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Battery size measurements
Submitted by reikiman on Fri, 08/15/2008 - 20:28
What size is that battery? How much 'juice' does a battery hold? The amount of electricity held by a battery is very directly related to the speed and/or range you can get with that battery. While you can think of a battery as being like a "fuel tank" it doesn't behave like the gasoline tank on a car or motorcycle. A battery weighs the same regardless of whether it's fully charged or fully discharged. This means you can't do a simple measurement like weight to tell when you're running low on fuel.
Electricity is stored in a battery using chemical reactions. Electrons are added to a battery while it's charging, and those electrons are bound up in the chemistry of the battery causing chemical reactions to store the electrons. Then when you connect a circuit to the battery other chemical reactions occur to release the electrons and give the current.
A battery comes with a pair of ratings, Volts and Amp-Hours. The voltage is also called a "nominal voltage" because the actual voltage is hardly ever the rated voltage. With most battery chemistries a fully charged battery will have a voltage well above its nominal voltage, and further during discharge the battery voltage will 'sag' due to the discharge. And as a battery discharges its voltage also decreases, and the most typical way to determine the charge of a battery is by its voltage.
An Amp-Hour is a measure of a number of amps for a number of hours. 1 Amp-Hour is one amp of current for one hour, 2 amp-hours are either 2 amps for one hour or 1 amp for 2 hours or some variant in-between which multiplies to '2'.
This might lead you to believe that a 12 volt 12 amp-hour will give voltage centered around 12 volts, and provide amps at some rate and time which multiplies to 12 amp-hours. However, life is not that simple.
The amp-hour rating is typically measured at the 20 hour rate, or C/20. C is a current rate which would completely discharge a battery in one hour. That is, for the 12 volt 12 amp-hour battery just mentioned the 'C' rate is a 12 amp discharge. Supposedly at a 12 amp discharge rate the battery will provide current for 1 hour (12 amps for 1 hour = 12 amp-hours).
However it is not that simple.
It's observed that, with most battery types, discharging it at a high rate will give you fewer amp-hours (and give a deeper voltage sag) than if it's discharged at a low rate. 'C' is the rate which would discharge the battery in one hour, and C/20 is the rate which discharges it in 20 hours. Since 12/20=.6 the C/20 rate for this 12v12ah battery is .6 amps.
Discharging this battery at a 12 amp rate will, as already said, provide less than 12 amp-hours. The exact amount it provides varies on a lot of factors.
The discharge rate can also be higher than C, such as 2C or 10C. At 2C the discharge rate of this 12v12ah battery would be 24 amps, at 10C it's 120 amps.
One interesting angle to look at the amp-hour capacity of a battery is to compare several batteries, their weight, and their amp-hour rating. Start with lead-acid batteries. You'll notice when looking at a chart of lead acid battery characteristics that there are several standard sizes to the battery, and the batteries of the same size tend to weigh the same amount. It's known that a certain weight of lead will hold a certain amount of electricity. However if you look at the right list you may find two batteries, the same size, the same weight, but a different amp-hour rating. What's going on?
Battery construction offers different ways to optimize a given battery. It can be optimized for large amp-hour capacity delivering current at a low rate, or it can be optimized for a high amp-rate and the total amp-hour capacity will diminish a bit.
Another important characteristic is whether it is a deep discharge battery, or not. Batteries meant to for starting a gasoline car are readily recharged while you're driving after starting the car. This means all they're called upon is to start the car, and then they're recharged right away. However in an electric vehicle you don't have a gas engine recharging the battery, instead you're going to travel as far as possible on whatever charge you have. This also holds true for marine use such as trolling motors. This leads to batteries optimized for the high current required to start a car (but have a shallow discharge) and batteries optimized for deep discharge over a relatively long time (such as in an electric vehicle or boat).