The most important terms to understand in an electrical circuit is voltage, resistance, current, and power. A simple analogy used to understand these terms. Think of the battery’s circuit as a pipe we are trying to pump water through, like the picture below:
- Voltage is the water pressure that is pushing the water down the pipe. Voltage is measured in volts and represented as V
- Resistance is the size of the pipe. If the pipe is wider, more water will flow through. When the pipe is smaller, there is more resistance. Resistance is measured in ohms or Ω.
- Current is how much water comes out of the pipe. Current is measured in amperes, amps or A.
If you know any of the 2 numbers you can calculate the third:
- Voltage = current x resistance
- Current (Amps) = voltage / resistance
- Resistance = voltage / current
Another important concept is electrical power. This is simply voltage x current. Think back to the water analogy: you’re taking the voltage, or the water pressure, and multiplying it by the current, or how much water is actually coming out. Power is a measure of how ‘hard’ the water is coming out of the pipe.
Why is this important? It tells us how much work we can actually do with the electricity.
Imagine you’re trying to put out a fire with a firehose.
- If a lot of water comes out of the pipe but comes out slowly (no pressure), then you can’t put out a fire.
- If the water comes out quickly but there’s very little water, then you can’t put out a fire.
- If the water comes out quickly (high voltage) AND there’s a lot of water (high current), then you can put out a fire.
Electrical power is like the ability of your firehose to put out a fire. Power is measured in Watts (W).
- Watts = Volts x Amps
- Amps = Watts / Volts
- Volts = Watts / Amps
Another important item to know about electrical systems is the difference between a series and parallel circuit.
- In a series circuit, the current through each of the components is the same, and the voltage across the circuit is the sum of the voltages across each component.
- In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component.
LiPo Battery Basics
Lithium polymer batteries, more commonly known as LiPo, have high energy density, high discharge rate and light weight which make them a great candidate for RC applications.
By learning the basics about LiPo batteries, you will be able to read and understand their specifications.
Battery Voltage and Cell Count (S)
LiPo batteries used in RC are made up of individual cells connected in series so the battery’s voltage is the sum of the voltage of the cells. Each cell in a standard LiPo battery has a nominal voltage of 3.7V. Therefore battery voltage is often referred to as how many cells in the battery (aka “S”).
1S = 1 cell = 3.7V
2S = 2 cells = 7.4V
3S = 3 cells = 11.1V
4S = 4 cells = 14.8V
5S = 5 cells = 18.5V
6S = 6 cells = 22.2V
For example, we call a 14.8V battery a “4-cell” or “4S” battery.
Voltage affects brushless motors RPM directly, therefore you could use higher cell count batteries to increase your quadcopter’s speed if your motor/ESC and other electronics support higher voltage. But a battery with more cells of the same capacity is heavier since it contains more cells.
A LiPo battery is designed to operate within a safe voltage range, from 3V to 4.2V.
Discharging below 3V could cause irreversible performance lost and even damage to the battery.
Over-charging above 4.2V could be dangerous and eventually cause fire.
However it’s advisable to stop discharging when it reaches 3.5V for battery health reasons. For example for a 3S Lipo, the max voltage is 12.6V, and you should land when the voltage reaches 10.5V (at 3.5V per cell).
LiPo Battery Capacity and Size
The capacity of a LiPo battery is measured in mAh (milli-amp hours). “mAh” is basically an indication of how much current you can draw from the battery for an hour until it’s empty.
For example, for a 1300 mAh Lipo, it would take an hour to be completely discharged if you draw a constant 1.3A current from it. If the current draw doubles at 2.6A, the duration would be halved (1.3/2.6=0.5). If you draw 39A of current non-stop, this pack would only last 2 minutes (1.3/39=1/30 of an hour).
Increasing your battery capacity might give you longer flight time, but it will also get heavier in weight and larger in physical size. There is a trade-off between capacity and weight, that affects flight time and agility of the aircraft.
Higher capacity could also give you higher discharge current as you will see in the next section.
C Rating (Discharge Rate)
Lipo batteries for quadcopters these days all come with a C rating. By knowing the C rating and capacity of a battery, we can calculate the safe, continuous max discharge current of a LiPo battery.
Maximum Discharge Current = C-Rating * Capacity
For example an 1300mAh 50C battery has an estimated continuous max discharge current of 65A.
Some batteries come with two C-ratings: “continuous” and “burst” ratings. The Burst rating is only applicable in short period of time (e.g. 10 seconds).
Although C rating could be an useful tool, it has become mostly a marketing tool and is not always accurately reported.
If C rating is too low, the battery will have a hard time delivering the current to your motors, and your quad will be underpowered. You could even damage the battery if current draw exceeds the safety rating.
Rule of thumb, the battery connector should match the one you are using on your copter. If you don’t own a quad yet, choose one, and stick with it.
All Lipo batteries come with 2 sets of wires/connectors: a balance lead and a main lead or discharge lead (Except for 1S batteries which only have a main lead). There are quite a few different connectors used in LiPo batteries. The main differences are shape, weight and current rating.
1S Battery Connectors
1S connectors are tiny and have very low current rating. They are commonly used in brushed micro quadcopters or “Tiny Whoop”.
|LOSI (Typically cheaper “toys” have this connector)|
|Pico blade (“The original “Tiny Whoop” connector)|
|JST-PH (Newer “Power Whoop” connector)|
2S-6S Battery Connectors
You will find a lot more different types of battery connectors in this category, in fact not all are listed here. But majority of them are not used that often so you don’t need to ever worry about them. For mini quad, the most popular connector is probably the XT60.
However since XT60 is only rated at 60A, and mini quad are running at higher and higher current and voltage, we might soon see a change in the popular connector used.
|XT30||Mainly 2S and 3S|
|XT60||Similar to the Above, only bigger||Mainly 3S, 4S|
|XT90||Similar to the Above but even bigger|
|EC5||Similar to the Above but bigger|
Balance lead is mainly used for balance charge to ensure all cell voltages are equal. It also allows you to monitor the voltage of each cell.
The number of wires in a balance lead starts at 3 for 2S LiPo, and for every increment in cell count, the number of wires also go up by 1.
LiHV is a different type of LiPo battery, HV stands for “high voltage”. They are more energy dense than traditional LiPo battery, and allow to be charged up to 4.35V per cell. However there are mix reviews out there regarding the longevity of LiHV, as they might have decrease in performance sooner than normal LiPo’s.
When you charge LiHV batteries like standard LiPo to only 4.20V per cell, they perform pretty much similar. However when you charge them fully to 4.35V per cell you get the following advantages:
- With a fully charged HVLi battery, voltage is higher than normal LiPo’s (on a 4S, HVLi is 17.4V, LiPo is 16.8V), therefore your motors will run harder at higher RPM, and your quad can fly faster theoretically
- Secondly, LiHV can store more energy than LiPo per weight, so theoretically (again) you get longer flight time. Hyperion (the company that makes these HVLi batteries I am testing) states there is a 10% increase in capacity than standard LiPo’s of the same size and weight
- Lastly, good quality HVLi has lower voltage sag on high throttle
Can you charge your normal lipo to 4.35V?
So you might wonder: “can I overcharge my normal lipo to higher voltage like 4.30V or even 4.35V, to get more power and longer flight time? ” The answer is NO!!! That’s extremely dangerous and very likely to cause fire because of the different cell chemistry.
How to Charge LiPo
Type of charging
- Balance charge – The charger monitors the voltage of each cell, and can charge them individually while trying to keep them at the same voltage level. This is the safest and most recommended way of LiPo battery charging
- Direct charge (fast charge) – You are charging through only the main lead, and the charger isn’t monitoring the voltage of each cell. This is normally faster, but it could result in unbalanced cell voltages and the battery might not be 100% charged
- Storage charge – The charger brings each cell of the battery to their storage voltage, which is 3.80-3.85V
- Discharge – The charger attempts to drain the Lipo battery (very slowly, even slower than charging)
Why Balance Charge?
Every cell in a battery is slightly different, after the battery is discharged, you might find that the cell voltages are all different.
If we were to direct charge this unbalanced battery without monitoring voltage of each cell, chances are some cells might end up under 4.2V (not fully charged), and what would be worse, some might go OVER 4.2V. If you remember, LiPo cells should never exceed 4.2V or they will become dangerous. Remember, overcharged = dangerous!
Most decent modern Lipo chargers are programmable and allow balance charging, and they should take care of this automatically.
Incorrect handling of LiPo batteries could potentially cause fire. Please take your time to read through these safety rules before handling/charging batteries.
- Pick up a LiPo battery by its body, not the leads – the wires could be pulled off from the fragile solder joints.
- Charge at safe places – It’s very important to find a fire-proof location to charge your batteries. Using a Lipo-safe bag is a good option, some even build a bunker for it. An ammo box is a cheap yet effective solution.
- Don’t charge your battery immediately after using it, wait until it has completely cooled down.
- It’s advisable to charge your battery at 1C or less.
- Never charge your battery unattended – regularly check if the battery is getting warm or starts to swell, if so stop charging immediately.
- Never use or charge a damaged battery – don’t charge if it is swollen (puffy) or has any other visible signs of damage.
- Ensure the number of cells and battery type are set correctly on your charger to match the cell count and type of your battery.
- Don’t overcharge, although this is normally taken care of by the charger, it is a good idea to check cell voltages regularly.
- Don’t leave a battery in the sun or a hot car.