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