Lithium-Ion (18650 cells) versus Lipo -- which is best?

Today we're talking about batteries, specifically focusing on how to get the maximum duration out of an electric model. We have two typical types we can use: LiPos and lithium-ion batteries.

Traditionally, we didn't use lithium-ion, because LiPos were basically the staple for RC model flying. But lithium-ion batteries are making a comeback. Let's look at an 18650 size battery (18mm diameter, 65mm length). These are the LG HG2 cells, which are quite good because they can handle 20 amps continuous current.

Capacity and Weight Comparison between Lithium-Ion and Lipo Batteries

The lithium-ion has a capacity of 3000mAh per cell, compared to the LiPo's 2600mAh. Putting them on the scales reveals a significant difference: A 3-cell LiPo weighs in at 218 grams, while the equivalent 3-cell Li-ion pack is just 151 grams. That's quite a significant weight difference, and remember - the Li-ion actually has more capacity!

Voltage Considerations

Both types have similar nominal voltages (3.7V for LiPo, 3.8V for Li-ion), and both charge up to 4.2V. However, there's an important difference in their minimum operating voltage. A LiPo is pretty much done when it reaches about 10.5V (3S pack), while Li-ion cells can go down to 2.5V per cell (7.5V for a 3S pack).

This means that a lot of the Li-ion's capacity will be at lower voltages. About 90% of a LiPo's capacity is available at 11V and above, while only 60-70% of a Li-ion's power will be at these voltages.

Power Output Limitations

The main trade-off comes in power output. Our LiPo is rated at 25C, meaning it can output 65 amps. The Li-ion cells are limited to 20 amps each. This means you won't be using Li-ion in high-performance, high-power models. For a 3S Li-ion pack, you're limited to about 120-130 watts. For endurance flying, you'll have a relatively underpowered craft because you're prioritizing fuel efficiency over raw performance. It's really "horses for courses" - you need to decide what you want from your battery.

Do you want a battery that will give you a huge amount of energy, albeit at a lower voltage once it starts to discharge, paired with a lower power motor? Or do you want something that can "burn up the sky"? Those are your choices.

However, there is a middle ground. You can make a 6-cell pack by arranging three cells in series and then putting another cell in parallel with each of those, making it a 3S2P pack. With this configuration, you could draw 40 amps out of it. Of course, this comes with additional weight, but you'll get more watt-hours in return.

Future Testing

I'm planning to make a 4S 6000mAh pack using these Li-ion cells and compare it with a similar capacity LiPo pack in the Rebel GT. We'll test which gives the maximum flight duration and compare their weights.

For my long-range FPV project, two of these cells would weigh about 90-91 grams with connectors, leaving enough weight budget for a sub-250g build. With 3000mAh capacity, we might achieve up to 40 minutes of flight time at a comfortable 30mph cruise speed.

Remember, it's all about choosing the right battery for your needs - whether you want maximum endurance with lower power, or high performance with more weight. Stay tuned for practical testing results!

Check out the full video here