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Getting the longest runtime & cycle life from your batteries

"Runtime" is how long a battery can power a device.  It's basically the same as "capacity".
"Cycle Life" is how many times a battery can be recharged before it won't charge any more, or its capacity has decreased too much.

Avoid alkalines for high-drain devices or for cold-temperature use

Regular alkaline batteries are terrible in high-drain devices (like cameras and motorized toys), because they can't pump the power out fast enough.  Rechargeable batteries will actually outperform alkalines in this application.  Yes, there are special high-drain alkaline versions available, but they cost more, and you're better off with NiMH which give great performance and which can be recharged hundreds of times, so you can stop buying batteries.

Ditto for use in cold temperatures, like bicycle lights in the winter.  Alkalines discharge fast when it's cold, but NiMH's are much more robust.

For high-drain and cold-temperature use, I like eneloop XX.

Charging strategies to maximize capacity and cycle life

Use a smart charger

Overcharging batteries hurts cycle life.  Use a smart charger which gives your cells only as much juice as they need.

Charge slow rather than fast

The faster you charge you cells, the lower the cycle life.  Four-five hours for a charge (regardless of battery size) is probably a good compromise between convenience and preserving cycle life.  I couldn't dig up really excellent figures, but here's a forum thread with some independent testing.

Recharge sooner rather than later

You'll maximize recharge cycles of rechargeable batteries by charging them before they're fully depleted.  You don't have to obsess about putting them in the charger after each time you use them, but getting them in when they're down to only 1.1V instead of 1.0V should give you a lot more cycle life.  That's true of all batteries, but it's especially true of NiZN and rechargeable alkaline.

Remove the batteries when they're done charging

Most chargers give a "trickle" charge after charging is finished.  Some chargers give too much of a trickle, and even those which give a reasonable trickle can still overcharge if the batteries are left in the charger too long.  A good compromise between convenience and not overcharging is to leave the batteries in the charger no more than a day after being charged.

The higher the discharge rate, the less the capacity

Realize that at faster discharge rates, you get less than the rated capacity

Capacity suffers a little bit when you use NiMH batteries in a high drain device vs. one that gently sips the juice slowly.  The Sanyo chart at right suggests a penalty of about 7% when discharging very fast vs. slow.  Here's another chart on Candlepower Forums.

I suspect that battery manufacturers test their batteries with a slow discharge rate so they can claim a higher capacity, but I haven't been able to find manufacturer's test methods, despite a lot of searching for them.  StefanV says the manufacturers use a 400 mA discharge rate for AA's, but doesn't cite a source.

How temperature relates to battery life

Heat kills batteries

Under hot conditions, all batteries have a shorter shelf life, and a lower capacity when in use.  For example:
  • Over 4 years, an alkaline loses only ~1% of its capacity at 0°C/32°F,  11% at 20°C/68°F, and 25% at 40°C/104°F. (Energizer, PDF, p. 12)
  • Over 15 years, a lithium will lose only 10% of its capacity at 68°F (20°C), but 60% when stored at 104°F (40°C). (Energizer, PDF, p. 14)

Refrigerating batteries doesn't really help

Refrigerating batteries make sense only if you're in a hot climate without air conditioning.  (But even then, manufacturers don't recommend that you put little chemical factories near your food.)  Here are the details.

Alkalines.  A long time ago the myth started that putting alkalines in the fridge or freezer would help them hold their charge.  But alkalines already have great shelf life anyway, so there's really no point.  That is, alkalines already hold their charge properly for long periods of time (around 80% after 5-7 years).  In 2000 Consumer Reports compared 432 Duracell AA, C, and D batteries in a refrigerator with some at room temperature. After 2.5 years, refrigerated AA's had kept their charge perfectly, but the unrefrigerated batteries lost only <4% of their charge. The unrefrigerated C's & D's lost only 10%. (CR has since removed the report from their site.)  Refrigerating alkalines makes sense only in very hot climates without air conditioning.  (They lose only ~5%/year at 85°F, but 25%/year at 100°F.) (Green Batteries) Though again, be careful before putting chemical factories near your food.

NiMH.  While the idea of refrigerating alkalines was always an urban legend, it actually does work well for NiMH, but there's not much point because you can simply use the low self-discharge flavor of NiMH's in the first place.  By freezing NiMH's you can cut the first-month self-discharge from 20% down to 10%. (Source: Green Batteries)

Use it or lose it?

Battery University says that NiMH's lose some of their capacity permanently if not used for long periods of time. (Battery University)

You can also see our charging tips.

(Google picks the ads, not me.)

Last update: January 2013

Michael Bluejay’s
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