How To Size A Battery Pack For A Project

The one thing to remember about battery selection is that there is no such affair as a perfect bombardment that works for every awarding. Selecting the right battery for your application is almost identifying the most important battery metrics and trading these off against others. For example, if yous need a lot of ability for your awarding, cell internal resistance needs to be minimized, and this is often done by increasing electrode surface area. But this also increases inactive components such as electric current collectors and conductive help, so energy density is traded off to gain power.

While your actual design goals on the bombardment may be lofty, you could have to give up some things in order to gain others when information technology comes to actual battery performance (Fig ane).

A lead acid bombardment works not bad in an automotive starter battery where it provides the required high rate capability. All the same, with its toxicity and low energy density it would be a terrible choice for a portable electronics awarding. And then, in this 3-part blog series, nosotros will look at how finding the right battery for your awarding is all about making the right tradeoffs. Office 1 discusses the of import considerations when selecting the right battery for a consumer application. These include rechargeability, energy density, ability density, shelf life, prophylactic, form factor, price and flexibility. Part 2 will look at how chemistry affects of import bombardment metrics, and therefore bombardment choice for your awarding. In part iii we volition look at common secondary bombardment chemistries.

Figure 1: Battery Pattern vs Performance

Some of import considerations in battery option are :

1. Primary vs. Secondary – One of the commencement choices in battery option is to decide whether the application requires primary (single utilise) or secondary (rechargeable) batteries. For the nearly part, this is an easy decision for the designer. Applications with occasional intermittent use (such equally a smoke warning, a toy or a flashlight), and disposable applications in which charging becomes impractical warrant the use of a primary bombardment. Hearing aids, watches (smartwatches being an exception), greeting cards and pacemakers are good examples. If the battery is to be used continuously and for long stretches of time, such as in a laptop, a cell telephone or a smartwatch a rechargeable bombardment is more suitable.

Primary batteries have a much lower rate of self-discharge - an attractive feature when charging is not possible or practical earlier first utilise. Secondary batteries tend to lose energy at a higher rate. This is less important in most applications because of the ability to recharge.

2. Energy vs. Ability - The runtime of a battery is dictated past the bombardment capacity expressed in mAh or Ah and is the discharge current that a battery can provide over time.

When comparing batteries of different chemistry, information technology is useful to wait at the energy content. To obtain the energy content of a battery, multiply the battery chapters in Ah by the voltage to obtain energy in Wh. For instance, a nickel-metal hydride bombardment with 1.two V, and a lithium-ion battery with 3.2 V may accept the same capacity, but the higher voltage of the lithium-ion would increment the free energy.

The open circuit voltage is commonly used in energy calculations (i.eastward. bombardment voltage when non continued to a load). Yet, both the capacity and free energy are both heavily dependent on the drain charge per unit. Theoretical capacity is dictated only past active electrode materials (chemistry) and active mass. Yet, practical batteries achieve only a fraction of the theoretical numbers due to the presence of inactive materials and kinetic limitations, which forbid full utilize of agile materials and buildup of discharge products on the electrodes.

Battery manufacturers often specify chapters at a given discharge rate, temperature, and cut-off voltage. The specified chapters volition depend on all 3 factors. When comparing manufacturer capacity ratings, brand sure you wait at drain rates in particular. A bombardment that appears to have a high capacity on a spec canvass may actually perform poorly if the electric current bleed for the application is college. For case, a battery rated at 2 Ah for a xx-hour discharge cannot deliver 2 A for 1 hour, merely will only provide a fraction of the capacity.

Batteries with high power provide rapid belch capability at high drain rates such as in power tools, or motorcar starter battery applications. Typically, high power batteries have low energy densities.

A good analogy for ability versus energy is to think of a bucket with a spout. A larger saucepan can hold more than water and is akin to a battery with loftier energy. The opening or spout size from which the water leaves the bucket is alike to power – the higher the power, the college the bleed rate. To increase energy, you would typically increase the battery size (for a given chemistry), but to increase ability you decrease internal resistance. Cell construction plays a huge part in obtaining batteries with loftier power density.

Figure 2: Battery Energy vs Ability

You lot should be able to compare theoretical and applied energy densities for dissimilar chemistries from bombardment textbooks. Still, considering power density is and then heavily dependent on battery construction you will rarely find these values listed.

3. Voltage – Battery operating voltage is another important consideration and is dictated by the electrode materials used. A useful battery classification here is to consider aqueous or h2o based batteries versus lithium based chemistries. Lead acrid, Zinc carbon and Nickel metal hydride all use water based electrolytes and take nominal voltages ranging from 1.2 to two V. Lithium based batteries, on the other hand, use organic electrolytes and have nominal voltages of 3.2 to 4 5 (both primary and secondary).

Many electronic components operate at a minimum voltage of three V. The college operating voltage of lithium based chemistries allows a unmarried prison cell to be used rather than 2 or three aqueous based cells in series to brand up the desired voltage.

Another thing to notation is that some battery chemistries such as Zinc MnO2 accept a sloping discharge bend, while others have a flat profile. This influences the cutoff voltage (Fig three).

Figure 3: Voltage Plot Based on Battery Chemistry

4. Temperature range – Battery chemistry dictates the temperature range of the application. For instance, aqueous electrolyte based Zinc-carbon cells cannot be used below 0°C. Alkali metal cells likewise exhibit a sharp refuse in chapters at these temperatures, although less than Zinc-carbon. Lithium chief batteries with an organic electrolyte tin can exist operated upwardly to -40°C but with a significant drop in performance.

In rechargeable applications, lithium ion batteries can exist charged at maximum rate but within a narrow window of about 20° to 45°C. Beyond this temperature range, lower currents/voltages need to be used, resulting in longer charging times. At temperatures beneath five° or 10°C, a trickle charge may be required in order to foreclose the dreaded lithium dendritic plating problem, which increases the risk of thermal runaway (nosotros take all heard of exploding Lithium based batteries which could happen as a result of overcharging, low or high temperature charging, or short circuiting from contaminants).

Other considerations include:

5. Shelf life – This refers to how long a bombardment will sit in a storeroom or on a shelf before it is used. Primary batteries have much longer shelf lives than secondary. Even so, shelf life is generally more than important for primary batteries because secondary batteries accept the ability to be recharged. An exception is when recharging is not applied.

6. Chemical science – Many of the properties listed higher up are dictated by prison cell chemistry. We will hash out ordinarily available battery chemistries in the side by side part of this blog series.

7. Physical size and shape – Batteries are typically available in the following size formats: button/coin cells, cylindrical cells, prismatic cells, and pouch cells (well-nigh of them in standardized formats).

8. Cost – There are times when you may need to decline a battery with better functioning characteristics because the application is very cost sensitive. This is particularly true for high volume disposable applications.

9. Transportation, disposal regulations – Transportation of lithium based batteries is regulated. Disposal of sure battery chemistries is also regulated. This may exist a consideration for loftier volume applications.

In that location are many considerations when selecting a battery. Several of these are related to chemical science, while others are related to bombardment design and construction. This makes it harder and sometimes meaningless to do a battery metric comparison without a more than fundamental understanding of the factors that affect that metric, a topic we'll explore in the second blog of this series.

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Topics: Battery Reliability