Once there were three little lithium cells..

"dbvanhorn"

• Errr - never thought of using just one cell ?

A bigger ( x3) capacity one.

...... Phil

Sorry, I forgot to mention that one additional constraint. :)

Yes, and no we can't do that either. The space fits three 123A cells, and not four. They have to be side-by-side, not in line, and since it's three, there's no way to do series/parallel combinations either.

"dbvanhorn"

Sorry, I forgot to mention that one additional constraint. :)

Yes, and no we can't do that either. The space fits three 123A cells, and not four. They have to be side-by-side, not in line, and since it's three, there's no way to do series/parallel combinations either.

** Then you are one a stupid ass who has painted himself into a corner.

Your problem - pal.

BTW :

only the biggest wankers use the " Royal plural".

... Phil

If you're turning the board, how about a little supervisory circuit? Three cells, three MOSFETs, one supervisory circuit that checks to see that _all_ the cells are in correctly, then turns on _all_ the MOSFETs.

You probably can't do this in a way that consumes no power, if so your 'on' switch will have to talk to the supervisory circuit which will have to be separately powered, probably from the same cells.

If you're really clever and your board environment is right, you may even be able to have the board come alive enough to pop a BIT fault (assuming you do any BIT at all) but not turn on the current-consuming stuff.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

How about something like this:

.------o-----------> V+ bus | + | --- | - '-----' (once per cell) | - - - Q1 | | ^ | '-------' '-+------> V- bus

That draws no power and at least prevents reversed-cell damage.

For a status signal, wire-AND some FETs:

Vdd B3+ >--. -+- B2+ >--. | | .------. | | | | + | | | .-. --- | | | | | R1 - '-----' '-----' '-----' | | | B1 - - - - - - - - - '-' | | ^ | | ^ | | ^ | | '-------' '-+--------+ '-+--------' '-+----+-----> Power fault+ Q2 Q3 Q4

If you really, REALLY wanted to you could probably conjure up a full-wave bridge variation of this general technique. I'm too lazy.

Cheers, James Arthur

Funny ,that. If I plug in the batteries on my camera the wrong way round it doesn't work, but then there's a big picture that shows me the correct way and I follow that. How easy is that?

"TT_Man"

** Well, the thing is as you must be aware, some folk just *never* seem to see the big picture.

..... Phil

I agree, for most devices this would be an acceptable solution.

This device needs to be easy to use under extreme stress.

There are a lot of reasons for why things are the way they are, and it's a long story.

I'm going to try to work out the fullwave bridge solution for a single cell this weekend. If that will work when the load has power on it from the other cells, then that's the solution that I need.

If I can't find a way to do that, then I'll have to do the simple version, but that means that if one cell is reversed, then the runtime will be about half normal which is a problem, but at least it's survivable.

Right, and that's my fallback position, but having two out of the three cells operating puts me in a bad spot. I can't easily tell that there's a problem based on battery voltage, and with the increased load on two cells, I'd expect about half-normal run time. That's a problem.

The ideal solution might be to do custom battery holders, but we're trying not to add any more mold tooling costs to the thing. We knew the cell holders were reversible, but we haven't found any that insert from the side that aren't. In the series configuration I had that problem solved.

I hear ya, it's making my head hurt too.. Maybe I'm looking at it the wrong way.

I solved that already. You snipped it.

James Arthur

"Tim Wescott"

** Psssssttt - all the OP needs to do it fit PTCs ( ie polyswitches) in series with each cell.

Litelfuse have a range of SMD versions.

But don't tell him I said so.

...... Phil

Have you talked to any battery manufacturers? Maybe they have a battery with a mechanical tab that will prevent improper insertion. You can't be the first one to have this issue.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what\'s the reason?
For if it prosper, none dare call it treason."

"Follow The Money"  ;-P

Can you add a circuit to sense the reversed polatity on cell(s) and create an error indication?

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what\'s the reason?
For if it prosper, none dare call it treason."

"Follow The Money"  ;-P

I have to work with "off the shelf" cells. If a special battery would work for the users, I would just put the cells in a keyed enclosure and call it a day. Then I would also be using prismatic cells to fill the space better. But special cells are ruled out.

Thought about it, but it doesn't solve the problem of getting half the life with 1 cell backwards, and PTCs don't work too well in high and low temperature environments. The current to keep the polyswitch fired in the reversed cell case is also an issue, but not a huge one.

Interesting. Can you point me to some docs on that?

Depending on which side of the situation you're on, it may well assist you in having a bad day. :)

Looks like this has been solved already, I thought it might have been.

I need to case it out with some real components, but this is more or less what I had in mind.

Google "cr-123 explosion"

I've seen a number of individual reports--you can peruse this one, for example:

These seem to happen with mis-matched or over-discharged cells, AFAICT, the result of a strong cell overpowering a weaker one.

A lot of the reports are about cells that ignite spontaneously hours or days after last use.

Those cells (above) were in series; parallel would at least preclude cell reversal and might be better, but me? I wouldn't do it. Nope.

Cheers, James Arthur

OTOH, here a single (overdischarged) CR-123 cell "vents with flame":

"You know, the most interesting thing is it happened without warning. The battery was in the flashlight for a couple of months now. I did not use this light much [...]. Yesterday night I took the [flashlight] for a walk and was a bit shocked that the battery is almost dead. Returning home i put in on a desk and switched on to see how much juice was left in it. After a few minutes the light started flickering. It was not hot at all.

I leaned over it to see what is going on, and this was when [t]he battery exploded. The light jumped off the desk, and piece of it hit my head. Interestingly the battery was left on the desk hissing and sparkling. Actually it burnt a hole into the tablecloth and left a mark in the wood itself."

--

Now there's a pocket-warmer !

--James Arthur