• Nickel-metal hydride (NiMH)
rechargeable batteries not only offer
better performance than NiCds, they
aren’t considered quite as poisonous to
the environment as NiCd cells. Today,
they are the premier choice in
rechargeable batteries. They are a bit
more expensive than most other
battery types; prices have come down
recently. As with rechargeable
alkalines, NiMH cells require a recharger made for them. As luck would have
it, most of the latest rechargers will
work with rechargeable alkalines,
NiCds, and NiMHs, so a single charger
may be all you need if you want to
experiment with different battery
types. It is important that you do not
use a standard NiCd recharger with
NiMH, or vice versa.
column, we’ll lump Li-ion and polymer
Li-ion cells together, but just know
there are differences between the two,
and if plan on using either one, that
you use the absolute correct recharger
or recharging circuit, and that you
observe all operating specifications.
Common Battery
Cell Sizes
• Lithium-ion (Li-ion) cells are frequently used in the rechargeable battery
packs for laptop computers and
high-end camcorders, and other
electronic devices. These batteries are
expensive and require specialized
rechargers — in fact, if you don’t use
the right recharger, you could end up
so severely overheating the battery
that it violently explodes, so be
careful! On the plus side, Li-ion cells are
surprisingly lightweight for the high
level of current output they provide.
• Sealed lead-acid (SLA) batteries are
much like the battery in your car,
except that the electrolyte is in gel
form, rather than a sloshy liquid of
water and acid. SLAs are “sealed” to
prevent most leaks, but in reality, the
battery contains pores to allow oxygen
into the cells. SLA batteries are
rechargeable using simple circuits,
and are a good choice for very high
current demands.
Not long ago, all household electronics used the same small assortment
of battery cells. These came in
standard sizes, most commonly AA, C,
D, and rectangular nine-volt (in the
old days, we called these transistor batteries, because they were commonly
used in small pocket transistor radios).
Table 1 shows the most standard
cell sizes, from N to D. Of these, only
N, AAA, AA, C, D, and nine-volt are
commonly available at the local retail
store. You can locate the other
fractional sizes online at various
battery specialty outlets. They are
typically used to build up battery packs
of different sizes and shapes of NiCd or
NiMH batteries.
Note that while the dimensions
of common battery sizes are largely
standardized, the weight can vary,
depending on the construction and
contents of the battery. The weight
provided in the table is representative
only for a cross-section of battery
chemistry types.
Cell voltage is especially critical
when used to create a battery pack.
Depending on the battery technology,
the combined output of a standard
four-cell AA holder may produce 4. 8, 6,
or 14. 8 volts. Obviously, this can have a
noticeable effect on anything you
connect to the batteries. With higher
voltages, many kinds of motors run
faster, but also hotter. An electronic
circuit may burn out if it gets too
much voltage, or may not operate if
the voltage is too low.
Forgetting about Li-ion cells for the
moment, the most common issue with
battery cell voltage is using NiCd or
NiMH rechargeables in a circuit that
was meant for alkaline batteries.
Rather than the usual six volts obtained
when using alkaline or other non-rechargeable cells, the circuit will get
only 4. 8 volts when using rechargeable
batteries. In some instances, the 1.2
volt decrease is not critical. However, in
other circuits — especially those
incorporating a five-volt linear regulator
— the 4. 8 volts is not sufficient to
power the device. Either the electronics
fail to operate completely or work only
sporadically.
The decrease in voltage also
affects motors. The higher voltage
means more current to the motor, so
the motor exhibits additional torque.
For non-stepping motors — such as
standard permanent magnet motors
used in gear motors and R/C servos
— the higher voltage makes the
And Now a
Word About
Battery Voltage
• Polymer batteries are among the
latest in rechargeable technology. They
are used for medium- to high-current
electronics applications like cellular
phones. These batteries use lithium as
a component, but they are not quite
the same as the Lithium and Li-ion cells
mentioned above. Polymer batteries
can be manufactured with thicknesses
as small as 1 mm wafers. In this
You already know that
different kinds of batteries
produce different voltages.
Even more, different battery
technologies produce different voltages for any given
standard size. For example,
an AA alkaline cell produces
a nominal (normal, average)
1.5 volts; for NiCd or NiMH,
the same size cell produces
1.2 volts. To confuse matters
even more, most AA-size
Li-ion cells produce 3. 7 volts.
Cell Size Diameter (mm) Height (mm) Weight (g)
N 12.0 30.0 5
AAA 10. 5 44. 5 12
1/3 AA 14.0 14.0 7
1/2 AA 14.0 17.0 14
2/3 AA 14.0 28. 3 14
4/5 AA 14.0 42. 2 23
AA 14.0 50.0 25
A 17.0 50.0 35
1/2 C 23.0 26.0 23
C 25. 2 49. 2 80
D 32. 2 60.0 150
Nine-volt 25. 7 x 17. 4 48. 2 45
TABLE 1
SERVO 09.2007 73