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schematic

simulate this circuit – Schematic created using CircuitLab

So, I have two 2000 mAh LiPo batteries that are going to be powering a GSM modem (on an arduino shield) a GPS and a motor. I considered just boosting to the motor (needs 6V), and using the batteries in parallel, but decided that it would make more sense to hook the batteries up in series and use LDOs to get down to the 5V needed for the shield and 6V needed for the motor.

The problem then arises: how do I charge the LiPo batteries back up, using only my external microcontroller and the charge controller that came with the first battery? The charge controller can charge at up to 500mA.

My gut instinct was to create a MOSFET controlled circuit to switch the batteries to a parallel configuration, and to then connect the charge controller to the batteries.

4 MOSFETS feels like overkill though. Does anyone have any better ideas?

Here are the specs for the charge controller

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    \$\begingroup\$ Watch the wire joins... \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 6 '14 at 4:48
  • \$\begingroup\$ looking at your charge controller, you have no choice but to charge those batteries on parallel. It won't do a series connection. I don't know enough about mosfets to help you there, but just a warning, two lipo batteries at 2000 mAh charged in parallel at only 500mAh charge rate will likely take well over 10 hours to charge. It depends on what the charge termination criteria is, but when it switches to the constant voltage charge phase, the charging slows right down. \$\endgroup\$ – Filek Apr 6 '14 at 6:52
  • \$\begingroup\$ Hey, I changed the schematic so that M1 is not shorted any more (oops). \$\endgroup\$ – macdonaldtomw Apr 6 '14 at 17:02
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The charge controller is described as a "Miniature Single-Cell, Fully Integrated Li-Ion, Li-Polymer Charge Management Controllers".

This means it can charge 1 (one) Li-Ion cell. If you want to charge two then buy another.

How to wire two together? It seems that the prog pin might help: -

The charge management controller can be disabled by allowing the PROG input to float.

Will this allow you to then connect the two batteries together without damaging the charger chip? It appears so but it's not absolutely clear. Possibly, by having a schottky diode connected from Vbat terminal to Vdd input power terminal this can be safely achieved. You will need to investigate this yourself.

So, if this works you can use two back-to-back MOSFETs (wired like a solid-state-relay) to link the two batteries together and then this combined battery can feed your boost circuit.

Regarding your circuit, M1 is shorted out and you need two charge controllers so I'm not attempting to analyse it further.

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  • \$\begingroup\$ I'm positive that I can charge both batteries at the same time with that charge controller, as long as they are being charged in parallel. Check this link for more info (another adafruit based single cell charger charging 2 cells): adafruit.com/blog/2013/02/28/… \$\endgroup\$ – macdonaldtomw Apr 6 '14 at 17:03
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    \$\begingroup\$ OK so what's the problem then? Just use a boost regulator to create the 6V and job done. \$\endgroup\$ – Andy aka Apr 6 '14 at 17:34
  • \$\begingroup\$ Using a boost will also allow the circuit to be operated while charging. Not always a requirement, but often nice to permit. \$\endgroup\$ – RBerteig May 6 '14 at 20:57

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