Low self-discharge AA comparison

Started Apr 25, 2007 | Discussions thread
OP archae86 Contributing Member • Posts: 658
Three month delay results--Hybrio and Eneloop much better than others

Here are the three-month readout capacities for individual batteries

TPE 1610 1600 1627 1524
ENL 1745 1744 1757 1739
HYB 1801 1807 1814 1809
ACC 1712 1679

Averaged over the batteries in each set, here is the three-month capacity as a percentage of the one-hour capacity
TPE 82.5%
ENL 88.2%
HYB 89.2%
ACC 83.9%

I think that for some hours, and possibly a few days, after charging there is a mechanism distinct from steady long-term self-discharge causing capacity loss.

So showing the three-month capacity as a proportion of the 1-week capacity averaged by set may better predict long-term relative capacity loss:
TPE 86.2%
ENL 94.5%
HYB 94.2%
ACC 89.3%

This round has a clear single sample loser--one of the Titanium Power Enduro batteries with just 1524 mAHr delivered only 80.5 % as much capacity at three months as at one week and so is by far the worst. This cell is showing much higher long-term self-discharge than the others in its category. As a single outlier, we can't say whether this is a bad sample or a possible sign of poor manufacturing control with excess variation.

The outlier bad TPE cell was an outlier in initial capacity. Oddly enough, it provided far more capacity on first discharge before any charging than the other three of the brand. That was the last good news however, as this cell has shown more loss from step to step than its companions, at each step from 1 week to four weeks, to three months.

The loser as a consistent set is now the Titanium Power Enduros, with even the non-outlier cells all individual below the worst individuals in the second-worst (Acculoop) set. With increasing time the test has split into two groups: frontrunners are Hybrio and Eneloop, laggards are Acculoop and Titanium Power Enduro.

At this round the Hybrio batteries are still the winners. They have really excellent within-set matching at all readouts, and the highest 1-week and 4-week and three-month capacity. While the Eneloops beat them by a small margin for loss of capacity between 1-week and 4-week readouts, the Hybrios won by an even smaller margin for loss between the 4-week and 3-month readouts. Probably within the repeatability of the measurements it is best to say ENL and HYB are matched for long-term rate of capacity loss, better than the other three low-loss brands, and very dramatically better than any of the traditional cells. Hybrio wins by virtue of higher initial capacity and better first-week loss than the Eneloops.

STANDARD NIMH BATTERIES

The three-month readout on these is:

(battery model, three-month mAHr, three month-as percent of one-hour, three month-as percent of 1-week)
KOD 1012 48.8% 52.8%
RAY 605 38.3% 41.1%
NEX 533 43.0% 44.0%
TI24 0 0.0% 0.0%

The Titanium 2400 reached three months with no remaining useful capacity (it could not provide even 1 mAHr at 200 mA).

CONCLUSIONS

Low self-discharge is not just a marketing slogan. The advantage of even the worst individual sample of the low-discharge cells in this test over any of the conventional cells gets more striking the longer the wait time.

If capacity past the first month, possibly even earlier, is of interest, then the low self-discharge batteries seem likely to do better than conventional cells with much higher initial ratings. They also beat older traditional cells with lower ratings.

On the results here, The Uniross Hybrio and Sanyo Eneloops are the clear winners. Hybrio wins at almost all times, and shares with the Eneloop an advantage in capacity loss vs. time which gives them an increasing advantage at longer times.

As the loss with additional time seems still to be dropping in the 4 week to 3 month interval compared to the 1 week to 4 week interval, it seems very clear that Eneloop and Hybrio woudl still have very substantial capacity after six months, very likely over 1700 mAHr for the Hybrio, and over 1650 for the Eneloop. The manufacturer's claim for substantial useful capacity after one year seem highly plausible.

The available charge as purchased (before any user charging) is an extremely bad predictor of the in-service performance, even for cells in the same batch of the same brand. Capacity loss in the first week is only moderately good at predicting later loss. The one-hour, 1-week, and 4-week readouts, taken together, give a pretty strong indication of which batteries will do best and worst at 3 months.

Caution: I suspect storage temperature and usage history both have an appreciable effect on the capacity loss with storage time. In particular, I'd not trust that a pair of these batteries which was first used heavily with many deep cycles for a year, then charged and left in a flashlight in the door pocket of a car parked in the Southwest sun would do very well after an additional year.

ANOTHER BRAND

When I started this test I was unaware of the Ray-o-Vac Hybrid batteries which compete in the low self-discharge segment. I purchased four, and have done the same tests, but results are delayed over a month compared to these. However, based on the readouts up through four weeks, it seems clear the Ray-o-Vac Hybrids will not approach the frontrunners, and are probably closer to the laggards, while still far better than any conventional.

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Peter A. Stoll
new 350D user July 1, 2005

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