Power Supplies, multiple output, negative included

[skimask]

Just a quick thought for you ski... properly charging that Battery-Pack is going to be more tricky than you think because the level of discharge across the entire pack isn't going to be constant... because of those taps you're going to be discharging some cells at a greater rate than others. So, if you're going to be charging the entire pack connected in situ, a lot of cells will reach full charge perhaps a very long time before others.

That's what I'm talking about. But as you implied back in post #2, ground is ground, wherever it is, it's ground, or at least it can be. So, I whip up a 36v battery pack, 40 cell to get 36v at end-of-charge .9v/cell, 36v total, with 4 regulators:
-28v -> 0v (i.e. ground), -6v -> +22v, 0v -> +28v, +5v -> 33v.

The unit, during testing, isn't connected to anything else besides the tester while be tested, so it shouldn't matter where ground is, as long as the right potentials are there to drive things, even the chassis is completely electrically isolated from the circuitry. Overall current draw is going to be far less than 3 amps, once in a great while peaking near 3 amps.
So using linear regulators to keep parts count and cost down, I regulate out the 33v direct from the batt pack, regulate 28v off the 33v, regulate 22v off the 28v...instead of putting all of the regulators in parallel (40v regulated to 22v using a linear is a lot of wasted heat), use pass transistors on all of them and I should still have enough overhead V+ to run them all, although the 33v might be pushing it at full load with a near end-of-charge batt pack.

Charging however, as you eluded to, will be another story. 40 cells would max out at around 58v, probably need something like 64v to push any current. Some of the cells will undoubtedly charge before others. So, either I go with a very slow charge (less than 1/10C) to keep out-gassing down, or I split the pack into smaller segments during charge. I've charged a 16 cell pack successfully before using a simple 'linear' (not sure what you'd call it) charger that charged the whole pack at once, started off with higher current, as the pack charged up, the current dropped to something like 1/50C. Probably not healthy for the battery pack, but it's still alive and kicking after 3 years.

Or I just skip the battery pack, and go with a unit that runs off the mains.

More thinking required...


PS.: Mel, and BTW, that fix involving the 2 swapped wires, was done while I was at Fairford, practically in your 'backyard'. And I couldn't have done it without a book from Mike M. at nikamelectronics, and PIC guy.