WORKSHOP
A
BC
Fig. A: Lead-acid battery. Fig. B: Lithium-ion battery.
Fig. C: Nickel metal hydride cell.
Power capability is the rate at which a battery can
deliver its power without wasting energy, denoted by
C and measured in amps. The C of a battery is proportional to the capacity, so a 1Ah capacity battery
with 0.1C capability can supply 100mA of current.
Stacked batteries should have matching C values,
because total capability is limited by the weakest link.
Capability depends on chemistry. Lead-acid
batteries have the highest capability, 10C or more,
which is why they’re used for cars and other
applications that require floods of power. Alkalines
are about 0.1C, and lithium cells have capabilities
of only about 0.01C — just enough trickle to run a
wristwatch.
Power density refers to how much power a battery
supplies for its weight, expressed in watt-hours per
kilogram (Wh/kg). Higher-density batteries are
good for projects that need to be lightweight.
Price tends to be proportional to capacity and power
density. The more power you want in a smaller,
lighter package, the more you have to pay.
Battery Chemistry Types
Lead-Acid
These workhorses are good for systems that need
plenty of power where weight is not important, such
as big motors, projectors, and loud amplification
(Figure A). Lead-acids are based on 2V cells, with
the most common voltages being 2V, 6V, 12V, and 24V.
152 Make: Volume 11
Pros: Rechargeable, cheap, powerful, high capability
Cons: Very heavy, batteries tend to be large because
power density is low
Price and capacity: $20 for a 12V battery with 7Ah
Power density: 7Wh/kg
Alkaline
These common disposables are great for projects
that need to be user serviceable (Figure D). Cells
are 1.5V, available in sizes from coin cells to AAAA
to D, so it’s easy to up-size to get more capacity.
Standard 6V lantern batteries (Figure E) and 9V
batteries (opening photo) are multi-cell alkalines.
Lantern batteries have massive capacity and terminals that are easy to clip or solder to. Standard 9V
batteries are common but have low capacity and
capability, are expensive, and can’t continuously
deliver more than 20mA.
Pros: Popular, safe, long shelf life
Cons: Non-rechargeable, low capability
Price and capacity: $1 for a AA cell with 3,000mAh
Power density: 100Wh/kg
NiCd (Nickel-Cadmium)
The original rechargeables, “ni-cads” are cheaper
than NiMH and are still used in cordless phones and
other products. NiCds also hold their charge longer
than NiMHs when not in use. Cells are 1.2V, often
bundled into 3.6V batteries (not pictured).
Pros: Rechargeable, inexpensive, long-lasting,
standard sizes
