Saturday, 4 February 2012

LiPo Batteries: All You Need To Know

Walkera V450D01 inverted hover (grass cutting!)
If there is one thing that has helped the hobby of flying RC helicopters explode to its current popularity it has to be the develoment of the LiPo battery, largely as the result of the drive to produce small light rechargeable power sources for the mobile phone industry. With not only their size and weight characteristics but also their power output and storage capacity being very favourable for use in RC flying models, where the power to weight ratio is so critical, everything was set for electric powered models to make huge strides forward. Whilst the only viable power source had been nitro/gas engines the hobby was always going to remain limited to the more serious enthusiasts; largely because of the generally perceived difficulty, messiness and expense of these models. Furthermore, the required size and noise of these models restricted when and where they could be used. With the rise of the LiPo battery the way was paved for electric powered RC helicopters and planes to rise to the fore and make the hobby much more widely accessible. Once radio controlled helicopters were able to enter the toy market the real surge in interest in the hobby side inevitably followed and from that rise in popularity we have seen not only a much wider range of manufacturers in the market but also manufacturers who have the financial means to invest in driving technological developments forward whilst dropping the prices of advanced products still further for the consumer. So the benefits of the advent of the LiPo battery have been wide ranging for the RC helicopter hobby.
Electric RC helicopters now have the power to perform the most amazing aerobatics even in large size models

LiPo’s, or more correctly Lithium Polymer, batteries are the most common way today of powering electric RC models. Planes and helicopters are biggest beneficiaries of the development of LiPo batteries because they are very light in weight compared to the amount of energy they can store. They also have high discharge rates compared to conventional rechargeable batteries (NiCad or NiMH) enabling them to provide the huge bursts of power required by high performance motors used in extreme flight manoeuvres. It is the innovation of the LiPo rechargeable battery that has enabled electric power to become so popular for RC helicopters, brought the cost down and opened the wonders of flying RC helis to a much wider audience who would never otherwise have entered the hobby.

The latest micro-technology together with the advent of LiPo batteries has enabled palm-sized Rc helicopters to be developed, now with highly agile performance, such as this 6-Channel RC helicopter from Walkera.

For all their advantages there are a few downsides to LiPo’s; namely their relatively high price, whilst having a lifespan that is quite limited. You need to care for them properly if you are going to get even 300 – 400 cycles out of them. In order to do this you need a little knowledge on how to care for them (another article needed for this). There is a degree of hazard involved with LiPo’s, the volatile electrolytes have been known to catch fire. This is not something that I would overplay but it is something that you need to be aware of and should always be kept in mind.
The risk of LiPo fires are sometimes overplayed but it should never be forgotten!
There are a few numbers on LiPo batteries that you need to particularly look out for, which determine its rating and suitability for your particular model. The main ones are the batteries voltage (V), its capacity (mAh) and its discharge rate or ‘C’ rating.

Confused by all the numbers listed for a LiPo?

So, the first of these – voltage (V). LiPo battery packs are built up of cells each with a voltage of 3.7v. Those used in radio controlled helicopters, planes and the likes will normally be 1 to 6 cells each of 3.7v. LiPo’s will normally state the number of cells (S) but do not always show the voltage. So a 1S pack will be 3.7v, a 2S pack is 7.4v and a 3S pack 11.1v, right the way up to a 6S pack at 22.2v. Why is ‘S’ used to represent the number of Cells? Well it is the number of cells in Series and ‘C’ is used to represent the discharge rate. Ok, ‘C’ for Discharge rate! Makes sense! Well actually there is sense in it as the discharge rate is a factor of the Capacity of the cell. This will all become clearer later. If there are a number of cells in Parallel this will be given a ‘P’ number, but other than very large capacity LiPo’s packs are usually 1P. So you might see “3S 2P” written on a LiPo, indicating that there are 2 lots of 3 cells in series hooked up in parallel.
The important thing though with voltage is simply to ensure that you have the correct voltage LiPo’s for your particular model. This will be determined by the motor and speed controller combination of the model. You simply follow that. So if it states you need an 11.1v 3S LiPo you get just that for your model. The capacity and discharge rate of the LiPo’s can be easily varied but the voltage must be right.
That then leads nicely on to deal with LiPo capacity. This is usually given in mAh (milliamp hours). This quantifies how many milliamps you could drain from it at a constant rate for an hour in order to fully discharge the pack. Thus if you could run a LiPo at 2200mA for an hour its capacity would be 2200mAh and it would have twice the capacity of a LiPo that could run at only 1000mA for an hour in order to completely drain it from fully charged. It is like the size of the fuel tank in your car. Obviously the larger the capacity of your fuel tank, or LiPo battery, the longer you can run for. With radio controlled models, especially RC helicopters and planes, the limiting factor is the weight and size of the battery pack. A larger capacity pack may give you longer flight times but being heavier it will adversely affect performance.
So now for the discharge rate (C). As noted before the ‘C’ is related to the capacity and here is how. The C number represents how many times faster than the Capacity you can discharge the pack safely at a constant rate. So a 2200mAh LiPo with a 20C rating can be safely discharged at up to a sustained load of 44000milliamps (44amps). From this you can also determine how long it would take to totally discharge the pack at that load. Since the capacity of a pack is quantified by how many milliamps it will take to totally discharge it in an hour, the time it will take to drain at the C rate will be 1 hour, or 60 minutes, divided by the C value. So a 2200mAh pack with a 20C rating would discharge in 3 minutes. In practice we never constantly apply anything like these levels of sustained discharge so the flight times a much longer.
Many packs will now show two figures for the capacity, eg 20C/40C. The first of these represents the constant discharge rate discussed above and the second the ‘burst rate’. As you might guess from the name the burst rate denotes the discharge rates for short bursts. 

Some manoeuvres require short bursts of high power. 
Pictured is the amazing Walkera V120D02S performing 3D

There are a number of factors to keep in mind when selecting the C rating for your batteries. Firstly it should be noted that in general the higher the discharge rate the more expensive the LiPo, although the difference is getting less. Secondly, you cannot use one that is too low; which would damage your pack and quite possibly ESC. For beginners and light sport or scale flyers with RC helicopters up to a 450 size 20-25C is ample. When you start doing more aggressive 3D manoeuvres or using bigger helicopters you need to look at stepping up to 35-45C rated packs. The best advice here though is that if you can afford to do so you should go for a little more than you need. A LiPo pack running close to its limits will get hot and this shortens its life. So you may pay more for a more generous C rating but it is likely to last you longer. A tip here is to test the temperature of your LiPo’s immediately after landing. If it is too warm to comfortably hold tightly in your hands then the pack is getting too hot. This means that either you need a higher C rated pack or your pack is getting old and needs replacing.
LiPo’s will also be overheated by running them too long. Never ever run them to a standstill. As they run down the voltage drops and the internal temperature rises, with 3 volts per cell being the critical point at which they become overheated and potentially damaged. Often quoted is the 20% golden rule. That is, you always have 20% of the LiPo’s capacity remaining at the end of the flight (Sometimes you will see this written as the 80% rule – obviously this being that you should not drain more than 80% of the capacity).  So a 2200mAh pack should always have a minimum of 440mAh remaining. A good digital charger will show you these figures. If you hook it up after a flight it will give you the capacity remaining in the pack and you can also read the voltage of each individual cell. Alternatively you can measure the voltage of the whole pack with a digital voltage meter. Look for reading of more than 3.75 volts per cell (ie 3.75 x the number of cells in the LiPo pack). So that would be 11.25v for a 3S pack. 
Conversely, at the other end of the scale you should never overcharge a LiPo. A 3.7 volt per cell LiPo is fully charged when it reaches 4.2 volts. Beyond this again it will overheat, shortening its lifespan and potentially they can set alight as the electrolytes they contain are volatile. A good quality charger which measure this and you will be safe so long as you set the correct number of cells or voltage on the charger (the correct voltage to select will be 3.7 x the number of cells in the LiPo pack).
It is also important to get the charge rate correct in order to maximize the life of your LiPo’s. Until recently this was always no more than the capacity of the LiPo (So a 2200mAh LiPo could be safely charged at a current of 2200mA). Now an increasing number of manufacturers are indicating that better quality packs can be charged at rates of 2-3 times the capacity or even more in some cases. The best advice here is to follow the advice given by the manufacturers of the brand that you purchase. The advantage of course of charging at a higher rate (current) is that the pack charges quicker; which equals less waiting time and more flying time.
Next it is important to balance a pack after charging. This ensures that every cell in a multi-cell pack has the same voltage. Your charger reads the total voltage across the whole pack, so that if it is set to charge a 3 cell pack it will stop charging when the voltage reaches 12.6 volts (3 x 4.2 volts). Now obviously that does not mean that each cell has 4.2 volts. One cell could have charged faster than the other two and have reached a higher voltage, and as discussed above it is critical that a cell does not get charged beyond 4.2 volts. Therefore, the pack should be balanced using a balance board, or today many better quality charges have a balance socket so that this can be done whilst charging.  
Charging using a 'blinky'

Alternatively, you can use a ‘blinky balancer’ which you can purchase separately if your charger does not have the facility. Cheaper balance chargers achieve the same effect through charging through the balance plug on the LiPo, but as this has narrow gauge wire only capable of taking a low current charging has to take place more slowly.

Charging through the balance plug

Finally a word on safe charging. It is always stated that you should never leave a LiPo unattended whilst it is charging. Since this is not very practical to stand watching them the whole time the following safety precautions are recommended. Firstly, check your LiPo’s regularly until the charge cycle is complete and never leave the house (preferably stay in the same room) during this time. It would be courting disaster to leave them whilst you go out and likewise do not go to bed with them still on charge, even though you know your charger will stop when they are completely charged. Charge your batteries in a fire proof container. The purchase of charging bags is highly recommended. Preferably charge your LiPo’s in a room fitted with a smoke alarm. Or even better buy one especially to site above where you recharge your batteries.

Look after your LiPo’s and they will repay you with good sustained performance over a long period of time. Charge safely and you will enjoy many years of flying.

 The power of electric flight - go anywhere, anytime - and now even fly onboard with FPV


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