The mobile phones we use every day, cool drones, and many small projects made by DIY enthusiasts are all powered by 3.7V lithium-ion batteries. This article is a comprehensive guide to 3.7V batteries, including the basic knowledge and common questions you want to know.
3.7V batteries are rechargeable with an average voltage of 3.7 volts, not the voltage value at a certain moment. There are two main categories of 3.7V batteries, lithium-ion (Li-ion) and lithium polymer (LiPo). The former is represented by steel-shell cylindrical batteries, and the latter is mainly represented by aluminum-plastic film shell soft pack batteries.
Different from the 1.5V AA/AAA alkaline batteries and 1.2V nickel-metal hydride rechargeable batteries (NiMH) that we use daily, it has high voltage and high power.
3.7V battery voltage
The full voltage of a 3.7V battery is not 3.7V, but the highest voltage, 4.2V. This is why lithium battery chargers are usually set to 4.2V.
The 3.7V cut-off voltage is generally set between 2.5V and 3V. During the discharge process of lithium batteries, the voltage will gradually drop, but it is not allowed to drop too low, otherwise it will cause permanent damage to the battery. The discharge cut-off voltage is also called the safety lower limit voltage, which is often determined by the battery protection board. So its core voltage parameters are as follows:
Nominal voltage: 3.7V
Charging voltage: 4.2V
Discharge cut-off voltage: 3V
Common 3.7V battery types and sizes
As mentioned above, 3.7V batteries are mainly cylindrical and soft-pack. Cylindrical batteries are represented by 18650, because they are so common that they can often be seen in flashlights, laptop battery packs, and power banks. There are also 14500, 26650, and 21700 batteries.
The other type is the square soft-pack battery, which is the flat, silver soft-pack battery commonly found in our mobile phones, tablets, and drones. Their advantage is that their shape is very flexible, they can be made thin and light, and adapt to various compact designs.
Of course, 3.7V batteries also have rechargeable button batteries. All 3.7V batteries are available with or without protection boards. The front-end is more focused on battery safety to avoid risks such as overcharging, over-discharging, and short circuits; the latter is cheaper and has the device motherboard to control its performance. Click here to read more about battery size information.
3.7V li-ion battery specifications
Capacity is the core indicator of how much power a battery can store, and the unit is usually mAh (milliampere-hour) or Ah (ampere-hour). The larger the capacity, the longer the battery life will be in theory.
The discharge rate is often expressed as C-Rate (e.g. 1C, 10C, 30C). 1C batteries are also called capacity batteries, and batteries with a rate greater than 1C are called power batteries. The rate represents how fast the battery can safely discharge electrical energy. Devices such as drone startup, power tool operation, and remote control model acceleration all require high-rate discharge. If the C-Rate is not enough, the battery may become hot, damaged, or even unable to drive the device.
Internal resistance is like the resistance inside the battery. The lower the internal resistance, the better. Internal resistance affects the discharge performance of the battery under high current (high internal resistance means faster voltage drop), and batteries with high internal resistance are more likely to heat up.
The charge rate, also commonly referred to as C-Rate, specifies the maximum current allowed when charging the battery. This is directly related to charging speed and safety. However, you must never exceed the recommended charge rate for the battery, otherwise it will severely shorten its life.
Cycle life refers to how many complete charge-discharge cycles a battery can go through while still maintaining a usable capacity (e.g. 80% of initial capacity). Common 3.7V batteries are marked with a lifespan of 300 or 500 cycles.
Connectors, especially lithium polymer batteries, often come with various connectors, such as JST, Molex, and XT series. You need to pay attention to whether the interface matches your device or charger.
Charging your 3.7V battery safely
The premise of safe charging of 3.7V lithium-ion batteries is to use a dedicated charger. Do not use any other charger to charge lithium batteries because they have a specific CC/CV charging mode.
You need to carefully check the output voltage of the charger to make sure it is designed for 3.7V lithium batteries. The charger output voltage is 4.2V. Using a charger with the wrong voltage will either fail to fully charge the battery, or it will be damaged or even dangerous.
The higher the charging current, the better. Each battery has a specification that specifies the maximum allowable charging current, usually expressed as C-Rate. The recommended charging current for a battery of normal capacity is 0.5C ~ 1C rate.
When charging, please place the battery and charger away from high temperature heat sources and flammable and explosive items. When charging a battery for the first time, it is best to have someone watching over it.
The risk of overcharging is high. If the charger fails or is set up incorrectly, and the battery is overcharged (voltage exceeds 4.2V), the consequences can be serious: the battery will heat up, swell, and in the worst case, leak or even catch fire. Read more about charging here.
3.7V battery safe use and protection
The safety protection of 3.7V lithium-ion battery mainly depends on the protection board (PCM / BMS), which is like a bodyguard for the battery, preventing dangerous situations such as overcharging, over-discharging, over-current and short circuit.
If you purchase bare cells with built-in protection boards for your DIY projects, you must add your own protection measures in external devices or circuits.
During use, please keep away from sharp objects to prevent puncture, and also prevent being squeezed or dropped by heavy objects. If the device allows, it is best to protect it with a shell.
Lithium batteries are most afraid of running out of power. You cannot use the device until it shuts down automatically or even leave it for a long time without charging. Over-discharging will seriously damage the battery life and may even make the battery unable to charge anymore.
It is absolutely necessary to prevent the positive and negative terminals of the battery from directly contacting each other, or indirectly contacting each other through metal objects such as keys and coins. Once a short circuit occurs, a large current and high heat will be generated instantly, which may cause smoke or fire.
Lithium batteries are very sensitive to temperature. Avoid using or storing batteries for long periods of time in extremely high temperatures (such as in a car under the sun in summer) or extremely low temperatures. High temperatures accelerate aging and increase risks, while low temperatures reduce performance and affect charging. Read more about storage.
If you find that the battery is bulging, this is a very dangerous sign that something is wrong with the battery and worse could happen at any time. Stop using it immediately, never charge it again, and dispose of it carefully according to safety regulations.
How to choose the right 3.7V battery
If you are looking for a replacement battery for your device, the easiest and safest way is to check the model number on the old battery. If there is one, give priority to buying the exact same battery according to this model, so that the size, interface, and performance can basically be guaranteed to match.
If you can’t find the specific model, you need to measure the size, voltage, capacity, terminals and other information to buy the battery that suits you.
If you are looking for batteries for a DIY project, you need to evaluate voltage, capacity, battery dimensions, discharge rate, protection board requirements, supplier reputation and other information.
3.7V battery pack
Sometimes, the voltage or capacity of a single 3.7V battery may not meet the needs of certain specific applications, which requires increasing the voltage or capacity to meet the needs, which in turn requires a series and parallel combination.
To connect in series, you connect the positive (+) terminal of one battery to the negative (-) terminal of the next battery, like train cars, so that the voltages add up and the capacity remains the same. 2 sets 3.7V batteries in series have a total voltage of 7.4V; 3 sets in series have a total voltage of 11.1V.
The batteries connected in series must be of the same model, capacity, preferably from the same batch, and in basically the same health status. If the cells are very different, it may lead to uneven charging and discharging, and some batteries may be overcharged or over-discharged, seriously affecting their lifespan and even posing safety risks.
Parallel connection means connecting the positive poles of all batteries together, and the negative poles of all batteries together, like several rivers flowing into a main river. When connected in parallel, the voltage remains unchanged, still 3.7V, and only the capacity is increased.
Similarly, batteries connected in parallel must be of the same model, capacity, and condition. Before connecting, it is best to ensure that the initial voltages of the batteries are as close as possible.
3.7V battery storage and maintenance
If you want to store lithium batteries for a long time (for example, if you don’t use them for more than a month), it is best to keep the battery charged at 40% to 60%. Do not fully charge the battery, nor completely drain it. Storing the battery fully charged will cause continuous pressure inside the battery and accelerate aging; storing the battery empty of power is risky, as the battery will slowly self-discharge, and over time the voltage may drop to an irreversible level.
The place where you store the battery is also important. It should be placed in a cool, dry place.
We recommend that users take it out and check the voltage every once in a while (3-6 months).
3.7V li-ion battery disposal and recycling
Lithium batteries contain chemicals and energy, and it is very dangerous to mix them with household garbage. They must be handled separately from other garbage. Short circuits or squeezing may cause fires and explosions, and the leakage of heavy metals and other substances inside will pollute the soil and water sources.
We recommend that you fully discharge the battery before recycling it, which can reduce the risk during transportation and handling. Of course, it is best to discharge it under the supervision of a professional.
Finally, send the discarded lithium batteries to a special recycling point, such as a hazardous waste recycling station or electronic waste recycling center set up by the community or government.
Advantages and disadvantages of 3.7V battery
3.7V batteries have many advantages, such as high energy density, high operating voltage, no memory effect, and light weight, which make them the mainstream choice for modern electronic devices.
The 3.7V battery also has obvious disadvantages, for example, it requires a protection circuit, poses safety risks, is relatively expensive, and has poor low-temperature performance.
Conclusion
3.7V lithium battery is a powerful and commonly used power source. You can maintain it scientifically from multiple angles such as charging, use, storage, etc. to extend its cycle life. You are also welcome to contact us for more information.
