I posted this in sci.electronics.design a few days ago with no feedback so I thought I'd try again here, sorry for the length...
After a career as a electronic design engineer and software author I retired a while back and recently took up R/C model flying. I've become frustrated with the available battery chargers I can find for my hobby and decided to look at designing my own universal solution. There are some units around providing some of these abilities, but none that I know of that provide them all. Anyway I can have some fun designing & building it during the wet 'n windy part of winter.
And yes I'm well aware that I can't build something for the price I could buy
- if it were available!
So basically I'm looking for design/topology suggestions and possibly some component selection advice from the mass of new available devices out there. I'd also like to find some better/lower parts count solution than the brute force one of just building several identical programmable current sources in hardware.
Requirements:-
- Powered from 12v leisure or car battery for field use or a 13.8v mains supply for domestic use.
- Capable of handling varied cell chemistry such as NiCd,NiMh,Pb,LiPo.
- Providing six programmable current sources capable of 100mA to 2A using step-down/buck technology only.
- One programmable current source 0.5A - 8A for 1 to 15 cells (say max
- Microprocessor controlled voltage & current monitoring/logging and setup via an LCD display.
- Smart enough to provide timed or automatic charging (delta peak etc) control for those cell chemistries that allow it.
- Battery capacity determination on-demand by charge/discharge cycling so you can tell if you should buy new batteries.
A PIC or AVR with perhaps 20k code space could supply the smarts to handle it. I'm not familiar enough yet with the range to make a sensible selection.
I suppose I could just use a bunch of the National SMPS regulator chips set up as current sources for the 6 channels. But then I have to provide some way of programming the charge current as well which may be harder than just monitoring it (which I have to do anyway unless I have accurate I sources) and making micro on-the-fly pwm adjustments. I even considered just using the pwm outputs of one of the micro variants to drive discrete switchers, and changing the effective charge current with a fast loop in the micro. There could be complications in providing fast enough s/c protection though. I haven't really thought that through.
Having a micro there could offer a lot of flexibility in handling different types of cell and could provide convenient automated charging so the batteries always ready during the flying season.
If I get anywhere with this, I think I'll put up the circuit and source code for others and perhaps arrange for a supplier to provide kits or assembled units for those who don't want to build. I've spoken to a number of folk who have similar needs to my own so there is interest out there, but I'm not looking to make any money out of it.
So grateful for any suggestions