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AA Cells and Mobile Power, by Brandon in Utah

The size AA battery is the ubiquitous form of mobile power that is presently available. There is a large amount of off the shelf devices that use AA cells. They are available everywhere at low cost. They are cost effective and very safe for lighting. The breadth and depth of equipment available in a portable format is unparalleled by any other type of battery. I will cover the known factors on how to care for and use this resource to help end users get the most out of their equipment.

To start, some general information that covers all types of cells. Cells do not like heat. Heat increases the chemical reactions occurring inside the cell, and thus the self-discharge and other chemical reactions in cell. A cell will lose it's charge and lower it's life span. Keep them cool.

Cells shouldn't get wet. Keep them away from moisture. You should avoid circumstances that will result in condensation on the cell.
Do not drop or roughly handle them. Especially in the case of rechargeables, you can break the separator inside the cell and you may end up with complete cell failure. Inside of a device they're a little more durable, your device will provide some impact protection and buffering.

Do not store your batteries inside of your device for long term readiness. There is a good reason they never come this way from the manufacture in the package. It's not good for your battery and you run a much larger risk of cells leaking or venting into your device. On a short term basis in a device that sees regular use, leaving the battery in is fine.

Matched cells perform better. A battery will only perform as well as the weakest cell. Avoid mixing brands, dates, and especially chemistries and you will get the most out of your cells. The more cells a device has, the more matches cells you need to provide. So it's easier to feed devices that use a smaller number of cells.
In general, take care of them and they'll serve you well.

Primary (use once) cells are the most straight forward. They usually have expiration dates printed on the cell or package. It's important to note that this date is an average amount of time for a specified failure rate. "Fail" is defined as having less than ~85% capacity (depends on manufacture), thought it can also mean complete failure with 0% recoverable capacity. The closer a battery is to it's expiration date, the less capacity it will have and the more likely you are to encounter completely failed cells. Even with expired cells though, they often work. I wouldn't choose to use them in really important applications, but they are still useful.

"Heavy duty" cells generally are not worth messing with - they are cheap, light weight, and low capacity. They seem to only be made to sell to the "lowest possible price" consumers. I would never buy or store them.

Alkaline are the best bang for the buck primary cells. You can pick up a pack of 48 cells for around $10 at COSTCO last I checked (Duracell is believed to be the OEM for Kirkland brand cells). The price has gone up approximate 10-15% in the last year, which seems likely to continue. Alkaline's are good performers under "average" conditions. They do not like low temperatures, and they do not like high current draw (cameras, some flashlights, and possibly other devices). Once you place a battery into a device, I recommend you use it up. Do not return [primary] cells into storage once you've started to use them.

Lithium cells provide the widest temperature and current rating of all primary cells, though you pay the most for the best performance. I do recommend having a few for important gear, red dot sights, night vision,and so forth, [reserving them] especially for emergency winter use.

Rechargeable cells are much more economical for the regular user. Unfortunately they require better understanding to maximize their useful life. So I'll go over NiMH extensively and also address NiCd.
In a quick overview of the current tech of AA cells. NiCd is the most durable battery chemistry, it has capacities ranging from 600-1000 [mil-Amp Hours] (mAH) It has the best temperature performance envelope, endures heat and over charge best, will operate with more cycles. NiMH is the most common consumer cell these days, mostly due to the capacity advantage which run in the 1800-2700 mAH range at present. NiMH also has a new variant on the market I will dub low self discharge (LSD) cells. LSD cells are in the range of 2000-2100 mAH as of this writing and have many advantages over traditional NiMH that mostly come from an effort to stabilize it. They are new, so some data points are not borne out over years, but current evidence indicates that they perform as advertised. I recommend LSD cells for most people over all other varieties, I'll go into more detail why below. First, the brands and types currently on the market. The top brand in my opinion is Eneloops (2000 mAH) from Sanyo, it simply does the low-self-discharge thing better than the competition. The rest of the field seems to originate from a single manufacture or the same licensed design, but there are a bunch of competing cells. Rayovac Hybrids, Hybrios, Titanium Enduros, and a bunch of others (2100 mAH). Given equivalent, or near equivalent prices, I'd pick the eneloops.

In both types of chemistry, the higher capacity cells are more fragile than the lower capacity cells. It's an engineering trade off. The 2700 mAH whiz bang top-of-the-line cells are not your best bet for good durable cells, they are actually fairly fragile (chemically and physically) because of this trade off. Around 2000 mAH is not only cheaper (usually) but yields a cell that will see a longer service life, more cycles, and less likely to fail if dropped. Lower than 2000 in NiMH does not appear to hold significant advantage in durability in most respects. LSD cells appear to be at least as durable as their 2000 mAH NiMH counterparts.

Standard NiMH cells have an approximately life span of 3 years. Cheaper brands may have less. NiCd cells have an estimated 5+ year life span. Much beyond these points or even before them (especially with high capacity cells), increased internal resistance, lowered capacities, and higher self discharge are the norm. NiCd doesn't exhibit a large amount of this and usually fails with internal shorts (complete failure) or excessively high resistance. These numbers are very temperature dependant, colder storage conditions will lengthen the time, warmer will lower it. LSD NiMH cells currently have no data in this regard, they're advertised as having better longevity than NiMH cells, and I would tend to believe them due to the engineering trade offs picked. However, they've only been out for about 1.5-2 years now. To date, my oldest cells (1.5 years old), lightly used, perform like new - so far so good.

Self discharge is one of the biggest inconvenient things about rechargeable cell use. NiMH cells discharge by themselves very quickly. They discharge on the order of a couple of months when new and the rate increases significantly with age and use. NiCd cells have about half the self discharge rate and this usually won't vary much up until cell death. LSD cells shine in this regard, the self discharge slows down after a charge to almost a stand still in a little over a months time. LSD cells will retain around 85% (Eneloops) to 80% (rest of the field) charge after a year of storage at around 70 degrees.
Keeping the voltages up during use is important for many devices and one of the principle reasons rechargeables deliver poor performance in some devices. Standard NiMH suffers from voltage sag over time. It will start out at a nice high 1.4 volts fresh off the charger. Soon it finds it's way to 1.3-to-1.2 v open voltage. If left on the shelf it will fall over time. Many devices require a minimum voltage to operate correctly, if this minimum is above what your battery can deliver under load your device will shut down (can be 1.2v per cell, and NiMH will often fail to meet this under less than ideal circumstances!) If you experience significant performance difference between primary cells and rechargeable cells (especially older ones) this is likely the problem, especially combined with self discharge "usable capacity" drops very quickly. NiCd cells can suffer from a form of voltage sag, it is not as pronounced as NiMH but it can also happen in mid-discharge and is related the over marketed term cell "memory". This problem can usually be corrected with a couple exercise cycles and a good top off charge. LSD cells retain their voltage very well on the shelf, like their charge, and also deliver better than average voltages in normal use anyway. You will usually see much better performance from LSD cells in these voltage sensitive devices than NiMH or even NiCd. If you've been frustrated with rechargeables in the past in some of your devices give some LSD cells a try!

The most common method to kill cells is poor charging practices. I can't stress this enough, especially with NiMH cells, buy a good ["smart"] charger. Usually cells are allowed to "cook" on a standard charger for far, far too long. Remember, heat is bad! It's normal for them to get warm at the end of a charge cycle (not burning hot!). If they continue to stay warm (or worse, hot) for several hours later, you have a [traditional "dumb"] charger that is cooking your cells. I recommend a Maha-C9000 as a good high end charger. On a lower budget I recommend a Duracell 15 minute charger. {To be ready for various circumstances,] I prefer to have both chargers available. The C9000 is a slower charger (relatively) but it will not cook your cells, you can leave them in the unit. The unit has options that allow you to easily exercise cells and see if they are improving. You can match cells to obtain the best performance from them and identify poor performing cells quickly. It also charges individual cells rather than pairs, which is better for them - especially a mismatched pair. The Duracell 15 minute charger is a quality unit that also allows "busy you" to not walk away for hours waiting for, and forgetting about, your batteries. You will be less likely to forget about them and allow them to be cooked on the charger. Some good charging technology goes into the 15 minute chargers, so while they are a little rough compared to a good slower charge - they are actually very good at what they do, especially compared to the cheap junk [chargers] on the market. Fast charging is also fairly energy efficient, reducing the power required to get a full charge. Both of these chargers run on 12 volt DC input so they can plug directly into 12 volt systems allowing for use in a car or directly off a battery based [alternative energy] system (PV, wind, etc).

Do not charge cells when they are below freezing (32 F/0 C). You will damage them. If you really need a charged cell, warm it up in your pocket (preferably the charger too) and use the 15 minute charger. The charge cycle should provide enough heat to keep it above freezing until it's done. Avoid chargers that come with your cells, generally they are poor.

When brought out of long term storage, cells will usually need "exercise". NiCds especially need fairly significant exercise before returning to full capacity. 5+ full cycles may be required, rule of thumb is exercise until you stop seeing capacity gains. This is easiest with a charger like the C9000 with capacity readouts. NiCds should be stored discharged. NiMH cells should be stored with a charge. LSD cells require significantly less maintenance and may not need any exercise at all and will likely have a serviceable charge intact after storage, depending on the length of time in storage and at what temperature.
NiMH cells like to be treated gently. When you're done with your device, recharge the cells. The more shallow the cycle the better. Full cycles will wear on them the most. Keep NiMH cells topped off and they'll last the longest. Occasionally you may need to perform a deep cycle to restore some performance if the cell appears to be waning. The more advanced NiMH care systems like on the Toyota Prius reportedly keep cells at 60-80% capacity and only use about 20% depth in discharge cycles, which seems to be the most chemically repeatable and stable region. NiCds stand up to abuse a lot better, in fact a regular full discharge is good for them and will help you avoid issues with the cells. It's not required for every charge, but once a month or so should keep it's performance high.

I suggest avoiding C and D size rechargeable cells. They are expensive, there are no LSD variants at present, your charging options are more limited, they take forever to charge, and there are adapter sleeves readily available to make AA cells fit these sizes. D sized alkaline cells are reasonable for storage and use for the price. C size cells are usually overpriced and are often repackaged AA cells anyway - use the adapters. COSTCO presently sells an excellent Eneloop kit that includes 8 AAs, 4 AAAs, 2 AA->C adapters, 2 AA->D adapters, and a cheap charger for $26.

Earlier generation NiMH cells had a very poor temperature envelope. There are evidences that this has improved and the LSD introduction advertised even better cold temperature performance. Unfortunately, to date, I am unable to find information or a datasheet to quantify this. I've done a bit of my own testing down to 0 F, the limit of my freezer, and have found no appreciable drop in capacity (old NiMH tech struggled below freezing). I can't really quantify if LSD NiMH is inferior or superior to NiCds at present, so suffice it to say they both do reasonably well in the cold (just remember not to charge them when they are below freezing).

In summary, I don't see any reason to buy any non-LSD NiMH cells these days. LSD tech has dramatically improve the performance and user friendliness of the cells, and hopefully longevity, durability, and cycle life too. However, it is new and relatively unproven tech. NiCd is the old known workhorse and there is good reason why power tools and similar equipment still ship with NiCd cells. It's worth having a few NiCds around as a backup because of their track record. For general use, the Sanyo Eneloops are the way to go.

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