altis
Newly Enlightened
Although I persist in trying to buy stuff that uses AAs I keep being told that I should 'get real' and use Li-ions instead. My hatred of the things started with a laptop that used a pair of camcorder batteries. These didn't seem to last very long and were very expensive to replace. With digital cameras, in particular, it's getting more difficult to find new models that don't use some proprietary Li-ion. The manufacturers are cottoning on to this money spinner. Is it time to bite the bullet?
Here's my take:
Availability
The AA format must be the most common battery shape on the planet. Replacements can be found almost everywhere. Many new cameras come with new Li-ion batteries and, for a while, the manufacturer is the only source. Later, as the clone makers bring at compatibles, there are other sources but there are so many models that most retail outlets are unlikely to stock but a few.
Price
Since the AA format is a common standard, the battery manufacturers are forced to compete on price. Proprietary batteries are much more expensive.
Spares Management
Looking after spares for a variety of battery-powered devices is made much easier if they all use the same battery. In established products AAs are probably the most common.
New Developments
Engineers are always developing new battery chemistries with better characteristics. The capacity of proprietary Li-ions hardly, if ever, changes but newer, better AAs are available from time to time.
Flexibility
Many different chemistries are available in an AA package. This allows the user to tailor the battery to the environment eg: rechargeable around town but lithium primaries while away from the mains. If the battery compartment is carefully designed it is also possible to use a CR-V3 (or a rechargeable version) in place of two AAs. With a proprietary battery you are stuck with what the manufacturer provides.
Self-Discharge Rate
This is the Achilles heel of ordinary NiMh cells but now we have Eneloops (and cousins) that hold around 85% after a year. This works out at 1.3% per month. Various rates between 1% and 5% per month are quoted for Li-ions (with the highest rate being most common).
Energy Density
This is difficult to calculate accurately from datasheets. Really, we need the area under the discharge curve under the same conditions but this is difficult to evaluate without actually measuring it. Perhaps the easiest way is to compare cells of the same size but with different chemistries. Using the data from GP Batteries, their '2700' high-capacity NiMh cells typically hold around 3Wh (calculated from 1.15V * 2600mAh). Their 14500 Li-ion cells have a capacity of around 2.5Wh (calculated from 3.6V * 700mAh). For the CR-V3 (double AA) it's around 4.1Wh (2.5V * 3300mAh / 2). For Energizer's e2 lithiums it's 4.5Wh (1.5V * 3000mAh) and for Sanyo's Eneloops it's around 2.5Wh (1.25 * 2000mAh). The non-rechargables have the highest energy density but, of the rechargeable, the high-capacity NiMh is best.
Of course, lithium-based cells are very light so, by weight, they have a better energy density. Energizer's e2s weigh only 14.5g so the gravimetric density is 0.31WH/g. For the CR-V3 (38g / 2) it's 0.22Wh/g and for the 14500 (19g) it's 0.13 Wh/g. For the 2700 (31.5g) it's 0.095 Wh/g and for the Eneloop (27g) it's 0.093 Wh/g. Advantage lithium - but the non-rechargables are by far the best.
Discharge Rate
Lithium-based cells tend to be rated conservatively to stop them overheating. The maximum recommended discharge current of GP's 14500 is 700mA whereas the 2700 NiMh cell is rated for 7.5A. Being able to draw lots of current is useful in applications like flashguns where this helps to reduce the cycle time.
Charge Rate
Likewise, lithium-based cells are not comfortable with fast charging. The fastest chargers seem to take about 2 hours but NiMhs will tolerate being charged in 15 minutes. Sure, this will reduce their cycle life if done repeatedly but not if you only do this from time to time.
Lifetime
Lithium-based primary cells have a tremendous shelf life but can only be used once. Li-ion rechargeables are limited by time since manufacture. We used to get 2 years but newer models last 5-7 years. NiMh cells are limited by number of cycles. Sanyo quote up to 1000 cycles for their Eneloops - equal to once a week for 20 years.
Safety
Batteries containing lithium have a tendency, albeit slight, to overheat and catch fire. There are many examples to be found on the web. A 48-pack of Energizer e2 lithium primaries that I recently purchased had a warning on the outside to say it was unauthorized for air transport. All spare lithium batteries now have to be carried in insulating containers on a plane. Alkaline and NiMh cells do not have such problems.
Datasheets:
GP 2700 NiMh
GP 14500 Li-ion
GP CR-V3 photo lithium
Energizer e2 lithium
Sanyo Eneloop
Here's my take:
Availability
The AA format must be the most common battery shape on the planet. Replacements can be found almost everywhere. Many new cameras come with new Li-ion batteries and, for a while, the manufacturer is the only source. Later, as the clone makers bring at compatibles, there are other sources but there are so many models that most retail outlets are unlikely to stock but a few.
Price
Since the AA format is a common standard, the battery manufacturers are forced to compete on price. Proprietary batteries are much more expensive.
Spares Management
Looking after spares for a variety of battery-powered devices is made much easier if they all use the same battery. In established products AAs are probably the most common.
New Developments
Engineers are always developing new battery chemistries with better characteristics. The capacity of proprietary Li-ions hardly, if ever, changes but newer, better AAs are available from time to time.
Flexibility
Many different chemistries are available in an AA package. This allows the user to tailor the battery to the environment eg: rechargeable around town but lithium primaries while away from the mains. If the battery compartment is carefully designed it is also possible to use a CR-V3 (or a rechargeable version) in place of two AAs. With a proprietary battery you are stuck with what the manufacturer provides.
Self-Discharge Rate
This is the Achilles heel of ordinary NiMh cells but now we have Eneloops (and cousins) that hold around 85% after a year. This works out at 1.3% per month. Various rates between 1% and 5% per month are quoted for Li-ions (with the highest rate being most common).
Energy Density
This is difficult to calculate accurately from datasheets. Really, we need the area under the discharge curve under the same conditions but this is difficult to evaluate without actually measuring it. Perhaps the easiest way is to compare cells of the same size but with different chemistries. Using the data from GP Batteries, their '2700' high-capacity NiMh cells typically hold around 3Wh (calculated from 1.15V * 2600mAh). Their 14500 Li-ion cells have a capacity of around 2.5Wh (calculated from 3.6V * 700mAh). For the CR-V3 (double AA) it's around 4.1Wh (2.5V * 3300mAh / 2). For Energizer's e2 lithiums it's 4.5Wh (1.5V * 3000mAh) and for Sanyo's Eneloops it's around 2.5Wh (1.25 * 2000mAh). The non-rechargables have the highest energy density but, of the rechargeable, the high-capacity NiMh is best.
Of course, lithium-based cells are very light so, by weight, they have a better energy density. Energizer's e2s weigh only 14.5g so the gravimetric density is 0.31WH/g. For the CR-V3 (38g / 2) it's 0.22Wh/g and for the 14500 (19g) it's 0.13 Wh/g. For the 2700 (31.5g) it's 0.095 Wh/g and for the Eneloop (27g) it's 0.093 Wh/g. Advantage lithium - but the non-rechargables are by far the best.
Discharge Rate
Lithium-based cells tend to be rated conservatively to stop them overheating. The maximum recommended discharge current of GP's 14500 is 700mA whereas the 2700 NiMh cell is rated for 7.5A. Being able to draw lots of current is useful in applications like flashguns where this helps to reduce the cycle time.
Charge Rate
Likewise, lithium-based cells are not comfortable with fast charging. The fastest chargers seem to take about 2 hours but NiMhs will tolerate being charged in 15 minutes. Sure, this will reduce their cycle life if done repeatedly but not if you only do this from time to time.
Lifetime
Lithium-based primary cells have a tremendous shelf life but can only be used once. Li-ion rechargeables are limited by time since manufacture. We used to get 2 years but newer models last 5-7 years. NiMh cells are limited by number of cycles. Sanyo quote up to 1000 cycles for their Eneloops - equal to once a week for 20 years.
Safety
Batteries containing lithium have a tendency, albeit slight, to overheat and catch fire. There are many examples to be found on the web. A 48-pack of Energizer e2 lithium primaries that I recently purchased had a warning on the outside to say it was unauthorized for air transport. All spare lithium batteries now have to be carried in insulating containers on a plane. Alkaline and NiMh cells do not have such problems.
Datasheets:
GP 2700 NiMh
GP 14500 Li-ion
GP CR-V3 photo lithium
Energizer e2 lithium
Sanyo Eneloop
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