## Latest from Test & Measurement

June 20, 2017

From cell phones to electric vehicles, every user is concerned about runtime. System designers work diligently to maximize runtime using one of two approaches: design the battery-powered system to consume electricity efficiently so the batteries last longer, or maximize the amount of energy available to the battery-powered system. To maximize available battery power, you can use a larger battery or a high-capacity smaller battery. Since most battery-powered systems are portable, weight and size are considerations. As such, using a larger battery somewhat defeats the goal of smaller and lighter.

So, when building a battery, you’re best served by building a battery with high capacity. A battery is built up from cells, placed in series to increase available voltage and in parallel to increase available current. Thus, high-capacity batteries are built up from high-capacity cells. Today, the lithium-ion cell is the go-to cell for most battery-powered applications, with a great balance of size, weight, available current, capacity, and cost.

The Capacity of a Lithium-Ion Cell

Lithium-ion cells, or any cell for that matter, have a capacity measured in ampere-hours (Ah). For review, one ampere-hour means that you can draw one ampere from the cell for one hour. So, ampere-hours is the product of amperes times hours. Likewise, 1 Ah also means you can draw 2 A for 0.5 hours, or 0.25 A for four hours.

Ah capacity is, in fact, a measure of stored coulombs. Looking at units involved in ampere-hours, one ampere is 1 coulomb per second. If you multiply amperes × time, you get coulombs. Given that one hour is 3600 seconds, then 1 Ah is 3600 ampere-seconds, or (3600 coulombs/second) × seconds, which equals 3600 coulombs of stored charge in the cell. Note that for smaller cells, you may find their capacity measured in milliamp-hours, (mAh).  For example, a typical 18650 lithium-ion cell will store around 3 Ah, or 3000 mAh.