Mobile Device Battery Management – Part 2, by J.M.

(Continued from Part 1.)

Rechargables

Rechargeable batteries come in a much wider variety of chemistries than primary ones, including nickel–cadmium (NiCd), nickel–metal hydride (NiMH) and various lithium-ion (Li-ion) and Lithium Polymer (LiPo) chemistries. It’s important to understand the ‘Li-ion’ and ‘LiPo’ aren’t specific chemistries, they’re categories of chemistries that use Lithium as one of the components of the electrolyte. Li-ion batteries use a liquid electrolyte, are usually cylindrical, and common chemistries include:

• Lithium Cobalt Oxide (LiCoO₂, LCO): Known for high energy density and used in portable electronics like smartphones, tablets, and laptops. It has a nominal voltage of 3.60V with a moderate number of recharges (500–1000 cycles), and a thermal runaway temperature of 150°C.
• Lithium Manganese Oxide (LiMn₂O₄, LMO): Offers high power and improved safety compared to LCO, often used in power tools and medical devices. It has a nominal voltage of 3.70V (3.80V), and a recharge cycle life of 300–700 cycles. These are frequently referred to as ‘IMR’ batteries.
• Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO₂, NMC): Balances high capacity and power, with a nominal voltage of 3.70V. It is widely used in electric vehicles (EVs), e-bikes, and industrial applications, with a recharge cycle life of 1000–2000 cycles.
• Lithium Iron Phosphate (LiFePO₄, LFP): Known for safety, long recharge cycle life (1000–2000 cycles), and a flat discharge voltage, and a nominal voltage (3.20–3.30V). It is used in stationary energy storage and high-current applications.
• Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO₂, NCA): Used in high-performance EVs, with a nominal voltage of 3.60V and a recharge cycle life of 500 cycles.
• Lithium Titanate Oxide (Li₂TiO₃, LTO): Features exceptional recharge cycle life (3,000–7,000 cycles), fast charging (up to 5C), and wide temperature range, but it’s currently the most expensive. It is used in UPS systems, EVs, and solar street lighting

Currently the most prevalent Li-ion chemistry in use NMC (Nickel Manganese Cobalt Oxide, or LiNiMnCoO₂), due to it offering a good mix of energy density, discharge capability and recharge cycles (600–1500+ cycles).

Lithium Polymer batteries (sometimes referred to as ‘pouch’ cells) differ from Li-ion in that they use a gel or solid material for the electrolyte and can be made in a thin form factor, usually in a rectangular shape. Common LiPo chemistries include:

• Lithium Cobalt Oxide (LCO): Provide a nominal voltage of 3.6–3.7V, making it ideal for compact devices like smartphones, laptops, and cameras. It has a moderate recharge cycle life (500–1,000 cycles), lower thermal stability, and requires careful management to prevent overheating.
• Lithium Manganese Oxide (LMO): Provides a nominal voltage of 3.7–4.2V, and moderate recharge cycle life (500–1,000 cycles). They provide good thermal stability and safety and are used in power tools, medical devices, and e-bikes.
• Lithium Iron Phosphate (LFP): Provides a lower nominal voltage of 3.2–3.3V, a very high recharge cycle life (2,000–5,000+ cycles) and good safety due to high thermal and chemical stability. Commonly used in solar storage, electric vehicles (EVs), uninterruptible power supplies (UPS), and robotics.
• Lithium Nickel Manganese Cobalt Oxide (NMC): Provides a nominal voltage of 3.6–3.7V and a recharge cycle life of 1,000–2,000 cycles. It offers a balanced performance, making it suitable for EVs, drones, power tools, and e-bikes.

Currently, the most prevalent LiPo chemistry in use for mobile electronics is Lithium Cobalt Oxide due to its low cost, but its shorter number of recharge cycles and thermal runaway issues are causing many manufacturers to start moving to Lithium Nickel Manganese Cobalt Oxide and Lithium Iron Phosphate. Since LiPo batteries can be made in almost any size and configuration there really isn’t any standard size naming convention, although some manufacturers encode the size in the model number – e.g. a 146074 battery will be 14mm thick, 60mm wide and 74mm long. Most LiPo batteries have two or more wires coming out of them with a connector on the end, with JST or DuPont connectors being the most common. Some devices combine multiple Lipo battery packs (called cells) into a single battery in order to provide increased voltage. A single LiPo cell has a nominal voltage ,of 3.7V, a ‘2S’ battery has two cells and provides 7.4V, ‘4S’ provides 14.8V, etc.

Although not as common, there are also rechargeable Li-ion versions of some common button cell batteries available, including CR1254, CR1654, CR2025, CR2032 and CR2450. When a button cell battery is rechargeable, the ‘CR’ in the model name changes to ‘LIR’ – e.g. an LIR2032 is a rechargeable version of a CR2032 battery.

One common issue that you’ll find when buying rechargeable Li-ion or LiPo batteries is that very few manufacturers specify the actual chemistry used – the usually just say ‘Li-ion’ or ‘LiPo’. You’ll need to do some research of contact the manufacturer directly if you want that detail.

Nascent Battery Technologies

There are some new rechargeable battery chemistries becoming available based on Sodium-Ion and Graphene that are worth keeping an eye on. Sodium-Ion batteries are already being manufactured at scale (primarily for large-scale fixed power storage), and since they don’t use expensive Lithium and other rare metals they can be less expensive to manufacture. They also aren’t subject to catching fire and exploding like Lithium batteries and can operate effectively at much lower temperatures (down to -40F), but they currently don’t have as much energy storage capacity as Lithium-based batteries. Graphene batteries on the other hand can provide up to four times the amount of energy storage as Lithium-based chemistries in the same form factor, up to five times the number of recharge cycles, they’re not flammable, and they can be recharged a lot faster. One manufacturer demonstrated a prototype graphene battery that could be recharged in 20 seconds, versus over an hour for a similar lithium battery. Graphene batteries are starting to become commercially available in limited form factors, but they’re very expensive.

Note that I’ve only scratched the surface of the available types, sizes and chemistries of batteries that are on the market, and in development.

Battery Sizes

How many LR6 batteries do you have in your house right now? If you said none you’re probably mistaken – ‘LR6’ is the official International Electrotechnical Commission (IEC) designation for an Alkaline AA battery. ‘HR6’ is the IEC designation for a NiMH rechargeable AA, and ‘KR6’ is the IEC designation for a NiCd rechargeable AA. Of course the American National Standards Institute (ANSI) has their own unique designations for AA batteries – ‘15A’ for alkaline, ‘1.2H2’ for NiMH and ‘1.2K2’ for NiCd. The UK, Japan, Russia and China also have different names for common battery types.

Having multiple designations for different sizes and chemistries for batteries can make things complex, but it can also allow to make sure you’re getting the right chemistry for the size of battery you’re looking for. Here’s a comprehensive chart showing most of the available battery sizes, designations and chemistries. (Note: While I despise Wikipedia’s politics, this is the most comprehensive chart of battery information I’ve been able to locate to date).

Battery Life

Regardless of the type (primary or rechargeable) or chemistry of a battery there are several factors that will impact how long it can continue to provide power. Understanding these factors can help you get the longest useful life out of your mobile electronics.

• Age – Batteries work on a chemical reaction – even if they’re not connected to anything batteries continue to produce power via the chemical reaction; this is referred to as self-discharge. Any chemical reaction will eventually run out of reactants, and when that happens the battery is dead. Primary batteries using Alkaline chemistries can even leak over time as the chemical reaction runs down, but newer formulations can be stored up to 10 years. Rechargeable batteries can typically be stored for a few years (Li-ion 2-5 years, NiMH 3-5 years, NiCd -3 years) before they’ll lose their ability to be recharged. LiPo batteries can swell up as they age and the internal chemical reaction starts to break down.
• Quality – Battery chemistries are very complex, and everything down to the size and mix of the individual particles used to make the nodes and electrolyte can impact the lifetime of a battery.
• Temperature – High temperatures generally cause the rate of self discharge to increase, decreasing the overall battery life for both primary and rechargeable batteries. Extremely high temperatures can cause some batteries such as Li-ion to catch fire or explode. While storing batteries at a low temperature (like in a refrigerator) may slightly slow the self-discharge rate, it also increases the risk of condensation forming inside the battery, which can significantly reduce the life. Extremely low temperatures can also slow down the chemical reaction, reducing the amount of power batteries can deliver (hello, EVs). The best temperature to store most batteries at is around 60F.
• Humidity – Humidity can impact battery life. High humidity causes moisture, which reacts with the chemicals in the battery causing them to degrade, and can cause corrosion on the metal components for the battery. Very low humidity can create problems with static electricity and cause the chemicals in the battery to dry out. The best humidity levels for storing batteries is around 40%-60%.
• Use – This may sound obvious, but every battery, both primary and rechargeable, has a certain amount of power it can deliver over its lifetime. The more you use it the less power will be available for future use.
• Chemistry – I discussed battery chemistries earlier, and as I pointed out different chemistries have different power storage capacities, both for primary as well as rechargeable. For example, a typical alkaline 1.5V AA battery has around 2000mAh capacity, while a lithium 1.5V AA can have around 3000mAh.
• Discharge rate – High-drain usage like turning on your flashlight at the million lumen setting can generate heat and internal stress on the battery, reducing it’s useful life. Some battery chemistries such as LiFePO4 and LTO are designed for high discharge rates and are frequently used in things like power tools.

Rechargeable batteries have a number of additional factors that can impact their lifetime:

• Recharge cycles – Based on their chemistry and usage, every rechargeable battery has a certain number of time it can be recharged. Older NiCd and NiMH rechargeable batteries sometimes suffered from ‘memory effect’ – if you consistently used the battery down to a certain level of charge and then recharged it, the battery would start to act like that level was the lowest it could go. This could be reversed by deep discharging and recharging. Newer Lithium-based chemistries don’t suffer from memory effect.
• Discharge – Consistently using a rechargeable battery until it’s completely empty before recharging can result in the battery ‘scavenging’ power from it’s internal components, much like the human body starts consuming internal muscle during extended starvation. You’re better off recharging batteries when the get down to around 10% or so.
• Charging – Providing the battery too much power when it’s charging is referred to a ‘overcurrent’, which can lead to excessive heat, internal damage, and safety hazards such as thermal runaway, fire, or explosion, especially with Lithium-based batteries. How the charger delivers power to a battery plays a huge role in the battery’s lifetime, and limiting your battery charge to 90% can potentially increase the number of available charges. Many smart battery chargers allow you to select how fast a battery is charged – anywhere from 300mAh to over 1000mAh – and the slower you charge it the less stress is placed on the battery.
• Storage charge – Long-term storage of rechargeable batteries when they’re fully charged or empty can significantly reduce the battery’s life and potentially cause permanent loss of capacity. The recommended charge capacity for long-term storage for rechargeable Li-ion/LiPo batteries is around 50%.

(To be continued, in Part 3.)