How To Look After LiPos Part 1

Lithium polymer battery technology has revolutionised electric model flying, particularly for helicopters, but they do require careful treatment if you are to get maximum performance and longevity from them. Jamie Cole offers some help and advice on how best to look after your precious power packs.

LookafterLiPos1

It doesn’t matter who you are or where you have been, unless it’s been on a desert island with no toys, you just cant get away from lithium battery technology. It has transformed our modern day living, mobilising all of us, from you mobile phone being so small with such a long talk time, through to a netbook with its lithium battery, or your Playstation 3 controller or your toothbrush or now even your drill. The technology in batteries has transformed so much around us and we in our hobby are no different.

Five years ago playing with a battery-powered helicopter was reserved for a few enthusiasts, with a 3D capable helicopter needing a bucket load of money being thrown at a bucket load of batteries weighing in as much as a bucket load of lead. That’s a lot of buckets for what was in my opinion not a lot of fun, or at least not as much fun as you could get with an IC nitro-powered helicopter.

So enter lithium polymer or LiPo batteries. They’re lighter, offer more volts and more capacity for the same size as a NiCad or Nickel Metal Hydride, they don’t suffer from memory effect and they don’t false peak when charging. Awesome, so what’s the downside?

Early adopters of the LiPo packs had some issues, but this was mostly down to poor cells, low C ratings (described later on), incorrect charging, incorrect cell count and damaged cell charging. With this in mind we aim to tackle a few of the myths, some of the basics and some of the ongoing points you need to be aware of.

LiPo Overview

There are currently three main types of lithium cells, these are lithium ion (Li-ion, 3.6V per cell) Lithium Polymer (LiPo, 3.7V per cell), Lithium Ferrite (Li-Fe, 3.3V per cell).

The main difference between lithium ion and lithium polymer cells is that the ion cells are in a conventional cell like cylindrical cell and the lithium polymer cell is in a foil like ‘flexible’ case, not exactly flexible to the point where you can fold one in half, but flexible to the point they are not in a hard case like a conventional cylindrical cell like a sub-C cell. If you read up on it then it is stated that LiPo cells offer 20% more energy density based on not having the hard outer shell saving space and weight.

In this article we will be covering the usage of lithium polymer cells or LiPo cells as we refer to them.

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S or P

LiPo cells are rated at a nominal voltage of 3.7V. You can join these cells in series (positive of one cell to the negative of another), I should probably rephrase this as purchase the cells joined together in series, to create a pack of the desired voltage. Two cells in series is therefore 7.4V (i.e. 2 x 3.7V) and three cells is 11.1V (3 x 3.7V). This gives us the ‘s’ or series of a pack. Hence a 4s pack is 14.4V or 4 x 3.7V.

The P rating of a pack is a rating of a packs capacity. If you have a one cell 3.7V and 1350mAh that is joined to another one cell 3.7 1350mAh cell in parallel you have a 3.7V 2700mAh pack (i.e. two x the capacity rating). This is the negative terminals of two cells joined together and then the positive terminals of a cell joined together. This is the P rating of cell. Therefore a 1s 2P 3.7V 2700mAh pack is made up of two cells of 1350mAh made up in a parallel configuration.

A 2sS2P 7.4V 2700mAh pack would be made up of four cells, a pair of cells made up of two pairs of parallel cells joined in together in series to make the complete pack. Commonly you see 6s2P packs.

Personally I am not keen on parallel configurations unless absolutely necessary as they become very difficult to keep in balance over a long cycle life. (explained in ‘charging’).

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Charging And Balancing

This is by far the most important step in owning LiPo cells, a good charger will give you a good cycle life and well balanced well maintained cells.

Charging a LiPo Cell is very different to charging a NiCad or a NiMh cell. With a NiCad or NiMh you mostly charge them with the ‘peak detect’ method, which in short means you feed them voltage and amps at the desired level until the voltage of the cells drop of slightly from it highest point, this because the cell will only take a certain amount of charge after this maximum point you will find the voltage will start to drop off, in effect when it is overcharged, this is peak detection mode. At this point the cells will normally warm up a bit. There are more complex ways of charging NiCad and NiMH cells but this is the basis of most chargers.

With LiPo technology the charger dictates the peak voltage per cell, based on the technology of the cell, this is normally around 4.2 volts per cell. LiPo cells should never go above 4.3V per cell as they will become unstable and you are at risk of damaging them. When the LiPo cells hit 4.2V a charger will normally back off the input amperage to keep the voltage below this point until such a time that cells are at 4.2V with little input. This then becomes a complete fully charged cell.

You can see the two methods differ greatly and that if you put a LiPo Cell on a NiCad charge the charger would be waiting for that ‘peak’ voltage drop but it never happens with a LiPo, the voltage will keep on rising until the inevitable happens. So in short! Don’t put a LiPo on a NiCad charge.

The above is a fantastic firework demonstration but above all is damn dangerous. The flames are actually quite scary and intense. This is not just the case for LiPos but also most other battery types, if you over charge them they will all go pop eventually. With the above in mind you can begin to understand the importance of cell balancing, or balance charging.

If you were looking at a 2s pack which is 8.4V fully charged then each cell would should be 4.2V per cell. But if you are charging the cell as a pack through the main output leads of the pack then potentially you could after use end up with the cells going out of balance yet ending up with a full charged pack.

 

As an examples with this 2s pack:

  • Pack voltage = 8.4V
  • Cell 1 voltage = 4.0V
  • Cell 2 voltage = 4.4V

 

Cell 2 would be well over voltage and at risk of being damaged, be this puffed or decreased long term capacity or worst case scenario even explode if left to continue going out of balance.

The answer to avoid the above is cell balancers, this is why most packs need cell balance leads to measure the voltage of each cell in a pack so you can connect to a cell balancer or balancing charger. Every time you use you packs they may well go in or out of balance so make sure when you charge them you balance them whilst charging or even best case scenario use a balancing charger such as the Bantams BC10’s.

There are also cell balancers that connect to the charger and then the cells, in effect sitting between the charger and the LiPo pack. The balancer will monitor the cell voltages and keep them in balance, should one of the cells go over voltage then the balancer will fail safe and cut the charge to the pack form the charger.

It’s also worth noting at this point that the quality of the packs you buy will have a bearing on how well you cells are matched or paired up. Good quality cells will ensure any charging and discharging has minimum impact on the cells going in and out of balance. Some cheaper cells are not matched through this stringent process leading to the cells going further out of balance. This is certainly the case in my experience of trying different packs and sets the higher quality packs apart from the cheaper counter parts.

So I guess in conclusion this is probably the most important part of owning a LiPo pack. Make sure you have a decent charger, and make sure the cells are being balanced whilst charging. If you stick to this formula you should be able to maximise the life of your cells.

There seemed to be a big fear of LiPos in the early days, but in my experience and from the ‘scare’ stories I have heard all of them were completely avoidable by charging them correctly with a good charged whilst balancing charging the cells.

The rate of charging of most cells is specific to the manufacturer’s guide lines, but if you are going for the branded packs such as FlightPower and the company’s latest EON cells you can even charge at 2C, i.e. two times its capacity. For example a 1000mAh pack could be charged at 2000mAh (whilst utilising a balance charger) meaning it would be charged in 30 minutes! Which is awesome news for those of you who are impatient.

My personal recommendation to get the most charge cycles would be to charge at 1C i.e. a 1000mAh pack charged at 1 Amp, meaning approximately an hour from discharged.

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C ratings explained

The ‘C’ rating is a reference to maximum discharge rate of the cells. It is basically the cell capacity multiplied by the C rating to give the maximum sustainable discharge rate.

Example:

  • 7.4V 2s1p 10C 2000mAh pack.

 This would mean the pack would give a maximum sustainable discharge rate of 20Amps, i.e. 10 x 2000mAh.

In the early days LiPos would be around this 10C figure, but now this has gone up to figures such as 33C contestant, and a burstable figure of 66C which is insane power from most of these packs and substantially more then NiCads and NiMH could every give.

In fact to give you an idea of how powerful the cells really are the EON 28 3s 2190mAh pack is typically used in the 450-sized machines is normally paired with a 40 Amp Controller or worst case a 60 Amp controller, this little pack can give out 61.3 Amp continuously which is above the rating of the speed controller by quite some margin!

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Applications of LiPos

The application of LiPos in the hobby of RC airborne flight is massive. It has enabled machines to go electric giving as much if not more power to weight ratios then IC/nitro machines.

It has also given us power in miniature such as the 450-sized helicopters like the Trex 450, Furion and even the T-Rex 250. Non of which were even thinkable before the LiPo batteries hit the mass market giving the power to weight ratio we needed to make it viable.

There has been an explosion of applications in the airplane market also giving way to a massive explosion of applications and new categories such as the Ikarus Shock Flyer indoor flying 3D plane.

From the point of view of helicopters there have been numerous competitions won by electric flying machines and they have even been used by the FAI guys such as Curtis Youngblood.

I personally think the LiPo could take over the RC scene altogether in the next few years, or at least become a dominant party over the Nitro machines. They are becoming more affordable and more to the point the latest generation is giving more charge cycles meaning you are getting even more value for money.

Charge cycles

Answering this question and tackling this subject is like answering the question does my bum look big in this, its subjective and there is no right answer, it depends!

This is an interesting point and relative to how you treat your batteries. If you follow some basic guidelines you should maximise the life of the cells.

The charge cycle is normally a measure of how many discharge and charge cycles can be achieved before the packs reach the end of there useful life. By useful life I would say that when cells come to the end of there useful life you will see the capacity drop off as well as how long the cells hold the peak voltage.

Out of all of my cells I can honestly say I have only killed two packs out of a collection of 20 or so that I have acquired over the last four years. The rest of them are still going strong.

The latest cells from FlightPower quoted as having an increased cycle life of over 300% compared to the previous cells. So what does this mean in real terms. Again this is down to how you treat your cells and would hate to quote a cycle life figure. To maximise your cycle life try to following the basic guidelines:

  • Make sure the application of the cells is appropriate to the voltage and capacity of the pack. If you are getting a two minute flight time I would say you have crossed the line and may decrease the life of the cells. If you put a 2000Mah pack in a machine that should have a 6000mah pack you are going to kill the pack in a few cycles.
  • Balance charge the cells
  • Make sure the voltage does not fall below 3.0V per cell
  • If you are storing them store them at a storage charge state
  • Don’t let them freeze!
  • Don’t let them get too hot
  • Try to charge them at 1C when ever you can

It seems basic, but sticking to the above has seen me with well over 100 cycles per pack and some are still going strong over 200 cycles.

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Speed Controllers And Discharging

There are still some non LiPo friendly speed controls out there. A LiPo friendly speed controller is basically a speed controller that wont allow the voltage of the pack to go below the equivalent of 3.0V per cell.

Taking a LiPo below 3V per cell can damage the cells and breakdown some of the core chemistry of the pack. If you do find a LiPo cell has gone down below 3V unloaded them I would probably bin it or at least be very wary of it.

A good LiPo friendly speed controller will give you a little notice if the voltage has dropped below the predefined point, normally in the form of decreased power to the motor. I would really advise a hard cut off in a heli as it may catch you short.

The voltage cut off point normally comes with a few options for the user to choose from, if you want to air on the side of caution I would opt for the highest voltage cut off point, normally 3.1V or higher.

However, if you are not so cautious I would advise the hard 3D guy to opt for the lowest voltage and the soft flier to opt for the highest voltage. This is quite simply because the harder flier will be likely to load the pack with bursts of power where the user is punching the pitch, causing the voltage to drop of during these loaded sessions. If the cut off voltage is set to a higher figure you would probably finish too soon, not getting the most out of the pack.

At the other extreme the softer flier is less likely to load the pack meaning by the time the voltage gets to the lower point it is likely to be very low and likely to get below that nominal 3.0V point.

LiPo storage

Many chargers will now offer a storage charge for your LiPo packs, this is quite simply a recommended charge state for the LiPo packs. If you are leaving them for long periods with out use, say more then a couple of weeks then its is recommended to storage charge the cells. Its normally around 3.7 Volts or 50 percentish charged.

From a users perspective and living with LiPos I would say that the cells go off when left fully charged for longer periods. When you come to use them for the first time after being stored with a full charge they don’t seem to give quite the punch they should. But one swift charge later they are back up to full performance.

LiPo Dos And Don’ts

Do

  • Use a good charger with LCD display showing capacity info, so you know how much is going back in to the pack
  • Balance charge them every time
  • Keep them in an appropriate container
  • Buy larger packs 8s and over in pairs, this way if you damage a pack not all cells are useless.
  • Use the right Speed controller, LiPo friendly
  • Do buy quality cells
  • Keep them in the car on a cold day

Don’t

  • Charge without balancing
  • Use a crappy charger
  • Let the voltage get too low, less then 3.0V per cell
  • Over charge the cells
  • Puncture a cell and then charge it
  • Take any risks
  • Use cold packs, i.e. leave them in freezing conditions
  • Leave them stored with a full charge

 

 

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