You have a 3.7V battery and a 4.8V charger that fits. The voltage is close, so it seems fine, right? But you hesitate, worried that this small difference could damage your battery or worse.
No, this is extremely dangerous. A 3.7V lithium-ion battery1 has a maximum safe charge voltage of 4.2V. Using a 4.8V charger will cause severe overcharging, leading to permanent damage, swelling, and a very high risk of fire or explosion.
As a battery manufacturer, this is one of the most critical safety warnings I give my clients. Voltage is not a suggestion; it's a strict limit. Think of it like water pressure. A little too much is fine, but 4.8V is like connecting a garden hose to a fire hydrant. The battery's internal chemistry simply cannot handle that much electrical "pressure." The only time this MIGHT work is if the "charger" is actually a smart power supply with a built-in circuit that regulates the voltage down to a safe 4.2V. But without that, you are creating a very dangerous situation.
What voltage should I charge a 3.6V battery?
You see batteries labeled 3.6V and others 3.7V, causing confusion. You wonder if they need different chargers, fearing you might use the wrong one and cause damage.
You should charge a 3.6V lithium-ion battery to a maximum of 4.2V. The terms 3.6V and 3.7V are both "nominal" voltages for the same type of battery, and they share the same safe charging voltage limit2.
In the battery industry, we use a few different voltage numbers to describe a single cell, which can be confusing. Let me break it down. The "nominal voltage" (3.6V or 3.7V) is just the average, working voltage of the battery. It's not the full or empty voltage. A standard lithium-ion battery has a voltage range it operates in. When you charge it, you are aiming for the very top of that safe range. For nearly all standard chemistries (like those used in vapes, flashlights, and laptops), that peak voltage is 4.2V. A dedicated charger is designed to hit that 4.2V mark precisely and then stop. Using a charger that goes even slightly over, like 4.25V, will shorten the battery's life. Anything significantly higher, like the 4.8V from the main title, is just asking for trouble.
| Battery State | Typical Voltage of a 3.6V/3.7V Cell |
|---|---|
| Fully Charged | 4.20V (This is the target for the charger) |
| Nominal (Average) | 3.6V or 3.7V |
| Fully Discharged | ~2.75V - 3.0V (Device should shut off) |
What voltage to charge a 3.7V battery?
You want to make sure you're charging your 3.7V battery correctly. You know the voltage is important, but you need to understand the right process to ensure safety and longevity for your batteries.
You must use a charger specifically designed for lithium-ion batteries that terminates at exactly 4.2V. These chargers use a method called CC-CV (Constant Current-Constant Voltage) which is essential for safely charging these batteries.
A proper lithium-ion charger is more than just a power supply. It's a smart device that follows a specific two-stage process called CC-CV. This is the only safe way to charge these cells.
First is the Constant Current (CC) phase. The charger pushes a steady amount of current (measured in amps) into the battery. During this stage, the battery's voltage steadily rises from its depleted state. I often advise my clients to use a lower current to extend the battery's overall lifespan. Once the battery's voltage reaches the peak of 4.2V, the charger switches to the second stage.
This is the Constant Voltage (CV) phase. The charger holds the voltage at exactly 4.2V. As the battery fills up, its internal resistance increases, and it naturally starts accepting less and less current. The charger simply holds the voltage steady and lets the current taper off. When the current drops to a very low level (usually about 3% of the initial rate), the charger's job is done, and it shuts off completely. This process ensures the battery is charged quickly but also safely, without ever exceeding its 4.2V limit.
Can I use a 5 volt charger for a 3.7 volt battery?
You see 5V on every USB charger and power bank. Since it's so common, you wonder if you can use it to directly charge a bare 3.7V lithium-ion battery.
No, you cannot connect a 5V power source directly to a 3.7V battery. However, you can use a 5V source to power a dedicated charging circuit, which then safely charges the 3.7V battery.
This is a fantastic question that gets to the heart of how modern electronics work. Your 5V USB wall adapter is a power supply, not a battery charger. The actual charger is a tiny, intelligent circuit inside your phone or other device. This circuit's job is to take the generic 5V from the USB supply and convert it into the precise CC-CV profile that the 3.7V battery needs. So, you are never directly connecting 5V to the battery. For hobbyists or engineers like my client Jacky who work with bare cells, you can buy these charging circuits as small modules. A very popular one is the TP4056 module. You feed it 5V from a USB source, and it outputs the perfect charging curve for a single 3.7V cell, cutting off at 4.2V. This is the safe and correct way to use a 5V supply. Directly wiring a 5V source to the battery terminals would be the same as using the dangerous 4.8V charger—it will cause overcharging and failure.
Can I charge a 3.7V battery with a USB charger?
You plug your phone, tablet, and headphones into USB chargers every day. You wonder if this same convenient method can be used for other devices or projects using 3.7V batteries.
Yes, you can, because the device itself contains the necessary charge management circuit. The USB port only provides 5V power; the device's internal electronics handle the actual charging process safely and correctly for the 3.7V battery.
This is how almost every modern portable electronic device is designed. The convenience of the universal USB standard is that it just provides a stable 5-volt power source. Every device designed to charge from it, from a power tool to a medical device, has a built-in charge controller. This controller is the "brain" of the charging operation. It negotiates with the power supply, draws the right amount of current, and most importantly, executes the CC-CV charging process to safely charge its internal 3.7V battery up to 4.2V. In many of the battery packs I design, we also include another layer of safety called a Protection Circuit Module (PCM)3 or Battery Management System (BMS). This small board is attached directly to the battery cells and acts as a last-ditch safety net. It will automatically cut the connection if it detects over-voltage (charging above ~4.25V), under-voltage (discharging below ~2.75V), or a short circuit. This multi-layered approach is why charging with a standard USB adapter is so safe and reliable.
How long does a 3.7V battery last?
You're trying to figure out if a 3.7V battery is right for your project. You need to know how long it will run your device before needing a recharge.
The runtime of a 3.7V battery depends entirely on its capacity (in milliamp-hours, mAh) and the power consumption of the device (in milliamps, mA). A higher capacity battery or a lower consumption device will result in a longer runtime.
This is a question I help my clients answer every day when we design new products. "How long it lasts" has two parts: the battery's fuel tank and the engine's fuel consumption. The "fuel tank" is the battery's capacity, measured in milliamp-hours (mAh). A 3000mAh battery can supply more energy than a 1000mAh battery. The "engine" is your device and how much current it draws, measured in milliamps (mA). A simple LED might only draw 20mA, while a powerful motor could draw 2000mA (or 2A). The basic formula is: Runtime (in hours) = Battery Capacity (in mAh) / Device Current Draw (in mA). For example, a 2500mAh battery powering a device that draws a constant 500mA will last approximately 5 hours (2500 / 500 = 5). This is a simplified calculation, as factors like temperature and battery age can reduce the actual runtime, but it's the best starting point for any project.
| Battery Capacity | Device Current Draw | Estimated Runtime |
|---|---|---|
| 1000mAh | 100mA | 10 hours |
| 2200mAh | 500mA | 4.4 hours |
| 3500mAh | 20mA (low power) | 175 hours |
Conclusion
Never use a charger with a voltage higher than 4.2V for a 3.7V battery. Voltage mismatch is dangerous. Always use the specific CC-CV charger designed for lithium-ion cells.
