Let's Make Robots!

Can I recharge a shared battery?

I just started messing around with a little RC boat (yet another way to emulate and test things for the bigger boat project I am part of) and it is getting the Fio that had been the brain in my AGV before I upgraded it. Anyway, the boat is small enough that to limit size and weight, I am going with one of those scary ol' LiPo batteries. One nice feature of the Fio is that it has an on board LiPo charger that will charge a 7.4v. I would like to take advantage of that, as that means as long as I make sure the mini USB port on the Fio is accessible, I can fit the battery in a hard to get at spot. A concern I have is that the battery will also be wired to the motor and to a 5v VR though. The motor will be MOSFET switched, but I am not sure I can do that with the VR. The VR is just for the steering servo right now, so I could use a resistor series and/or diodes if that helps with the charging question, which is - can I charge the battery with it being shared?

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You have been my high tech guardian angel lately. I believed a post elsewhere about it working with 7.4s but after reading your response I went back and looked at the SFE listing and both batteries listed under related products are indeed single cell 3.7v. I guess I will have to make sure a connection can easily be accessed even if I leave the battery tucked away.

This may actually push me toward a different uC that is lying around doing nothing - a 5v 32u4 breakout. Then I would just have 5v to worry about for system and servo and I have an RF receiver that might work well for it that requires 5v (have trouble with getting older Nordics to work with current Arduino code anyway). When Ipiece together personal projects with cast offs, I have to be flexible...

I do what I can =)

32u4 you say? If only out of interest, take a look at this application note from Atmel.

 

http://ladyada.net/products/atmega32u4breakout/

And I am cheating and using the TeensyDuino libraries. You just have to hunt down the hex file after compiling (it's in a temp folder) and run an AVRDUDE command line and do a little pin mapping. There is some cool stuff for it to appear as various devies on the USB, but I really don't have a current plan. I got it emulate a pure ATMega ship in breadboarding.

It's hard to give a definitive answer without drawing up a circuit diagram. There are two main issues you have to address in this case:
• The charging voltage will be a good couple of volts higher than your battery's nominal voltage - make sure any components exposed to this are able to handle it.
• Your USB will only supply so much current to the charging circuit (100mA in this case), so you don't want any of that going to waste or your battery will charge slowly, or not be able to charge completely. Ensure that the circuits powered by the battery are able to remain in a low-power/standby/idle state while charging, or failing that isolate them with a relay/transistor/switch.

Looking into it further, the Fio is equipped with a single MAX1555-based charger, which can only charge a single LiPo cell. Your 7.4V pack has two cells in series however, which the MAX1555 cannot charge by itself. Either you'll need to supplement the Fio's onboard charger, or you'll need to add a discrete charger that can handle two cells.