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The lowdown on batteries: Specs, terminologies

Posted: 24 Jan 2014  Print Version  Bookmark and Share

Keywords:lead-acid  battery  power management  resistance  Impedance 

In this series of articles, we will consider a variety of different battery technologies, ranging from lead-acid, to zinc-based, to lithium-based, to all sorts of weird and wonderful implementations. Also, in each article I will be providing a couple of tips and tricks.

Tip No. 1: Start Selecting. I always begin the selection process with the environment. Often, the search space can be pruned by the environmental constraints. For example, why waste time looking up lithium batteries when the battery must operate at 325°C, where standard lithium dare not tread and sodium-sulphur lives? Another example is the old-style (non-GEL) lead-acid battery that periodically needed water—this battery type is automatically eliminated in the case of a fully-sealed environmental requirement.

What is a battery?
The traditional view of a battery is as a contained chemical reaction that produces electricity, but this definition is blurring in these modern times. This could provide an interesting blog topic in itself—perhaps we can challenge each other with a good discussion at the end of this series.

Tip No. 2: Energy vs. Power. To keep from getting confused, it is important to note that a battery stores potential energy. When load current flows, this potential energy is converted to work in the form of electric power, which is amps at some voltage, which is watts (amps x volts). The total energy delivered to a load is the instantaneous volts multiplied by the instantaneous amps, or v(t) x a(t), integrated over time. This is because the unit of energy is the joule, which is watts multiplied by seconds.

Specifications and terminology
We seem to always want a battery that has the biggest "wallop." But at what cost? For what size? And at what weight? Obviously there are always trade-offs, and we need to know how the manufacturers specify all of this stuff. We will be covering some of these items in more detail in future posts, but for the moment here are some terms and specifications:

Primary and secondary types: Primary batteries are those that are producing a voltage and are basically ready to be used (not charged) as soon as they are assembled. These are mostly not rechargeable. Secondary batteries need to be charged before first use after being assembled. These are generally rechargeable.

Smart type: This battery type includes a device or circuit used to monitor the battery and communicate its status to an external device. Typically, the circuit reports things such as state-of-charge, temperature or environment, health, age, serial number, model number, etc. Some can be commanded to perform certain power management tasks and include safety features such as current limits and performance thresholds. If a battery has four or more connections, there is a good chance it could be a smart battery. There is a great deal of information associated with this topic, so we will consider it in greater detail in a future post.

Internal resistance and maximum internal resistance: The internal resistance of a battery, measured in ohms, is the effective DC resistance acting in series with the voltage-generating part of the battery (for the AC resistance, see Impedance below.). The maximum specification is the worst-case internal resistance for a charge or discharge when operating the battery safely and within the manufacturer's recommendations. To measure the DC internal resistance, I use the two-tier DC-load method. Select a load resistance that will lightly load the battery, apply the load for 10 seconds, then measure the terminal voltage V1 and the current I1 with the load still connected. Then apply a higher load (lower resistance) for three seconds and measure the terminal voltage V2 and current I2. The DC internal resistance = (V1—V2)/(I2—I1). The two loads must be different enough to produce a very clear voltage delta while remaining inside the manufacturer's recommendations. Confidence in the number increases by making an additional measurement using another completely different resistor-pair. Note that the internal resistance (and impedance) of a battery are the primary causes of loss-of-capacity and inefficiency.

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