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Batteries demystified – part.1

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By

Tim Mackey

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This is the first in a two-part article all about cells and batteries; the various types, how to choose and use them, charge them, discharge them and heck, in these green times – even how to dispose of them!

In this first part I am going to kick things off with a description of the modern energy source that is the Lithium chemistry based cell. Borrowing a little from my previous RCM&E articles “Electrickery” (which can be read on the site here, in the archives, link below)  I shall endeavour to explain how to get the best from these cells, and their unique properties compared to nickel based cells.

Now I am not a chemist, scientist, or even particularly technically savvy about the manufacture of these little marvels, but I have been a user of them for well over 5 years and have over a hundred different types and sizes in my arsenal, these fulfill many different roles not only in powering my electric models and  radio equipment, but likewise, assorted auxiliary gear onboard my aeroplanes. The latter includes sound effects, simulated guns, and lighting while I’m “currently” (dreadful pun intended) experimenting with making a power supply unit from the newer lithium based M1 cells from A123 technologies, to feed my hungry chargers when out at the field. I have old ones, new ones, good ones and some bad ones, I’ve charged and discharged these power-packs literally thousands of times and all without incident.

Voltage increases in line with the cell count.CHOOSE AND USE

Lithium based cells and batteries have been with us in radio control model land, in various forms, for several years now, and by far the most popular variant is the Lithium Polymer, more commonly referred to by its abbreviation Li-Po. A single Li-Po cell has a nominal voltage (think of this as sort of an average voltage) of 3.7V and they come in many different capacities – from as low as around 30mAh right up to 5000mAh or more.

The voltage and capacity (think of capacity in terms of duration) of the Lithium cell is much higher for the same weight, than older nickel based cells, so they make great sense in applications which require high power and low weight such as electric powered models. Unlike most nickel cells, they also have a very low self discharge rate.

Another of the great things about them is that we can happily join identical cells together in either series or parallel, or indeed a combination of the two, whereas this practice is not recommended for the older type of nickel based cells.

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This combining of cells in various configurations is where the pack designations come into play. If a 4-cell 2100mAh Li-Po battery is connected in Parallel to another identical battery this results in 4 cells in Series, and 2 batteries in Parallel, or, in other words 4S 2P. The nominal voltage remains the same as one battery, which in this case (4 cells @ 3.7V each) would be 14.8V. However the capacity is doubled and this newly formed 4s 2p battery now  boasts 4200 mAh capacity. To increase the volts, we join in Series, to increase the capacity, we join in Parallel. Combining both increases both.

A 3-cell pack on charge, note the balancer board connection which connects to and enables the use of the the charger’s on-board balancerC RATINGS
One thing that confuses many beginners is something called the ‘C’ rate. If a battery is described as 14.8V 2500 mAh, this means it can theoretically supply 14.8 volts, at 2500 milliamps (2.5 amps) for 1 hour. If the pack is also designated as 10C this (again, theoretically) means it can supply its power 10 times faster, so in this example 10 x 2.5 amps, which is 25 amps. However it’ll only supply this for six minutes; 1 hour (or 60 minutes) divided by 10 = 6 minutes.

One should treat manufacturer’s claims of very high ‘C’ rates with a degree of scepticism, indeed apart from anything else, running the packs at maximum ‘C’ rate will shorten their life.

It’s good practice to try and operate at about half the claimed maximum ‘C’ rate. If I need a pack to supply 40 amps then ideally, I will look for a pack which claimed to have a capacity of say 4000mA and a ‘C’ rate of 20. The ‘claim’ is that this pack can actually supply its 4 amps at 20 times that rate (80A) but personally I would run it at half that rate (10C) giving me the 40A I need. Some packs even claim rates as high as 30C – but remember if you actually did run them at that for the whole flight, they would last for just two minutes! (60 minutes divided by 30).

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CHARGING

When it comes to charging, Lithium based batteries and cells must only be replenished using a purpose designed charger, as the charging algorithms used to handle these cells are different to that of other cells, and using a ‘normal’ charger could be very dangerous. There are many suitable chargers available these days ranging from around £20 to upwards of £200 depending on the capability and versatility of the unit. Li-Po cells should only normally be charged at a maximum rate of 1C, so a 2000 mAh pack will be charged at 2000 mA (2A), giving an approximate charge time of 1 hour.

The correct terminal voltage on completion is 4.2V per cell max but slower charging and to a reduced level may well be advantageous in prolonging cell life – a Li-Po cell at 4.16V is over 90% charged, and will still give excellent performance.

Charging through an in-line balancer unit, in this case a FlightPower V-BalanceAs mentioned earlier, one of the benefits of Li-Po cells is their very low self discharge rate, and to many people this can be a great attraction, as it effectively means that a battery could be fully charged and then left until required – at which point you load up the car and go fly! Now this is indeed possible, and I confess to doing this myself with some of my packs, but – and there is a ‘but’ – best practice with Li-Po cells suggests that they benefit from being stored only partially charged to around 80% (4V) – the exact level is not critical, but this increases the overall life span, and potential cycles available from the pack.

Longer term storage should be at even lower levels of 60%, around 3.9V or so. Therefore to obtain optimum performance and longevity from your packs, store them partially charged, and then finish off just prior to a flying session. Many of the better chargers have a ‘storage’ setting for this very purpose. Any good quality LiPo charger should be capable of determining the correct charge cut-off point, even if the battery was not empty when put on charge – so don’t worry too much about the oft heralded warning about not topping up your packs.

STORAGE

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Lithium batteries are best stored in a dry cold environment of between 0° and 15° C. There is some evidence that storing them in the fridge, at just above freezing will considerably improve their lifespan and capacity retention but they must be protected from moisture. Fully charging them in a cold environment is not good practice, and if the temperature is lower than 10° C then charge to no more than 4.15V per cell. This is because the internal resistance of the cell reduces as it warms up and voltage then increases, so if fully charged to 4.2V when cold, and then brought into a warmer environment, the voltage will effectively rise slightly, and could get higher than 4.2V which is very bad for the cells.

Most packs come with heat-shrink protected loose ends or increasingly ‘Deans’ connectors like thisSome of the more intelligent chargers now have inbuilt ambient temperature sensing, and will automatically stop charging at a reduced level when a low temperature is detected. If charging cold and planning to take the packs out into a cold winter flying session, then only charge to around 90% capacity.

Li-Po batteries in common with most cell types perform poorly when cold, and will work a lot better if their temperature is around 30° to 40° or so. Remember also that internal temperatures will rise considerably during use, especially at high discharge rates, so allow adequate cooling to ensure they don’t get too hot. As with any battery, they shouldn’t be left in places that are subject to high temperatures such as car dashboards or rear parcel shelves, if cell temperature is allowed to get to 70° or more, then they will probably suffer permanent damage.

I normally top up my packs at ambient temperature, just prior to leaving for the field, and then transport them in an insulated carry pouch that my wife very cleverly made from an old quilted blanket, only fitting them just prior to flight. Good quality packs, properly treated could deliver 200+ cycles, but if you always store your batteries at high room temperatures and charged to full capacity then expect to see a loss of capacity of up to 40% a year, and if abused through charging whilst still hot, and constantly pushed to maximum C rate etc, then be prepared for a lifespan of maybe only 50 – 60 cycles – if you’re lucky!

The Li-Po label should provide all the info you need to assess its current handling capabilitiesSAFETY

Charging should only be carried out under supervised conditions, and many people advocate the use of fireproof containers for storage and charging of Li-Po batteries. Here, several purpose made container designs are now available including Li-Po sacks which will contain any fire that may happen if the charge process is not completed correctly.

It should be borne in mind however, that in almost every single case of reported problems with Lithium based cells, operator error was found to be the cause. On a personal note, I have had no such incidents at all – believing that a careful and regimented procedure when handling these batteries is paramount. Of course, extra safety precautions can only ever be a good thing, although as usual, the final decision is down to the individual user. If any cell or pack should suffer damage EG: in a heavy crash or as a result of similar physical abuse, then it must be inspected carefully. Any puncturing or noticeable crushing of the soft outer pouch should be considered serious. Any cell that exhibits signs of damage such as leaking electrolyte, bad odours, puffing or swelling should be disposed of as detailed later.

Li-Po cells should never be short circuited as extremely high currents could be passed which can cause excessive heat leading to rupturing of the packaging, so be especially careful with any terminals used for connectors – and ensure these are insulated when not in use – a careless moment in a pocket of loose change for instance could prove disastrous. Finally, Lithium based cells will not self ignite or vent unless you are doing something to them so those stories of spontaneous combustion whilst just sitting on a shelf are simply untrue.

A BALANCING ACT

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Unbalanced cells within a pack can ruin a battery. It is a common misconception that Lithium cells suffer this phenomenon more than other types – they do not, and in fact are actually better at holding balance than many of their counterparts. However, the result of an unbalanced Lithium pack can be more dramatic and could in some circumstances lead to the cell venting. It is important therefore to provide a method of ensuring that each cell in a battery is properly balanced with each of its neighbours.

So just what constitutes an acceptable level of balance and how do you achieve it?  In an ideal world, all cells in a battery would be perfectly matched and perfectly balanced, however it’s not a perfect world, and even the most finely matched cells will still result in some unbalance after a time. Many chargers now have cell balancers built in, and every Li-Po pack available today has a balance lead and plug together with the main power leads.

A good balancer should ensure that each cell remains within a few millivolts of the others, and this can vary from around 1mV or better, right up to 100mV depending on the accuracy of the unit. Anything greater than around 20mV (after charging) should be investigated and steps taken to improve the levels. Incidentally, checking the cell balance of an ’empty’ pack is of little use.

Balance boards like these mean the charger should be able to accommodate all balance plug typesSome batteries can tolerate unbalance better than others, and the way that the battery is used can also have a significant effect on cell balance, with packs pushed at high discharge rates and close to their specified ‘C’ rate faring worst. One continuing problem with balancing packs is the still, as yet, non standardization of the balance plug types fitted to different branded batteries. Thankfully this is slowly improving, and many of the more popular packs seem to have chosen the JST XH style of plug, accordingly this socketry is now found on many of the charger /balancer units available today.

People often ask about the best method of charging and balancing multiple packs. Well, provided the packs are of the same voltage, brand, and in the same state of charge to begin with, then connecting them in parallel, together with a suitable harness to connect the individual cell balance plugs in identical parallel configuration also, is the best way to accomplish this.

Put simply, two cells in parallel with each cannot be at a different potential, and will immediately ‘level each other out’ to one common voltage. Therefore, if you had for instance four identical 3s 2000 mAh packs connected as suggested here, you would effectively be charging them as one large 3s pack, and using just one 3s balance plug connected to your balancer. Of course, in order to retain the normal charge time of around one hour, the charger must be capable of supplying the necessary power, including a relatively high 8A output current.

DISCHARGING

A very important factor in Li-Po cells is the voltage level that they are allowed to drop to in use. This figure is generally accepted to be an absolute minimum of 2.6V under load and better still a little higher, at 2.8V or even 3.0V. This is where the choice of ESC (Electronic Speed Controller) is important, and good ones will ensure that the LVC (Low Voltage Cut-off) is set correctly to protect your particular battery combination. Deeply discharging cells below 20% capacity generates excessive heat and is what causes cell unbalance.

Remember also that any flight batteries installed in a model should be disconnected completely when not in use – even leaving them connected to an ESC or regulator that is idle, will slowly drain the pack, and could result in them being discharged too low and almost certainly lead to their demise. Complete disconnection is also a wise safety precaution in ensuring that motors cannot suddenly start up without warning, and besides, Li-Po packs should always be charged outside of the model so a method of disconnection and easy removal is required for this too.

There are four main balance plug types – clockwise from top – FlightPower type, Polyquest, JST-XH and JST-EHRDISPOSAL
As you follow the electric flight path you will almost certainly accumulate a number of LiPo packs; eventually ending up with a number of spent ones. Here then, is the correct procedure for the disposal of a pack.

1.     Completely discharge the cells down to 0V by connecting to a low resistance load such as a car headlamp bulb or similar.
2.     If possible, make small cuts with a sharp knife along the side seams of the cell envelope.
3.     Soak the packs in a bucket of salt water for several hours to ensure complete discharge.
4.     Either take the depleted packs to a proper re-cycling facility – or simply dispose of them in your regular rubbish bin. Depleted Li-Po packs are not considered environmentally hazardous.

That just about wraps things up for Pt.1. In Pt.2  we’ll discuss the new kids on the block – LiFePO4 cells, A123 Technologies Nanophospate based cells, and the slightly less popular Li-Ion cells, followed by a round up of the older school nickel based NiCd and NiMH.   PART 2


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