Back to back AAA cell whim, bad idea?

[flippineck]

I have a laser spirit level which takes two AAA cells. they slide lengthways into the side of the level, one positive end first, one negative end first. the battery compartment cover has a sheet of copper which connects the two exposed cell ends together, so that the cells end up in series once installed.

It occurred to me that with a 7 year old lad wandering round, I might be sensible to avoid leaving a laser device around that could be easily turned on and stared into.

so, I took out one cell and turned it round so that both cells had the same polarity end facing the cover. I can't recall if I had the + or - ends both sticking out.

figured, it might prevent an inadvertent retinal burn whilst still leaving the level ready for use at the drop of a hat, with the only inconvenience being, having to reconfigure the cells before use.

at this stage I thought, hell, what you doing messing around, just stick the darn thing in the shed & lock the door, problem solved, no need to worry

It did start me thinking though.. what actually would happen if one did that and then left the device lying around for say 6 months, with the device's 'on' switch in either position?

simple thinking, two back to back fresh cells of the same type would cancel out, no voltage across the resulting battery, no current flow, the cells would just sit there and do nothing, as if they were entirely disconnected. they'd maybe discharge slowly at the normal self-discharge rate but otherwise would remain as ready for action as if they had been left in the unopened packet.

but i bet it's not as simple as that.. I mean if one cell were slightly more highly charged than the other then you'd have some small voltage being applied to the device, not enough to turn it fully on maybe, but perhaps enough to allow a nonsignificant flow of current.

is that likely to be a situation that would be self extinguishing as the more highly charged 'winning' cell discharged more rapidly than the other? or would it go the other way with one cell doing some kind of runaway self-flattening?

Could you end up damaging the device doing this, I guess the worst you could end up with would potentially be 1.5V the wrong way round across a circuit expecting 3V in the other direction (maybe for an extended time)

Is it a recipe for a leak in the battery compartment?

Thoughts?

 

[(*steve*)]

Yeah, at worst the batteries are in parallel.

As long as both batteries have a similar voltage, the circuit should not see anything that would damage it.

However, if the device uses the centre tap between the cells, some part of the circuit will have a negative voltage applied to it.

On that laser, if it's a class 1 laser, your blink reflex will save you from any damage.

 

[flippineck]

Cheers Steve. I reread the manual & saw it actually does say (in the disclaimers section) that if one looked into the beam & had a failure of one's blink reflex there could be a problem.

My lad loves 'Power Rangers' as do his friends, I had horrible visions of what might happen!

 

[ChosunOne]

Just now saw this thread and decided I'd throw in my 2 cents:

Assuming alkaline cells, with the 2 cells trying to push current in opposite directions, and assuming there are no barriers to current flow in the direction of the highest-voltage cell (e.g., a protective diode to guard against putting batteries in backwards), then what will likely happen is that one cell will have slightly higher voltage (tenths/hundredths of a volt?) and will push a very small current, a few mA at most, through the device--unlikely to have any effect on the circuitry.

And then what will happen in a very short time is that the current will stop; because the two cells' voltages will equalize exactly, as the stronger cell charges the weaker cell until they reach equilibrium; and all current flow will cease.

All battery cells, including primary cells, will recharge at least a smidgen when current is run through them in reverse. I used to play with dry cells to see how much of the original capacity I could get them to recharge, and how many cycles I could squeeze out of them. It's a long story, but the knowledge actually came in handy once when we couldn't replace a 2-cell ("BC" size) carbon-zinc battery for over a week and had to recharge the existing battery until then. It was a complicated mess--we used Telco line voltage (48 VDC, pulsed) to recharge it. The system held up with no problems until we could get the replacement.