Multiple output isolated DC/DC supply

Hi, I need to charge lithium batteries (3 to 12 cell packs) and
prevent individual cells from exceeding the desired voltage (4.2v
each). This is most commonly done in hobby circles using a charger
that takes the overall pack up to the desired total voltage (eg: 12.6v
for 3 cells) and a separate balancer circuit to try to keep each cell
the same.

What I would prefer is a charger than can charge each cell
individually to 4.2v and eliminate the need for the balancer circuit.
Single cell lithium charging chips are readily available, and if I
were to supply each of these from separate 5v mains power supplies I
could daisy chain any number together and achieve my objectives. An
example of what I mean is shown in this post if I'm not clear: http://
www.rcgroups.com/forums/showpost.php?p=6049417&postcount=4

However, I would like to do this from a single 12v car battery source.
So I need a step-down design with up to 12 isolated outputs (~5V
each). I need at least 3A each, preferably more. Does anyone know of a
elegant way of achieving this please?
Thanks, David.

 

Usually, that for isolating AC. Very difficult to isolate DC in this
situation.

Alternative is to switch cells in/out of charger or to switch by-pass
loads.

 

Off the top of my head I think thats 180w. You could probably get
that with one of those Topswithc GX controllers from Power
Integrations. 12 secondary windings would track reasonably well.

Although I think you said a car battery for the input.

Then I would probably just use a homebrew open loop half-bridge to get
the isolated supplies.

then drive some battery charger chips. I think TI has benchmark or
use a simple switcher and homebrew the output side too.

many ways to skin this cat.

 

why use separate supplies? just use one high current source....

 

The 12 cell pack is my biggest problem. Its not practical to parallel
them for charging purposes.
You've given me a few ideas to pursue.
Thank you, David.

 

I'm fast becoming an expert on these chargers. I didn't know what a
Li-ion battery pack was two weeks ago. This stuff is great fun. Have
you tried those A123 systems batteries. I have wallpaper on my work
machine of a guy doing a burnout on a 9 second electric bike powered
by Li-ion.

Check it out.

http://www.electrifyingtimes.com/a123/KillaCycle.html

regards,
NEO

 

Yes I'm using A123's and LiPo's. The A123's are more forgiving but
still need balancing if taken to their boundary voltages. Regards,
David.