Speak the Language

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Speak the Language: Understanding the Specifications of Batteries

By: Eric Wasson at Battery Systems

For those who know a lot about the technical side of batteries, the jargon associated with them is common knowledge that is shared by those in the field. But, for those of us who may not be as well-versed in the terms that battery experts use, this can get very confusing, very quickly. Hopefully this guide will act as your translator, helping you better understand what is being said by those you talk to who are a bit more knowledgeable in this field.

Batteries usually are described and specified with regard to three characteristics: chemistry, voltage, and specific energy.

As for chemistry- batteries are typically either lead, nickel, or lithium. Each battery type has its’ own charger. Also, when shipping and disposing of batteries, each chemistry has different regulatory requirements, so pay attention to regulations, and make sure you follow them correctly.

Batteries are also denoted by voltage. Batteries are marked by nominal voltage. They also might specify open circuit voltage (OCV) on a battery. The OCV is usually about 6% higher than whatever the nominal voltage is. The closed circuit voltage (CCV) is also slightly different. The CCV is the operating voltage for a battery.

Lastly, capacity is used often in specifications. Capacity is represented by a few different keywords. The most common term thrown around with regard to capacity is ampere-hours (Ah). Ah is the discharge current a battery delivers over time, or the amount of energy charge in a battery that allows one ampere of current to flow for one hour. Usually, Ah is compared based on a discharge over 20 hours. For example, a 100 Ah battery discharges 5 Ah for those 20 hours. European starter batteries are marked in Ah. Reserve Capacity (RC) is a North American rating for discharge for starter batteries that is also commonly used. It measures the number of minutes a fully charged battery at 80°F will discharge 25 amps until the battery drops below 10.5 volts.

There are a few other specifications that batteries may include. I will go through these other terms you may hear or read, and help break down what they mean.

  • CCA: is the cold cranking amps. It is a measure of the discharge load in amperes that a new, fully-charged battery can deliver for 30 seconds, while still maintaining a terminal voltage equal to, or higher than 1.20 volts per cell. The key to this is that the battery is tested in a cold environment (0°F/-18°C) to mimic starting a vehicle in harsh cold weather that can impact a weak battery’s performance.

  • MCA: is the marine cranking amps of a battery and is derived from the same test as the cold cranking amps test, except that it is performed at 32°F (0°C). The cranking amps number will be higher than the CCA number, because of the warmer temperature, which helps the battery perform slightly better.

  • CA: is the same as MCA; it is the cranking amps measured at 32°F.

  • C- and E- rates: The C-rate is the measure of the rate at which a battery is discharged relative to its maximum capacity. The C-rate helps allow comparison between batteries that have very different battery capacities. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hours, this equates to a discharge current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would be 50 Amps. Similarly, an E-rate describes the discharge power. A 1E rate is the discharge power to discharge the entire battery in 1 hour.

  • State of Charge (SOC) (%): is the percentage of the present battery capacity out of its maximum capacity. SOC is generally calculated using current integration to determine the change in battery capacity over time

  • Depth of Discharge (DOD) (%): is battery capacity that has been discharged as a percentage out of maximum capacity. A discharge to 80 % DOD or more is called a deep discharge.

  • Terminal Voltage (V): is the voltage between the battery terminals with load applied. Terminal voltage varies with SOC and discharge/charge current.

  • Open-circuit voltage (V): is the voltage between the battery terminals with no load applied. The open-circuit voltage depends on SOC, and increases with SOC.

  • Internal Resistance: is the resistance within the battery. Internal resistance is generally different for charging and discharging and is also dependent on the battery’s state of charge. As a battery’s internal resistance increases, the battery efficiency decreases.

  • Cycle Life (number for a specific DOD): is the total number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of cycles and by other conditions such as temperature and humidity. The higher the DOD, the lower the cycle life of your battery.

  • Float Voltage: is the voltage at which the battery is maintained after being charged to 100 percent SOC. The float voltage is the voltage needed to maintain that capacity by compensating for self-discharge of the battery.

This article was written using information from MIT’s 2008 Electric Vehicle Team’s guide, as well as from Optima’s article on Cold Cranking Amps.

 

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