F
D
E
G
Fig. D: Alkaline cell.
Fig. E: 6V alkaline lantern battery.
Fig. F: Lithium coin cell.
Fig. G: Lithium battery.
Cons: Low power density, require periodic full
discharge/recharge cycles to reduce “memory”
(caused by crystals growing on internal plates),
contain toxic cadmium metal
Price and capacity: $1 for a AA cell with 1,000mAh
Power density: 60Wh/kg
NiMH (Nickel Metal Hydride)
With cell voltage of 1.25V, these rechargeables
(Figure C) are a good replacement for alkalines.
They like to be charged at about 0.1C but can be
discharged at 0.2C.
Pros: Rechargeable, high power density, standard
sizes, higher capability than alkalines
Cons: More expensive and shorter service life than
NiCd, self-discharge quickly
Price and capacity: $2 for a AA cell with 2,500mAh
Power density: 100Wh/kg
Li-ion (Lithium-Ion) and Li-poly (Lithium Polymer)
These are the latest rechargeables, and are now
standard for portable consumer electronics (Figure B).
They are very light and have high power capability
and density, but also require special circuitry to
keep them from exploding! That’s why you can’t buy
general-purpose Li-ion cells in standard sizes.
To use these, your best bet is a camcorder (large),
cellphone (medium), or R/C airplane (small)
battery with its matching charger. Cells are 3.6V,
commonly packaged as 3.6V and 7.2V. They can
easily provide 1C of current, and some deliver 10C!
Pros: Rechargeable, ultra-light, high cell voltage,
high capability, high density
Cons: Expensive, delicate, can explode if misused!
Price and capacity: $10 for a replacement cell-
phone battery with ~750mAh
Power density: 126Wh/kg for lithium-ion, 185Wh/
kg for lithium polymer
Lithium Batteries and Coin Cells
Lithium batteries have a voltage of 3V per cell
(Figure G). Most are in coin/button form (as in
Figure F, but coin cells can also be alkaline, or zinc-air “hearing aid batteries” — all 1.5V). These are
great for small, low-power devices, but they can’t
be recharged and provide only 0.005C of continuous current (although you can draw more in pulses).
One popular cell is the CR2032, which measures
20× 3.2mm and provides 220mAh. Coin cells can
get as large as the 24×8mm CR2477, and the 3V
lithium CR123 is a bit thicker and shorter than a AA.
Pros: Light, small, inexpensive, high cell voltage,
high density, easy to stack, long shelf life
Cons: Non-rechargeable, low capability, need a
special holder
Price and capacity: $0.35 for a CR2032 with
220mAh; $1.50 for a CR123 with 1,300Ah
Power density: 270Wh/kg
Limor Fried makes nifty things and writes about them on
ladyada.net.
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