A guide to Lithium Polymer Batteries

[teslahed]

Why lithium polymer rechargeables?

If you are familiar with your standard duracell AA rechargeables you may be wondering what all the fuss is about with lithium polymers.

The short answer is they store more energy and provide more power than any other battery technology currently available. If you want an electrically powered aircraft to perform well for more than a couple of minutes you need to be able to store a lot of energy without relying on a heavier battery and access it very quickly without the battery burning out. Lithium polymer batteries will store more energy for a given weight and allow you to discharge that energy more quickly (they are more powerful) than anything else currently available. This is particularly important when flying multirotors as the whole aircraft relies on the power of the battery to stay airborn.

So how does this translate into real world figures?

mAh (milliAmp hours)

Charge capacity is measured in terms of mAh (milliAmp Hours). This is the total amount of charge the battery can store at one time. The higher the mAh rating the more energy the battery will supply, but also the larger and heavier the battery will be. A 1000mAh battery can supply 1000 milliAmps for 1 hours. In terms of multirotor use a 1000mAh battery is more likely to supply 10 amps for 0.1 hours (commonly referred to as 6 minutes). If it can. Which leads us to;

C rating

The C rating of a battery is the amount of energy it can supply in a given amount of time - it's maximum discharge rating. Power in other words. The C value is multiplied by the mAh rating of the battery to find it's maximum discharge rating. So a 30C 1000mAh battery can supply 30 x 1000mAh (1 amp) or 30 amps before it's at risk of failing. Because multirotors are totally dependent on their motors and props to stay in the air this is much more important than it would be for a radio control plane. If your multirotor (quadcopter) consumes a maximum of 5 amps per motor (and you have four motors) then with a 1000mAh battery you will need at least a 20C rating to be sure your battery doesn't fail and your quadcopter fall out the sky, burst into flames, or both.

Often batteries will give a 'sustained' as well as a 'burst' C rating. If they say something like 35-50C then you can be sure the battery will provide 35C continuously and as much as 50C for brief periods. This is more useful than it sounds because you'll often be hovering at half power and only use full power for brief periods whilst accelerating or decelerating. Stunt fliers will require higher C ratings and will be especially attracted by batteries with high burst as well as high sustained C values.

If your C rating isn't high enough your battery will start to swell and eventually be at risk of catastrophic failure - poor quality lipos should be avoided.

Cell count (voltage)

Lithium polymer battery packs are made up of a number of cells. The cell count varies depending on the voltage you require; different motors will be rated for different voltages and higher voltages will, if all else is equal, produce more thrust. If you supply more than the rated voltage to a motor or electronic speed controller then you will very quickly destroy components.

The most commonly used voltage is 11.1 volts - that's 3 x 3.7 volt cells. Some small multirotors use 2 cell lithium polymers (7.4 volts) and a few microquads use single cell batteries to save on weight, and some larger quadcopters use 4 cells or higher to get more power to large motors. 3 cells is a good starting point for the average F450 style quadcopter that many people fly.

Each cell is nominally rated at 3.7 volts. In reality a fully charged cell is 4.2 volts and a fully discharged cell will be about 3.3 volts. This may sound counter intuitive - isn't fully discharged going to be 0 volts? If you allow a cell to get below 3.3 volts your battery will be destroyed; this is a quirk of the chemistry involved with lithium polymer batteries. They are much less forgiving than the usual NiMh AA rechargeables most people are familiar with.

As you approach 3.3 volts the rate at which the voltage drops will increase exponentially. This means you don't want to set your lipo alarm for 3.3 volts or you will have no safety margin in which to land. A sensible minimum voltage to set you alarm to is perhaps 3.5 volts. This means you will have plenty of warning before your voltage starts to drop rapidly and your multirotor drops out the sky. It will also give your lithium polymer batteries a longer lifespan.

Higher quality LiPos will hold their charge better and their empty voltage will be higher. This means if you are used to using cheap LiPos and have set your voltage alarm accordingly, when you upgrade to better ones you will need to set a higher voltage alarm. If you try to fly down to the same low voltage as your cheaper batteries went to, you'll be sucking out more mAh than the battery contains which will damage it. In theory a perfect LiPo would be 4.2 volts fully charged and 4.2 volts empty. You'd need to fly with a mAh counter to know when to land as the voltage wouldn't tell you.

Batt	Nominal Voltage	Charged Voltage @	Safe minimum Voltage @
2s	7.4	8.4	6.8
3s	11.1	12.6	10.2
4s	14.8	16.8	13.6
5s	18.5	21	17
6s	22.2	25.2	20.4

The graph shows how the voltage drops off with mAh used on a typical 4200mAh battery - as the amount used approaches the total capacity available the voltage starts to drop much more rapidly. Ideally it would be safest to use no more than 3600mAh of the total 4200mAh available to ensure that the voltage doesn't reach the point where it starts to drop catastrophically, potentially leading to the multirotor losing lift and control and falling out of the sky. This is why people set their lipo voltage alarms to go of prematurely.

Lipo Alarms

Most multirotor fliers use low voltage alarms to prevent their lithium polymer batteries from dipping below the critical 3.3 volts. They can be set to go off at around 3.5 volts so that you know to land before you irreparably damage your batteries. Also if you allow your voltage to dip below 3.3 volts you will start to lose all power and control, so it's not just the battery that will be damaged but also the multirotor (and anything underneath too!).

Some low voltage alarms have LEDs, some have digital readouts that display actual voltage readings and most have buzzers. It's up to you what you prefer to use but loud buzzers are particularly useful as they allow you to hear the alarm even when the quadcopter is too far away to see the LED or digital readout.

Once you've managed a few flights with the low voltage alarm you will get a feel for how long you quadcopter stays up for before the alarm goes off (a stop watch will help here). If you know your quadcopter flies for 10 minutes before the alarm sounds you can set a timer on your transmitter to beep after 9 minutes. This gives you a bit of extra warning and means you can land with an extra 10% of safety margin. This will prolong the life of your lithium polymer batteries; generally it's better to buy one more battery pack and not squeeze every last drop of energy out of them on every flight - you'll find they last longer before they start to wear out.

Other factors

Cold weather will decrease the performance of lithium polymers. You will find they don';t last as long or provide as much power. Keep them in a warm pocket until you are ready to use them.

Older lithium polymer batteries will also perform less well. Most people think you';ll get 100 flights and recharges or so before you start to see a slow decrease in peformance.

Good quality chargers will 'balance' the individual cells. Over time the separate cells of the lithium polymer battery pack will start to drift out of alignment, so one cell of a 3 cell lipo may be 4.2 volts when fully charged and the others 4.1 or 4.0. This will result in reduced performance. Balancing chargers will charge the cells individually to ensure they are all the same voltage when it's finished. Cheap chargers just look at the total voltage and leave the individual cells to drift - so get a good charger and your batteries will last longer.

Safety

Lithium polymer batteries store a lot of energy. It is still nowhere near that of an equal weight of petrol but it's much higher than any other battery. The result is that a failure or short circuit can result in a catastrophic fire or even a small explosion.

Lithium polymers shouldn't be left unattended whilst charging. Ideally they should be charged in a lithium polymer charging bag or similar flame proof container. Lithium polymer charging bags are easily available from most places that sell the batteries. Some people prefer to use a metal or ceramic container with a lid - (empty) ammo tins that can be bought from army surplus stores are also ideal. If you leave a lithium polymer battery to charge unprotected anywhere around flammable items found in most houses, like carpets, a small failure can turn into a large house fire. It's not common but it has happened so don't let it happen to you.

Lithium polymer batteries should also be stored somewhere away from flammable objects. Outside or in a garage is ideal, failing that the charging bag or ammo tin or similar flame proof container should do the job. 

If you damage a lithium polymer by puncturing it it's likely to catch fire - it can be quite dramatic and dangerous. Again, keep them away from flammable objects and small children likely to poke them with scissors. If you have a bad crash and your battery looks damaged then dispose of it safely - don't recharge it or keep it in the house.

If you short circuit a lithium polymer battery it will try to discharge it's entire store of energy in one go. Whatever it's C rating you will exceed it and as the battery tries to empty itself it will swell up and possibly catch fire. Even worse, the contacts are likely to weld themselves together with a big fat spark making breaking the short circuit even more difficult. When soldering new battery connectors to your lithium polymer battery you should only cut, strip and solder one terminal at a time. This may seem obvious but it's a surprisingly easy mistake to make, especially when you are new to the subject (don't ask me how i know this :embarrassed). If you have both wires stripped at the same time then make sure you have one sellotaped back again the battery out the way of the other. It only takes a split second for the two to contact each other - then it's game over for the battery and you may have a small fire to put out.

The best way to dispose of old or damaged lithium polymers is to put them in a bucket of salty water, outside, and leave them there until the bubbles stop - preferable a day or two to make sure.

It almost goes without saying, but i will say it anyway. Unlike an old square 9 volt battery it is definitely NOT SAFE TO LICK THE TERMINALS TO CHECK IF THE BATTERY IS CHARGED. At least not if you like having a tongue

http://youtu.be/D8OEWg_4T10

[teslahed]

This guide is new. Any comments, questions or corrections would be most welcome.

[neagron]

  well done Chris,Marty

[Smeagol]

Good info there, particularly the tongue retention feature, i';m sure the world being as big as it is does have a few lipo tongue testers 

[teslahed]

particularly the tongue retention feature, i';m sure the world being as big as it is does have a few lipo tongue testers

That was mostly a joke, but on reflection i am sure you are right...

When i was (very) young i did once lick a scaletrix track to see what would happen. I probably shouldn';t have admitted to that...

[mickyt]

Well i stand by my comment, yet another well thought out and well put together post by yourself well done that man and thanks for the info

[Gav]

When i was (very) young i did once lick a scaletrix track to see what would happen. I probably shouldn';t have admitted to that...

My dog put her wet nose across my sons scalextric tracks once LoL.  That was funny.

[teslahed]

I bet she didn';t do it again though (I certainly didn';t and i was at least sensible enough not to try it on full speed...)

[QuadBod]

Nice job. The graph could do with an explanation though.

[teslahed]

Nice job. The graph could do with an explanation though.

Good suggestion, thanks. Also - welcome to the forum

The graph shows how the voltage drops of with mAh used on a typical 4200mAh battery - as the amount used approaches the total capacity available the voltage starts to drop much more rapidly. Ideally it would be safest to use no more than 3600mAh of the total 4200mAh available to ensure that the voltage doesn';t reach the point where it starts to drop catastrophically, potentially leading to the multirotor losing lift and control and falling out of the sky. This is why people set their lipo voltage alarms to go of prematurely.

Added.

[QuadBod]

Thanks, and thanks. 

That helps, thanks. "drops of" -> "drops off", and maybe change the label on the bottom axis to "Capacity used"

[teslahed]

Changed the spelling mistake, thanks, and I';ll look at improving the graph tomorrow. All good suggestions 

[dp106]

This is really useful information for someone starting out. Personally I could really use some help with chargers as well. Ideally I want one that I can charge up more than one at a time as quickly as possible without them exploding :-)

[guest325]

This is really useful information for someone starting out. Personally I could really use some help with chargers as well. Ideally I want one that I can charge up more than one at a time as quickly as possible without them exploding :-)

Be careful with charge rates, any more than 1C will reduce their life regardless of what the blurb says!
1C for a 2200 mah battery is 2.2 amps; 3300 is 3.3 amps etc regardless of how many cells. Best thing to do is get enough batteries so you don';t need to keep charging them, if you charge them straight after use you increase the chance of a fire - they should have cooled before you charge them again.
You don';t have to pay a lot of money for good batteries, local model shops charge silly money for them, I pay about £13 for Gens ace 3S2200 25C';s which are ideal for a 450 size quad - they are very good and will stand up to being left fully charged for a couple of weeks without and problems, and with hardly any voltage drop; I get mine from Giant shark.

[dp106]

I think that';s one of the issues I';ve had in the past in that I';m not sure how long they will last after being charged. I';ve not wanted to charge them until I know the weather is good enough and then have to wait hours for them to charge. Here in the UK it may be at least 2 weeks before there is a clear day :-)

[dp106]

Also another thing, when is it ok to recharge a previously charged battery that hasn';t been used?

[guest325]

I don';t recharge any of mine unless they drop below 90% but make sure you allow less flight time.

[teslahed]

Also another thing, when is it ok to recharge a previously charged battery that hasn';t been used?

Yes it';s perfectly fine to recharge a battery that isn';t fully discharged - better in fact.

You should store your batteries at about 50% charge so there is nothing wrong with leaving them less than fully charged in between flights. You are more likely to damage a battery by allowing it to over-discharge than you will overcharging it (if you use a good lipo charger).

[Tony Campbell]

Good guide guys!!

One thing I would say, is that normally, buying cheap is not a good idea. You';ll usually end up buying twice. Strange thing is, with lipos, it isn';t always true!
Some of the expensive brands are absolute rubbish. Flightpower being one that springs to mind. Often the cheaper lipos can be very, very good. GensAce being a good example of this. They are one of the much cheaper options out there, and usually available from Hobbyking UK. So, Lipos, if you aren';t used to buying them, are one of the items that you should check with other forum members, as to their quality. Some others, like Turnigy Nanotech, can be very good........ if you end up with a genuine one! There have been reports from some battery suppliers of fake ones doing the rounds, which do not live up to the ';C'; rating printed on the packs. This normally results in puffed packs, from people drawing more amps than the cells can handle. Beware of what you are buying, and ask around. As usual, the forum members can be trusted to point you in the right direction. 

[Gaza07]

Excellent post Tony and your right Flightpower are crap I have a bunch of them here that have a higher internal resistance from new than some of my cheap HK packs do after quite heavy use,
Ive always found the zippy flight max packs to be good, having said that I have received them new and puffed 

[Ross]

I think that LiPos don';t last irrespective of type when people fly in the manner that sucks the guts out of them.  Do it a few times and a duff battery happens.   Sedate flying makes them last longer.

Agree tho'; a few crap batteries out there...experience is the only way to tell.

[guest325]

In the space of a month I';ve had 3 Gens ace lipos go down on me, all middle cell drooping stupidly on load resulting in hasty landings. Yet my cheap as chips Turnigy blues are 100% reliable; always balance charge at 1C never discharge below 25%; and I do know of other people who are having the same problems with Gens ace.

[jwl940]

Sat at home waiting for the first job of the week to be phoned through I gravitated to this thread, saw the video and WOW . I am (was) used to seeing electrolytics explode and the wire wool and batteries demonstration but that demo was on a completely different level.  Loved your mum';s input but she';s wrong, doing what you did and videoing it has certainly given me more respect for these batteries than any amount of reading would ever do.  Thanks.

John

[teslahed]

Loved your mum';s input but she';s wrong, doing what you did and videoing it has certainly given me more respect for these batteries than any amount of reading would ever do.  Thanks.

LOL - yes i can';t blaim her for being concerned. I should have explained what i was doing first. But it';s definitely an effective demonstration...

[G60jet]

Awesome thread.  Loads of useful info


Sent from my iPhone using Tapatalk

[Martinus]

Thanks very much. This is my go-to thread for battery info. :)

[all4cam]

Very helpful guide.
Thanks 

[Rich]

great post 

very informative  thank you

[Area21]

Do lipo users still apply the 80 / 20 rule leaving 20% as a redundancy ?

[guest325]

Do lipo users still apply the 80 / 20 rule leaving 20% as a redundancy ?

I usually go 70 / 30 just to allow a bit of leeway for unforeseen circumstances.