Battery Capacity Understanding
1.The essential definition of battery capacity
Battery capacity is the core parameter to measure the battery’s ability to store charge, commonly used in milliampere-hours (mAh) or ampere-hours (Ah), defined as the product of rated discharge current and discharge time (capacity Q = current I× time t),for example, a 100Ah battery discharging at 10A current can theoretically work for 10 hours,it should be noted that the capacity is divided into rated capacity (design nominal value) and actual available capacity, the latter is affected by operating temperature, discharge rate, aging degree and other factors.
2.The key effect of battery capacity on performance
2.1 Endurance and energy density
2.1.1 Directly determine the battery life of the device
capacity is the basis of battery life, the same power consumption, the larger the capacity, the longer the battery life. However, it is necessary to combine the battery voltage (energy = capacity × voltage), for example, the 4000mAh/3.7V battery energy is 14.8Wh, while the 3000mAh/4.2V battery energy is 12.6Wh, which is small in capacity but not necessarily lower in energy.
2.1.2 The balance of energy density
If the volume weight of high-capacity batteries is not well controlled, it will lead to a decrease in energy density (Wh/kg or Wh/L), affecting the design of portable devices (such as mobile phones, drones). For example, when electric vehicles pursue high capacity, they need to take into account the lightweight battery pack, otherwise the improvement in battery life may be offset by the increase in vehicle weight.
2.2 Discharge characteristics and internal resistance performance
2.2.1 High current discharge capacity
High-capacity batteries usually use larger electrodes or more parallel cells (such as power batteries), may have lower internal resistance, and support high current discharge (such as fast charging of electric vehicles, camera flash). However, excessive pursuit of capacity may lead to increased electrode material thickness, longer ion diffusion path, and significant capacity attenuation at high rate discharge (such as a battery 0.5C discharge capacity of 1000mAh, 2C discharge may drop to 800mAh).
2.2.2 Self-discharge and storage performance
The larger the capacity, the higher the surface area of the electrode material or the total amount of active material, which may exacerbate the self-discharge (for example, the larger the capacity of the lead-acid battery, the proportion of monthly capacity loss when standing may be slightly higher), long-term storage needs to control the state of charge (SOC).
2.3 Cycle life and aging rate
2.3.1 Capacity attenuation mechanism
Battery cycle life (number of charge and discharge) is closely related to capacity,in the case of lithium-ion batteries, for example, high-capacity designs may use a higher nickel content positive electrode (such as NCM811) or a thinner diaphragm, which increases capacity but accelerates the growth of the SEI film in the cycle and the loss of active lithium, resulting in faster capacity decay. For example, the capacity of a 3000mAh mobile phone battery remains 80% after 500 cycles, while the same 4000mAh battery may be compromised by design, and the capacity will be reduced to 75% after 300 cycles.
2.3.2 Deep discharge effect
Low capacity batteries if frequent deep discharge (such as discharge to 0%), the plate or electrode material damage is more significant, and high capacity batteries in shallow charge (such as 20%-80% SOC) longer life, but need system management cooperation.
2.4 Charging efficiency and safety
2.4.1 Charging time and current limit
Large-capacity batteries require a larger charging current (such as 4A current for a 4000mAh battery when charging at 1C), but the charger power and battery internal resistance will limit the speed,during fast charging, insufficient heat dissipation of a high-capacity battery may cause the temperature to be too high (for example, more than 60 ° C), trigger the protection circuit, or accelerate aging.
2.4.2 Overcharge risk
When the capacity calibration is not accurate or the protection circuit fails, the overcharge of the high-capacity battery can lead to the precipitation of lithium metal (lithium battery) or the decomposition of electrolyte (lead-acid battery), increasing the risk of short circuit or fire.
3.Temperature and environmental adaptability
3.1 Low temperature capacity attenuation
All batteries at low temperatures (such as -20 ° C) capacity will decline, high-capacity batteries due to higher electrolyte viscosity, slow ion migration, attenuation amplitude may be greater (such as lithium battery low temperature capacity remaining 70%, and small capacity battery remaining 75%).
3.2 High temperature capacity maintenance
Under high temperature environment (such as 55 ° C), the internal chemical reaction of high-capacity batteries is intensified, the self-discharge rate increases, the long-term storage capacity loss is faster, and the heat dissipation system (such as the liquid cooling plate of electric vehicle batteries) is required.
4.Key suggestions for user perspective
4.1 Take a rational look at nominal capacity
Pay attention to the actual available capacity (such as the mobile phone marked 4500mAh, the actual fast charge/low temperature can be used for about 90%), and refer to the manufacturer’s measured battery life data (rather than simple capacity comparison).
4.2 Avoid extreme use cases
Avoid long-term full charge storage of high-capacity batteries (it is recommended to store SOC 40%-60% for lithium batteries), and avoid frequent large current discharge (such as connecting a charging bank when playing high-quality games with mobile phones to reduce battery burden).
4.3 Matching equipment requirements
For scenarios requiring high rate discharge such as drones and power tools, a battery with “moderate capacity + high discharge C rate” (such as a 2200mAh lithium battery with 25C discharge) is preferred, rather than simply pursuing the maximum capacity.
5.Conclusion
Battery capacity is the “genetic parameter” that determines performance, but not the higher the better,it has a complex game relationship with energy density, discharge characteristics, life and safety, which needs to be balanced in combination with material system, structural design and application scenarios. As a user, understanding the nature of capacity and the engineering compromise behind it can be a more scientific choice and use of batteries to maximize their performance.