Strange AA NiMH battery issue

Equipment Radios & Power
1

My GPS uses 2x AA cells and I use Ikea LADDA 2450 NiMH cells. They are 6 years old and work brilliantly. One set will power the GPS for about 24hrs before needing a recharge.

Last weekend I had recharged 4x AA cells and put them in the SOTA box along with the GPS. I didn’t need the GPS for Friday’s walk so it stayed in the car. It was below zero rising to about 6C during the day. Overnight the temperature dropped to -5C. Bottles of water in the car did not freeze so the temperature in the car did not drop below zero.

Saturday morning I powered up the GPS whilst getting ready mainly to record the route. When I came to set off there was no display. I repowered the GPS and it worked for about 1min before shutting down. I swapped to the other batteries and stood about for 5mins or so and the GPS was fine. This happened once before in December 2024, GPS would not stayed powered and swapping cells fixed the issue.

My thought was one or both cells cannot provide current when cold. I placed both cells in a LED torch that draws 45mA and it stayed powered for 39hrs in the shack (14-20C). After this use the cell voltages are 1.23 and 1.24V. Maybe it was the cold. I placed both cells in the deep freeze (-16C) for 1hr and tried and the GPS powered up and stayed powered. The GPS pulls around 100-120mA without backlight.

A new pack is only £6 and I have 6 more LADDA cells that work so I may just drop these into the recycling centre. It probably makes more sense not to have dodgy cells in the SOTA box.

My main question is just what is failing? The cells work fine now at 45 or 100mA load and fine when damn cold. But this is twice batteries left in the car overnight in Winter have failed. That’s once too often!

Any suggestions as to what is happening?

2

I’m not using NiMH cells anymore, having changed to Alkaline in the GPS and LiFePO4 batteries for my TX-500 or KX3 when doing SOTA. Even so I carry the GPS and the LiFePO4 batteries under my anorak in the pockets of my fleece jacket to keep them warm

3

Below 0C the internal resistance increases rapidly. Normal performance returns when the temperature rises.

4

So the batteries which failed when they had been to near 0C didn’t work. I think internal resistance could be a failure mode. But other identical batteries stored in the same conditions work fine. Then the batteries that failed when cooled well below 0C (towards -16C) still work. There is something else I think.

5

I activated Place Fell in full winter conditions, I estimated -10C but it might have been lower. The 817 started off normally but faded quickly and died on me. Later in the climbing hut the batteries recovered and worked normally. These were 2500mAh NiMH. Whatever happens takes time, I doubt that a solution of KOH freezes at such temperatures, it is some form of gradual passivation over time. Whatever the mechanism, it is something that is recognised and NiMH batteries that work reliably below 0C are available.