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My setup is simple: I have this bluetooth module with a built in single-cell Lithium ion/polymer battery charger. I have this double cell LiPo battery and I also have an arduino nano that I will have to supply with power.

The bluetooth module runs fine on 3.7volts so that would be fine with a single cell battery but the arduino needs 7volts to operate.

So basically how can I charge both the cells without adding any more major components? (Or how can I power this setup?)

Any help is appreciated as I am a beginner (über noob).

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  • \$\begingroup\$ It would be helpful to add a sketch or diagram to better illustrate your setup. \$\endgroup\$ – Adam Head Apr 7 '14 at 21:18
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So basically how can I charge both the cells without adding any more major components?

You severely underestimate the complexity of two-cell LiPo charging. A specialised IC designed for two-cell Lipo charging is recommended, as there is a complex sensing and 'balancing' process that needs to happen during charging to ensure both cells are changed at the same rate/voltage, else problems can occur.

A possible IC for charging 1,2, or 3 Cell lipo batteries is the Texas Instruments BQ24133 and similar devices. They have a large range of battery managment ICs, check it out.

arduino needs 7volts to operate.

Well that is not entirely true - it needs 7 volts because it has a crappy cheap linear regulator on the Vin input jack. It has a large drop-out margin, so needs the 2V gap between the 5V output and the 7V+ recommended input. You can bypass this with your single cell lipo by putting it directly into the VCC pin (V5+ pin). PLEASE NOTE YOU MIGHT DAMAGE YOUR ARDUINO, AND YOU CANNOT RUN THE ATMEGA328P AT 16MHZ ANYMORE!!

Basically, you will need to somehow get a 3.3V, 8Mhz version of an arduino (Like the arduino Pro [mini] with 3.3V 8Mhz setup on the board) and do what I mentioned above. Please be careful, there is no reverse protection or over-voltage protection on this pin, as it's assumed to be nicely regulated and protected by then. Don't say I didn't warn you!

(Or how can I power this setup?)

I suggest you just get a much larger capacity single cell Lipo pack and let the bluetooth module's integrated charger do it's thing. Nothing wrong with 3.7V. Also your 2-cell Lipo will need regulating down to 3.3V somewhere, so there is losses involved with this as well. You must check that if you will use a single cell lipo charged fully to 4.2V, if the system will not be damaged by this - check max ratings for all devices in their datasheets. Maybe you should look at getting a 3.3V LDO (low dropout regulator) and plug the single cell lipo into this, which will nicely drop out the system at 3.4~3.5V and remain very efficient the whole time.

I suggest a Micrel MIC5205 for that purpose I mentioned above.

There are benefits to doing high voltage (like lower input currents), for motor driving etc and avoiding regulation of those high current driven devices, but for a low-power embedded system I suggest just staying with a single cell lipo with high capacity.

good luck!

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    \$\begingroup\$ pffff... I forgot to mention, that my problem with adding additional major components was that I ran out of space. But I can put the charger where ever I want.. So yeah.. problem self-solved. However, your answer is full, well understandable and generous (suggested components) so I am grateful, and accept your answer. Thank you! \$\endgroup\$ – 19greg96 Apr 8 '14 at 16:58
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I'm gonna bet your arduino will work fine on a single cell LiPo down to 3V if you skip the regulator. Check Atmega328P datasheet. You might lose the USB though, but for a stand-alone application, it is not needed.

Check datasheet page 299. You may need to reduce operating frequency, but that is not usually a problem.

You could also use a 3.3V variant of the arduino.

If you do this, however, remember to disconnect the battery when you plug your arduino into a USB port, or else the USB +5V will go straight into the LiPo...

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