A lot of folks have trouble understanding the energy characteristics of batteries and the differences between them. Why does one light take 123A batteries and another take AAA? What's the deal with lithium AA? What makes one battery better than another? Well, here are some answers for you.

I'm going to focus on lithium and alkaline batteries here and add in rechargeable near the end.

It is very hard to explain the energy capacity and characteristics of batteries, but I'll use an analogy that seems to work pretty well.

Think of it this way:

• You have a bucket of marbles. The marbles run out through a spigot at the bottom. The marbles then flow over a waterwheel that they spin.
• The bigger the bucket, the more marbles you can hold.
• Big marbles spin the wheel faster than small marbles, but any particular bucket is designed to hold only one size of marbles.
• The bigger the spigot, the faster the marbles can pour out and the wheel spins faster. However, the bucket also runs out faster.
• Each bucket is fairly fragile and can only be used once.

Makes sense doesn't it? Now let's apply the analogy.

The energy characteristics of a battery are usually described as Amp Hours, Voltage, and Internal Resistance. All three of these factors affect how the battery works.

The waterwheel? That's the work that the energy is doing.

The bucket? That's the battery. The "bigger" the battery (bucket), the more electricity it can hold. The size of the battery is measured in Amp Hours (Ah for Amp Hours or mAh for milli-Amp Hours).

The size of the marbles? That's the voltage. Bigger marbles = higher voltage. The higher the voltage, the more effort it can put into getting the work done.

The size of the spigot? That's the internal resistance. Lower internal resistance (big spigot) means that the battery can dump out more of its electricity over any given period of time. Higher internal resistance (small spigot) keeps the electrictiy from flowing out of the battery as quickly.

Now let's apply this knowledge.

Amp Hours capacity:

D size cells are bigger (have greater mAh) and hold much more electricity (hold more marbles) than a AAA battery, so can do the same work much longer. Larger batteries are needed for when you need longer runtimes.

Voltage:

The higher the voltage, the more "effort" the electricity can exert and the more work it can do at any given moment (bigger marbles make the wheel spin faster). Using the same batteries, a 3 volt light will not be as bright as a 6 volt light. The batteries are doing more work (spinning the wheel faster) and giving us a brighter light as a result. We can either get a higher voltage battery or we can tie a bunch of batteries together to increase the voltage.

Amp Hours vs. Voltage:

Remember, though, that any given bucket can only hold one size of marbles. When we compare the 123A and the AA, the 123A has twice the voltage but only has half the capacity as the AA. The 123A holds fewer large marbles while the AA holds more smaller marbles.

Internal Resistance:

Lithium batteries have lower internal resistance (bigger spigot) than alkalines (smaller spigot) and can dump out all of their electricity (also called: "deliver high currents") much faster. This makes them good for when you need your effort (voltage) applied VERY QUICKLY. Very high output lights and electronic devices take advantage of this characteristic of lithium batteries.

Amp Hours vs. Internal Resistance:

The larger the alkaline battery (bigger bucket), the lower the internal resistance (bigger spigot). So the spigot on bigger alkaline batteries is naturally larger than the ones on smaller alkaline batteries. Why is this important? Because some devices RELY on the internal resistance of the batteries to limit the amount of Amps that are sent through the device. Reduce the internal resistance and the device cooks itself. This is why you absolutely do not want to replace alkalines with lithium batteries unless the manufacturer specifically says you can. With Lithium AA available from Energizer, some folks think you can just plop them into anything that takes AA batteries. Not true! Always verify that it is OK to do so first with the manufacturer. Otherwise your expensive electronic device may turn into an expensive paperweight!

On the subject of Lithium AA batteries... Lithum AA also carry higher voltage (bigger marbles) than alkaline AA cells. They're 1.7 Volts each, not 1.5, and they maintain more of that voltage even under load. This can cause serious problems in devices designed to use Alkalines which dip to about 1.2 Volts under load. So the lesson here is: be extra careful as to what you stick Lithium AA batteries into and always get manufacturer approval before doing so.

But what about rechargeable batteries?

Rechargeable batteries include NiCad (Nickel Cadmium), NiMH (Nickel Metal Hydride), and Lithium Ion.

If we apply the bucket analogy to these batteries:

• They all have big spigots, are easily refilled, but they all leak!
• If we empty them out too much they won't hold the marbles any more. We have to leave a minimum number of marbles in the bucket to keep it working.
• Every time we refill the bucket it gets damaged. The faster we dump in more marbles, the more damage is done. Dumping in marbles slower generally minimizes the damage.
• Regardless, eventually the bucket is damaged to the point where it won't hold marbles anymore.

Let's apply this information in real terms:

All three of these types of rechargeable batteries are capable of delivering high currents (dumping their electricity out) very rapidly.

They don't like to hold a charge when not used for a long period of time (the bucket leaks). They lose some of their charge on a daily basis. Lithium Ion is affected the least by this characteristic.

All of them should not be over-discharged (don't completely empty the bucket) or they can be damaged and won't hold a charge well any more (bucket gets damaged).

Generally a slower charge rate is better for making them last longer overall (refill the bucket slowly to avoid damaging it).

And that's rechargeable!

Well, it's a simple analogy, but hopefully it makes sense and brings some clarity to the relationship between batteries and their electrical characteristics.