I. Main factors affecting the self-discharge rate of lithium-ion batteries:
Cell self-discharge rate and battery storage temperature;
The battery management circuitry within the battery pack draws current.
When the battery is installed inside the main unit, the device's static current generates the current consumption.
When the battery is connected to the charger, the charging circuit generates a current consumption due to its static current.
The battery capacity includes the number of cells connected in parallel within the battery pack;
II. What is the self-discharge of a lithium-ion battery?
Because lithium-ion batteries spontaneously generate physical and chemical discharges, the cells slowly self-discharge, losing charge and chemical energy. Typically, the self-discharge rate of a lithium-ion battery is 0.5% to 3% per month. The storage temperature has the greatest impact on the self-discharge rate; higher temperatures intensify internal chemical reactions, leading to more self-discharge. Therefore, the storage environment significantly affects battery lifespan.
III. Self-consumption of power by the Battery Management System (BMS)
In addition to the battery cells, smart batteries also require a battery management system (BMS) to monitor the battery current, voltage, and temperature in real time, providing safety protection and power calculation for the battery. Besides the self-discharge of the battery cells, these electronic components also consume additional current in the battery product.
The battery management system is meticulously designed to minimize current consumption and features different operating modes with varying current consumption. Large standard smart batteries typically have the following operating modes:
Normal mode: <500uA (battery charging/discharging state or communication state)
Sleep mode: <200uA (automatically enters sleep mode after 20 seconds if there is no battery charging/discharging current or communication).
Power-off mode: <10uA (cell voltage below 2.2V or activated by host command)
Based on the above data, the theoretical battery capacity consumed by the battery management system circuit during a month of static storage can be calculated as follows: 0.2mA * 24h * 30D = 144mAh. (200uA = 0.2mA)
Assuming the battery is stored under optimal conditions, the following is a reference for the performance of a lithium battery pack: BMS self-discharge plus cell self-discharge (at 1%/month):
From the above diagram, we can draw the following conclusions:
Battery capacity has a direct relationship with battery storage shelf life. Smaller capacity batteries can be stored for a shorter time, while larger capacity batteries or batteries connected in parallel can be safely stored for a longer time.
The battery's charge level is directly related to its shelf life. Before long-term storage, the battery should be charged to at least 30%.
The battery should be checked every 6 months. If the capacity is below 30%, it should be charged to above 30%.
IV. Power Off Mode
The Battery Management System (BMS) features a shutdown mode to minimize current consumption. When the cell voltage drops below 2.2V, the BMS enters shutdown mode and shuts down the battery output. In this mode, the BMS's self-discharge is reduced to approximately 1uA, preventing permanent battery failure due to severe undervoltage. This mode requires charging to activate.
If the customer requires long-term storage or extended sea transport, the host can proactively send a command to put the battery into shutdown mode. This minimizes battery self-discharge and extends battery storage time. Shutdown command: Send the 0x0010 command twice to battery address 0x16 within 2 seconds.
V. Transportation Safety
Since 2016, IATA/UN/DOT have limited battery charge levels to <30%. We must comply with these requirements, so all our batteries are shipped with less than 30% charge. We recommend charging the batteries promptly upon receipt. If you do not use the batteries for an extended period, the charge may drop to 0%. If the batteries are not charged within 6 months of their manufacturing date, there is a risk of permanent battery failure.
VI. Full charge storage
Considering battery self-discharge, some customers may store batteries fully charged for maximum storage time. However, lithium-ion batteries should not be stored at 100% charge. Continuous storage at 100% charge will cause lithium-ion batteries to lose some capacity. This is common in UPS applications and laptops. When continuously kept fully charged, lithium batteries will lose 5%-10% of their original capacity. We recommend storing batteries at around 50% charge for long-term use.
VII. Calendar lifespan
Even if the battery is not charged or discharged, it will still suffer some loss due to the aging of the chemical substances inside the cell and long-term storage. Our experience value is 5% per year.
Under normal use, the empirical value is that the battery capacity will decrease by about 5% every 100 cycles.
VIII. Summary
Many factors influence the actual shelf life of lithium-ion batteries. For general maintenance of lithium-ion batteries, we recommend:
• Check the stock batteries every 4 months; especially those that have been installed inside the equipment and have not been used for a long time, as they may not be being charged.
• Under low SOC conditions, lithium-ion batteries should not be stored for more than 6 months.
• Storage temperature has a significant impact on batteries. Store batteries at room temperature or slightly below room temperature (ideally 10-20°C).
• When batteries need to be transported, their charge level should be kept below 30% to comply with regulatory requirements.
• Our products require a capacity check every 6 months, and should be charged promptly if necessary.
Note: The above are conservative rules of thumb, intended to provide guidance on the use and maintenance of lithium-ion battery packs, and do not constitute a commitment or warranty.
