custom li-ion pack charging

[manhackman]

First of all, I should let you know I am looking for a cost effective way of doing this, and I have absolutely no exp. with battery charging and circuit design... But I'm handy, have done IC projects from kits.
I would like to make a custom battery pack from surplus LI-ION batteries wired together to produce a greater voltage and Ah.  The batteries are 7.2 volts, 2000 mAh, and I'll probably put them in sieries and parallel to get 21.6v 4000 mAh (6 bateries).  Then DC-DC converters to get my desired voltages... 12v and 20v (or transitors?)
The batteries are quite cheap ($3 can ea.) and I've seen some universal chargers that could charge the individual batteries, but nothing that could handle the whole pack.  
Is there a product I could buy? Could I build my own charger relatively easily?or reverse engineer one?
any help/thoughts would be nice!

 

[manhackman]

I found this today!!! I think it should do the trick
https://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2393

 

[FoxyRick]

Hi,

Please don't take this as patronizing, it's meant for your safety!

Li-ion cells are EXTREMELY dangerous. I.m not being over-cautious - you could lose your life from a fire or bits of you (like fingers or eyes) from an explosion.

I have seen examples of them blowing apart a heavily-built aluminium flashlight and burning the insides of the cupboard it was in, just because it was accidentally left on with unmatched cells in it.

Note the *Lithium* - it is a highly reactive, low melting-point metal (I am also a chemist). I have jars of the stuff, stored under oil. You don't want to mess with it. In a cell it is placed in close proximity to something it really wants to react with. If this reaction goes out of control, a *lot* of energy is released very quickly.

Li-ion cells are the easiest of all cells to charge, but you must get it right.

You must make sure that the cells have not been over-discharged, otherwise they can burst and ignite. This is called "venting with flame".

The cells nearly always have a built-in protection circuit, that protects them from over charge, over discharge, overheating etc. But some battery packs may have the protection circuit in the *pack*, so the individual cells are *not* protected. You can also buy bare, unprotected cells from some sources (like surplus!), although they are not usually made available easily due to the danger.

I have given you links to some protection circuits at the end of this post, but before that I strongly advise you to read the comments about them on the CandlePower forums in this link:

http://candlepowerforums.com/vb/showthread.php?t=106242

These people like to use the highest energy-density cells they can get to run their eyeball-seering lights (me too!) and they have a lot of experienced people there. You will find a lot of info about Li-ions, safety and charging them if you search there.

Also, you must watch this video, and maybe the second one as well. It's fun 8O

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Here's a document about safety, then the protection links I promised:

http://proceedings.ndia.org/5670/Lithium_Battery-Winchester.pdf

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Best of luck with your charger, and don't burn yourself.

FoxyRick.