Wiring 2 LiPo chargers in series?
Moderators: adafruit_support_bill, adafruit

Wiring 2 LiPo chargers in series?

by webrender on Tue Dec 04, 2012 1:23 pm

Hi all, I'm working on a portable raspberry pi project which I would like to be battery powered. I would like to keep the batteries in the system, such that when AC power is plugged in the batteries are being charged and the pi is powered, and when unplugged the batteries keep the system powered.

I have 2x 3.7v batteries. My plan was going to be to buy two of these chargers:

http://www.adafruit.com/products/280

and to hook one of them up to each battery. So that takes care of most of the equation. My problem comes when I get to wiring the batteries in series - I need to provide 7.4v to power my display. Just wiring the batteries in series would be no problem, but I'm not just wiring the batteries together - I've got the outputs from the battery chargers to work with. Is it possible to wire two of these chargers up in series, so that they provide a combined voltage? Is there another chip or part I'd need to facilitate this?

Don't have a ton of electronics/power experience so any advice is appreciated.
webrender
 
Posts: 5
Joined: Fri Nov 30, 2012 3:28 am

Re: Wiring 2 LiPo chargers in series?

by adafruit_support_bill on Tue Dec 04, 2012 1:47 pm

Charging that way is entirely possible if all the chargers do not share the same ground (e.g. via the same power supply). It is not really a good idea to use the battery if one or more cells are at different charge levels. Multi-cell chargers are designed to monitor and balance the charge level of all the cells in series.
User avatar
adafruit_support_bill
 
Posts: 29712
Joined: Sat Feb 07, 2009 9:11 am

Re: Wiring 2 LiPo chargers in series?

by webrender on Tue Dec 04, 2012 3:32 pm

is there anyone who can link me to an example of these mythical multi-cell chargers? I've been looking around for something that will charge the batteries, but I havent had much luck. The one thing I've found is this:

http://www.microchip.com/wwwproducts/De ... e=en543956
Image

Which looks promising with one glaring fault: this appears to only be a battery charger. I am unsure if I could use this circuit to charge the battery, and also power the device, at the same time.
webrender
 
Posts: 5
Joined: Fri Nov 30, 2012 3:28 am

Re: Wiring 2 LiPo chargers in series?

by adafruit_support_bill on Tue Dec 04, 2012 4:44 pm

Which looks promising with one glaring fault: this appears to only be a battery charger. I am unsure if I could use this circuit to charge the battery, and also power the device, at the same time.

That is the way our LiPo chargers work (except for the solar version). The Load is connected in parallel with the batteries.
User avatar
adafruit_support_bill
 
Posts: 29712
Joined: Sat Feb 07, 2009 9:11 am

Re: Wiring 2 LiPo chargers in series?

by webrender on Tue Dec 04, 2012 6:17 pm

Thanks for the quick responses, adafruit_support. Unfortunately, I'm still a little lost as to the best way to wire up my project. I understand that the load is connected in parallel with the batteries, but your posts raise 2 questions:

1) you mentioned that the chargers should not share the same ground. does that mean that it would not be possible to supply power to both chargers with one wall adapter?

2) The batteries being charged by the LiPo chargers are 3.7v - but I need to run the batteries in series so that i can get >6v power. What is the safest way to do this? Do I wire the power directly to the batteries, rather than the charger? Do I wire the opposite terminals of the chargers together, like batteries in series? My gut tells me that both of these methods aren't the proper way to wire it up.

At this point, I've only purchased the batteries and none of the charging/power equipment. If anyone has an alternative way to solve my problem it's completely welcome: It must be 1 plug used for charging, it must supply over 6v of power (or use a step up), and it must be able to charge while running.
webrender
 
Posts: 5
Joined: Fri Nov 30, 2012 3:28 am

Re: Wiring 2 LiPo chargers in series?

by adafruit_support_bill on Tue Dec 04, 2012 6:36 pm

1) Yes. If they used the same wall adapter for power, they would have a common ground and you couldn't connect them in series.
2) The circuit you posted appears to be designed for that purpose. Connect he charger as shown and connect the load in parallel with the batteries.
User avatar
adafruit_support_bill
 
Posts: 29712
Joined: Sat Feb 07, 2009 9:11 am

Re: Wiring 2 LiPo chargers in series?

by Cerin on Tue Jan 29, 2013 11:34 am

adafruit_support wrote:1) Yes. If they used the same wall adapter for power, they would have a common ground and you couldn't connect them in series.
2) The circuit you posted appears to be designed for that purpose. Connect he charger as shown and connect the load in parallel with the batteries.


This is unfortunate. I'm also looking for a 6-7V rechargable lipo supply, and was thinking of just buying two of those boards and wiring them in series, but since you've said this isn't possible, and you have no other products, I'll look elsewhere.
Cerin
 
Posts: 3
Joined: Fri Jan 18, 2013 9:28 am