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Old Nov 07, 2011, 07:33 PM
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Joined Nov 2011
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Help!
Need help adjusting balancer charger circuit for 2x LiPo

Hi,

I would like to request some help with a circuit: I am currently following these 2 guides here for a netduino LED circuit powered by 2 LiPo batteries.:

Original Guide Hackerspace/Coding4fun
Thundergod Guide using netduino mini

But from what I found out so far about LiPo charging ... the solution for the charger in the circuit I currently have is not very safe and quite old (design from 2004 or something like that)

Here is the original design of the whole circuit:



From the original I need to alter the following:

- balance charger for LiPos
- add more LEDs: 50 total now (easy and already updated in the new schematic below)
- add a step down module from 7,4V to 5V: original design was used with a netduino, but i use a "netduino mini" to fit everything into a smaller space and the netduino can only handle 200mA ... so I have to route a seperate power supply to the max7219 instead of using the netduinos 5v output
- remove the charging pins (probably will simply add a small sized connector to the disk instead)

Here is a download (Eagle .sch) and an image of the whole circuit I currently have:

I hope the charge detection part can stay as it is, but I wouldn't mind if it had to go. Ideally the circuit would be able to charge and still power the LED circuit, so I could use the disc as an ambient light at home ... i guess it is possible since most cellphones work that way (still work while beeing attached to a charger).



General Data that might be relevant:

requirements:
LEDs 50x20mA = 1000mA
netduino ~80mA
max7219 ~80mA
step down module ??? afaik there is some loss here during converting

Lipo Battery I wanted to use: 2x 850mah 2c 3.7V

Oh and almost forgot to mention: I did search the forums (as well as via google) but I could not find any premade solution in form of a breakout charger or similar, only for single cell ... did find some balancers and chargers here on the forums (also read the lipo stickies) but they were 4.2V or seemed to have other usages that wouldntt fit the circuit i need ... Also I have no clue about electronics, but I learned quite a lot about general stuff in the last week

So after that long introduction: Can anyone provide or point me towards a circuit I could use please?

Any help appreciated and thanks in advance

Edit: I already got a suggestion on the netduino forums to use 2x premade single cell charger, which I will do if all else fails, but I would prefer a circuit that works while beeing charged, similar to the one in the original deisgn

Edit 2: found a balance charge on the forums and was suggest to adjust it for my circuit new schematic and eagle file below)
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Last edited by bpkdasbaum; Nov 10, 2011 at 10:30 AM.
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Old Nov 08, 2011, 05:16 AM
Southern Pride
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Haralson County GA. USA
Joined Oct 2004
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I bet yyou have no idea that there is a forum area just for such DIY electronics projects.

http://www.rcgroups.com/diy-electronics-199/

Charles
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Old Nov 08, 2011, 07:11 AM
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true, i have no idea, thought since i have a battery problem i post it in the battery forum

but thx ill check the forum section out you mentioned

if a mod reads this and my post does not fit here, please move it

edit: reported thread to be moved to the DIY section
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Old Nov 10, 2011, 10:16 AM
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Update:

Can anyone tell me if it will work like this? I was already told i need to "adjust/change the resistors divider of LM2577" in the charging circuit , no idea how to do that yet is that done by the potentionmeter? and all the other resistors can stay as they are?

eagle schematic here

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Last edited by bpkdasbaum; Nov 10, 2011 at 10:32 AM.
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Old Nov 10, 2011, 02:08 PM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
2,700 Posts
The first thing you need to understand is that 7.4V is the nominal voltage. The operating voltage range of a lipo cell goes from 4.2V when fully charged down to around 3V which is the lowest voltage they should ever be discharged to. Lipos are charged to 4.2V per cell so any lipo 2 cell balance charger would charge them just fine.

Have you built that charger circuit yet? It seems extremely complex for someone with "no clue about electronics", and, I see at least a couple problems with it. You would be much better off just buying a 2 cell lipo balance charger.
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Old Nov 10, 2011, 04:20 PM
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thx for the info about the nominal voltage

and nope, didnt build it yet, only trying to figure out how to charge my batteries ... throwing ideas around and so on ... and the charger is a copy paste from another thread here (altered it from 3 modules down to 2), i asked the guys involved in the making of it, they said it could be used for my circuit.

another scource actually discoureged me from using that circuit though, since it is rather complex.

I was also told to go the IC route. There are supposed to be easy usable ICs for lipo dual cell charging. Guess I have to find one, any recommendations are welcomed
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Old Nov 10, 2011, 11:03 PM
Stuart
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UK, Cardiff
Joined Dec 2008
3,166 Posts
"Also I have no clue about electronics"

I would stay well away from a DIY Lipo charger then.

Lipo charging needs great care to reduce the substantial risk of serious fires if it goes wrong.

Very definetly NOT a beginners project.

Where are you based, no where near me I hope ?
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