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I know that dead batteries can leak and the alkaline ones are specially corrosive.

Alkaline batteries self discharge even when not installed in any device.

I've already opened a four battery pack with one battery leaking, but this situation is by far more uncommon than finding leaking batteries in forgotten devices.

Nowadays some appliances have standby power that possibly in a long time can drain the batteries and consequently provoke a leakage.

But why alkaline batteries with the same age, brand and charge installed in simple flashlights with a mechanical switch seem to leak even when the switch is off and the flashlight is not used by a long time?

(This is a suspicion i have. I don't have enough data so i may be wrong)

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  • \$\begingroup\$ Some modern alkalines have electrode coatings that only break down to fully expose them in initial use. In broad terms the chemistry changes and the seals age but it would be interesting what more specific may be known. \$\endgroup\$ – Chris Stratton Mar 8 '19 at 18:20
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    \$\begingroup\$ I'm going to attest this is confirmation bias. Put 2 AA's in a sealed plastic box not connected, and connected in a series battery pack not connected to anything (or connected, then disconnected, they'll corrode the same. \$\endgroup\$ – yhyrcanus Mar 8 '19 at 19:08
  • \$\begingroup\$ I have drawer with used alkaline batteries with several years and there's no sign of leaking. In other hand a have several flashlights damaged by batteries en the conditions described in the question. \$\endgroup\$ – AmssmA Mar 8 '19 at 19:24
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    \$\begingroup\$ You're right i don't have enough data. It's just a suspicion i have. That's why i asked the question. \$\endgroup\$ – AmssmA Mar 8 '19 at 20:24
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    \$\begingroup\$ Dave Jones of the EEVblog on youtube has just started an experiment where he took 7 different brands of AA size alkaline cells and subjected half of them to a load and the other half unloaded. He will be observing them for signs of leakage over probably the next several months. \$\endgroup\$ – Barry Mar 8 '19 at 22:18
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Some switches themselves are to blame. Many cheap slide switches are simply two metal plates that slide over each other. Sometimes one is shaped with a curve to add a bit of spring action.

This type of switch is rife with friction, and sometimes there is dirt build-up and such. The device may turn "off" (a motor stops spinning) but there can actually still be leakage current because the switch plates didn't completely separate. They also may have completely separated, but over time the curved plate may flex due to vibration, temperature.

Moisture/humidity is another culprit that can affect these switches because they are not themselves enclosed.

Diagram of cheap sliding switch

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  • \$\begingroup\$ I think this is a plausible situation for the damaged flashlights. Answer accepted! \$\endgroup\$ – AmssmA Mar 9 '19 at 0:13
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Batteries installed in devices are almost certainly partially discharged. Nobody puts fresh batteries in a device and doesn't immediately use it, at least a little bit.

Any discharge at all produces hydrogen gas, which raises the internal pressure of the cell. This can only accelerate any leakage.

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This is a suspicion i have. I don't have enough data so i may be wrong.

Alkaline batteries leak due to self discharge. This is well known. Therefore, it follows that dead cells do not self discharge, because they are already depleted. There is nothing left to discharge. Therefore dead cells don't leak. Of the 2 dozen discharged cells (1.1V - 1.25V) lying around for 6 months or so, none of them have leaked. Leaked cells are obviously dead, because they leaked, created a short and discharged completely, not the other way around.

If alkaline cells leak due to self discharge, it follows that self discharge will be maximum when cells are new, and when in a humid atmosphere. Hot weather also increases cell voltage. Additionally, hot weather can allow greater relative humidity to be achieved, the opposite of cold weather where humidity decreases. Therefore hot weather is more likely to increase self discharge by increasing cell voltage and by lowering self discharge resistance via increased humidity. Indeed, for me, new cells seem to leak regularly in their packages in summer in a high humidity environment. If caught in initial stages one can see leaked cells still showing 1.5V.

It follows that new cells in low drain or infrequently used devices have a greater chance of leaking in summer. How to reduce this? For low drain or infrequently used devices, use alkalines only in cold climate (not very efficient as cell capacity drops in cold). In hot humid climate, use up alkalines in a couple of weeks, e.g. in high drain devices (not efficient as alkaline voltage drop is high during high drain). Also possible in humid climate, discharge in high drain device and then use in low drain device (should work theoretically without leaking, but I have not tested). Obviously in humid climate, buy new cells only when required.

Obvious way to reduce headache: switch to rechargeables.

PS: zinc chloride carbon will not eat away the zinc container after depletion since, if the zinc container is thick enough, the HCl produced will be converted to ZnCl, and there will be no rupture or leakage. Usually a good manufacturer will calculate and design the zinc canister properly. I have never seen one spill in last few decades, mainly due to the additional plastic casing. They also do not require valves, unlike alkalines, so are truly sealed. The acid (ZnCl and HCl produced) is also much milder, if at all they do leak. The cells are sufficient for low drain devices like wall clocks, weighing machines, multimeters... and also cheap, even the top quality panasonic ones, and last a long time with no ill effects. Also, due to the construction, gap between +ve and -ve terminals is much more, so humidity affects them much less.

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