the down position, power to the Arduino is disabled & power from the charger is applied to both cells in parallel.