Cheetah62
02-23-2010, 01:33 PM
NOTE: This was originally posted by me in an earlier post on HeliFreak.com - it has since been edited for clarity.
"LiPo" is short for "Lithium Polymer" and describes the electro-chemical composition of the cells that make up the battery.
LiPo batteries are measured and rated by three basic specifications:
Voltage (V), capacity (mAh) and discharge rating (C).
Voltage (V) for a LiPo battery can be represented directly (e.g. 22.2v) or more often as an "s" value (e.g. 6s).
The "s" stands for "series" and the integer value represents the number of LiPo cells wired in series to make up the battery.
The voltage of a battery is a measure of its electromotive force, and can be thought of as analogous to "torque".
The higher the battery voltage the more "force" it has to deliver its stored energy to the power system.
A LiPo cell has a "nominal" or "average" voltage of 3.7 volts. The actual working voltage range is 3.3v - 4.2v.
When expressed in "s" terms, the "nominal" voltage rating of a battery is calculated as s x 3.7v.
This is the voltage that will be printed on the battery, or on a device designed for use with a LiPo battery.
For example a 2s battery is rated at 7.4v (2 x 3.7v) and will be labeled as 2s or 7.4v or both.
The same 2s battery when fully charged will have an actual voltage (indicated by a charger) of 8.4v (2 x 4.2v).
Since LiPo cells permanently break down below 3.3v it is critical that you never completely discharge a LiPo battery.
The accepted practice is to follow the "80% rule" which means that you can safely discharge 80% of the rated mAh capacity.
This protects the LiPo cells from falling below the safe minimum voltage.
Capacity is measured in mAh (milli-amp-hours). It is a measure of how much total energy the battery will store.
Using the fuel analogy it is a measure of how big the "tank" is. The larger the mAh value the "bigger" the battery, both figuratively and literally. Storing more energy requires larger cells and more material, so the battery is larger and heavier.
More stored energy means you can get either longer flight time or more power, or some intermediate combination.
You can discharge all the energy quickly and get a lot of power, which we do in helicopters, or you can discharge it more slowly and get longer run time.
Current is a measure of electrical flow, like the flow of water. It is measured in Amps. A milli-Amp or mA is 1/1000 of an Amp.
The units of capacity measure, "milli-amp-hours" (mAh), indicate that the energy stored by the battery is equivalent to a constant current discharge measured in milli-Amps over a period of one hour. It is a unit measure because the time value is one hour.
The one-hour current discharge value is the mAh value of the battery minus the "h": for example 5000mAh --> 5000mA = 5.0A.
Simply put, a fully charged battery with a XXX mAh capacity will fully discharge at a current of XXX milli-amps in one hour.
A higher discharge current will result in a proportionally shorter discharge time.
A constant discharge current of 10 x XXX will fully discharge the battery in 1/10 of an hour, and so forth.
Discharge current rating (C) is a measure of the rated safe maximum continuous discharge current for the battery.
It is expressed as an integer multiple of the capacity value (for example 20C).
The continuous discharge current is equal to (C-value x XXX mA).
A battery rated at 1C will allow a discharge of 1 x XXX milli-amps and will take one hour to discharge.
A battery rated 10C can safely discharge at 10 x XXX mA and will discharge in six minutes (1/10 hour).
A high-performance battery rated at 40C may deliver huge power, but if operated at that level it's all over in 90 seconds!
For example, a 5000mAh capacity battery with a 20C rating can safely discharge at (20C x 5000mA) = 100,000mA = 100 Amps.
In normal use peak currents will be higher than this and are allowed to exceed the C-value by some margin.
The C-rating on the battery is important because it tells you how much power the battery can safely deliver.
Note that none of this has anything at all to do with voltage or s-rating. It applies to batteries of all voltages.
Now for the power: power is expressed in watts and is equal to volts x Amps.
For a given battery voltage (remember, this applies to all voltages) controlling the discharge current directly controls the power output, which is exactly what an ESC does. An Electronic Speed Controller (ESC) is a current controller which regulates the current flow and thus the power delivered to your motor. The motor converts the electrical power to mechanical power.
A high-C rating on a battery indicates that it can discharge its energy quickly, at a high current, and thus produce a lot of power. This is why helicopter batteries, particularly for high-performance 3D flying, are always of a high C-value, typically 30-40C.
They can deliver a lot of current and produce a lot of power, but at the expense of reduced flight time.
Another big reason to use high-C batteries in high-performance helis is that in real life you run at power levels that are dictated by your ESC and motor, so running a high-C battery leaves some extra performance "overhead" in reserve, reducing battery stress, heat production, and ultimately preserving the service life of the battery.
It is this high current capability along with light weight that makes LiPo a popular battery chemistry for powered models.
Balancing:
LiPo batteries are made up of individual cells, averaging 3.7v but ranging from 3.3v to 4.2v.
The total voltage of the battery is the sum of all the individual cell voltages, which is why you can describe the battery voltage using either the total voltage or the "s" rating . Conventionally if you multiply the individual cell voltage (3.7v average) by the number "s" in series you get the rated battery voltage. The actual battery voltage is the sum of the actual cell voltages at that moment.
When batteries are charged and discharged the individual cells tend to end up at slightly different voltages. Small voltage variances are normal and do not cause problems. However, if the voltage on one or more cells drops significantly below the rest, there is a danger that these cells may fall below the lower voltage limit and fail. Also, the total voltage of the battery becomes difficult to manage as the cells vary from cycle to cycle.
To manage this, an electronic tool called a balancer is used when charging LiPo batteries. The job of the balancer is to monitor the individual cells within a battery and correct any variations so that all of the cells have the same voltage. The flat white plug on your battery that has a bunch of wires going into it is the balancing plug, and it plugs into the balancing port on your computerized charger, or perhaps into a separate balancer if your charger does not provide balancing. You can quickly determine the number of cells in your battery - the "s" value - by counting the wires in the balancing plug. There is one common wire for the battery and one separate wire for every cell, so you will have s+1 wires in your balancing plug. For example, a 6s battery will have 7 wires, while a 2s battery will have 3 wires, and so on.
Most computerized chargers will keep the cells balanced to within less than 0.01 v, which is a very close tolerance. Although you do not have to balance your batteries on every charge, conventional wisdom says to do this at least every ten cycles to prevent 'cell dropout" and battery failure.
*** AN IMPORTANT NOTE ON CHARGING ***
DO NOT CONFUSE THE C-RATING ON YOUR BATTERY WITH CHARGING BEHAVIOR!
The C-rating is for discharge performance only and should never be used as a guide for charging rate.
As a general rule battery charging currents should be limited to 1C values unless the battery is specifically labeled as capable of tolerating a higher charging current (maybe 2C). Even then, many people believe doing so will shorten the life of your battery.
This applies to all LiPo batteries, regardless of voltage, capacity or C-rating.
The 1C charge current is easy to calculate - it is simply the mAh capacity of your battery, minus the "h".
For example, a 3000mAh battery can be charged safely at a 1C rate using 3000mA of charge current.
This is the value you should program into your charger.
Oh, BTW, when charging multiple battery sizes and types remember to correctly re-program the voltage and capacity as well.
Always double-check the charger settings before starting a charge cycle.You can set a LiPo pack on fire if you're not careful.
OK, confused yet? Let's run some typical examples through the pencil…
Let's use our previous 6s battery with a 5000mAh capacity and a 30C rating, common for a large electric like the TREX 600.
To calculate the voltages:
Nominal or "rated" voltage = 6s x 3.7v = 22.2v <-- this will be the voltage printed on the battery
Minimum or cutoff voltage = 6s x 3.3v = 19.8v <-- this is a voltage you hope never to see!
Peak fully charged voltage = 6s x 4.2v = 25.2v <-- this is the voltage your charger will display when charging is complete
To calculate currents:
Rated continuous current = 30C x 5000mA = 150,000mA = 150 Amps <-- this is a lot, and you'll probably never use it
Safe charging current = 1C x 5000mA = 5000mA = 5.0 Amps <-- this is what your charger is programmed for
Using the 80% rule:
80% energy discharge = 0.8 x 5000mAh = 4000mAh <-- this is the amount of energy you can safely use in flight
<-- it is also the amount of "put back" you want to look for when charging
Calculating actual flight performance:
I fly my TREX 600ESP for six minutes of moderate "sport aerobatics" and then recharge the battery.
When the charger stops, it indicates that 3000mAh of energy have been restored to the battery.
I can calculate what my safe maximum flight time would be using the 80% rule and flying at the same level of performance, using the 80% energy result from above.
My average discharge current = 3000mAh / 0.1 hour (six minutes) = 30,000mA = 30 Amps <-- well below the 100A ESC rating
The actual "C" value for the flight = 30,000mA / 5000mAh = 6C <-- this is well below the 30C battery rating, so it stays cool…
Rate of energy consumption = 3000mAh / 6 minutes = 500mAh per minute <-- this is a good number to keep track of...
Maximum flight time for 80% rule = 4000mAh / 500mAh per minute = 8 minutes <-- set your flight timer no higher than this
Hope that helps. :shock:
</rant>
"LiPo" is short for "Lithium Polymer" and describes the electro-chemical composition of the cells that make up the battery.
LiPo batteries are measured and rated by three basic specifications:
Voltage (V), capacity (mAh) and discharge rating (C).
Voltage (V) for a LiPo battery can be represented directly (e.g. 22.2v) or more often as an "s" value (e.g. 6s).
The "s" stands for "series" and the integer value represents the number of LiPo cells wired in series to make up the battery.
The voltage of a battery is a measure of its electromotive force, and can be thought of as analogous to "torque".
The higher the battery voltage the more "force" it has to deliver its stored energy to the power system.
A LiPo cell has a "nominal" or "average" voltage of 3.7 volts. The actual working voltage range is 3.3v - 4.2v.
When expressed in "s" terms, the "nominal" voltage rating of a battery is calculated as s x 3.7v.
This is the voltage that will be printed on the battery, or on a device designed for use with a LiPo battery.
For example a 2s battery is rated at 7.4v (2 x 3.7v) and will be labeled as 2s or 7.4v or both.
The same 2s battery when fully charged will have an actual voltage (indicated by a charger) of 8.4v (2 x 4.2v).
Since LiPo cells permanently break down below 3.3v it is critical that you never completely discharge a LiPo battery.
The accepted practice is to follow the "80% rule" which means that you can safely discharge 80% of the rated mAh capacity.
This protects the LiPo cells from falling below the safe minimum voltage.
Capacity is measured in mAh (milli-amp-hours). It is a measure of how much total energy the battery will store.
Using the fuel analogy it is a measure of how big the "tank" is. The larger the mAh value the "bigger" the battery, both figuratively and literally. Storing more energy requires larger cells and more material, so the battery is larger and heavier.
More stored energy means you can get either longer flight time or more power, or some intermediate combination.
You can discharge all the energy quickly and get a lot of power, which we do in helicopters, or you can discharge it more slowly and get longer run time.
Current is a measure of electrical flow, like the flow of water. It is measured in Amps. A milli-Amp or mA is 1/1000 of an Amp.
The units of capacity measure, "milli-amp-hours" (mAh), indicate that the energy stored by the battery is equivalent to a constant current discharge measured in milli-Amps over a period of one hour. It is a unit measure because the time value is one hour.
The one-hour current discharge value is the mAh value of the battery minus the "h": for example 5000mAh --> 5000mA = 5.0A.
Simply put, a fully charged battery with a XXX mAh capacity will fully discharge at a current of XXX milli-amps in one hour.
A higher discharge current will result in a proportionally shorter discharge time.
A constant discharge current of 10 x XXX will fully discharge the battery in 1/10 of an hour, and so forth.
Discharge current rating (C) is a measure of the rated safe maximum continuous discharge current for the battery.
It is expressed as an integer multiple of the capacity value (for example 20C).
The continuous discharge current is equal to (C-value x XXX mA).
A battery rated at 1C will allow a discharge of 1 x XXX milli-amps and will take one hour to discharge.
A battery rated 10C can safely discharge at 10 x XXX mA and will discharge in six minutes (1/10 hour).
A high-performance battery rated at 40C may deliver huge power, but if operated at that level it's all over in 90 seconds!
For example, a 5000mAh capacity battery with a 20C rating can safely discharge at (20C x 5000mA) = 100,000mA = 100 Amps.
In normal use peak currents will be higher than this and are allowed to exceed the C-value by some margin.
The C-rating on the battery is important because it tells you how much power the battery can safely deliver.
Note that none of this has anything at all to do with voltage or s-rating. It applies to batteries of all voltages.
Now for the power: power is expressed in watts and is equal to volts x Amps.
For a given battery voltage (remember, this applies to all voltages) controlling the discharge current directly controls the power output, which is exactly what an ESC does. An Electronic Speed Controller (ESC) is a current controller which regulates the current flow and thus the power delivered to your motor. The motor converts the electrical power to mechanical power.
A high-C rating on a battery indicates that it can discharge its energy quickly, at a high current, and thus produce a lot of power. This is why helicopter batteries, particularly for high-performance 3D flying, are always of a high C-value, typically 30-40C.
They can deliver a lot of current and produce a lot of power, but at the expense of reduced flight time.
Another big reason to use high-C batteries in high-performance helis is that in real life you run at power levels that are dictated by your ESC and motor, so running a high-C battery leaves some extra performance "overhead" in reserve, reducing battery stress, heat production, and ultimately preserving the service life of the battery.
It is this high current capability along with light weight that makes LiPo a popular battery chemistry for powered models.
Balancing:
LiPo batteries are made up of individual cells, averaging 3.7v but ranging from 3.3v to 4.2v.
The total voltage of the battery is the sum of all the individual cell voltages, which is why you can describe the battery voltage using either the total voltage or the "s" rating . Conventionally if you multiply the individual cell voltage (3.7v average) by the number "s" in series you get the rated battery voltage. The actual battery voltage is the sum of the actual cell voltages at that moment.
When batteries are charged and discharged the individual cells tend to end up at slightly different voltages. Small voltage variances are normal and do not cause problems. However, if the voltage on one or more cells drops significantly below the rest, there is a danger that these cells may fall below the lower voltage limit and fail. Also, the total voltage of the battery becomes difficult to manage as the cells vary from cycle to cycle.
To manage this, an electronic tool called a balancer is used when charging LiPo batteries. The job of the balancer is to monitor the individual cells within a battery and correct any variations so that all of the cells have the same voltage. The flat white plug on your battery that has a bunch of wires going into it is the balancing plug, and it plugs into the balancing port on your computerized charger, or perhaps into a separate balancer if your charger does not provide balancing. You can quickly determine the number of cells in your battery - the "s" value - by counting the wires in the balancing plug. There is one common wire for the battery and one separate wire for every cell, so you will have s+1 wires in your balancing plug. For example, a 6s battery will have 7 wires, while a 2s battery will have 3 wires, and so on.
Most computerized chargers will keep the cells balanced to within less than 0.01 v, which is a very close tolerance. Although you do not have to balance your batteries on every charge, conventional wisdom says to do this at least every ten cycles to prevent 'cell dropout" and battery failure.
*** AN IMPORTANT NOTE ON CHARGING ***
DO NOT CONFUSE THE C-RATING ON YOUR BATTERY WITH CHARGING BEHAVIOR!
The C-rating is for discharge performance only and should never be used as a guide for charging rate.
As a general rule battery charging currents should be limited to 1C values unless the battery is specifically labeled as capable of tolerating a higher charging current (maybe 2C). Even then, many people believe doing so will shorten the life of your battery.
This applies to all LiPo batteries, regardless of voltage, capacity or C-rating.
The 1C charge current is easy to calculate - it is simply the mAh capacity of your battery, minus the "h".
For example, a 3000mAh battery can be charged safely at a 1C rate using 3000mA of charge current.
This is the value you should program into your charger.
Oh, BTW, when charging multiple battery sizes and types remember to correctly re-program the voltage and capacity as well.
Always double-check the charger settings before starting a charge cycle.You can set a LiPo pack on fire if you're not careful.
OK, confused yet? Let's run some typical examples through the pencil…
Let's use our previous 6s battery with a 5000mAh capacity and a 30C rating, common for a large electric like the TREX 600.
To calculate the voltages:
Nominal or "rated" voltage = 6s x 3.7v = 22.2v <-- this will be the voltage printed on the battery
Minimum or cutoff voltage = 6s x 3.3v = 19.8v <-- this is a voltage you hope never to see!
Peak fully charged voltage = 6s x 4.2v = 25.2v <-- this is the voltage your charger will display when charging is complete
To calculate currents:
Rated continuous current = 30C x 5000mA = 150,000mA = 150 Amps <-- this is a lot, and you'll probably never use it
Safe charging current = 1C x 5000mA = 5000mA = 5.0 Amps <-- this is what your charger is programmed for
Using the 80% rule:
80% energy discharge = 0.8 x 5000mAh = 4000mAh <-- this is the amount of energy you can safely use in flight
<-- it is also the amount of "put back" you want to look for when charging
Calculating actual flight performance:
I fly my TREX 600ESP for six minutes of moderate "sport aerobatics" and then recharge the battery.
When the charger stops, it indicates that 3000mAh of energy have been restored to the battery.
I can calculate what my safe maximum flight time would be using the 80% rule and flying at the same level of performance, using the 80% energy result from above.
My average discharge current = 3000mAh / 0.1 hour (six minutes) = 30,000mA = 30 Amps <-- well below the 100A ESC rating
The actual "C" value for the flight = 30,000mA / 5000mAh = 6C <-- this is well below the 30C battery rating, so it stays cool…
Rate of energy consumption = 3000mAh / 6 minutes = 500mAh per minute <-- this is a good number to keep track of...
Maximum flight time for 80% rule = 4000mAh / 500mAh per minute = 8 minutes <-- set your flight timer no higher than this
Hope that helps. :shock:
</rant>