Power Supplies, multiple output, negative included

[T.Jackson]

Here I was thinking that Melanie was merely referring to a balance type charging arrangement -- whereas each cell is individually charged to ensure optimal life expectancy, by eliminating the possibility of overcharging. But don't take my word for it, I'm just a pathological liar.

[skimask]

Here I was thinking that Melanie was merely referring to a balance type charging arrangement

Yes, I know, charge balancing, always the best way to go, with an unlimited budget, loads of time, a nice looking PCB, and so on and so on...(i.e. educational environment)
Then there's the practical way of doing things (i.e. welcome to the real world)...

I'm not planning on using rechargeable li-ion batt's (which would be the obvious way to go unless I want to start semi-contained chemical reactions, i.e. fires). I've also got to think about the (and I use this term loosely) morons (read that as uneducated persons who don't know the ins and outs of various battery types) who are going to be using this tester long after I'm gone. Therefore, Ni-Cad's are out due to the memory effect (these guys won't know how to do the charge/discharge thing for max life, even if it's specifically stated in the manual).
So, I'm basically left with NiMH if I want a rechargeable setup. Memory effect is practically nonexistent, low temp performance isn't the best, negative delta-V during charge is small, offset that with a low charge rate and a timer.

I took a good hard look at the box I'm testing. There's only one component in there that's polarity sensitive...and that's dump diode across a single relay coil. That one thing ruins everything for me. Without that one diode, I could easily use N-ch MOSFETs under PIC control to self-test the whole thing for me and tell me where the fault is, if there is one. And it turns out I only need 28v. The extra 5v section is completely separate from the rest of the box. Figure I only need 33v (28v + 4v of regulator overhead with pass transistors plus some fudge factor), brings me down to a nominal 32 cell pack, 4 packs of 8 cells. The circuit can tolerate a couple volt drop at end-of-charge on the pack.

So, it's time to figure out how to get rid of all those rotary switches and replace them with banks of P- & N- channel MOSFETs and automate something...

P.S.

I'm just a pathological liar.

Don't be putting words in my mouth there sparkles.