Alkaline vs. Li-Ion/Polymer Battery Testing

What Is Alkaline Battery?

Alkaline-manganese, also known as alkaline, is an improved version of the zinc-carbon battery and delivers 1.5V. Lewis Urry invented alkaline in 1949 while working with the Eveready Battery Company laboratory in, Ohio, USA. Alkaline batteries are used in many household items such as MP3 players, CD players, digital cameras, pagers, toys, lights, and radios, to name a few.

What Is Lithium Polymer Battery?

Lithium polymer (Li-poly or LiPo) and lithium ion (Li-ion), is quite different from the more commonly used NiCd and NiMH. There are many things to consider before using lithium cells for eflight. But none is more important than safety. While all cells must be treated with respect due to the energy they contain when fully charged, lithium cells have the highest energy density.

Li-Ion/Polymer Battery Characteristics

The nominal voltage of a Li-Po battery cell is 3.7V (about 4.23 V when fully charged). Two and three cell batteries are available giving us a choice of 7.4 or 11.1 volts. Li-Po batteries can supply substantial current, 6A continuously and 12A for short (30-second) bursts. Li-Po cells have a flexible, foil-type (polymer laminate) case. Since no metal battery cell casing is needed, Li-Po batteries are very light. Because of the lack of metal casing and less space used in intercell spacing, the energy density of Li-Po batteries is over 20% higher than that of a classical Li-ion battery and store more energy than nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries of the same volume.

To the left is my Mini Pulse XT aerobatic airplane. It uses the 450 motor and a gens ace 3s lipo 11.1 V, 2100 mAh, Li-Po battery. Futaba 4-channel radio.

Early lithium batteries had a rather high internal resistance, and had rather low discharge rates. As with all technology that is doggedly pursued, significant improvements have been made to the point that the contemporary Li-Po batteries may be substituted in most systems for the original NiCad or NiMH batteries.

Alkaline vs. Lithium Polymer Testing

We wanted to characterize the batteries for both electronics and pyro uses. The electronics battery must supply between 7 and 12 volts at 100 mA for several hours. (We budgeted 2 hours per launch.) The pyro battery needs to provide higher current for a short period of time. Typical e-matches need 1.5V @ 1A for less than one second. Previous experiments showed that a standard 9V Alkaline could provide power for greater than ten (we stopped at 10, the battery's charge was still very near full voltage) e-match ignitions. However, the new electronic release device we are developing requires an 18W heating element to be powered for about 15-seconds. The element would draw about 2.3 Amps from a 7.4V battery.

Batteries tested:

Alkaline Batteries - baseline case

    Three Duracell MN1604 Copper-Top 9V batteries that were new (dated to expire 48 months after the test date).

    Two Duracell MN1604 Copper-Top 9V batteries that were at expiration (dated to expire the month we tested). These batteries were stored for years at room temperature, but never used.

We selected Duracell batteries due to the recommendation of the manufacturer of our flight electronics. Apparently, their welded cell interconnect construction makes these batteries much less prone to drop-outs caused by high G-forces.

Li-Ion/Polymer Batteries

    Four Gens ace NL606290M-3S 7.4V 2200mah lipo (10C) Li-Po batteries

    One Gens Ace 11.1V  800mAh (10-15C) (no model number printed on battery).

    Recharging was performed with a Tenergy Universal Smart Charger (TLP2000, for 1 to 4 cells, non-balancing).

A concern we have regarding Li-Po batteries is that their technology is evolving very quickly. The battery models that we tested last month may no longer be available this month. For example, we ordered two 11.1V 500mAh (10C) batteries and received the 11.1V  800mAh (10-15C) batteries that had now replaced that "old" product line (at the old price point). We were also concerned that the manufacturer does not label their batteries with model numbers. The rapid advancement in battery technology is great, but it makes it difficult to use components that have been well characterized.

Test Equipment for 100mA discharge test:

West Mountain Radio - CBA II - Computerized Battery Analyzer and supporting software.

Test Equipment for 2.3A discharge test:

Keithley 2100 6.5-Digit USB Digital Multimeter and supporting software.

3.44 Ohm precision power resister

Procedure for the 100 mA test:

The discharge rate of our flight computer (waiting for launch) is 100mA, so that is the discharge rate we programmed into the CBA analyzer. We attached the battery to the analyzer and recorded the discharge data. The test was continued until battery voltage dropped below 7V (9V for the 11.1V batteries). Batteries were rested at least 24 hours between discharge and charge operations. During our first test using the CBA analyzer, the computer attached to it froze, resulting in the deep discharge of 7.4V Li-Po battery #1. The deep discharge damaged the battery (would not take a full recharge) so we excluded it from the results discussed below. A second similar incident occurred with 7.4V Li-Po battery #2. We reset the computer in time to save the battery, but its first test results were lost. After that we monitored the test very closely and were able to detect and correct hangs before they effected the tests. We also switched to a faster dedicated computer. This change eliminated the hang problems.

We tested one new 9V Alkaline battery, one nearly expired 9V Alkaline battery, and all the LiPo batteries using the above method.

Procedure for the 2.3A test:

The thermal element of our pyro event device is equivalent to a 3.4-ohm resistor. We wired the Keithley meter to measure the voltage across the resistor and report it to the computer every 500 milliseconds. The voltage recording was started. We then connected the battery to the resistor for a period of about 20-seconds. Then the battery was disconnected and allowed to rest for 40-seconds. This sequence was repeated 10-11 times.

Our first high current test using Alkaline batteries was performed on the nearly expired Alkaline battery. The 9V Alkaline batteries are not designed for high current use. The 100mA test is at the limit of its rated performance. The battery has a relatively high internal resistance, so trying to draw over an Amp from the battery significantly reduced the voltage at its terminals.

After the first test, it was clear that a single 9V Alkaline battery would not be able to power our 18W heater, so we placed two new 9V Alkaline batteries in parallel for the next test.

How LiPo Battery Packs Are Sized

There are lipo battery sizes ranging from tiny one cell packs for micro flyers to mulit-cell batteries for large scale electric powered planes and heli's. The voltage and capacity of any size battery pack depends on the capacity of each cell and how they are wired together.

To understand how LiPo battery packs are sized, you need to have a basic understanding of voltage and capacity. If you're not that familiar with batteries, that's OK. Just think of voltage as the "muscle" and capacity as the "energy" of a battery. At firs, you should know what is the lipo battey voltage.

LiPo Cells in Series Increases Voltage



To build voltage, cells are added together in series. The voltage (muscle) of the LiPo battery pack will continue to increase as more and more cells are added in series.

Battery packs achieve the desired operating voltage by connecting several cells in series; each cell adds its voltage to the total terminal voltage. Parallel connection attains higher capacity for increased current handling; each cell adds to the ampere/hour (Ah) count.

Some packs may consist of a combination of serial and parallel connections. Laptop batteries commonly have four 3.7V Lipo cells in series to achieve 14.4V and two in parallel to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning four cells in series and two in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short.

When connecting cells in series, it is the other way around, where tight capacity tolerances (mAH) are HUGELY important, while voltages are not. For instance, a 2200mAH Gens ace 3s lipo can be connected in series with a 1S 2200mAH battery of the same age. the 2S battery is made up of 2 cells in series, and thus the whole system is like a 3S battery. However, again do not use different battery chemistries or capacities in this config. Also if you plan of charging or discharging at high currents, you will need to keep the batteries balanced since there is always a bit of play in the capacities of the cells and one cell will always die or charge faster than the others. In series, that is especially bad if nothing is done about it, it means the battery with the least capacitance will be stressed the most, causing it to become even worse and possibly damage it or destroy it! (it is a positive feedback loop or a chain reaction type effect). Just make sure to keep them balanced.

LiPo Cells in Parallel Increases Capacity

To build capacity, cells are added together in parallel. The capacity (energy) of the LiPo battery pack will continue to increase as more and more cells are added in parallel.

But the voltage (muscle) will NOT increase as cells are added in parallel.

Total Voltage = 3.7 V
Total Capacity =2100mAh + 2100mAh + 2100mAh = 6300mAh

With Gens ace 5000mah packs in parallel you would have one 3000mAh 20c pack. The C rating does not double like mAh in a parallel battery circuit. You can load up the batteries more in this configuration though. Where, individually, the 1500mAh 20C packs have a constant discharge rating of 30 amps, the "new" 3000mAh 20c pack has a constant discharge rating of 60 amps. 

Right Size Pack for Your RC model

Choosing the correct battery pack is as easy as 1-2-3. Most of the current LiPo battery packs of the countless manufacturers sport common dimensions meaning you can easily replace that gnarly NiMH pack with a super-duper LiPo battery. Many model manufacturers offer their own line of hop-up packs to switch from NiMH to LiPo with no problems. Often, when you look closely they offer some sort of 'hop-up' sections where they promote their products on their websites. Just have a look first and see what the model manufacturer has to offer. Most of the time you find optional battery packs very easy and you are also ensured that the pack(s) you buy fit your models chassis just right.

The next tricky part in choosing the correct pack are the overall dimensions of the battery. If the battery you want to replace is of the usual 7.2V Sub-C style than the alternatives are quite clear. You need a pack not longer as 140 mm, not wider as 46 mm and not thicker as 24 mm. These standard dimensions are featured by most battery packs and make swapping easy. If you need special batteries, for example for smaller 1/16 or 1/18 scale models than 85 x 29 x 17 mm are the numbers you need to take care of. But remember: what we tell here is just a rough guide line! Always double check with the actual batteries you use to make sure to order the correct items.

If you look for a battery pack to suit your monster or Short Course truck or even the new generation of 1/8 scale electric off-road buggies the choice is quite easy as these often have rectangular battery compartments. Just measure what the maximum dimensions of the battery compartment are (length/width/height) and have a go on whatever product suits your needs. When it comes to battery packs for the above models many manufacturers offer special solutions for trucks like the Traxxas E-Revo, Mini Revo etc. Again, just have a look around the specific websites and most of the time you will find what suits your needs.

LiPo Battery Cell Count Charge Supports

There are maximum and minimum LiPo cell count, the battery charger can handle. For instance some battery charger supports up to 6S Lipo, some even up to 8S Lipo, but they might not be able to charging 1S LiPo. Ensure you understand what cell count the charger supports.

Charge Current Rate

Choosing the correct charge current is also critical when charging RC LiPo battery packs. The golden rule here is "never charge a LiPo or Li-Ion pack more than 1 times its capacity (1C)" . For instance a 2000mAh pack could be charged at a maximum charge current of 2000 mA or 2.0 amps.

But things are changing and many battery manufacturers allow higher charging °C current. Most LiPo experts feel that you can safely charge at a 2C or even 3C rate on quality packs that have a discharge rating with a minimum of 20C or more, with little effect on the overall lifetime of the pack, so long as you have a good charger with a good balancing system.

For instance a 2000 mAh pack, could be charged at a maximum charge current of 2000 mA or 2.0 amps. Never higher or the life of the pack would be greatly reduced. When you purchase a charge rate significantly higher than the 1C value, the battery will heat up and could swell, vent, or catch fire.

Remember, the three main things that shorten LiPo battery life are:

• Heat
• Over Discharging
• Inadequate Balancing

Maximum Charge Voltage and Current

A 3.7 volt LiPo cell is 100% charged when it reaches 4.2 volts. Charging past that voltage will destroy the cell, and possibly cause it to catch fire. This is really important to note and keep in mind all the time. A computerized charger will stop the charge process when the battery reaches 4.2V per cell.  A balancing computerized charger will do this for each individual cell.

It is critical that you use a charger specified for Li-Po batteries and select the correct voltage or cell count when charging your LiPo batteries if you are using a computerized charger. If you have a 2 cell (2S) pack you must select 7.4 volts or 2 cells on your charger. If you selected 3s 11.1v lipo battery by mistake and attempted to charge your 2S pack, the pack would be destroyed and most likely catch fire.

LiPo batteries are often recommended to be charged at 1C current rate for various reasons, although some more expensive LiPo batteries these days are advertised as fast charging, which can be charged at 2C or even higher. The key reason for charging at lower current is safety, and to avoid the battery gets too hot, which can cause the battery to go puff and shorter battery life.

Basically, to charge at 1C, it means if you have a 3s 2200mah LiPo battery, your charge current would be 1 x 2200mA = 2A; But to charge at 2C, the charge current is 2 x 2000mA = 4A.

When selecting a battery charger, The manual should specify what the maximum charge current is. If a low charge rate charger is used, charging will take longer. It's completely fine if you don?ˉt mind spending more time waiting.

Sometimes the charger might say the max charge current is 6A, and your battery is also fine to be charged at 6A, but it doesn?ˉt necessary means you are able to charge at this rate. It also depends on your charger power and the cell count of your LiPo battery. In the next chapter we can find out about why and how to choose charger power wattage.

A Revolution in RC Battery Technology

The revolution motors was accompanied by a revolution in battery technology. The energy density of modern lithium-polymer (LiPo) batteryies, combined with the amount of power they can provide at very cheap prices, has opened the door to a new class of aircraft. With this comes a warning: Lipo batteries can catch fire in very dynamic ways from charging, physical damage, or a change in the wind. A great idea is to have a collection of clay flowerpots in which to store and charge batteries. Don't let the batteries get hotter than 120 degrees Fahrenheit or freeze.

A common way to destory a battery is to leave it plugged in to the speed control when the plane is not being flown. If the battery is discharged too deeply. It will destroy the battery.

At the Brooklyn Aerodrome, we have settled on the 1800-milliamperehour (mAh) or 1.8-amperehour two-cell battery pack because such battery packs are cheap and are around the correct weight to balance the Flack. For the more creative aircraft, such as the Bat, we double up battery packs for nose weight and will add lead tire weights if needed. It is easier to have a standard size and work around it than to have varying capacities that inevitably will have you at the flying venue with the wrong-sized battery pack. The other considerations around batteries are the speed-control connector and the balancing connector.

Recommend LiPo Battery pack

Genstattu.com has the least-expensive battery packs historically, and they perform okay. I use very high-quality Thunderpower packs that have not puffed out despite being five years old, but that is the difference between an $8 to $9 battery and a $30 battery. I suggest going cheap with low expectations. Lots of brands are available, but try to buy from a U.S. warehouse to minimize shipping costs and transit time.

3S Gens Ace 5000mAh

Currently, I use the Gens Ace Gens Ace 3s lipo 1800mAh 20C (U.S warehouse) at $8 to $9. This battery lasts about one to two years if treated well and has shown itself to be crash-tolerant. It has a JST-XH connector for charging and a male XT60 connector for powering the airplane. Be mindful of these connectors when buying chargers, and you will have to buy and solder a male XT60 connector onto your speed control.

Alternative Batter Packs

Any two-cell LiPo pack with a 20C or greater rating will do. BP Hobbies offers the Cheetah Packs 7.4V lipo battery 5000mah 35c LiPo battery at $16 with JST-XH connectors for charging and Cheetah 4.0-mm controller connectors.

Be aware that lithium-iron-polymer (LifePo) and lithium-iron (LiFe) batteries are now being sold that look very much like LiPo batteries but differ in a few ways. They have a lower voltage (6.6 volts versus 7.4 volts for LiPo), so if you go with these, be sure to go on ghe higher end of kilovolt ratings for the motor - 1800 kilovolts is about right - and consider using a 10-inch prop where I have been recommending a 9-inch prop. These batteries also will require a charger that is designed for them. A major advantage of LiFePo batteries is that they are less likely to catch fire.

Discussion About Lipo Battery Internal Resistance

IR is a window to the inside of your battery and tell you the overall battery health. It wilt el you how strong or weak it is, how it will perform, and when your battery is sick and dying in need of replacement. OK for Ri to be useful you have to establish a Baseline Reference. To do that certain conditions have to be met in order for the readings to be meaningful and useful. All that means is we are going to start with a new battery, fully charged, well rested, at a specific temperature like room temperature. Don’t worry if the battery is not brand new, you can still do this, it just will not be as meaningful starting with a tired battery because you will not know what it was when at peak or new condition.

A new battery contrary to what you may have heard needs to be broken-in by a few gentle charge discharge cycles. By gentle I mean 1C charge/discharge rate. Take your new battery and fully charge it up at 1C allow it to rest to come to room temp, measure the Ri with your charger, record the reading. Now discharge it down to 3.2 vpc and note AH capacity. Recharge full, rest, measure Ri, and repeat a few times. What you will notice is the Ri will slightly decrease and stabilize, and the AH capacity will slightly increase and stabilize. This is your Baseline Reference of when you battery is new in 100% tip top shape.

How to Measure Internal Resistance in Packs

If you have, for example a 3s 11.1v lipo battery, you need to load it to a couple values to get an accurate internal resistance. First you need to put a relitively small load on the battery, around 1C, and measure the actual current from the battery and the actual voltage across the battery. For a 3-cell 1000 ma battery, you would need to use a load with a value of around 12 ohms. Since the voltage of a freshly charged 3-cell Li-Po is about 12 volts, a 12 ohm load will pull about 1 amp which is 1C for a 1000ma battery. The load will need to dissipate at least 12 watts, so you would need to use a resistor rated for at least 20 watts to keep it from overheating.

Lets say that this test gives you a voltage of 11.20 volts and 0.98 amps of current.

Next you want to load the battery close to it's maximum C rating. To load our gens ace 3s lipo 1000ma battery to 10C we would need to pull 10 amps, since the 3-cell battery loaded down this much will probably have a voltage of around 10 volts, it would take a 1 ohm load to pull 10 amps. 10 amps at 10 volts is 100 watts, so it will take a pretty big resistor to dissipate that much heat. Again you will want to measure the voltage right at the battery and the current being pulled from the battery during this test.

Let's say that in this test we measure 9.97 volts and 10.11 amps. Now that we have these 2 data points, we can calculate the internal drop of the battery and calculate the internal resistance.

The difference in voltage between the 2 tests is 11.20 - 9.97 or 1.23 volts. The difference between the current in these 2 tests is 10.11 - 0.98 or 9.13 amps. Now using Ohms Law, we can calculate the resistance. In this case it would be the voltage difference divided by the current difference or 1.23/9.13 which equals 0.1347 ohms for the entire pack. Dividing this value by 3, since there are 3 cells in the pack, yields an internal resistance of 0.0449 ohms or 44.9 milli-Ohms per cell.

Does LiPos with Higher IR Affect Capacity?

The internal resistance (IR) will not affect the capacity of the pack but rather its capability to deliver at higher rates. Well, here is an example for you. You have a 1000 gallon water tank with a 3" drain valve (low resistance) and a 1/2" drain valve (high resistance). You use this tank to water your 20 horses. The capacity of the tank remains 1000 gallons regardless of which drain valve is open but the 3" valve can water all 20 horses simultaneously while the 1/2" valve can water only 1 horse at a time. Obviously that is not a technical description and there are dozens of factors and exceptions to every rule but that is it in a nutshell without all of the technical jargon.

I have tested one pack that was identical in weight to a "standard" 3S 2200 that delivered over 2800mAh at very high rates (approx 30C) BUT it got very hot.

The verifiable fact is that the IR of LiPo packs will vary enormously with temperature and different packs have different curves. And they may cross. One pack that is physically identical in size and weight to another can have an IR that is much higher at low temperatures (say 20˚C) than the second but that is significantly lower at high temperature (say 60˚C). Yet at the same time their capacity is identical over a range of discharge currents.

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).

Lipo Battery Load Testing You Should Know

Load testing is used to verify that the battery can deliver its specified power when needed. The load is usually designed to be representative of the expected conditions in which the battery may be used. It may be a constant load at the C rate or pulsed loads at higher current rates or in the case of automotive batteries, the load may be designed to simulate a typical driving pattern. Low power testing is usually carried out with resistive loads. For very high power testing with variable loads other techniques may be required. A Ward-Leonard controller may be used to provide the variable load profile with the battery power being returned to the mains supply rather than being dissipated in a load.

Note that the battery may appear to have a greater capacity when it is discharged intermittently than it may have when it is discharged continuously. This is because the battery is able to recover during the idle periods between heavy intermittent current drains. Thus testing a battery capacity with a continuous high current drain will not necessarily give results which represent the capacity achievable with the actual usage profile.

Load testing is yet another way of testing a battery. Load test removes amps from a battery much like starting an engine would. A load tester can be purchased at most auto parts stores. Some battery companies label their battery with the amp load for testing. This number is usually 1/2 of the CCA rating. For instance, It is 4000mah 80C made 2011 weight 127 gr/cell full discharged 200 Amps. It is 5000 65C made 2012 weight 133 gr/cell full discharged 275 Amps. A load test can only be performed if the battery is near or at full charge.

We decided the only way to get some answers was to put a few batteries under a serious high load and see what squeezes out. Regardless of what the manufacturers imply with their high C ratings, even a gold plated battery pushing 200 amps through 10 gauge wires will melt the solder off the junctions. A case in point is the first test of Gens Ace lipo battery 5000mah, 65C, 6S $250 Lipos showed that the 4mm bulleted split pack link connector melt off their wires at the equivalent of 32C.

Higher charge rates will only improve performance if you run the pack as soon as it's off the charger as this will result in slightly higher cell temps which reduces the IR. You could charge a pack at 1C and then put under some lights or other heating source and get the same results as charging at higher rates depending on how hot your heat source is getting the pack. I have come to the conclusion that in a hardcase 2S pack the maximum C rate possible is 35 to 40C if the pack has 2s 5000mah lipo. If the pack has 6500mAh it will have 25 to 30C.

Voltage available under high load was another area of question. I've seen ESC's cut out when the throttle is opened past 75% because a crap battery couldn't keep up with a motors current demand so the ESC shuts down on low voltage. So what's a "respectable" percentage of voltage a battery will hold under load? Does one hold 5% voltage reserve at WOT conditions while another will hold 25%?

To get a start at some answers I built a box that puts a constant resistance across the batteries terminals. Using OHM's law and 6S as my standard I calculated what the nominal current should be and provided a number of different "taps" or resistances I could plug into. The resisters are large spiral types that act like a powerful heater so 1200 SCFM of air is blown over them during testing. While the batteries are under load the voltage, current and temperature are recorded. A common lipo tester is plugged into the balance taps so the actual percentage of voltage remaining is displayed.

There are the caveats: This is called "resistive" loading. When we run our motors we are putting them under an "inductive" load which is a different kind of load. A resistive load will tell you alot but there will be difference in the results. Also differences whether the load is pulsed or sustained, etc. Therefore the second half of this posting will be with an inductive load producing tester.

Why Lipo Battery Is Different from Ohter Battery Types

Modern Lithium Polymer batteries (LiPo, Li-Poly) are able to store and deliver large amounts of energy from light-weight packs. Think of and treat LiPo batteries as fuel. Lithium Polymer cells, as with any high energy source (petrol, electricity, gunpowder etc) must be handled with appropriate precautions and care. Lithium Polymer batteries have been proven world-wide to be a practical and enjoyable power source for model aircraft.

LiPo batteries may take a wide variety of shapes due to the gelling agent separating the cells. Machines and tools with strange sizes and shapes often use LiPo batteries for this reason. However, because they are stronger and potentially more dangerous than other rechargeable batteries like NiCd or NiMH batteries, special care must be taken.

What is the composition of a Lipo Battery?

LiPo battery consists of one cell or 2 or more cells connected to provide a specific voltage and/ or current capacity.  The battery is interfaced into an AV using a basic connector, such as a Deans. The connector consists of a red (+) and black (-) lead as well as a Cell Balancer Connection.
 

Each cell in a multi-cell LiPo pack is rated for 3.7 volts and requires a charging voltage of 4.22 volts. When operating with a  LiPo battery it is EXTREMELY important that you do not allow any cell to drop below 3 volts. Discharging a LiPo cell to less than 3 volts can cause irreparable damage to the internal chemistry causing dramatic reduction in battery life, charge capacity, and discharge time.

It is not uncommon for individual cells in a battery pack to discharge at different rates over the duration of a flight.  Cells may vary in voltage by a couple tenths of a Volt during discharge.  It is important to program your transmitter or Autopilot to alert you before a cell drops below 3 volts. A LiPo cell has a nominal voltage of 3.7V. For the 2s lipo 5000mah above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a "2S" battery pack - it means that there are 2 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.

Special Considerations when Using LiPo Batteries

The main concern that people using LiPo batteries have is that they have a tendency to explode. This is an obvious problem and means that extra care must be taken to prevent this from occurring.

General usage tips:

1.Lithium batteries don't work well in cold air. If you are flying in the winter keep the batteries in your car for best performance.
2.Don't let the batteries overheat. Try and keep them under 140-160 degrees F. This will prolong your battery life.
3.Don't push the batteries past their rated maximum C rating. This will damage the battery and the apparent capacity of the batteries will drop. If when you recharge you are only putting ½ to ¾ of the rated capacity back into the batteries you are probably pushing them too hard.
4.If your building your own cells then put spacing between each cell in the pack to help cooling of the pack. This is most important when building packs larger than 2 cells.
5.Some LiPoly cells use aluminum tabs that you must solder to. Normal soldering procedures will not work on aluminum. You'll need to purchase aluminum soldering paste. The vendor where you purchased your aluminum tab cells should stock this paste.

Lipo battery Voltage and Capacity

When talking about a LiPo, the primary characteristics to understand are the battery’s voltage and capacity. This is typically noted in a shorthand such as “4S-2200”. In this example, “4S” denotes that the battery has four cells in series. The nominal voltage of each cell is 3.7 volts (4.2v fully-charged), so the total pack voltage is:

4 cells x 3.7v = 14.8v.

The second number denotes the capacity of the battery in milliamp-hours (mAh). A fully charged 2200mah is rated to provide a current of 2200 milliamps (2.2 amps) for one hour before it is fully discharged. This capacity value is completely independent of how many cells are in series. In simple terms, the capacity value allows you to estimate how long a battery will provide useful power in a given application. In practical terms for RC use, the capacity rating is typically only helpful for rough comparisons of different batteries. i.e. a lipo 3s 5000mah battery will provide about double the run time of a 2S-2500 lipo in the same RC car.

A 4S2P-2200 battery would consist of two 4S-1100 batteries wired in parallel to provide a total 2200mAh capacity. All other things being equal, you would care for and use this battery the same as you would the previous 4S-2200 example (which is really a 4S1P-2200, but we ignore the 1P). There may be a difference in physical size, but a 4S-2200 and a 4S2P-2200 are functionally equivalent. The differences will really only matter to the guy at the factory who has to assemble the battery.

Check for Lithium Cell Balance

Before charging a new pack, check the voltage of each cell in the pack individually by using the taps. Keep a record and check every tenth use cycle. An unbalanced pack, or a pack with voltage levels not within 0.1 volts of each other, may catch fire. If the pack becomes unbalanced after every discharge, it should be replaced.

Do Not Charge Batteries Unattended

LiPo batteries don't take too long to charge, but there should always be someone near the battery and charger to make sure nothing bad happens. This is very different from other rechargeable batteries, which are more set-it-and-forget-it products. Charge in an open, ventilated space on a safe, fire-retardant surface such as a fire-safe Pyrex dish with sand or a fireplace. Keep a bucket of sand nearby in case a fire starts. If it does, dump the sand on the fire to put it out.

Life of Lithium Polymer Batteries

A Lithium Polymer battery needs to be replaced when it holds 80% or less of its capacity. If used at the maximum continuous discharge rate on every cycle and every charge is at 1C or greater and it’s used down to 3.00V per cell under load (when your LiPo compatible ESC cuts power to the motor) then don’t expect to get  more then 40 or 50 cycles from your LiPo Pack. Please see my guide to prolonging the life of your LiPo’s.