Lithium polymer batteries and balance chargers for multirotors
I get a lot of questions about multirotor equipment so I thought I’d start creating some posts about that. Please bear in mind that I learn a lot from reading around the blogs and forums so any inaccuracies are completely due to my own misunderstanding. By reading this you accept that, although I hope it will aid with your own understanding, it is written in good faith and I can’t take responsibility for any damage to yourself or your equipment that occurs through the use of this information. If you see anything that I have got wrong or you feel is dangerous, please contact me through the HexCam website or Facebook page and I will rectify it as soon as possible. Please treat lithium polymer batteries and multirotors with same respect you would give to other electrical and mechanical equipment. Look after them and they will look after you.
You also need to know the following: I am not paid to review or endorse any products but I do have a partnership arrangement with Versadrones to act as their demonstrator and trainer in the UK.
It worries me that people are prepared to skimp a little bit on their lipo batteries. It is worth bearing in mind that anything that can fly can fall, so I tend to look around for the best batteries I can within my budget. However, there are now a vast number of different lipo batteries, chargers and power supplies out there which leads to some confusion. I tend to buy my batteries from Hobbyking who now have a UK warehouse and have come to prefer the Turnigy and Turnigy NanoTech batteries. My reasons being that they hold good cell balance and seem to maintain battery health for a good period of time whilst being reasonably priced. I’m going to try to keep this as non-technical as possible, but when talking about batteries you have to get a bit technical. As a result I suggest you get a coffee (or maybe a beer if it is late enough in the day), maybe a notebook and possibly some biscuits as well. Sitting comfortably? Then I’ll begin…
Lithium polymer battery basics
What on earth is 3S, 4S, 6S etc?
I’m not going to go into the chemistry of lithium polymer batteries in this blog, but the basic unit of the battery is the cell. A battery is generally made up of between 2 and 10 cells in series. Each cell has a nominal voltage of 3.7 volts and a fully charged voltage of 4.2 volts. So 3S means 3 cells in series. Putting cells in series multiplies the voltage of each cell by the number of cells. A 3S battery has a nominal voltage of 11.1 volts (3 times 3.7V) and a fully charged voltage of 12.6 volts (3 times 4.2V). A 6S battery (now standard on a lot of larger multirotors) has 6 cells in series so has a nominal voltage of 22.2V and a fully charged voltage of 25.2V.
With me still?
But I saw 6S2P!!! What is that?
OK. Unlike most other types of battery, lithium polymer batteries can be connected in parallel (P) too. This is incredibly useful as it means by having two batteries the same connected in parallel you can maintain the original voltage but double the capacity. 6S2P means we have two 6S batteries connected in parallel. So, if each battery is 5000mAh, by connecting them as 6S2P we now have 10000mAh at 22.2V. In some uses this would effectively double the life of the battery. When flying, you have to take into account the extra weight you are adding with the extra capacity. As a result, if a single battery gives you 10 minutes of flight time running two in parallel will give you around 16-17 minutes. So, you lose some flight time but gain a degree of redundancy in the system in case of one battery failing. Your call! I prefer to fly with two batteries where possible.
This one is a bit simpler, The mAh or Ah refers to the capacity of the battery. As a compromise between weight and power we tend to use 5000-6000mAh batteries and connect them in parallel. So a typical hexacopter will run on 6S2P with each lipo having a 5000mAh capacity, giving 10000mAh to play with, of which we recommend about 8000mAh is actually used. Leaving 20% in your batteries not only gives you a safety barrier, it also means that if you do suddenly need that last bit of power on landing, it is there. Discharging a battery repeatedly below 20% will cause damage to the battery chemistry which could then fail during flight or charging.
Here is a video of one of our hexacopters. You can see the two 6S 5000mAh Turnigy Nanotechs on top. They are connected in parallel to give 6S2P at 10000mAh:
I found some more numbers! 25-35C. What’s that!?
Lithium polymer batteries almost always have a “C” (capacity) value associated with the other battery information. In fact, they may have two, one for charge rate and one for dicharge rate. The C value is the recommended or maximum rate at which current can flow through the battery without causing damage to the battery chemistry. These days batteries tend to start at around 20C for discharge (use) and can go up to around 100C. So what would a 1C rate look like? The easiest way to think of it is a 1C rate would use the capacity of the battery in 1 hour. So a 5000mAh battery used at 1C would be down to 0% after an hour. If you double the C rate it halves the time. So at 2C it would last 30 minutes and at 4C it would last 15 minutes. Most of my copters tend to use power at around the 5-6C rate when hovering, so a single 6C battery will last around 10-12 minutes. However, if I suddenly put my copter into full vertical lift it will draw more power and so the C rate will shoot up. If a battery says 25-35C, it means that the continuous discharge it can cope with is 25C with a peak discharge of 35C that should only be reached for seconds at a time. It is recommended that you choose a battery where all your usage will be at or below the continuous discharge rate. For most hexacopters 25C continuous discharge should be perfectly adequate. The discharge rates going up towards 100C are really aimed at pilots carrying out extreme flying with very rapid changes in power output, for example 3D helicopter flight. With aerial filming, the majority of flight is gentle movement and hovering.
The charge C rate
Conventionally, it is recommended that batteries are charged at the 1C rate. This means that a battery will take around an hour to charge and a little longer with balancing. However, you will find that batteries now have charge rates of up to 5C. I tend not to charge my batteries above 1.5C. Rapid charging will eventually degrade battery life. Of course if you are using batteries in 10 minutes and then they take an hour to charge, there is a mismatch in supply and demand if you are out on a long shoot. At this point you have a few options:
- You carry enough charged batteries to do the whole shoot
- You carry fewer batteries but have a charger solution that allows you to match supply and demand
- You have an intermediate amount of batteries and accept that as the day proceeds demand may exceed supply!
This conundrum leads me into chargers and home and portable power supplies, which I will blog about in the next instalment.
I will also talk about lithium polymer battery safety soon as, although the safety has come on a long way in the last couple of years, lithium polymer technology should still be treated as potentially volatile, particularly when it comes to storage, transport and treatment after a crash.