There are a few different ways to 'measure' batteries, here are the ones I will be comparing
Since this is a particularly confusing part of measuring batteries, I'm going to discuss it more in detail.
Power capacity is how much energy is stored in the battery. This power is often expressed in Watt-hours (the symbol Wh). A Watt-hour is the voltage (V) that the battery provides multiplied by how much current
(Amps) the battery can provide for some amount of time (generally in
hours). Voltage * Amps * hours = Wh. Since voltage is pretty much fixed
for a battery type due to its internal chemistry (alkaline, lithium,
lead acid, etc), often only the Amps*hour measurement is printed on the
side, expressed in Ah or mAh (1000mAh = 1Ah). To get Wh, multiply the Ah
by the nominal voltage. For example, lets say we have a 3V nominal
battery with 1Amp-hour capacity, therefore it has 3 Wh of capacity. 1 Ah
means that in theory we can draw 1 Amp of current for one hour, or 0.1A
for 10 hours, or 0.01A (also known as 10 mA) for 100 hours.
However, the amount of current we can really draw (the power
capability)
from a battery is often limited. For example, a coin cell that is rated
for 1 Ah can't actually provide 1 Amp of current for an hour, in fact
it cant even provide 0.1 Amp without overextending itself. Its like
saying a human has the capability to travel up to 30 miles: of course
running 30 miles is a lot different than walking! Likewise, a 1Ah coin
cell has no problem providing a 1mA for 1000 hours but if you try to
draw 100mA from it, it'll last a lot less than 10 hours.
For
example, in this image, a coin cell can drive a 3.9Kohm resistor and
provide 230mAh (which is what its rated for) before dropping to 2V, but
if its a 1Kohm resistor, it will only provide 125mAh (image from
http://biz.maxell.com/en/product_primary/?pci=9&pn=pb0002)
's. A
is the Amp-hour capacity divided by 1 hour. So the
of a 2Ah battery is 2A. The amount of current a battery 'likes' to have drawn from it is measured in
. The higher the
the more current you can draw from the battery without exhausting it prematurely. Lead acid batteries can have very high
Lead Acid batteries (
image above)
are the workhorse batteries of industry. They are incredibly cheap,
rechargeable, and easily available. Lead acid batteries are used in
machinery, UPS's (uninterruptable power supply), robotics, and other
systems where a lot of power is needed and weight is not as important.
Lead acid batteries come in 2V cells, that means you can have a battery
with an even number of volts. The most common voltages are 2V, 6V, 12V
and 24V.
Pros: Cheap, powerful, easily rechargeable, high power output capability.
Cons: Very heavy, batteries tend to be very large bricks because power density is very low.
Prices: A 12V lead acid battery with 7Ah of charge should run about $25.
Power Density: 7 Wh/kg.
Alkaline
Alkaline batteries are the most common batteries you will come
across. They are the ones sold in every store, so they're great for
projects that need to be 'user serviceable.' They have higher power
density than NiCads and slightly better power density than NiMH.
However, they are one-time use. Cels are 1.5V, and availble in sizes
from coin cells to AAAA to D cell. One nice thing about having multiple
sizes with a standard voltage is that you can always just specify the
next size up when you need more capacity and capability. You'll get the
published capacity rate if they are discharged at about 0.1C.
6V lantern batteries (image above) are very large alkalines made of a couple large
cells, they're rather convenient in that they're available in many
stores, have massive capacity and capability and you can clip/solder
onto their tabs pretty easily.
9V batteries are a strange case: they're actually made of 6 very
small 1.5V batteries, pretty much the size of coin cells. As a result
they have very low capacity and capability and are very expensive. If
you are drawing more than 20mA then they are probably not a good idea to
use.
Prices: AA size battery costs about $1 and has up to 3000 mAh of charge capacity.
Power density: 100 Wh/kg
Pros: Popular, well known, safe, long shelf life
Cons: Non-rechargeable, low-capability
Ni-Cad Batteries (Nickel Cadmium)
These are the older rechargables that were popular for a long time.
They come in 'standard small' battery sizes like AA, AAA, C as well as
recangular shapes that make them easier to embed in an enclosure
(see image above).
They are not used as much these days because NiMH batteries have much
higher power density. However they are cheaper, and are still used in
many cordless phones, solar lights and RC cars where performance is not
as important as price. Another nice thing is they discharge slower than
NiMHs. (That is, left alone, they will retain their charge longer.)
Battery cells are 1.2V, often bundled in "packs" of 3 to make 3.6V.
Pros: Inexpensive, rugged, come in "standard" sizes, easy to recharge.
Cons:
Lower power density, requires "full discharge/recharge" cycles every
once in a while to reduce 'memory effect' (the growth of crystals on the
battery plates), contains toxic metal.
Prices: AA size battery costs around $1 and has up to 1000mAh of charge capacity.
Power Density: 60 Wh/kg
