Battery Power Density vs Energy Density: Which One is Important for Your Application?

Your application may impose several requirements on your battery specifications including safety, size, weight, and cost constraints. So, this brings the question–is your application better suited for high energy or high power batteries, and what are your cell chemistry choices?  How do all these factors impact cell architecture?  This blog will help you make informed choices in selecting the best battery for your device. While this blog article is mostly focused on drop in battery solutions, if you have custom battery development needs, many of the underlying principles still apply.

Before delving into this topic, let’s define terms so everyone’s on the same page. You can also check out our blog on battery terms explained for more detail.

  • A battery comprises one or more cells, regardless of size; multiple cells in a battery will be linked together by electronic circuitry and/or wiring.

  • Cell capacity is a measure of how much current (electron flow, in ampere-hours) a cell provides, and is determined by the limiting electrode material’s capacity. The limiting electrode is the electrode with the lesser capacity; in the case of lithium-ion cells, this is the cathode. 

  • Electrode material capacity is measured two ways: milli-ampere-hours/gram (mAh/g, more common and easier to quantify) and milli-ampere-hours/cubic centimeter (mAh/cc, more important). The former method of quantifying capacity (mAh/g) is referred to as gravimetric capacity, while the latter method (mAh/cc) is referred to as volumetric capacity.

  • Battery capacity values (ampere-hours) are constrained by cell and equipment designs. Cells are available in numerous sizes (for example, AA, AAA, D, 18650, and custom form factors), but within a given apparatus, must conform to fixed dimensions; that is, the available volume for materials in a cell is a constant. Therefore, if either electrode material is at the low end of the density range, it will require more of that material to offer competitive capacity with its higher-density competition. But there’s no room to fit it in! This is why cell manufacturers want maximized densities for electrode materials.

  • Energy is calculated by multiplying capacity (ie, current-time) and voltage, and expressed as Watt-hours. High energy batteries are associated with long run times between charging in the case of secondary batteries, and with a large capacity prior to disposal in the case of primary batteries.

  • Power is calculated as energy divided by time, expressed as watts. High power batteries with rapid discharge capability are used for bursts of energy, such as in vehicular acceleration and portable tools.

  • Material density is a critical aspect of both cathode and anode materials that may be overlooked; substandard values are often purposely omitted from data sheets.

  • Energy density is expressed as gravimetric (mW-h/g) or volumetric (mW-h/cc). Since size constraints are more common than weight limitations, equipment manufacturers use volumetric energy density as the critical metric. 

  • Power density is similar: gravimetric (mW/g) or volumetric (mW/cc). Since this usually refers to bursts of energy that have little to do with cell weight, volume is the more important variable.

Application Power Requirements

In selecting batteries, it is necessary to understand what your application demands. If it is long run time, then you want a high-energy battery. But be careful: don’t automatically choose based on the capacity (Ah) rating, but factor in the cell voltage (energy = Amp-hours x Volts).  For example, Nickel metal hydride  (NiMH) and Li-ion cells may have similar capacity/amperage ratings, but the higher Li-ion voltage (3-4-3.7V versus 1.2V) means nearly 3X as much energy (and 3X the run time).

If the application is acceleration-oriented (like portable tools), then go for the power batteries. These ratings are harder to find, in fact, are rarely listed because they are so use-dependent, so you must depend on cell chemistry choices.  Li-ion batteries with names/descriptions that include “phosphate” or “manganese” are good choices, and nickel metal hydride cells are OK, but require more maintenance (recharging after several days of non-use).  Having said that, almost any battery chemistry can provide short bursts of energy, but cell working life will be diminished, and cells will overheat, perhaps dangerously so, if pushed too hard.

The following graph compares energy outputs for several commercial battery chemistries in terms of gravimetric energy densities.  Obviously, Li-ion technology leads in performance.  Note that cobalt was the energy generator in the original Sony cells (1991) and Nickel cobalt aluminum or NCA cells (7100 of them) are what powers Tesla Motors’ electric car.

Figure 1. Comparison of energy performance from different cell chemistries.  PbH+ = lead-acid, NiCd = nickel-cadmium, Mn = manganese, PO4 = lithium iron phosphate, Co = cobalt, and NCA = nickel/cobalt/aluminum.  2014 data.

Battery Type

Cost, $/Wh

Energy, Wh/kg

Energy, Wh/L

Pb-acid

0.17

41

100

Alkaline*

0.19

110

320

Carbon-Zn*

0.31

36

92

Ni-MH

0.99

95

300

Ni-Cd

1.50

59

140

Li-ion (Co)

0.47

128

230

Table 1.  2011 cost/energy comparisons of common battery types.  *Limited cycle life.  Note cobalt-based Li-ion output has increased >35% in 5 years (Graph 1), dropping cost to $0.34/Wh.  Little change with other entries.

Bottom line: you, the user, must determine which is more important, energy density or power density.  Long battery life may be attained 2 ways: slow discharge (up to several hours) to maximize output with high energy density cells, or pulse discharge of power cells (several amperes for periods down to a few milliseconds).  Remember, the longer the pulse, the more stress on the cell chemistry.

Effect of Cell Architecture

In general, prismatic cells offer better power density: the rectangular design allows heat to dissipate faster, which is a safety factor and also extends working life (performance-damaging side reactions are accelerated at higher temperatures). Cells for high energy applications are usually cylindrical, as relatively slow discharge does not overheat the units. The larger the cell, regardless of architecture, the more difficult it is to control heat build-up during operation.

Other Application Requirements

Perhaps there are limits on circuitry in your widget that restrict voltage to 1000F can cut cycle life in half, or worse for secondary cells.

If you are looking for more information about batteries, check out our battery resources or battery fundamentals sections. Please contact us for any application or flexible battery development questions that you may have.

About the Author

John

Hi, I'm John, editor-in-chief of an Flexel Battery online magazine!

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