How to prevent NiMH batteries from leaking?

Question

I have a low-power device (2mA max), wherein my NiMH batteries tend to leak. Following the advice of How do I prevent batteries from leaking?, I now have Low Self-Discharge batteries; however, they still leak when in this device. Why can this be?

• I suspected this might be due to overdischarging before being put in the charger; however, the battery that leaked today measured 1.16V when taken out, so this is not why.

• I have other ideas of why this can be (e.g. I have to hit the device to get the battery to pop out; maybe the reason is purely mechanical?); however, I don't want to lose a battery for every idea that turns out to be wrong.

What are other reasons NiMH batteries can leak in such a scenario? I would like some opinions, so I can assess which of those are likely, and test the most likely ones first.

p.s., I have all sorts of battery test equipment; so if some idea of yours would require something, you can assume I have it.

Answer 1

NiMH cells should not leak unless abused. The following can damage them, causing leakage.

• Soldering, which overheats the polymer seal.
• Operation at very high or low temperatures, which causes expansion of the electrolyte and melts the seal or makes it less flexible and more brittle.
• Operation in harsh environments, such as water, which causes damage through electrolysis or corrosion.
• Overcharging. Both charging current and voltage must be limited.
• Complete discharge, which can cause dendrite growth and cell swelling.
• Excessive discharge current, which causes internal heating.
• Placing cells in series and discharging too much, which causes weaker cells to reverse-charge.
• Excessive mechanical stress, such as a very tight-fitting battery box.

A likely suspect is charging rate. From this manufacturer's specifications:

• Slow charge at 0.1C (1/10 cell current rating) for 16 hours to a maximum of 1.46 V at 23 °C.
• Trickle charge at 0.03C to 0.05C indefinitely if this is a modern NiMH cell which self-catalyzes H2 / O2 recombination.

N.B. Battery position is irrelevant, since almost all the electrolyte is absorbed in the separator -- there should be none sloshing around.

Answer 2

NiCd and NiMH cells have a vent. If the cell is place in a position where the internal electrolyte level is above the vent, it may leak even when not charging or discharged (charging is actually more likely to result in a leak, as it generates gas in the cell). If you can, reposition the device so the positive terminals of both cells are up, which will tend to keep the electrolyte away from the vent (which is at the positive end).

If, like most devices, the cells are in opposite directions in the device, you may need to switch to rechargeable alkalines, which have less gas production (and, as a bonus, higher capacity on a charge) -- or replace the device with one that has a built-in rechargeable battery, which ought to be positioned to prevent leakage.