Panasonic and Sanyo AA NiMH cells compared

[LeifUK]

As an owner of Panasonic and Sanyo NiMH AA cells, I thought I'd carry out some simple tests for my own benefit to see how they performed. All I did was charge the cells, leave them at room temperature for a few months, then fully discharge them in a Fenix L2D Q5 on turbo mode. I then compared with the runtime for freshly charged cells. In each case I used cells that had already had quite a few charge/discharge cycles ensuring they had been broken in.

Here are the results:

Panasonic AA 2600mAh:
Fully charged 15 January 2010.
Discharged 4 April 2010 at 16:00, dim at 17:03, very dim at 17:13.
There was considerable heat build up and a significant internal pressure increase, with the rubber switch cover expanding outwards, preventing the torch from tail standing.
Recharged and discharged. Started 20:40, dim at 22:43, very dim at 22:45.

Panasonic AA Infinium
Runtime on freshly charged cells ~ 1hour 40 minutes.
Fully charged 6 January 2010.
Discharged 4 April 2010 at 17:15, dim at 18:45.

Sanyo Eneloop
Run time on freshly charged cells ~ 1 hour 40 minutes.
Fully charged 12 January 2010.
Discharged on 4 April 2010 at 18:50, dim at 20:21.

Summary:
~~~~~~~~~~~~~~~~~~:~~~~~~~~Runtime~~~~~~~~~~~~~~~~~~~: Storage time
Cell Type~~~~~~~~~: Freshly charged : After storage ~:
Sanyo Eneloop~~~~ : 1 hour 40 mins, : 1 hour 31 mins : 79 days
Pansonic Infinium : 1 hour 40 mins, : 1 hour 30 mins : 88 days
Pansonic 2600 mAh : 2 hours 03 mins,: 1 hour 3 mins~ : 82 days

There is nothing to choose between the Eneloop and Infinium cells, whereas the Panasonic 2600 mAh cells have lost almost 50% of their charge in 82 days i.e. a bit more than 0.5% per day assuming a linear self discharge.

Clearly there are numerous caveats here, most importantly I only tested one pair of each type of battery. And the runtimes are accurate to within a few minutes only, as the runtime was measured by eye as the time until the torch significantly dimmed. However, the Fenix L2D has good regulation with a sudden and noticeable drop in output once the cells get low.

If you think these tests are too uncontrolled for your tastes, please do your own tests, and report back. As for myself, I conclude that in future I will choose between Infinium and Eneloop on the basis of price alone as they are as good as identical performance wise. I have some non LSD Panasonic cells, which I will use in my running torches, as they perform better when freshly charged, but I won't buy any more due to the poor storage behaviour. Bear in mind that the Panasonic 2600 mAh cells cost £9 for 4 compared to £6 for 4 Infinium or Eneloop cells.

Finally, I do wonder about the pressure build up seen with the non-LSD cells. It is conceivable this was due to heating of the air in the torch. At least I hope that was the cause, as surely discharge of gas from the cell must indicate some damage.

 

[VidPro]

There was considerable heat build up and a significant internal pressure increase

if you get that again, after the test measure the voltages of the cells. it could tell you something. mabey even measure the voltages after each of the tests, might tell you somthing anyways :-)

 

[Mr Happy]

There was considerable heat build up and a significant internal pressure increase, with the rubber switch cover expanding outwards, preventing the torch from tail standing.

:eek: Don't do that!

Finally, I do wonder about the pressure build up seen with the non-LSD cells. It is conceivable this was due to heating of the air in the torch. At least I hope that was the cause, as surely discharge of gas from the cell must indicate some damage.

Yes! Damage. Bad, bad thing to happen.

This is why we always advise people to stop discharging NiMH cells in a multi-cell light as soon as any loss of brightness is seen, and not to discharge until the light goes dim.

What happens is one cell discharges down to 0 V before the other one, and then the empty cell gets reverse polarized, being charged in reverse by the other cell that still has charge. This reverse charging causes irreversible chemical damage to the electrodes in the cell due to the wrong voltage potentials, and it also breaks down the electrolyte by electrolysis into hydrogen and oxygen that then escapes through the pressure vent. This outgassing represents a permanent loss of water and a gradual drying out of the cell.

Suffice to say, these effects are quite damaging to the cell. One member recently reported that after doing this accidentally just one time an Eneloop was reduced in capacity by about 25%.

 

[LeifUK]

This is why we always advise people to stop discharging NiMH cells in a multi-cell light as soon as any loss of brightness is seen, and not to discharge until the light goes dim.

There was no (obvious) loss of brightness despite the pressure increase. It was later on that it dimmed, I suspect it was expansion of the air inside due to heating, as the torch ran hotter with the 2600 mAh cells for some reason. Having just checked the cold torch, I can see that there was some expansion with the other batteries too, just not as much. And the fact that both cells once fully recharged powered the light for 2 hours (roughly as expected) suggests no real damage if any.

I wonder if a diode inserted between two batteries in series, or even inside a battery, could prevent reverse charging without consuming too much power?

 

[Mr Happy]

I wonder if a diode inserted between two batteries in series, or even inside a battery, could prevent reverse charging without consuming too much power?

A diode could not help as the current does not reverse direction.

Here is the thread I was thinking of:

Only had to charge eneloop AAA backward once to ruin it (unintential)

 

[LeifUK]

A diode could not help as the current does not reverse direction.

Here is the thread I was thinking of:

Only had to charge eneloop AAA backward once to ruin it (unintential)

Yes, I'm quite familiar with the problem you are talking about. It is widely discussed here. I don't think that is the cause of the excess pressure.