Power? Energy? Doesn’t My Battery Deliver Both?

Oct 11, 2018 8:30:00 AM / by Steve Jennings

Batteries are like tools: you must get the right one for the job. You wouldn’t use a sledgehammer to drive a nail in a wall, for obvious reasons, and you wouldn't use a AAA battery when a D battery was required. Fortunately, most consumer devices are built so you can't insert the wrong battery, but in data center and other commercial situations, we are sometimes using the wrong batteries without even knowing it. One fundamental operating differentiator for industrial batteries is power versus energy. Every battery delivers both, but each type is better at delivering one or the other. The kind of battery you need depends on your application and, like a sledgehammer vs. a hammer, you get better results when you choose the right one.

Energy versus Power

We'll reference Battery University to define energy and power as they apply to batteries.

Specific energy, or energy density, is the total amount of energy (watt-hours or Wh) that a battery can deliver relative to its weight (Wh/kg) or volume (Wh/l). Batteries requiring long runtimes at moderate load are optimized for high specific energy.

Specific power indicates loading capability: how much power (in watts) the battery can deliver at a time. Batteries for power tools, for example, are built for high specific power (loading) and lower specific energy (capacity). You don't use your battery-powered drill for hours at a time, but you need it to have enough power when it is running.

The figure on the right illustrates the relationship between specific energy (the total water in the bottle) and specific power (the spout, which determines how much water the bottle can deliver at a time).

The Right Battery for the Job

Now that we've defined our terms, let's look at applications for each battery type. An all-electric car needs to run for hours at a time on battery power for about 5,000 charge cycles, so it needs an energy battery. A micro-hybrid vehicle stops and restarts the engine at every stopping point. This type of vehicle needs large amounts of energy in 3-second bursts for about 250,000 charge cycles, so it needs a power battery.

Similarly, in a cell phone or mobile computer, the device needs to run for hours on battery power, so an energy battery is needed. During a utility power outage, mission critical facilities such as data centers require back-up power systems to deliver high amounts of power for short periods, until utilities are restored or back-up generators can be brought online. 

Choosing for Energy or Power

To answer the original question: yes, all batteries can be described in terms of both energy (capacity) and power (loading capability), but each type of battery is optimized for one or the other. Lead-acid and lithium-ion batteries are energy batteries, built to support lower loads over longer periods. Nickel-zinc is an example of a power battery, built to efficiently handle large loads for short periods in start-stop, power back-up, and other "power" applications.

In choosing one type of battery versus another, you make trade-offs in battery safety, cycle life, and recyclability. For example, using energy batteries for data center backup is not space-efficient and creates environmental impacts (lead-acid batteries) or safety risks (lithium-ion batteries), whereas using a power battery such as nickel-zinc minimizes all these issues. So, whether you need energy to run your smartphone or Tesla all day or reliable power for data center backup or to start and stop a hybrid bus 500 times a day, you always want to find the best, most reliable, and safest battery for the job. We'll talk more about all the characteristics and trade-offs of different battery types in future blogs.

Topics: UPS, Power, batteries, power battery, energy battery, data centers

Steve Jennings

Written by Steve Jennings