Wednesday, November 19, 2014

All About Lipo Batteries

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Why Should You Use Lipos?

It all comes down to useable power.  Lipo batteries (an abbreviation of Lithium Polymer) are a rechargeable battery that are very popular -- especially in the RC world -- because of their power to weight ratio. In other words, more electricity in a smaller and lighter package. Obviously, this is ideal for anything you’re trying to get in the air, or for moving faster or farther on the ground.

Another advantage is the high discharge rate – which is another way of saying they can deliver large amounts of power at once.

Of course, higher performance comes at a higher price, so you may be surprised the first time you see the price of lipos. On the other hand, prices are dropping and you can often get great deals on lipo batteries at places like Amazon.  I've had a lot of success with Turnigy batteries and feel comfortable recommending them.  HobbyKing is another source for deals on Turnigy. This is one of my favorite all-purpose Turnigy packs, and I've ordered a number of them.   And I've also enjoyed the performance of Zippy FlightMax batteries from HobbyKing.  Before you get too hung up on the price, remember that if you take good care of them -- which this guide is intended to help you with -- they'll last a long time.

How To Choose The Right Lipo Battery

There's no quick and easy answer... but to make it easIER, let's review some of the basic things you should know.

Li-po Voltage

The most important thing you need to know when purchasing lipos is what voltage you require and what voltage the battery supplies.  A 7.4 volt lipos won't do much good on a 14.8 volt motor... and an 11.1 volt will quickly burn out a 3.7 volt motor.

What Does C For A Lipo Battery Mean

This next part might start sounding like a science or math lecture.  You may need to read it a couple of times before it clicks.  In short, the “C” represents capacity -- and the capacity is how much load or drain you can put on the battery before totally discharging it in one hour.

To figure out the maximum discharge current for a specific batteries, multiply the discharge rating by the capacity of the battery in amps. If you have a 25C 4000mAh battery…

25 x 4.0 = 100.

Based on that, the maximum you should ever continuously draw from this is 100A.

But it's better if you work backwards. In other words, choose the motors you want first, then pick ESC’s (Electronic Speed Controllers) to match, and then make sure your batteries are up to the job.

Most of the examples on this page are based on a lipos for a quadcopter, which has four brushless motors.  By using four motors in these examples, it's easy for you to scale down for 1 to 3 motors (like in a plane or car), or up for 5 or more (like in a hexacopter or octocopter).

Let's say your motor’s maximum draw is 19 amps, at the very least you’ll want a 20 amp (20A) ESC (electronic speed controller). Most people would go to 25A or 30A to be safe.  Always practice safe specs!  For a quadcopter, you have four motors and four ESC’s. At four times 19A, the most your motors will draw is 76A. So, the 4000mAh 25C lipo used in the example above would work since, at 100A, it can supply more than your maximum load.

If you already own batteries and want to know what kind of motors you can buy, multiply the discharge rating by the capacity of the battery amps, and divide that by four (because each motor gets 1/4 of the available current).

Again, using that 4000mAh 25C battery as an example…

25 x 4.0 / 4 = 25. So, ideally, your motors’ maximum draw should be less than 25A.

NOTE: All the above calculations assume your motor is running at full throttle. This will rarely be the case for any aircraft (if it is, you miscalculated something), but you should still make your calculations on that assumption to provide a margin of safety.

A more detailed explanation comes later.

The above is best explained with some examples:

  • A 1000 mAh (milliamp)  battery with a 1000 mAh draw on it would be completely drained in one hour.
  • A 1000 mAh  pack with a 500 mAh load on it would be completely drained in two hours.
  • A 1000 mAh lipo with a 5000 mAh draw would be completely drained in 12 minutes.

At the time this was written, Turnigy 2217 motors are a popular choice for quadcopter builders.  They're extremely affordable and dependable.  I used them for my first quadcopter builds and they survived a lot of hard landings. If you look at the provided specs, the operating current load is between 6 and 17 amps. As far as quadcopters go, you're generally shooting for the ability to hover at approximately half-throttle -- so let’s go with an average of 10 amps per motor, which gives us 40 amps in total. 

A 2000 mAh lipo battery with a 40,000 mAh (40 amps) load would be totally discharged in 3 minutes. But that doesn’t really give you your actual run time, because of…

The 80% Rule

Many types of batteries can be fully discharged while using them. As a matter of fact, NiCd’s  (or nicads) need to be fully cycled to keep them at their best. But all Lipos have a minimum voltage limit – and going below it can irreversibly damage your battery. This minimum voltage is typically 3.0 volts, but it can (rarely) vary with different packs.

That’s why the vast majority of users follow the 80% rule.  Take a normal fully-charged cell of 4.2 volts x .8 (.8 = 80%) to get 3.36 volts. You may choose to be even more conservative and make it an 85% (3.57 volts) or 90% (3.78 volts) rule (a lot of people do), but let’s go with 80% for the time being.  Many low voltage alarms are preset for 3.3 volts but allow you to adjust them.  The voltages give above are "under load".  If the alarm goes off and you shut the motors off quickly, you might even notice the alarm shutting off as the power pack "rests"  (a very non-technical term, but you get the picture).  Don't restart the motor as your pack is still in need of recharging.  If you're measuring unloaded, 80% works out to about 3.7V.

Another good reason to follow the 80% rule is to protect whatever you're flying. Lipos under load give you fairly flat or consistent voltage until you’ve drained about 90% of their capacity, at which time it drops very quickly – which is what your airplance, copter or quadcopter will also do if it happens to be in the air when it happens.

So, to calculate your approximate air time, you can use this following formula:

80% of the battery’s capacity, divided by the load, and then multiplied by 60 to give you minutes.  Which looks like:

.8 x battery mAh / motor mAh x 60 = estimated flight time.

If we use the Turnigy 2217’s mentioned above, with each drawing 10,000 mAh, resulting in total of 40,000 mAh, on a 2000 mAh battery… it gives you:

.8 x 2000 / 40,000 x 60 = 2.4 minutes.

But let's say you buy a battery with more capacity – I prefer the weight to power of a 5000 mAh -- and you get:

.8 x 5000 / 40,000 x 60 = 6 minutes of approximate run time.

Now, let's throw in yet another factor!  The weight of your quadcopter, plane, or vehicle will determine how much power your motors draw. For example, if you have a 2-kilogram quadcopter that hovers at half throttle, a 1-kilogram quadcopter using the same motors, propellers, and battery will hover about 50% longer.

So referencing the last example above, let’s say the overall weight is brought down to the point where the quadcopter hovers at a total 32,000 mAh of draw from all four motors…

.8 x 5000 / 32,000 x 60 = 7.5 minutes of approximate flight time.

For those of you who want to get even MORE detailed… the given specifications for the Turnigy 2217’s tell us that if you're using a 10x7 propeller and pulling 16,300 mAh, the motor will give you 970 grams of lift. Mulitply that by your four four motors,and you get 3.88 kilograms or approximately 8.5 pounds.

.8 x 5000 / 65,200 x 60 = about 3.6 minutes of flight time.

Bear in mind, most self-built quadcopters are usually under half that weight. 

I barely got through high school math, so I don’t have a clue about calculating a motor's draw based on the thrust it’s providing but, at the time I put this information together, the link below has a detailed calculator you can experiment with.

NOTE: You should invest in a battery monitor – preferably one with an alarm that goes off when the battery (or one of its cells) reaches a specific voltage. This inexpensive piece of equipment will save you from destroying an expensive battery and/or quadcopter. They usually connect to the balancing plug, and are available from a number of sources.  This lipo battery low voltage monitor and alarm is highly rated.

What Does S For A Li-po Battery Mean?

Unlike the usual battery packs that typically have 1.2 or 1.5 volts per cell, lipos offer 3.7 volts per cell. As well, most LiPo’s have two or more cells connected together to create higher voltages or longer lasting packs. The number of cells is usually represented by a number, followed by an "S" in the battery description.  S stands for the number of cells in the pack connected in series.

1S = 3.7 volt lipo pack
2S = 7.4 volt lipo pack
3S = 11.1 volt lipo pack
4S = 14.8 volt lipo pack
5S = 18.5 volt lipo pack
6S = 22.2 volt lipo pack

What Does P For A Lipo Battery Mean?

Depending on the lipo battery you're shopping for, you might also see a “P”.  The "P" tells you how many cells are connected in parallel. For example, 3P would equal 3.7 volts, but would run longer than a regular 3.7 voly lipo. 3S2P would be two 3-cell packs connected in parallel to give you 11.1 volts but with a longer run time than just a 3S. But before you assume the more P's you get the better, remember that the battery is heavier because of the extra cells -- so a 2P rating won't automatically double your driving or flight time.

Charging a Lipo Battery

A 3.7 volt li-po battery cell is fully charged when it reaches 4.2 volts. If you tryand charge it beyond 4.2 volts, you will destroy the cell and quite possibly cause a fire.

So it's very important that you use a charger designed for lipo batteries, and to choose the right voltage and/or cell count settings on the charger. If your lipo is a 2S pack you must choose 7.4 volts or 2 cells. If you accidentally choose 11.1 volts (or 3S), you’ll ruin the pack and may cause a fire. If you can afford it, buy a computerized charger that will automatically warn you if you choose an incorrect cell count. This lipo battery charger balancer  will check the number of cells you enter, and then check the number of cells in the battery pack.  It will only start charging if both numbers match.  (It also has a lot of other helpful functions).

When you're recharging a li-po, a constant current is being applied to the cells during the first part of the cycle. As the battery voltage gets close to 100% of the charge voltage, the charger (mentioned above, as well as others) will start reducing the charge current. The charger will then completely stop charging at the correct time.

Many chargers will let you select the charge current. The setting most recommended by lipo battery manufacturers and experienced users is 1C or, in other words, 1 times its capacity. Using that guideline a 3000 mAh pack would be charged at 3A. A 4000 mAh battery would be charged at 4A. 

Some people choose to charge li-pos at even a little less than 1C. The theory is that, even though the charging takes longer, the battery may have a longer overall life.  i haven't been able to find any conclusive test or results on the subject though.

Note that charging at more than 1C may possibly cause a fire.

That being said, many lipo experts now say a 2C or even 3C rate is possible on high-end packs rated 20C or higher. Check the specifications for your particular battery before attempting this and remember that 1C is safer and anything higher increases the possibility of a fire.

If you don't already have one, I recommend buying this Lipo Battery Balance Charger.  It comes with a number of adapters to fit all the different batteries out there, and includes the 110-volt "wall wart".  Most chargers that you buy require hooking up to a car battery, which is great when you're out flying, but a pain for charging at home.  Whatever model you end up buying, make sure you get the wall adapter with it,

What Is Lipo Balancing?

As mentioned earlier, most lipo batteries (except for the 3.7 volt 1S types) are made of mulitple cells.  For your li-po battery pack to perform at it's best and to give it a long life, the individual cells in the pack need to be charged equally. That type of equal charging is called balancing.

A lipo battery charger shouldn't supply the same charge to all the same cells in a pack. let's say you have a 3S lipo and the cells are discharged to 3.1V, 3.3V and 3.7V respectively.  If you plugged the batteru into a non-balancing charger, the 3.7V cell would reach the maximum 4.2 volts way before the other two cells. And then, as the other two cells continue to charge,  it would exceed the maximum – wrecking the cell and possibly causing a fire.

That’s why every li-po battery with more than one cell 
will have a balance plug (sometimes called a balance tap). The balance plug sends individual cell information to the charger to keep everything safe, and to maximize the lipo's power when charged.

Lipo Battery Charging Safety

  • Let me state, for the record, that you should never leave a charging lipo battery pack unattended. That being said, many people do.
  • It's best to charge your lipo’s in a fire resistant container (an old metal toolbox is ideal) or a special lipo charging sleeve, envelope, or bag , and well away from combustible materials.
  • A fire extinguisher should be kept in the charging area.
  • If you want to be extra safe, you can install a smoke detector in your charging area. If that happens to be in your garage, you may want to take the battery out when you’re not charging so that your car exhaust doesn't set it off.
  • Li-po batteries tend to get warm while you're using them. Give them time to cool off before recharging.

Don’t get all paranoid, but don’t get careless or over-confident either. A good reality check is to go online and search for lipo fire videos.

NOTE: I’ve read that lipo's are non-toxic and can be thrown out in the regular trash after being completely discharged. However, my strong personal preference and recommendation is that you take it to your local hazardous waste depot or battery disposal.

Puffy Lipo Battery Packs

You might notice a li-po battery pack puffing up or swelling slightly -- especially if it's being used hard enough to heat it up. I don't have an expert opinion, but most of the articles I've read say that minor swelling will go down as the pack cools and it's nothing you have to worry about. If the puffiness doesn't go down, and you've gotten a lot of use out of it, the battery is probably approaching the end of its usable life. 

Storing A Lipo Battery

Remember that if a lipo battery cell's voltage drops too low, you've probably ruined it.  Even though the self-discharge rate of a li-po is significantly much slower than other battery types, they still do self-discharge. As a result, you don’t want to store them away if they're close to their minimum voltage.

But you also don’t want to store lipo's at their maximum voltage. There's a chemistry reason for this, but it's way over my head.  Just trust me when I say that a full-charge is bad for any lipo battery storage longer than three or four days.  Most computerized chargers (including the one Iinked to earlier) have a storage program or setting – and that will take care of all the calculations for you. If your charger doesn't have that option, just charge to what most manufacturers recommend -- 40-50% (about 3.85 volts per cell).

(if you find this information useful, feel free to share it by linking to it)