There’s no such thing as a “perfect battery.” The best way to find an ideal battery for a given application is to find one that offers the right combination of duration, cost, and performance. There are a number of important questions to ask when trying to find the right battery for your needs.

Primary vs. secondary batteries: What’s the difference?

  • Primary batteries are “one time use” batteries. Once the chemical reaction between the cell’s electrode materials has ended, the cell loses its ability to produce energy. It must be removed and replaced with a fresh cell/battery.
  • Secondary batteries are “rechargeable batteries”. The chemical reaction that takes place within the cell is reversible and repeatable for hundreds or thousands of cycles, depending on the chemistry.

What is the drain rate of a battery?

  • The term “drain rate” refers to how rapidly energy is released from a battery. The drain rate is the speed at which its electrical charges is depleted.
  • Different chemistries and cell types (bobbin versus spiralwound, for example) provide better drain rates than others.
    • For example, lithium coin cells are designed for low loads and long calendar life, while lead acid and NiCd cells are good choices for high rate applications.

Discharge modes

A battery may be discharged under different modes, depending on the equipment load. The discharge mode selected will have a significant impact on the service life of a battery in a specified application. The three typical modes under which a battery may be discharged are:

  • Constant Resistance ("R"): In this mode, the resistance of the equipment load remains constant throughout the discharge
  • Constant Current ("C"): In this mode, the current drawn by the device remains constant during the discharge
  • Constant Power ("P"): In this mode, the current during the discharge increases as the battery voltage decreases, thus discharging the battery at a constant power level (power = current × voltage)

What is self-discharge?

“Self-discharge” can refer to the shelf-life of a primary cell or the rate of loss of charge in a rechargeable battery. A battery is said to be “no good” when cannot deliver at least 60-80% of its rated capacity, depending on the battery type.

  • Many primary cells will last for many years in storage at room temperature:
    • Carbon cells last about two to three years on the shelf
    • High quality alkaline cells should generally last 10 years
    • Lithium cells can last for ten years or more depending on the type
  • Rechargeable or secondary batteries lose their charge more quickly when in storage:
    • Lead acid batteries will last about six months
    • Lithium ion batteries are best stored at a partial state of charge (30-50%), and self-discharge about 2-4% every month
    • NiCd cells generally maintain 80% capacity for about 30 days
    • Some types of NiMH batteries can lose their charge in three to four weeks

Note: Higher ambient temperatures during storage will significantly reduce the shelf life of all batteries.

What is a battery’s charge rate?

For secondary, or rechargeable, batteries the charge rate refers to how long it takes to recharge a battery.

  • Lead acid batteries (C/300) and some NiCd and NiMh cells (C/20 to C/30) are designed for float or trickle charges for “stand by” applications.
  • A 14-hour charge (C/10) is considered “standard” for sealed lead acid, NiCd, and NiMH batteries.
  • Seven-hour fast charges are possible for most secondary chemistries.
  • Li-ion and lithium polymer batteries typically require roughly three hours to complete charging.

Can I use any charger to charge a rechargeable lithium battery?

  • Not all lithium batteries are the same! Never use a charger that was not designed for the battery in question. Every charger is specially tailored for use with a specific type of lithium chemistry.

How does temperature affect batteries?

Temperature is an often overlooked aspect of battery performance. However, it should be a major consideration when selecting a battery. Batteries do not charge or discharge well at low temperatures, and high temperatures are detrimental to shelf life (self-discharge—see below).

  • Most batteries perform best in the 20° to 60°C (68° to 140°F) range.
  • Lithium primary cells do better than most chemistries at both temperature extremes
  • Some lead acid batteries, such as Enersys Cyclon, can perform relatively well at -20°C (-4°F) or below
  • Special “high temp” NiCd, NiMh, and lithium cells are available that are designed to perform at higher temperatures

What is a fuel gauge?

  • A fuel gauge is an integrated circuit that can be installed in a lithium ion or other type of battery pack that keeps track of the amount of charge remaining.

Why are rechargeable lithium batteries shipped with such low charge level?

  • As of April 2016, the United States Department of Transportation requires that all lithium secondary batteries be transported at a state of charge (SOC) of no more than 30%. To harmonize manufacturing, all rechargeable cells leave manufacturing facilities at less than 30% charge.

What is the best/safest way to store rechargeable lithium batteries?

  • Because the self-discharge rate of all battery and battery cell types increases with ambient temperature, it is best to store batteries in a cool, dry environment where temperatures do not exceed 25°C (77°F).
  • For best possible cell life, a lithium battery pack should ideally be stored with a SOC of roughly 50%.

What’s the difference between differently shaped rechargeable batteries? (cylindrical vs. flat)

  • Cylindrical secondary batteries are typically enclosed in a steel tube that is 18mm in diameter and 65mm long. These are commonly referred to as 18650 cells.
  • Flat secondary cells are generally made from plastic or polymer materials, and can be manufactured in many different sizes. The dimensions of a flat cell are limited only by the imagination of the manufacturer.

What does 2S4P Pack mean? Or 3S1P Pack? etc.

  • This nomenclature is used to define a battery pack’s configuration.
    • For example, 2S4P means 2 cells in Series, and 4 cells in Parallel. This particularly configuration would yield a 7.4V output with four times the capacity of a single cell.

Do all rechargeable lithium batteries require circuit protection?

  • Yes. For safety reasons, all rechargeable lithium cells must include a protection circuit that prevents over-charging, over-discharging, and over-current (short circuit).
  • Over temperature protection is also a common feature.

Why are lithium rechargeable batteries used in most handheld devices?

  • Rechargeable lithium cells offer greater energy density than most other battery chemistries, such as nickel metal hydride or lead acid.

Are batteries recyclable?

  • Currently, the only two battery chemistries that must be recycled by law are lead acid and NiCd. These batteries must be clearly marked with the “chasing arrows” symbol and a national 1-800 phone number that end users can use to locate their local recycling center.

How important is the cost of batteries?

  • Cost can be a huge factor when selecting a battery type. In many cases, a higher initial battery cost easily pays for itself over the long term. And, of course, some batteries and battery packs provide faster ROI than others. Ultimately, lower cost is not always best.

How important is weight?

As portable electronics continue to evolve, the weight of devices continues to shrink. In many applications, the battery or battery pack is the heaviest component in a portable electronic device.

  • In general, carbon-based batteries are lighter than alkaline batteries, but provide lesser performance.
  • Lithium battery cells are lighter than other primary chemistries. For example, a AAA alkaline battery weighs roughly 12 grams, while a AAA lithium battery weighs only 8 grams. Additionally, lithium cells generally provide superior shelf life and performance
  • Li-ion and lithium polymer batteries are far lighter than other rechargeable batteries of the same size, and have helped improve the portability of many electronic devices