A usefull little nugget to help with lipo battery condition / quality monitoring

[Stealthkiller]

Pulled this usefull battery info of one of the rc forums , INTERNAL RESISTANCE

Another rating??? Yep, the first 3 are industry standards and as was mentioned with that last one (C discharge ratings), is used by the manufacturers to market their product or justify a higher price and realistically can't be verified, but they are still a good general guide line when choosing a pack.

Internal resistance to the rescue! This one is verifiable and one of the best ways to monitor your RC LiPo batteries condition. Most decent higher capacity and higher discharge rated LiPo cells will have roughly 2 to 6 milliohms (0.002 to 0.006 ohms) of internal resistance when brand new. To calculate the total internal resistance of a series wired pack, you would then add these numbers together so a 4S pack with each cell having 4 milliohms of resistance will show a total internal resistance of about 16 milliohms (0.016 ohms).

As I mentioned, as packs age, the internal resistance goes up and the warmer they run. Lower discharge rated packs and small capacity packs will generally have higher internal resistance readings. It is not unusual to measure internal resistance numbers in the region of 200 milliohms on smaller 100 to 200 mAh micro park flyer LiPo packs when they are brand new for example.

So the best way to use internal resistance (if your charger supports this very useful function) is to take an IR reading of your LiPo/s when it/they are brand new.



As seen here, I will then write that number (or the IR of all the cells in the pack) somewhere on the pack with a permanent marker so I will always have a brand new IR base reference for that particular battery. As the packs age, I can simply reference how the resistance is increasing or if one cell for some reason is getting bad.


How do you measure internal resistance? This again is where good computerized chargers come into play. The good ones that support this feature with built in balance boards will check the "IR" of each cell as well as the entire pack. Pictured above I am taking the IR reading of each cell in this new 6S Turnigy LiPo. It is hard to make out in the photo, but the IR of cells 1-6 are 2,2,1,1,1,2 milliohms each giving a total IR for the entire pack of 9 milliohms - pretty respectable!

Internal resistance really opens up a huge and complex topic of how to accurately calculate voltage drop in the pack and the total amount of watts being expended in the form of heat within the pack. I am not going to get into those calculations here for the simple reason I am not qualified enough to explain them.

[ste887]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

6S? 22V is a lot, your AEG must hate you :P

[Snowman]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

The man sounds like he's an RC dude, higher voltages are more common for them.

Also, not seeing a picture on my end?

But this is all very true, this is (one of the) reason LiPos are so awesome, their internal resistance is much lower than nickel-based battery chemistries. Internal resistance is reflected in the C rating for a pack. Bit of a faff unless you're a battery nerd like me, if you have a fancy charger that can measure it then great (I don't).

You might want to be careful with using IR as an abbreviation though, I and R in electronics stand for current and resistance respectively, and often appear multiplied together - for example in Ohm's law, V=IR (voltage equals current times resistance).

With that in mind, the calculations actually aren't that complicated either - V=IR so voltage drop across the pack is simply the battery's internal resistance times the current flowing through it. This tells you how many volts are being 'wasted' at that current. For 'wasted' wattage it's that same voltage drop times the current through the pack, or alternatively current squared times internal resistance (pretty sure that's right, electronic engineering student and all-round nerd here).

The only other thing I'll add is don't go trying to measure your battery's internal resistance using a multimeter in resistance mode! The meter will have a very low resistance (to get an accurate reading) and is essentially shorting the battery - which for a LiPo is particularly dangerous! With any luck you'd blow the fuse in the meter before serious damage was done but if you were 'clever' enough to use the unfused high current setting, well...

[seansamurai1]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

Then we get to a whole different kettle of fish.
You aint supposed to measure resistances on a 'Live' circuit using Ohms, mili Ohms etc etc. A live circuit is what a battery is (unless its totally dead, and LiPo doesnt like that).

[Fumps]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

As long as you pop your lipo on storage condition after each skirmish day the cells will last for years.

My best mate races 1:4 scale brushless RC cars. he is a battery geek.

[Boo-Sabum Ben]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

They really aren't that much more complicated. Don't let them get empty, and always balance charge them unless you're in an absolute rush. And don't set fire to them.

That's it really.

[Fitch]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

"Storage condition" - I've seen that setting on my lipo charger - what does this do exactly?

I tend to balance charge mine after a shoot and then rebalance when it comes to the next skirmish so they are topped up.

Should I be storage charging them instead?

[Stealthkiller]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

Lipo battery cells will start to oxidise if left fully charged for long periods of time , a storage charge is usually 50 percent of a battery's full charge . Another fact worth knowing is that lipo cell life can be quite substantially reduced if the battery is stored / used in extremely cold or hot conditions . .

[Boo-Sabum Ben]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

As above. If out in to storage they're supposed to be at about 50% full

[Fumps]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

"Storage condition" - I've seen that setting on my lipo charger - what does this do exactly?

I tend to balance charge mine after a shoot and then rebalance when it comes to the next skirmish so they are topped up.

Should I be storage charging them instead?

Hi Mate
My mate explained this in great detail (I nearly fell asleep lol) So please bear with me.

Basically there are a set amount of cells in each battery. I like to think of them as bottles with liquid in them. the battery works at a more optimum level if all the bottles are at the same level so the power (Liquid) can be drawn off them quicker.

A balance charge will charge one cell & then take some off the other to equalize the cells & put the battery to the optimum standard. Hence the term balance charge because it balances the battery. So that explains why when your doing a balance charge there is no power going into your battery because in some cases it's drawing the power out of the battery.

A Lipo battery does not like it when it's totally flat & in most cases being flat can damage the cells inside. instead the battery will have a storage mode where each cell will fill to (Roughly 50-60%). If you put each battery to this level when you have finished using it the battery will last for years. when you abuse them the individual cell can become damaged which lowers their service lives.

I hope that make sense....sometimes it's hard to decipher his knowledge into English


OOo I forgot to add, If you think about the above. If you say want to just nip out into the yard for a bit of plinking, Don't bother charging the battery, your already on 60%. Just when you get back in pop it onto storage again & your done.

[Fitch]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

Superb stuff hope this is helpful to others too.

[Snowman]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

Then we get to a whole different kettle of fish.
You aint supposed to measure resistances on a 'Live' circuit using Ohms, mili Ohms etc etc. A live circuit is what a battery is (unless its totally dead, and LiPo doesnt like that).

What? That makes no sense. The Ohm is the unit for resistance.

[Stealthkiller]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

LiPo

A true LiPo battery doesn’t use a liquid electrolyte but instead uses a dry electrolyte polymer separator sheet that resembles a thin plastic film. This separator is sandwiched (actually laminated) between the anode and cathode of the battery (lithium carbon coated aluminum & copper plates) allowing for the lithium ion exchange – thus the name lithium polymer. This method allows for a very thin and wide range of shapes and sizes of cells.

The problem with true LiPo cell construction is the lithium ion exchange through the dry electrolyte polymer is slow and thus greatly reduces the discharge and charging rates. This problem can be somewhat overcome by heating up the battery to allow for a faster lithium ion exchange through the polymer between anode and cathode, but is not practical for most applications.

If they could crack this problem, the safety risk of lithium batteries would be greatly reduced. With the big push towards electric cars and energy storage, there is no doubt some pretty huge developments will be made in ultra light weight dry and safe LiPo’s in the coming years. Seeing that theoretically this type of battery could be made flexible, almost like a fabric, just think of the possibilities.

LiPo Hybrids

All RC LiPo batteries out there at the time of this write up (January 2013) are actually a hybrid lithium polymer battery. The correct name for this type of battery is lithium-ion polymer, but the battery world of today simply calls them lithium polymer even though they are not a true dry type LiPo battery.

By introducing a gelled organic/solvent based electrolyte to saturate the polymer separator, the lithium ion exchange rate between anode and cathode is improved immensely. LiPo hybrids like Li-Ion can still burst and catch on fire if over charged, shorted, punctured, or incinerated.

When first introduced, LiPo batteries were more expensive than Li-Ion because they are more labor intensive to manufacture. Fortunately prices have dropped substantially since they have become as, if not more popular than Li-Ion battery technology. This holds especially true for electric powered RC aircraft and the real driver behind LiPo battery research – portable communication/entertainment devices.

LiPo hybrids use the same flat cell structure as their dry counter parts meaning they have the same flexibility with sizes and shapes allowing for very specialized shaped battery packs perfect for use in our RC models.


Almost every RC LiPo battery cell is packaged in a foil pouch coincidentally called a pouch cell. The picture to the right shows a typical 2 cell LiPo RC battery pack.

Pouch cells are the perfect solution for building multi celled battery packs since the flat pouch cell can be stacked with no wasted air spaces like found within round celled battery packs. Of course since LiPo’s use this light weight pouch instead of a metal can, less weight is the result making LiPo’s the preferred choice over Li-Ion in a weight conscious application such as RC aircraft.

If you ever open up a LiPo foil pouch cell, this is what you will find. A long piece of very thin plastic film (the polymer) with the thin lithium carbon coated aluminum & copper anode & cathode electrodes laminated in an alternating pattern on the front and back side of the polymer separator film. The works will be saturated with the greasy solvent based organic electrolyte.

This long film (over 7 feet long in the case of this 5000 mAh cell), is then folded accordion style back and forth upon itself. The entire folded cell matrix is then heat sealed into the foil pouch along with the gelled electrolyte which incidentally has a very sweet solvent smell much like nail polish remover/acetone.

If you're wondering what the burnt hole is in the center of all the cell folds, I purposely drove a nail through this cell to discharge it rapidly & watch the fireworks. The cell rapidly ballooned out, burst, and vented a fair amount of flammable electrolyte but never caught on fire. On the positive side, if it would have burst into flame, I wouldn't have this picture to show the "guts". I only did this because I dropped this heavy 6S 5000mAh LiPo pack on the hard concrete floor (yes - very dumb & costly butter finger moment) and one cell was damaged in the process. Lesson learned, don't carry more LiPo's than you can safely hold!

Here's a good video of the processes involved in manufacturing LiPo cells.



One interesting characteristic hybrid LiPo batteries share to an extent with their dry counterparts is they do get more efficient at ion exchange once warmed up. If you have ever noticed your RC model seem to gain a little more power a minute or so after working the battery; what you are experiencing is the increase in ion exchange efficiency once the battery chemistry warms up.

This should have you thinking that if you fly your electric RC helicopter or plane in the winter time, you might want to keep your RC LiPo battery packs in a warm place prior to the flight.

LiPo RC Battery Ratings

Now that I have bored you to death on RC LiPo battery basics, time to get into the main topics at hand. First are ratings, specifically voltage and capacity. These are the two main numbers you will need when going battery shopping.. There is a third number you will also need to be aware of which I will get to in just a bit.

VOLTAGE

Unlike conventional NiCad or NiMH battery cells that have a voltage of 1.2 volts per cell, LiPo battery cells are rated at 3.7 volts per cell. The benefit here is fewer cells can be used to make up a battery pack and in some cases on smaller micro sized RC aircraft like Blade mCX's, mSR/X, mCPX, or 120SR one 3.7 volt cell is all that is needed to power the model.

Other than the smallest of electric RC models, RC LiPo battery packs will have at least two or more cells hooked up in series to provide higher voltages. For larger RC models that number can be as high as 6 cells and even more for larger birds or HV (high voltage) applications. Here is a list of LiPo RC battery pack voltages with cell counts. If you are wondering what the 2-12S in parenthesis means; it is a way the battery manufacturers indicate how my cells hooked in series(S) the battery pack contains.

3.7 volt battery = 1 cell x 3.7 volts (1S)
7.4 volt battery = 2 cells x 3.7 volts (2S)
11.1 volt battery = 3 cells x 3.7 volts (3S)
14.8 volt battery = 4 cells x 3.7 volts (4S)
18.5 volt battery = 5 cells x 3.7 volts (5S)
22.2 volt battery = 6 cells x 3.7 volts (6S)
29.6 volt battery = 8 cells x 3.7 volts (8S)
37.0 volt battery = 10 cells x 3.7 volts (10S)
44.4 volt battery = 12 cells x 3.7 volts (12S)
I should point out you may run across packs or cells hooked up in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 2S2P would indicate two, two celled series packs hooked up in parallel to double the capacity (2S2P is actually a popular configuration in high capacity LiPo receiver packs).

So, those are the voltages you need to know and each RC model or more specifically, the motor/speed controller combination will indicate what voltage is required for correct operation/RPM. This number has to be followed to the letter in most cases since a change in voltage equates to a change in RPM and will require changing the gearing but more likely the motor to a higher or lower Kv rating - not something I want to get into in this write-up. If a model calls for a 3 cell (3S) 11.1 volt battery – lets just say that is what has to be used unless you want to open a whole new can of worms.

A quick word on motor ratings...

Many people new to electric flight get confused by brushless electric motor ratings, specifically the Kv rating thinking Kv = kilo-volts (1 kV = 1000 volts). This is not the case at all. The Kv rating of a brushless motor refers to how many RPM it turns per volt. An example might be something like a 1000 Kv motor with a voltage range of 10 - 25 volts. That would mean this motor will turn at about 10,000 RPM @ 10 volts up to around 25,000 RPM @ 25 volts.

I don't want to start into motor ratings; battery ratings are plenty to get through... I just thought I would make mention of it since I do get that "Kilo-Volt" question often.

CAPACITY

Capacity indicates how much power the battery pack can hold and is indicated in milliamp hours (mAh). This is just a fancy way of saying how much load or drain (measured in milliamps) can be put on the battery for 1 hour at which time the battery will be fully discharged.

For example a RC LiPo battery that is rated at 1000 mAh would be completely discharged in one hour with a 1000 milliamp load placed on it. If this same battery had a 500 milliamp load placed on it, it would take 2 hours to drain down. If the load was increased to around 15,000 milliamps (15 amps) a very common current drain in a 400 sized RC helicopter while hovering, the time to drain the battery would be only about 4 minutes.

As you can see, for a RC model with that kind of current draw, it would be very advantageous to use a larger capacity battery pack such as a 2000 mAh pack. This larger pack used with a 15 amp draw would double the time to about 8 minutes till the pack was discharged.

The main thing to get out of this is if you want more flight time; increase the capacity of your battery pack. Unlike voltage, capacity can be changed around to give you more or less flight time. Naturally because of size & weight restrictions, you have to stay within a certain battery capacity range seeing that the more capacity a battery pack has, the larger and heavier it will be.

DISCHARGE RATE

Remember that third number I was talking about when you go RC LiPo battery shopping? Yes, discharge rate is that number. This one is probably the single most over rated & miss understood of all battery ratings.

Discharge rate is simply how fast a battery can be discharged safely. Remember that ion exchange thing further up the page? Well the faster the ions can flow from anode to cathode in a battery will indicate the discharge rate. In the RC LiPo battery world it is called the “C” rating.

What does it mean?

Well Capacity begins with “C” so that should give you a pretty good idea. A battery with a discharge rating of 10C would mean you could theoretically & safely discharge it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C pack = 20 times more, and so on.

Using our 1000 mAh battery as an example; if it has a 20C discharge rating, that would mean you could pull a maximum sustained load up to 20,000 milliamps or 20 amps off that battery (20 x 1000 milliamps = 20,000 milliamps or 20 amps). From a purely theoretical time stand point, this equals 333 mAh of draw per minute so the 1000 mAh pack would be completely exhausted in about 3 minutes if it's exposed to the maximum rated 20C discharge rate the entire time. Calculation as follows: 20,000 mA divided by 60 minutes = 333 mAh which is then divided into the 1000 mAh capacity of the pack giving us 3.00 minutes).

Most RC LiPo Battery packs will show the continuous C rating and usually a maximum burst C rating as well. A burst rating indicates the battery discharge rate for short bursts of extended power that. An example might be something like “Discharge rate = 20C Continuous/40C Bursts”

The higher the C rating, usually the more expensive the battery. This is where you can save some money. Getting an extremely high discharge rated pack when there is no way you could possibly pull the full amount of power is not required but it won't hurt either. The most important thing is you can't go with too low a discharge C rating or you will damage your battery and possibly your ESC (electronic speed control).

So how do you know what C rating to get when purchasing your LiPo RC Battery Pack? The easy answer most will give is to get the largest C rating you can... If money is not an object I agree with that 100%; but for most folks, especially beginners & intermediate or scale fliers who won't be performing power hungry 3D maneuvers and drawing much current - stretching your RC battery budget by purchasing lower C rated packs when you're first learning so you can get a few extra packs makes much more sense in my opinion.

As a very general guide line, 25C to 30C discharge rated packs are the norm for most 250-400 size electric helicopters with general to light sport flying in mind. For larger birds, 25C to 30C discharge rated packs are a safe bet (again for normal to light sport). Once up to aggressive sport or 3D, that is where the 35C and up discharge rated packs come into play.

All this said, RC LiPo packs are coming down in price all the time. If you find a 35C pack for the same price as a 25C when that is all you need, go for the 35C pack - it will run cooler and have a longer life span. Like most things, pushing a Lipo pack hard close to its limits will wear it out and reduce it's useful capacity in very short order. If however you get a pack with a C discharge rating at least double of the maximum you intend to pull out of it, with proper care, there's no reason you shouldn't be able to get at least 400 charge and discharge cycles out of it with little degradation.

One interesting point I should mention about selecting discharge ratings seeing that HV (high voltage) electric RC aircraft (usually defined as using LiPo packs over 8S) are becoming more and more common place is the reduced current that HV provides. This of course is another topic, but for many HV applications, you can get away with lower C ratings since the models won't pull as much current as a similar size/powered model running on a lower voltage pack. The flip side of course is most folks who are running HV birds are also pushing them to the limits and will still need high discharge rates... I just wanted to point out why higher voltage can be advantageous (less current = less heat).

[Coski]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

My brain hurts.

[Stealthkiller]

Re: A usefull little nugget to help with lipo battery condition / quality monitoring

It was the best/ simplified article I could on lipo battery specs / use , long winded but very informative ..