The higher the internal resistance, the lower the maximum current that can be supplied. For example, a lead acid battery has an internal resistance of about 0.01 ohms and can supply a maximum current of 1000 amps. A Lithium-ion battery has an internal resistance of about 0.001 ohms and can supply a maximum current of 10,000 amps.
Don’t allow the battery voltage to drop below 3.0V as it can damage the battery Lithium batteries will often have a specified maximum discharge current of say 2C, which means 2x their mAh rating. For example a 120mAh battery with a 2C max discharge current would only allow you to draw up to 240mA continuous operating current.
Occasionally lithium battery cells are marketed with just a C rating and not a maximum current rating. This can make it easier to compare the power level of battery cells of different capacities. As long as you know the capacity of the cell, you can use the C rate to quickly calculate the maximum current rating of the cell.
Like all batteries the Li-ion battery also has a voltage and capacity rating. The nominal voltage rating for all lithium cells will be 3.6V, so you need higher voltage specification you have to combine two or more cells in series to attain it. By default all the lithium ion cells will have a nominal voltage of only ~3.6V.
The capacity of lithium battery cells is measured in amp-hours (Ah) or sometimes milliamp-hours (mAh) where 1 Ah = 1,000 mAh. Lithium battery cells can have anywhere from a few mAh to 100 Ah. Occasionally the unit watt-hour (Wh) will be listed on a cell instead of the amp-hour. Watt-hour is another unit of energy, but also consider voltage.
The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery. Li-ion battery has a higher cut-off voltage of around 3.2 V. Its nominal voltage is between 3.6 to 3.8 V; its maximum charging voltage can go to 4– 4.2 V max.