TMI about batteries (TMI = Too Much Information)

[oh_blinding_light]

It's time for another installment of OBL's sleep inducing lecture series on bicycle lights. So pull up a comfy chair, a pillow and prepare to doze. Today's snooze inducing lesson will be on batteries. I've been getting a lot of questions about which batteries to use with my home-built LED lights, so I figured another post was in order.

Some terms:

Amperes, amps, amp-hours, mA, or mAh: These terms all refer to current. Current is a measure of the number of electrons that flow thru a wire. 1 ampere is about 6.242 × 10 to the 18th power electrons (that's 6242000000000000000 electrons). Current is usually measured over a period of time, so an amp-hour is 6.242 x 10 to the 18th electrons flowing thru a wire over the course of an hour. Most people use amps, amperes, and amp-hours interchangeably, which isn't technically correct, but we'll ignore that for now. mA stands for milliamp and it means a thousandth of an amp. mAh stands for a milliamp-hour. Batteries are often marked in terms of ampere-hours (abbreviated Ah) or milliampere-hours (mAh). A battery with a marking of say 2500 mAh can deliver a current of 2500 mA for one hour, or a current of 1250 mA for 2 hours, or a current of 625 mA for 4 hours. Well, you get the idea.

Volts, mV: These terms refer to volts, which technically is defined in terms of current and power. If you never took Electrical Engineering 101 you'll have no clue what P = I x V means, so I'll use an easier analogy. Amp-hours is a measure of how many electrons go thru a wire in an hour, you can think of amp-hours as being like the measurement of water flowing thru a pipe (say gallons per hour). Following this analogy, volts is sort of like pressure. There's a more technical explanation of this at: http://en.wikipedia.org/wiki/Hydraulic_analogy

Watts or Power: The hydraulic analogy helps explain power. Think of a pipe with water flowing thru it hitting a waterwheel. The more water flowing thru the pipe and the higher the pressure, the more power the waterwheel can produce. If you harness the waterwheel to some mechanical contraption, say an elevator, with more current at a fixed pressure, the more weight you can raise with the elevator. Likewise with a fixed amount of water flow, and a higher pressure, the more weight your elevator can lift. In electrical terms, power is often expressed as watts. The formula that relates power to voltage and current is P (watts) = I (current) times V (voltage). Watts is a good way to measure how long you can run your light, especially with LED lights because the electronics that drive them can magically convert a range of volts and amps to allow the LED to run off many different types of batteries. A good approximation of the watts in a battery is to multiply its voltage times the current it is rated for. An example: an AA alkaline battery can produce about 1.5 volts and deliver around 3000 mAh of current (or 3.0 Ah). Doing the math we get about 4.5 watts. If you had 8 of these, wired in a pack, you could in theory draw about 36 watts. And if you had a LED headlight like my BFL-1000, that draws 10 watts of power, wired to your battery pack, you could run your light for about 3.6 hours. Please note: this is a rough approximation and we'll cover why later.

Types of Batteries: There are basically 3 battery chemistries used for bicycle headlight these days: alkaline, nickel, and lithium.

Alkaline - we've all used these disposable batteries in flashlights, toys of kids, cameras, etc. They are not rechargeable. You will from time to time find somebody selling a charger for them, but in my limited experience these rechargers don't do a very good job and may cause your cheaper batteries to leak acidic goo onto the electronics of your expensive digital camera. For mountain biking, they make good batteries for your backup flashlight (you do carry one don'y you?). Some of my lower power lights can use these as backup batteries. More info: http://en.wikipedia.org/wiki/Alkaline_batteries

Nickel - You usually see these as either NiMh (nickel metal hydride) or NiCad (nickel cadmium). Really old lighting systems (like from before 1999) will use NiCads. Modern systems that use Nickel batteries use NiMh. NiMh typically hold more power and are lighter, so the energy density is better, you can carry more watts with less weight. Also Cadmium is pretty toxic. One should never throw old NiCad batteries in the trash, always recycle them. It's also not a bad idea to recycle NiMh batteries, but if some wind up in your trash it isn't the end of the world. More info: http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery

Lithium - The rechargeable batteries used by bike lights that contain lithium are typically referred to as Lithium Ion batteries. You will see a lot of different abbreviations: Li-Ion, LiPo, etc. If a battery's abbreviation has Li in it it is most likely some form of Lithium Ion battery. There are 3 main types of Lithium batteries in use in bike lights today. Li-Ion usually refers to a battery pack built up of 18650 cells (18 mm in diameter, 65 mm long). These cells have a core electrode of elemental Lithium, which is very reactive. It bursts into flame in the presence of oxygen, it will even disassociate the hydrogen and oxygen that make up water if it gets wet. The lithium combines with the oxygen releasing a lot of heat and hydrogen, this can cause further problems if there is air present as then the hydrogen burns. LiPo (Lithium Polymer) batteries also use lithium. These batteries impress lithium, carbon, and electrolyte into a polyester weave material. The resulting battery is slightly flexible, and can be shaped to fill a small flat space. LiPo batteries are popular in cell phones, mp3 players, etc. Here's a reference for folks who want more info about lithium ion batteries: http://en.wikipedia.org/wiki/Lithium_ion

Arghhh! my head is going to explode! What type of battery do I really want?

We'll get to that in part 2 of this lesson. For now I need to solder up some new LED driver boards.

OBL
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Nimium est melior!

[mtbxtc]

V for volts also is

V=IR

Were I is current in amps
and R is resistance

if P = IV
substituting V with IR you get

P = IxIxR
or I squared times R.

Good writeup
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Everyone is entitled to be stupid, but some abuse the privilege.
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