Guide to Understanding LiPo Batteries

by Richard Warrender on 1 March 2018
Estimated reading time 6 minutes

Lithium-ion polymer batteries or lipos as they are often called are super light-weight batteries. Most drone or RC aircraft these days use them because they're incredibly affordable and relatively safe - if you treat them correctly that is! Read on to get a deeper understanding of lipo batteries.

Components of a battery

Technically the very essence of a battery involves a number of chemical reactions between two electrodes (positive and negative) and an electrolyte. This is known as the battery "cell". The chemical reactions between the metals used produce a given average voltage for the cell which is different for given chemistries. So for example, Nickel–Metal Hydride (NiMh) batteries produce 1.2v per cell, while Lithium-ion polymer (LiPo) batteries produce 3.7v per cell. The maximum voltage for a freshly charged battery tends to be 4.2v however after a few minutes of usage this drops on average to 3.7v, hence why this is the battery voltage written on the side of the battery packet!

Choosing a voltage

How do you get powerful batteries such as 11.1v? Bigger battery cells result in longer lasting batteries but don't actually increase their voltage. What battery manufacturers do is stack cells together in series to get the desired voltage. An 11.1v battery is actually made up of 3 cells - 4.7x3 = 11.1. If you've ever seen 2S or 3S next to a battery this is what it means, S standing for series.

Your voltage requirements will depend on the motors and ESCs (speed controllers) you've used so best to check the manuals or risk blowing up your shiny drone!

Understanding Current, Capacity and C-Ratings

Earlier it was mentioned bigger battery cells don't produce higher voltage batteries, they usually produce longer lasting batteries! One of the first things to understand is current - this is how much electrical energy you are using from your battery at a given moment in time. The other thing to understand is capacity - this is how much electrical energy is actually stored in the battery.

Current is often measured in Amperes aka Amps. Capacity is often stated by declaring how much current you would have to use continuously for an hour until the battery died - hence the name Ampere Hours (AH) or more commonly milliampere Hours (mAH).

Batteries also a limit to the maximum amount of current you can use at any one time before the battery will be damaged. If you have very large motors that draw a lot of power this could draw more current that the battery can provide. However you'll rarely see the current limits on batteries, instead, you'll see the continuous and burst discharge C rating which you'll have to use along with the capacity to calculate the maximum current.

The formula is to take the battery capacity measured in milliampere Hours and divide that by 1000 to give Ampere Hours. Then multiply the Ampere Hours by the C rating.

Continuous C-Rating

Example:
7.4v 2S 1100mAH Battery
60C Continuous, 120C Burst

1100mAH / 1000 = 1.1 Amp Hours
1.1 Amp Hours * 60C = 66 Amps Continuous

For a 1100mAH battery if it says Continous Discharge at 60C that means it can deliver under normal conditions 66 Amps of current.

Burst C-Rating

Example:
7.4v 2S 1100mAH Battery
60C Continuous, 120C Burst

1100mAH / 1000 = 1.1 Amp Hours
1.1 Amp Hours * 120C = 132 Amps Burst

Another number you'll see is Burst Discharge such as 120C. This means that for short periods you can pull 1.1 x 120 = 132 Amps out of the battery but if you do it for long periods you could damage the battery. What can cause bursts? Activating flaps on an RC aircraft or increasing power to climb suddenly on a drone are usual suspects. One of the best ways of finding out how much power your drone or RC aircraft is using is to use a watt meter and power analyser connected between the aircraft and the battery.

Charge C-Rating

Another C rating to be aware of is the charge rating. This is usually always 1C for most lipos as they are very sensitive and require slow charging. If the batteries capacity is 1100mAH then you can only charge at 1.1 Amps. Sometimes you can charge at a faster rate but there is the risk that the battery will overheat and catch fire or explode.

Estimating running time

So now you know what the capacity of your battery is, you can use this along with an average of how much current you'll use to estimate how long a battery will last.

For example, if you know a battery has a capacity of 1100mAH then you know using 1100mA of current will mean the battery lasts an hour. Using a power analyser you discover that on average you use 550mA when flying your drone. This means that in an hour you'll have used half the batteries total capacity.

Example:
Your battery has a capacity of 1100maH 
Your drone uses 550mA of current

1100maH / 550mA = 2 hours

Connectors

Most lipo batteries have two connects - the main connector for accessing all the cells together as one "battery" and another JST connector for balance charging.

With regards to the main connector there are many different types you can use:

  • EC2
  • EC3
  • Deans
  • XT30
  • XT60

Mostly this will depend on your drone or RC aircraft but something to consider is that the connectors have different max current ratings e.g. 20 Amps. If you use more current than the connector is rated for there is a risk of it getting hot or even melting.

The JST connector is a special connector and is designed to tap into each battery cell for balance charging.

Balance charging

Due to manufacturing differences, cell placement, varying resistance in the cells, etc that each cell actually discharges at a slightly different rate. If you charged the battery up with all the cells connected in one go there is a risk that some cells could be overcharged while others would be undercharged. The solution is to balance change, which is where a special charger charges each cell individually. Luckily nearly all lipo chargers balance charge. You just need to hook the battery up as it says in the manual (usually JST + main connector).

Fast charging

If you're at the field and you know your batteries are in good condition you can fast charge. Balance charging will charge all the batteries to 80% of their capacity and then trickle-charge each cell until full. This trickle charge can be slow therefore when out in the field check your charger for a "fast charge" option - this will just charge the combined cells to 80% of their capacity ready for using out in the field. Obviously, you'll have reduced capacity but you'll be going much quicker than waiting for the balance charge to complete. Just make sure that when you get home you do balance!

Summary

Hopefully, you understand how lipos work now and some of the estimates and calculations that can be done to ensure you're using the battery at it's best. If you enjoyed this, please tweet or share with friends.