lithium ion (li ion) batteries come in a variety of chemistries that will influence their exact properties. Review the manufacturer’s informational/data sheet that explains the exact part number of the component you are going to use for it’s specific properties.

18650 lithium ion cells from a reputable source are mostly safe as long as you don’t damage them (or other components and connections connected to them).

  • Avoid high heat (short circuits and other excessive current causes). Keep loose metal away from connective points and never let any parallel batteries be inserted backwards. Be aware of the current handling abilities of the entire circuit and stay well below those levels.
  • Avoid overcharging and/or overdischarging the battery: That will alter it’s chemistry in a way that is likely to start a fire or cause an explosion. High heat can cause thermal runaway (the high heat ignites the chemistry, which keeps creating heat until the fuel runs out).
  • Some sellers repackage used and possibly abused batteries or sell batteries that don’t as high of safety standards as reputable sellers and manufacturers. They often exaggerate the abilities of the batteries and sell them at lower prices.

Below, I will be listing typical properties of li ion batteries as a starting point.

The 18650 is the most commonly used package for a single li ion cell. It looks like a standard AA or AAA battery, but is larger 18mm width x 65mm length (18650. I’ve seen both explanations but neither from an official source (such as from a datasheet) yet.

  • Secondary – li ion batteries are rechargeable (secondary). Single use/non rechargeable batteries are referred to as primary.
  • Nominal Voltage – Typically given as 3.6V or 3.7V.  Will be about the approx. average voltage of the cell during a commonly charge to discharge cycle of 4.2V to 2.5V (reminder! it’s very dangerous to over charge or over discharge!).
  • Fully charged voltage – Typically 4.2V. As you charge a secondary battery, the voltage rises accordingly. More voltage = more stored charge, but over charging can cause damage (fire hazard, even in the future)
  • Fully discharge Voltage – Although cells are typically rated for a 2.5V discharge, this may damage the cell. Going below 2.5V is very dangerous (possible fire hazard, especially at a future time). It is better for the life of most li ion batteries to not discharge below 2.7-2.9V.
  • Constant current charging – A way to limit current when the voltage of the charger (probably 4.2V) is a bit higher than the battery being charged.
  • Constant voltage charging – Once voltage between the charger and the battery being charged are close to equal, applying exactly the maximum voltage that the battery will be charged to will prevent the battery from charging above that voltage. The voltage difference needs to be small enough that current is reduced to a safe level.

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  • This website is only intended to provide supplemental information to those already studying electronics and takes no responsibility for how that information is used or injuries/damage that may arise. Electronics projects have a possibility of causing injury and or property damage. Make sure to learn how to safely work with electricity and consult the component datasheets, manuals for equipment, etc. before starting electronic projects.