There’s no denying that Lithium Polymer (LiPo) batteries have revolutionised electric flight. Gone are the days when getting an electric model to fly above head height was considered an achievement. Electric flight is now a direct (some say better) alternative to i.c.; today’s LiPo-powered models can easily climb vertically and have ample power for all manner of aerobatic shenanigans. 

Lithium polymer battery chemistry isn’t without its foibles though and, as a result, our LiPo packs need careful attention, treatment and monitoring. The first visible sign that something is wrong is when a pack starts to swell, or puff. In this article we’ll look at what’s going on, what we need to do to look after our packs, how to get good long service out of them, and what to do with a failed pack.

 

WHY HAS IT PUFFED?

LiPo puffing is a clear sign that a pack is nearing the end of its useful life. There are several reasons why a pack puffs and there’s generally no consistency in what causes this to happen. Here (in no particular order) are the main culprits, any combination of which can lead to puffing:

• Excessive heat

• Excessive current draw

• Over charging

• Low voltage

• Long-time storage while fully charged

• Increased internal resistance

• Age 

• Manufacturing fault

 

A nearby photo shows two swollen LiPo packs (donated by clubmates John Saunders and Andy Syme). They’ve had vastly different lives. John’s blue pack had regularly been stored fully charged for long periods and used at quite high current draw in a 3D model. In contrast, Andy’s (older) yellow pack had a gentler life; always stored at 50% capacity and used with more throttle control, it was subject to less peak demand. The likelihood is that a different combination of the above culprits has occurred in each, but with the same end result - puffing.

 

THE SCIENCE BIT

Look closely at an electric flight LiPo pack and underneath the colourful exterior you’ll see a collection of foil packages stacked up, each of these foil packages is a single LiPo cell. The number in a LiPo pack designation: 3S, 4S etc. is the number of cells in the pack. A 3S pack has three cells, the ‘S’ simply means the cells are connected in series. Like all electrical cells there are three main components in a LiPo cell: a cathode, an anode and an electrolyte. In a LiPo cell there are multiple anodes and cathodes arranged in layers separated by the electrolyte which is impregnated into an insulating separator to stop the layers touching. It looks a bit like a lasagne, as can be seen in Fig.1 below. 

 

    All the cathode layers are connected together to form the positive terminal and all the anode layers are connect together to form the negative terminal. The anode and cathode layers contain the lithium mixed up molecularly with other elements (depending on the manufacturer) and the electrolyte is a conducting polymer that allows the lithium ions to flow from the cathode to the anode when charging and the anode to the cathode when discharging.

So, what’s going on inside those little foil bags to cause puffing? When a pack is abused by either overcharging, excessive discharging (both current and low voltage), overheating or storing fully charged for long periods, various chemical reactions take place within the cell which allow lithium atoms to break away from the cathode and the anode and oxygen atoms to break away from the cathode, anode and the electrolyte. Once these atoms are free there’s no going back, and they play no further part in the storage or delivery of electricity. Worse than this, if excessive heat is applied these free atoms start to join together to form some very unhelpful molecules.

When two lithium atoms and an oxygen atom get together they form lithium oxide - Li2O (aka lithium rust, similar to iron oxide rust on steel), which builds up on the cathode and anode. This lithium oxide blocks the flow of electrical energy, thereby increasing the pack’s internal resistance (more on that later) and reducing capacity. The increased internal resistance generates more heat, which exacerbates the situation still further. The other oxygen atoms just collect with others (and the odd carbon atom from the anode) to form oxygen, carbon dioxide and carbon monoxide - the gasses that cause puffing.

Lithium is an alkaline metal, which means it has a violent reaction with water. This isn’t really a problem for modellers as the lithium is sealed inside the foil bags that make up a typical LiPo pack. However, if water vapour gets into the pack during manufacture, or through a tiny hole made in the foil bag following an accident, then the puffing process is accelerated. Once water vapour (H2O) gets in and the lithium atoms start to break away, the extra oxygen atoms speed up the lithium oxide production and hydrogen atoms join with lithium atoms to form lithium hydroxide – another gas for the puffing cocktail. Water vapour getting into a pack during manufacture might be an explanation as to why pampered packs seem to puff easily.

If the chemical reactions get out of hand, usually by serious overcharging (either too much current or too-higher voltage) then more lithium and oxygen atoms are released, more lithium oxide is produced, the internal resistance increases and so heat builds up, releasing more lithium and oxygen atoms... an increasing spiral of doom ensues, or thermal runaway. In this extreme case, pressure inside the cell can get to a point where the foil bag bursts and the cocktail of lithium and oxygen gas mixes with more oxygen and water vapour in the air, to spontaneously combust.

These extreme conditions only apply when a LiPo pack is abused. Follow the few simple rules that follow later and a pack should give long service. In my opinion the benefits of LiPo chemistry far outweigh the imperfections.

 

What we’re seeking to avoid. An extreme case perhaps but when your LiPo fails, then so might your model. 

 

 

WHAT’S A PUFFED LIPO?

Check out a new LiPo pack – it’ll be solid and square, with no sign of swelling or puffiness on any of its faces; lay it on a table and it will sit nice & stable with no rocking from side-to-side. As long as it’s well cared for there’s no reason why it shouldn’t give long service, however, over time a slight bulge will start to appear; this may get a little worse immediately after a flight when it’s warm, but will shrink back as it cools. Put this one on the table and it’ll rock from side-to-side; the classic tell-tale sign of puffing.

    A common question is, how puffed is too puffed? The simple answer is that any puffing is a sign that all is not well, but some mild puffing can be tolerated as long as the pack is only used in low power situations (powered gliders, for example) and rigorously checked and monitored, particularly cell balance, internal resistance and pack capacity.

    My view is that up to 2mm of puffing is tollerable; any more, and the pack gets scrapped. I also regularly monitor balance, capacity and internal resistance, which get factored into the decision when to retire a pack.

When testing for puffiness, never use a fingernail as this will dent the cell, which will then start to degrade around the trauma. Always use a fingertip, and squeeze gently when testing. 

 

HOW TO DELAY PUFFING

Here are some recommendations on how to deal with the puffing culprits In order to delay the demise of a pack. 

 

Avoid excessive heat

Heat is the no.1 enemy of LiPo packs. It’s not only the generated heat during charging and discharging we need to keep an eye on but also where the pack is stored. The generally accepted safe maximum temperature for a LiPo is around 55 - 60°C; anything over this increases the chemical reactions that cause puffing and can lead to thermal runaway. So never leave packs in a hot car or hot shed or garage, and if your packs are hotter than a cup of coffee when you land then take a closer look at what’s causing the excessive heat.

    Personally, I get concerned if my packs are warm (40 - 45°C) when I land and, needless to say, always let a pack cool down before recharging.

 

Avoid excessive current draw

All LiPo packs have a maximum labelled discharge rate, usually a multiple of the capacity (C). A 2200mAh capacity pack might have a labelled discharge rate of 25C, which means a maximum current draw of 2.2A x 25 = 55A (I mention the discharge rates as being ‘labelled’ because manufacturers do sometimes exaggerate discharge rates). To increase the longevity of a LiPo, always try to limit the current draw - ideally 50%, to an absolute maximum 80% of the labelled rate. In our example above this would mean a motor and prop combination that requires 28 - 44A maximum.

 

Avoid overcharging

There are two things to consider here; charge rate and maximum voltage. Modern packs are supposedly capable of high charge rates of up to 5C, but there’s overwhelming evidence that this will deteriorate a pack very quickly and cause puffing. It’s far better to limit the charge rate to a maximum of 1 – 1.5C. Taking our 2200mAh example, this means a charge current of 2.2A & 3.3A respectively.

Always balance charge your LiPos to ensure that the cells in the pack stay below the maximum 4.2V. Even this 4.2V figure needs a caveat, as the maximum safe voltage of a LiPo cell is temperature dependent; 4.2V is relevant only at room temperature. As cell temperature drops, so does the maximum allowable safe voltage. For example, at 10°C the maximum safe voltage for a LiPo cell is 4.1V. If a cell is charged to 4.2V at room temperature and then taken out to a freezing cold flying field it will effectively become overcharged, so keep ‘em warm on a cold day. If your charger has the facility to adjust the peak cell voltage, setting it to 4.15V (lower in Winter) will maximise the life of the pack and you really won’t notice the slight drop in capacity. 

 

Avoid low voltage

LiPos should never be discharged below 3V per cell; always try to land a model with at least 20% capacity left in the pack. The resting voltage of a LiPo cell after a flight should be 3.7V minimum. Any lower than this will mean that the cell voltage will most likely have dropped below the 3V minimum under load whilst flying.

An invaluable tool in the flight box of an electric flight enthusiast is a battery checker. Simply plug the LiPo’s balance plug into this after a flight to quickly check the state of the pack. Any over-
consumption will quickly be picked up and flight times can be adjusted to suit.

 

Careful storage

Always store packs at 50% capacity. Most better-quality chargers have a storage charge facility that either charges or discharges a pack to the ideal 3.85V per cell. Try to get into the habit of charging or discharging your packs to storage capacity after flying. This will have two benefits – increasing the life of the pack and reducing charge time for the next session.

 

MANUFACTURING FAULTS

As consumers there’s little we can do here other than buy LiPos from a reputable supplier, preferably with a warranty agreement.

 

INTERNAL RESISTANCE

In most cases, internal resistance (IR) goes unchecked in a LiPo pack, however the implications of its presence are quite serious. IR inevitably increases with age, and that increase is accelerated if the pack is abused by any of the conditions listed earlier.

Generally speaking, the larger the pack, the lower the initial IR. A typical 2200mAh cell in good condition might have an IR of around 10 - 20mΩ; this is by no means a fixed value, it varies with temperature and storage state.
The IR of each cell in a pack is added together to give the total pack IR. If a 3S pack has individual cell IR’s of 12, 13 & 15mΩ, then the pack’s total IR would be 40mΩ. As packs age, IR increases due to the build-up of Li2O. John and Andy’s puffed packs discussed earlier
have IR’s of around 35mΩ per cell; being 3S packs, the total IR of each is around 105mΩ –
a near three-fold increase from ideal. 

An increase in IR has two significant implications:

 

Reduced power

Resistance in an electric circuit restricts the flow of current. In our models this manifests itself as reduced voltage at the motor terminals, which results in a lower prop rpm. The voltage loss equation used to calculate
the effect is:

 

Voltage drop = Amps x Resistance

Taking John and Andy’s puffed packs as an example, assuming a current consumption of 35A and an original pack IR of 40mΩ, the voltage drop is therefore:

 

Voltage drop = 35A x 40mΩ = 1400mV or 1.4V. 

This means that only 11.1 - 1.4V = 9.7V leaves the battery.  With the packs in their puffed state and a pack IR of 105mΩ the volt drop is:

 

Voltage drop = 35A x 105mΩ = 3675mV or 3.7V.

This time only 11.1V – 3.7V = 7.4V leaves the battery. In reality the increase in IR would reduce the maximum current drawn so the voltage drop would be slightly less than given above, but the principle is the same. 

 

• Increased heat

IR in a LiPo pack generates heat inside the pack, right where we don’t want it. Heat energy is measured in Watts and the relevant equation is:

 

Watts = Amps2 x Resistance 

With the same example LiPos as above, using the original pack IR of 40mΩ, the heat energy generated is:

 

Heat energy = 35A x 35A x 40mΩ =
49,000mW or 49W

With the packs in their puffed state and an
IR of 105mΩ, the heat energy generated is: 

 

Heat energy = 35A x 35A x 105mΩ = 128 625mW or 128.6W

That’s 128W of heat building up inside the pack!

 

MEASURING IR

As can be seen, IR has a significant effect on the performance and longevity of our LiPo packs. So how can we keep tabs on it? First of all, we need to apply some discipline. As IR varies with temperature and charge state it’s necessary to always test the packs under the same conditions each time the IR is measured. I always test at room temperature, around 20°C and at 50% capacity, i.e. storage capacity.

    There are two methods that can be used. Many better quality chargers have an IR measuring facility. This is by far the quickest way to check a pack’s condition; the charger gives the individual cell resistances and some also add them together to provide the pack’s total IR.

If this facility isn’t available, another way to measure IR uses a test rig along with a multimeter and battery checker. My rig consists of an old, brushed ESC and a servo tester to adjust the current draw through some 12V bulbs, the principle being to measure the current and pack voltage at two different conditions: Condition 1 is at around 1 - 2C discharge current, Condition 2 is around 2 - 3C. The IR is then determined as:

Internal resistance = difference in voltage ÷ difference in current

 

TESTING THE THEORY

I’ve always suspected that both charger and test rig methods only give an indication of IR so, to test the theory, I used both methods to check the IR of the same 3S 2200mAh pack. Using two different chargers I got two similar results, one example (shown in the nearby photo) indicates a pack IR of 28mΩ. Using the test rig method the results for the same pack were:

 

 

Deriving IR from a test rig, here at rest prior to test.

 

Condition 1

Voltage = 11.28V

Current = 2.85A

 

 

Readings at Condition 1 (see text).

 

Condition 2

Voltage = 11.22V

Current = 4.57A

 

Readings at Condition 2 (see text). 

 

Internal Resistance
=
(11.28 – 11.22) ÷ (4.57 – 2.85)

= 0.06 ÷ 1.72

=35mΩ

 

As can be seen, there’s a slight difference between the results from the charger and test rig. I found that repeated tests using each method gave slightly different results after each test. So whichever method you decide to use, always adopt the same method on subsequent tests. That way at least a comparison can be made, irrespective of the actual figures. 

 

DISPOSAL

No matter how careful you are, puffing will eventually start to happen and you’ll have to retire and dispose of a LiPo pack. The safest method of disposal is to first discharge it to 0V, either using a charger with a discharge facility or by connecting it to a suitable bulb (a single 12V bulb is fine for a 3S pack). Constantly monitor the pack during the discharge and, once it’s at 0V, cut off the plug and short out the +ve and –ve leads. This renders the pack safe for disposal at your local recycling centre.

    Under no circumstances be tempted to stick a pin in a hot, puffed LiPo to relieve the pressure. I know someone that tried this, the result within the space of a few seconds being a 2 - 3” pressurised, blowtorch-like flame and a room full of acrid smoke; he managed to throw it outside, where it continued to fizz for a few more minutes. You have been warned. 

 

 

With the best will in the world, packs will get warm, especially on a Summer’s day, so try and ensure that airflow cooling is maximised.

 

CONCLUSION

As we’ve seen, LiPo puffing might be inevitable but, with a little care & attention and some periodic testing, it should be possible to delay the effect for long enough so that you can enjoy the maximum use.  ✈