The maximum voltage for a 18650 cell is typically 4.2 volts and a minimum of 2.5 volts because most 18650 cells use NMC chemistry. For an LFP cell, the maximum voltage is 3.7 volts, and a minimum of 2 volts. 

Maximum and Minimum Voltage For NMC 18650 Batteries

The NMC (Lithium-Nickel-Manganese-Cobalt-Oxide) type of 18650 batteries are the most common ones on the market. They have a nominal voltage of 3.6 or 3.7 volts and a maximum charge voltage of 4.2 volts. When discharging, these batteries should not drop below a minimum voltage, usually around 2.5 volts, to prevent damage to the battery. This is one of the reasons choosing a good BMS (battery management system) is required.

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What Happens If You Charge An 18650 Past 4.2 Volts?

Overcharging an NMC 18650 beyond 4.2 volts will lead to something called overpotential. This is a state in which the voltage of the cell is significantly higher than its equilibrium voltage. When this happens, the excessive voltage will drive unwanted reactions within the cell.

One of these reactions is the breakdown of the electrolyte, which is what separates the anode and the cathode. This leads to the formation of gas and heat which will eventually cause the battery to swell and, in extreme cases, vent with flame or even explode. Overcharging also leads to the plating of lithium metal on the anode, which creates internal short circuits and poses significant safety risks.

Overcharging contributes to faster degradation of the battery's capacity, a process known as capacity fade. This will cause the cell to degrade more rapidly over time. The higher voltage range accelerates the breakdown of the active materials within the cell, reducing their ability to store and deliver energy over time.

What Happens If You Discharge An 18650 Below 2.5V?

The minimum voltage for NMC 18650 batteries is about 2.5 volts. A BMS will actively work to prevent a cell from going below 2.5v by putting the battery pack into safe mode. Any lower than around 2.5V, and irreparable damage in the form of lithium plating will occur within the battery.

If your cells run lower than 2.5V for a short period of time, then the amount of damage that happens is minimal. If, however, they are left that low for a long time, it can totally destroy the cells.

LFP 18650 Batteries: The Lower Voltage Alternative

On the other hand, LFP 18650 batteries, while less common, offer a few unique advantages, including a longer cycle life and higher thermal stability. These batteries have a lower nominal voltage, usually around 3.2 volts, with a maximum charge voltage of approximately 3.65 volts. The minimum voltage for LFP 18650 batteries is around 2.5 volts, although most manufacturers recommend not discharging below 2.5 volts to maximize cycle life.

The same kind of bad things will happen if you overcharge or over-discharge an LFP cell, but at a slower rate because LFP chemistry is more resilient to heat than NMC chemistry. There are several other key differences between NMC and LFP chemistries besides just their voltage ranges. 

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What Is The Maximum Storage Voltage For 18650 Batteries?

When dealing with the most common type of 18650 batteries, it is best to store them at a minimum voltage of 3.6 or 3.7 volts per cell, or about 40-60% of their full charge. This helps to minimize capacity loss during storage through self-discharge. Also, make sure to store the batteries in a cool, dry place to avoid any adverse conditions that could impact their performance.

Do 18650 Batteries Automatically Prevent Overcharging?

No, but some 18650 batteries come with a built-in protection circuit. This is a tiny BMS that is built into the cell packaging that safeguards the battery against common issues like overcharging, over-discharging, and short-circuiting. This circuit, if present, is always added additionally after the cell is manufactured. This sounds like an ideal solution, but it's only good for a few limited applications as a BMS that small cannot support very high currents.

Another issue arises when dealing with putting batteries like this in series. The MOSFETs in a 1S BMS generally only support a few volts between their gate and source pins. This becomes a problem when adding cells in series, as the overall voltage of the battery pack goes far beyond that which the BMS MOSFETs can support.

Do 18650 Batteries Automatically Prevent Over Discharging?

No, 18650 cells don’t prevent over-discharge on their own. If, however, they are the type of cells that have a small protection circuit built in, then they can. The same issue as above arises when trying to use these types of cells in any large battery projects.