Aqualab: Research: Which Kind of Battery?

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Research Topic: Which Battery Will Do?

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Overview & Terms
8 March 2012

Single Use
25 August 2011

Rechargeable
29 May 2009

Battery Analyser
13 March 2012

Battery Shopping
5 May 2012

Original article by Ian Mander, 22 July 2002

Single Use Test
6 November 2007

Rechargeable Test
13 March 2012

Test Procedure
4 June 2011

When Battery Testing
Goes Bad -
Consumer Magazine
27 November 2011

More Info & Links
29 February 2012

LSD Shootout
11 March 2012

When Battery Testing Goes Bad - Consumer Magazine

First posted 12 October 2011.

The point of a good test is to allow buyers to make an informed decision about what is best to buy. Consumer has failed to do this. In its October 2011 issue Consumer published a rechargeable battery test titled "On and on ..." by Paul Smith which drew flawed conclusions from the report's own test data about which NiMH batteries are the best. This resulted in poor quality batteries being recommended by Consumer.

I've taken a closer look at Consumer's results for two of the batteries included in their test - the Energizer Recharge and the Sanyo Eneloop - and drawn my own conclusions about what Consumer's results actually mean. I'll also explain how they should actually have tested certain battery characteristics such as self discharge.

Faulty Understanding of How Batteries Work

The first problem with the Consumer report is the Consistency score, which accounts for 30% of the total score for each battery. Their idea of "consistency" for a battery is "measured by the amount of running-time the battery loses over its life. A high score is good." (Italics in original.)

Actually no. A high score is bad.

Consumer's test procedure defined the end of life of their batteries to be when "the battery capacity is reduced to 50 percent of its starting charge" - by which they probably mean 50% of its initial capacity. That's OK - the test has to finish sometime - but the article claims that the Energizer Recharge was "the most consistent performer, losing only 21 percent of its running-time by the end of its life."

How is it possible for a battery to lose 50% of its capacity but only 21% of its runtime?

Clearly the discharge did not involve a constant current discharge, which would always give 50% runtime for 50% capacity. The simplest way to explain it is if the test involved a constant resistance load, for example, by using a 1 Ω resistor. The current the resistor draws will depend on the voltage of the battery according to Ohm's Law, V = I * R (or I = V / R). Because the resistor is 1 Ω, V = I. The battery condition deteriorates during the testing, so at the end of the test the battery cannot deliver the same voltage as when it was new. Less voltage means less current, and because the current has decreased, runtime is extended compared with what it would have been with a constant current test. A standard resistor across the battery terminals also allows easy calculation of capacity using a voltage logger and Ohm's Law.

Ideally a battery's voltage would not deteriorate at all over its usable life, and like a constant current discharge it would also always give 50% runtime for 50% capacity for any given resistive load. Unfortunately Consumer would give that ideal battery only 5/10 for Consistency.

In other words, Consumer's Consistency figure is actually a Crappiness figure. The higher the Consistency figure the worse a battery performed, because the extension in runtime is a direct result of an undesirable lower voltage.

Practical Consequences of Crappy Batteries

Electronic devices that are particularly voltage sensitive such as digital cameras possibly won't even turn on because the batteries don't hold a high enough voltage.
Even if a device does turn on, its performance is likely to be impaired. For example, a digital camera will take significantly longer to charge its flash, it may turn off as soon as you try to take a flash photo, and its screen may be disabled; a walkie talkie may be unable to transmit; incandescent torches will be much dimmer (and operate less efficiently); radio contolled cars will crawl instead of honking along.
Many battery chargers will reject the batteries due to their high internal resistance. This causes a lot of frustration, especially if few cycles have been completed. (This could significantly reduce the number of cycles achievable by several of the batteries tested.)
Battery chargers that do not reject old batteries are likely to miss termination - they don't stop charging when they should because the sight drop in a bad battery's voltage that indicates it's full is too small to detect. This causes very hot batteries which damage the batteries further (internally and sometimes the label externally as well).

If runtime at the cost of voltage really was a desirable thing then all batteries would have a resistor connected in series. It would lower the voltage available to the device using the battery, especially at high current (because there would be voltage drop across the resistor), but that would reduce current and thus extend runtime. Imagine a torch being run on this sort of battery - it would last longer but would never give adequate light. With its Consistency score Consumer would have us believe this would be a good thing.

How Good Are the Energizer Recharge Cells Really?

From Consumer's results the Energizer Recharge got 2455 mAh at the start of the test with an initial runtime of 138 minutes, meaning an average 1.07 A discharge rate. 50% capacity at the end of the test, or 1227 mAh, with a 21% lower runtime, or 109 minutes, means an average of just 0.68 A. The significantly lower average discharge rate at the end of the test is a direct consequence of the lower voltage that the battery can maintain in its aged state. This is not a battery in a good condition; it would be rejected by many smart chargers.

Let's compare with the Sanyo Eneloop. It got 2068 mAh at the start of the test with an initial runtime of 111 minutes, which works out to an average 1.12A discharge current. 1034 mAh at the end of the test works out to an average 0.93 A at the end of the test. This seems pretty good for a battery with only 50% of its original capacity left. Whether it was good enough to still use would probably depend on the purpose.

Battery	Initial Capacity & Runtime	Initial Average Current	Final Capacity & Runtime	Final Average Current	Final Current %
Energizer Recharge	2455 mAh 138 min	1.07 A	1227 mAh 109 min	0.68 A	64%
Sanyo Eneloop	2068 mAh 111 min	1.12 A	1034 mAh 67 min	0.93 A	83%

These Final Current % figures give a better idea of the state of the batteries than the figures Consumer presented, and are roughly the opposite of Consumer's Crappiness figures. What difference does it make to Consumer's Overall Scores using these figures instead of Consumer's Crappiness figures? They're no longer incorrectly running neck and neck.

Battery	Consistency/ Crappiness	Original Overall Score	Corrected Consistency	Corrected Overall Score
Energizer Recharge	8.0	69	6.4	64
Sanyo Eneloop	6.0	70	8.3	77

Most of the LSD batteries in Consumer's test are pretty consistent - as indicated by low Consumer Crappiness scores - so were incorrectly rated too low in their Overall Score. Clearly, the more consistent batteries are the ones that suffered less voltage loss and are thus able to maintain a higher average current over the battery's life.

Runtime and Cycle Life

Runtime is certainly something to take into consideration in a rechargeable battery, but how justified is a simple comparison of runtime retention, especially when it's over the life of batteries that achieve considerably different numbers of cycles? For example, is the Energizer Recharge retention of about 80% of its runtime after 105 cycles really better than the Sanyo Eneloop retention of 60% of its runtime after 314 cycles? This is another serious failing of the Consumer report.

What would have happened to the Eneloop's runtime after just 105 cycles? As it turns out, not a lot. Accelerated test data from Sanyo (something more battery makers should be unafraid to publish) shows Eneloops are still going strong with roughly full capacity up to about 200 cycles, a point when all of the other batteries in the Consumer test were already dead or well on their way out. The capacity of Eneloops also actually improves a little in their first few cycles. The capacity of ordinary (non-LSD) NiMH batteries starts decreasing right from their first cycle. The Energizer Recharge would have started deteriorating quickly after just 50 cycles, and had less capacity than the Eneloop after only about 60 or 70 cycles. How can Consumer consider that a more consistent battery?

This graph is not from the Consumer test data (which they did not publish with their report). It is based on Sanyo's fast cycle test data, and is supported by fast charge data from SilverFox on CandlePower Forums. Remember, Consumer thought these two batteries rated only 1% different from each other.

From the graph, since the capacity of the Sanyo Eneloop is basically unchanged at 105 cycles it's reasonable to assume that the Eneloop's runtime will also not be significantly decreased at 105 cycles. 80% of the Energizer Recharge's original runtime would be less than the Sanyo Eneloop runtime at 105 cycles, even without considering the very undesirable loss of voltage meaning the Energizers are crappy batteries by that stage. If Consumer was running a fair test the Eneloop - and probably most of the LSD batteries - would have rated at or near 100% for consistency at 105 cycles. Let's see how this changes things at the point the Energizer Recharge got to.

Battery	Consistency/ Crappiness	Original Overall Score	Corrected Consistency @ 105 cycles	Corrected Overall Score @ 105 cycles
Energizer Recharge	8.0	69	6.4	64
Sanyo Eneloop	6.0	70	10.0	82

The simple truth is that having a little bit extra theoretical runtime when a battery is new is of no real value if batteries quickly become unreliable.

Cost

Consumer paid lip service to cost by listing the prices of single batteries, but there's strangely no indication whether this contributed to the final score for each battery, and no attempt was made to compare the cost effectiveness of the batteries over their life. Not surprisingly Consumer did not consider the cost of frustration with crappy batteries either.

There's also a problem with the price Consumer listed for a 4 pack of AA Sanyo Eneloops. They are commonly available nationwide with free delivery for $24.99 from Dick Smith, or just $22.89 from PB Tech - significantly less than the $28 Consumer mentions. (If you don't mind waiting for international shipping they're also available for about $22 here or about $20 here.) Price comparison web sites PriceSpy and PriceMe don't list any for $28, so where did Consumer get them from? Evidently not a major chain. Unless... Consumer has used Dick Smith's 2xAA price of $13.99 and doubled it. (It's possible that's all they got and just used one cell for cycle testing and the other for the LSD test, but in the article they included a photo of a 4 pack.)

Battery	Cost per Battery	Cycles	Cents per Cycle
Sanyo Eneloop	$5.72 or $6.25 (not $7.00)	314	1.8 or 2.0
Duracell Active Charge	$5.25	196	2.7
Energizer Recharge	$7.00	104	6.7
Vapex	$5.75	53	10.8

The Cents per Cycle figures do not include the cost of electricity required to charge these batteries, which is almost negligible. At just 23.5 c/kWh (NZ$) it costs less than 0.1 cents to charge even the highest capacity battery, even allowing for charging inefficiency and the power the charger itself uses. The Duracell Active Charge (a LSD battery) had the second lowest cost per cycle. The Vapex was one of the cheapest batteries in the test but the most expensive per cycle. Paying extra is no guarantee of quality.

The Sanyo Eneloop offers the best value for money.

Self Discharge

Self discharge is the tendency of NiMH batteries (and other sorts of batteries as well) to go flat while sitting around doing nothing, whether on a shelf, down the back of a sofa, or in a digital camera. It's a problem because when they're needed, a battery can be completely flat because of self discharge. Particularly bad batteries can lose most of their charge overnight, meaning they always have to be charged immediately before use. It's like a car with a hole in the bottom of the fuel tank.

Low self discharge batteries - unfortunately abbreviating to LSD - were invented to address the problem, with the first being the Sanyo Eneloop. At last there was a NiMH battery that didn't need to be charged immediately prior to every use!

The concept has been so popular that all battery makers - with the notable exception of Energizer - now have LSD batteries amongst their products. LSD batteries are normally sold as "ready to use" or "pre-charged" because thanks to their LSD ability they can be sold with a partial charge. Normal NiMH batteries are completely flat when bought.

The self discharge figures in the Consumer article were probably measured when the batteries were brand new. While it's interesting to know, this is next to useless information as the self discharge characteristics of NiMH batteries can change significantly after a bit of use. The test thus favours the non-LSD batteries. Give them a couple of dozen cycles and then see how well they do for self discharge! This would be a a far more realistic test and give far more useful information.

In the comments for the report on the Consumer web site doubt has also been raised about the validity of the self discharge test results. The 12 week duration of the self discharge test should have been long enough to clearly show the benefit of LSD batteries. Consumer's results did not show that. Why not?

Many people think the capacity of a rechargeable battery is the most important characteristic. It's a single number that's easy to understand and allows batteries to be easily compared. Unfortunately the claimed capacity is seldom the actual capacity, especially after the battery has been used for several cycles. It also makes no consideration of voltage under load, which affects the power the battery can supply, or how reliably the battery will maintain its charge if left for several days.

Whether they realise it or not, low self discharge is far more attractive to the average user than high capacity. This is because most people want a reliable battery that will have a good charge when they come to use it. They don't want to worry about high self discharge, and thus having to plan when to charge batteries so they'll be ready just before they're needed. This is one of the main reasons that people don't use rechargeable batteries.

Low self discharge batteries are what most users should be buying.

Batteries for an Emergency Pack

Alkaline batteries were recommended by Consumer as "still best for an emergency pack because they lose very little charge when stored for extended periods." It was a strange recommendation because the test was exclusively for rechargeable NiMH batteries; no test data or other supporting evidence was included or even referred to to support any use of alkaline batteries. The truth is that alkaline batteries may hold their charge, or they may be unusable after a relatively short period of storage. Alkaline batteries can go flat or even leak in storage, and exposure to heat is particularly bad for contributing to this. Alkalines should be stored in a cool location. Even without heat, the voltage that alkalines can provide under load slowly deteriorates, so alkalines stored for long periods may be rejected by high drain devices such as digital cameras.

It's a shame Consumer apparently didn't bother assessing how good LSD batteries might actually be in an emergency pack. Some LSD batteries presently on sale are rated for 70% charge after 3 years. That is easily enough to provide a useable amount of runtime.

Also, of course, LSD batteries are fully rechargeable at any stage, and usable for any temporary task at any time without worrying about having to buy a new set of batteries to replace them with in the emergency pack. (This is a significant improvement in convenience.) Rechargeable batteries only have to be used a handful of times before they're more cost effective than alkalines - less than 3 times for the top brands of alkalines.

LSD batteries are now a real option for emergency pack batteries.

FWIW the next version of Eneloop was announced earlier this month - they will retain 90% charge after 1 year and 70% charge after 5 years, and claim 1800 (slow) recharges. Initially they'll be sold only in Japan, starting on 14 November 2011.

Other Notes

The Kodak batteries were labeled Digital Camera Batteries and it's not surprising to see that they got the equal lowest Consumer Consistency score, meaning they held their voltage in the Consumer test quite well over their life.

Conclusions

Most battery buyers should get LSD batteries.
High capacity batteries have a short usable life (managing few cycles before useless) and are likely to become unreliable after just a few dozen cycles. They have their uses, but are not well suited to the average user. Capacity at the expense of reliability is not necessarily a good thing.
The Sanyo Eneloop has by far the longest cycle life of commonly available batteries and is the least expensive per cycle.
LSD cells may be the best option for emergency pack batteries (most convenient, most cost effective, least likely to leak).
Consumer has failed to run a fair or accurate battery test.

Responses

"Consumer is in la-la land if they think there's only 1% difference between Eneloops and Energizers."
"They're misrepresenting Sanyo's product. They've misrepresented them so badly it's verging on libelous. Are they going to publish a correction?"

Updates

November: The November Consumer magazine has a paragraph about report writer Paul Smith. "His passion is product quality and he abhors badly designed products." I have to ask - why doesn't Paul Smith abhor badly designed tests that can't identify quality products?
27 November 2011: A simple battery test of some crap Energizer cells supports what I was saying above - the claim of 105 cycles for the Energizer Recharge 2450mAh cell is ridiculous because with its high crapiness value it would be useless for many important tasks long before it got to that many cycles.

Yesterday I was handed a pair of Energizer 2500mAh cells to charge because they had been rejected by their normal charger. It turned out they were almost fully charged, and yet they had also been rejected by a digital camera; hence why their owner wanted to charge them.

The Energizer 2500mAh has a very bad reputation for being a crap NiMH cell so I offered to give them a full cycle on my Maha MH-C9000 battery analyser to see how much capacity they now have.

The MH-C9000 test cycle showed they have 83% of the claimed 2500mAh capacity, with quite a low mid-point voltage. The Consumer test last month ran cells until they had only 50% of their capacity left, but even with 83% these Energizer cells still couldn't power a digital camera when nearly fully charged.