Reserve Batteries: How to Address their Specific Energy and Energy Density

 Reserve batteries or deferred-action batteries are primary cells intended for emergency use. In such power sources, the electrolyte is typically stored separately from the electrodes, which remain in dry inactive state. As a result, the active materials are not degraded over conventional storage, and no self-discharge of the battery occurs prior to battery activation. Hence, reserve batteries possess a quasi-unlimited (“infinite”) shelf life. They are activated on demand, by introducing the electrolyte into the cell. Allowing the access of the electrolyte between the electrodes, establishes the internal circuit of the cell, and the reserve battery gets activated, starting to deliver power.

The different types of reserve batteries are: (i) ampoule batteries, in which the electrolyte is stored in a separate container located in the battery case; the battery is activated by breaking the ampoule, which enables the dispensing of the electrolyte into the cell; (ii) thermal batteries containing a solid electrolyte, which is dormant at ambient temperatures, becoming active upon melting; so, their operation is triggered by heat from an external source; (iii) water-activated batteries using seawater or some other water (brackish water or wastewater) as the electrolyte. This electrolyte is supplied by the surroundings of the battery, when the cell is deployed. This kind of operation offers the advantage of not having to transport the heavy electrolyte with the cell. (For additional information see: “Reserve Batteries” in Electropaedia)

Some advantages of water-activated batteries are their reliability, safety in operation, light weight (without the electrolyte), high power density and energy density, instantaneous activation, infinite shelf life, and no need for maintenance. Among the few shortcomings one should mention that after activation they undergo fast self-discharge, and once activated, they need to be replaced.

Here below we will discuss the specific energy and energy density of seawater-activated batteries. The anode material of such reserve batteries may consist of magnesium or a magnesium alloy; these are the magnesium-air batteries. Aluminum is also widely used as the anode. After activation, magnesium and aluminum dissolve over time; so, they are sacrificial anodes. Cell operation ends at the time, when no more metal is left in the cell. Therefore, such reserve batteries are anode limited.

Specific Energy and Energy Density of a Magnesium-Air Battery

Prismatic Reserve Battery

As an example of calculation, let us consider a prismatic reserve battery, with the specifications listed in Table 1. The specific energy listed below does not take into account the mass of the electrolyte, which is supplied by the natural environment, where the cell is being deployed. Hence, the total mass considered is the sum of masses of electrodes and battery mechanical frames. By contrast, the volume used for energy density calculations includes the void space between the electrodes, where the electrolyte gets access upon deployment.

Table 1: Specifications for prismatic reserve battery

Battery Specifications

Unit

Value

Length

[cm]

20.

Width

[cm]

12

Height

[cm]

25

Volume

[L]

6.0

Mass (total)

[kg]

1.2

Mass (Magnesium anodes, Mg)

[kg]

0.8

Theoretical Capacity (from Mg)

[Ah]

882

Nominal Voltage

[V]

1.5

Theoretical Energy

[kWh]

1.32

Specific Energy*

[Wh/kg]

1.1

Energy Density**

[Wh/L]

0.22

* Total mass taken into calculations is the sum of masses of electrodes and battery mechanical frames; no mass of electrolyte is being added

** Total volume includes the space between the electrodes, where the electrolyte penetrates upon deploying the reserve battery

Cylindrical Reserve Battery

The energy density calculations become more complicated for a pipe-shaped reserve battery (schematic shown in Figure 1), which has the anodes sandwiched in between two cylindrical cathodes. In this case, the mass of the cell can be weighed in a reliable manner, but its volume is more difficult to be delimited. The question here is, whether the volume to be considered for energy density calculation should be the total (external) volume, according to Equation (1), or the difference between the external and internal volume, as shown by Equation (2)?  (All parameters of the equations are defined in Figure 1.)

V (external) = π (L/2)2 x H      (1)

V (external) – V (internal) = π (L/2)2 x H – π (l/2)2 x H      (2)

As an example of calculating the energy density in both assumptions, we will use data listed in Table 2. While the specific energy of the battery should be the same (given the constant mass of the reserve battery), the last entries of Table 2 reveal that the energy density is significantly different for the two volumes considered.
  

 

Table 2: Specifications for cylindrical reserve battery

Battery Specifications

Unit

Value

External diameter

[cm]

12

Internal diameter

[cm]

8

Height

[cm]

20.

Volume (external)

[cm3]

2,262

Volume (external) – Volume (internal)

[cm3]

1,257

Theoretical Capacity (from Magnesium)

[Ah]

220

Nominal Voltage

[V]

1.5

Theoretical Energy

[Wh]

330

Energy Density (external volume, Eq. 1)

[Wh/L]

146

Energy Density (volume difference, Eq. 2)

[Wh/L]

263

As revealed by Table 2, considering the volume in between the electrodes, and neglecting the innermost volume of the reserve battery, yields 1.8X greater energy density than using the whole cell volume. The innermost empty space/room of the reserve battery may accommodate a device to be powered, i.e., the battery may be built around the device; in such scenarios Equation 2 (smaller volume considered) may be more appropriate for energy density calculations than Equation 1 (where the entire volume is used in calculations).

Conclusions

Reserve batteries supply power on demand, typically in emergency situations. The main advantage of water-activated batteries is that their electrolyte is supplied by the environment, where they get deployed; hence, only the electrodes and battery frames need to be transported, rather than additionally carrying the aqueous electrolyte.

While the specific energy of reserve batteries can be determined unambiguously, their energy density calculation needs a clear definition of the considered battery volume.

About the Author

John

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

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