infodragon
Rock Stacker
For the first part of this write up we are going to ignore voltage as it simplifies the discussion.
The battery is the fuel tank of your vehicle. They are typically rated in mAh, milliamp hours. A milliamp is 1/1000 of an amp and an amp hour is the ability of a battery to produce a certain amperage for an hour. For example a 5,000mAh battery is a 5 amp hour battery. It is capable of producing 5 amps for one hour. To follow the the fuel tank analogy you can think of milliamps as gallons.
If your rig lasts one hour on this battery it is consuming 5,000mAh/60minutes mAh/minute (83.3mah/minute). This is like the mpg on a 1:1 car. MPG changes with driving style such as city or highway driving.
The C rating is how fast you can fill up and how fast you can empty your tank, aka battery. 1C stands for complete drain/fill up in 1 hour. In the above example your 5,000mAh battery is drained in one hour which means your average discharge is 1C. Sometimes it may be much higher, when you gun it, and other times it's much lower, idling.
In my research I have found that all brands exagurate about C rating because people want a "betterrerrer" battery. It's simply a marketing ploy that one company started then others had to follow to be able to compete. To read more about the phenomenon of a better number read this as it relates to price. Price-Quality Relationships by Steven M. Shugan
Now to get a bit more specific...
C is a multiplier of capacity. If you have a 5,000mah battery with 20 C it will discharge at 100 amps. A 2,500 mah at the same C rating will discharge at 50 amps. This is *CONSTANT* discharge. Peak discharge which lasts about 10 seconds is much more important when crawling and is about 2x of the batteries standard C rating.
To put it another way, a battery discharged at 30C will have a 2 minute life time. Discharge at 1C = 60 minutes, 2C = 30 minutes, ..., 30C = 2 minutes, ..., 60C = 1 minute, ..., 120C = 30 seconds. The math is 60(minutes)/C no matter the capacity.
In practical terms if you have a 30 minute drive time on a battery you have an average discharge of 2C. If you are driving a 5,000mah battery then you have an average discharge of 10 amps, 10,000 ma. If you have a 2,500mah battery then you have an average discharge of 5 amps, 5,0000 ma.
People have complained about the "punchiness" of a battery with a lower C rating. However their ESC can only handle 50 amps, for example, and the battery is more than capable of exceeding that. We are humans and are subjective. Many experiments have been done about this and we perceive better performance with a "better number." Refer to the link above.
If you really want to compare the quality and capability of batteries get the internal resistance from the manufacturer. The lower the better and that is the REAL measure of quality for a lithium based battery and is verifiable. Feel free to read about it here Battery Testing, Test Methods and Procedures
In summary if you have a 50amp ESC and a 5,000mah battery you can't use more than 10C.
Now, on to voltage... Voltage is the "pressure" at which the tank can empty and is the second half of how much power is delivered through the ESC to the motor. 5v at 10 amps is the same amount of power as 10v at 5 amps. Volts * Amps = Watts. However motors turn at a specific rate per volt. So this complicates the gas tank analogy so I'll try to use voltage as the octane rating. The higher octane gasoline has the higher the compression ratio the engine can be set achieving more power delivered to the drive train.
Back to motors turn rate. A 2000kv motor will rotate at 2,000 rpm per volt. So at 3 volts it will turn at 6,000 rpm. If there is more load, a heavier rig vs a lighter rig, then more amps (gallons) will be drawn draining the tank faster. Climbing a hill vs flat land, drag brake or binding in the drive train will increase the amount of work the motor needs to do which drains the battery faster.
Now you may be thinking that if I have a 7.4v battery how does the ESC work to make sure I have a wide range of control over the speed of my rig? The ESC sends power in on/off pulses to the motor achieving a lower average voltage. This happens about 8,000 times per second in a brushless system. Sending power one pulse every 8 opportunities will achieve 12.5% throttle. The motor will operate as if it's receiving 12.5% of 7.4v or 0.925v. At 0.925v a 2000kv motor will rotate at 1,850 rpm. Another example would be sending power every 80 pulses, the motor would turn at 1.25% of 7.4v or 0.0925v. At that voltage the motor would turn at 185rpm. The ESC does all the calculations to give you a very wide range of control over a motor with a constant rate of voltage from the battery. To follow the analogy the ESC changes the octane rating of the gasoline on the fly so the motor can operate at a higher or lower compression ratio to give you the speed/power/acceleration you need when you need it.
Now to get a bit more technical about the battery you may notice that lithium batteries have an S and P designation. S is the number of cells in series and P is the number of cells in parallel.
A cell is just a single lithium battery, think of a AA battery. A battery pack is multiple cells operating together, most TV remotes use two AA batteries. Those two cells becomes a pack.
By connecting cells in series each cell adds it's voltage to the next. A LiPo cell is 3.7v nominal (average) so a 2S LiPO is 3.7*2 or 7.4v. By connecting cells in parallel each cell adds its capacity, mAh, to the pack. So if we have a 2P LiPo it still is 3.7v but if each cell is 1,000mAh it creates a 2,000mAh pack.
Having 4 cells of 1,000mAh we can create 3 different types of packs...
1. A 1S4P pack, this would be a 3.7v 4,000mAh battery
2. A 2S2P pack, this would be a 7.4v 2,000mAh battery
3. A 4S1P pack, this would be a 14.8v 1,000mAh battery
As another example imagine we have four 9v batteries, the type that would go in a smoke detector, and we create three different packs as above. For the sake of the example they have 1,000 mAh capacities. The poles are denoted + for positive and – for negative. Poles in parenthesis are where the battery pack would connect to the ESC. The | denotes internal connection in the battery. S is a space that the site doesn't deal with very well at the beginning of a new line
Configuration 1: 1S4P (9v 4,000 mAh)
S+(-)
S| |
S+-
S| |
S+-
S| |
(+)-
Configuration 2: 2S2P (18v 2,000 mAh)
+(-)
| |
+-
|
-+
| |
-(+)
Configuration 3: 4S4P (36v 1,000 mAh)
+(-)
|
-+
S|
+-
|
-(+)
Each pack has exactly the same amount of power but is configured to provide that power differently. Remember V*A = Watts. 3.7*4 = 7.4*2 = 14.8v*1. By connecting in series you are increasing the octane of the gasoline, by connecting in parallel you are increasing the size of your tank. Not a perfect analogy and I'm open to other ideas!
The battery is the fuel tank of your vehicle. They are typically rated in mAh, milliamp hours. A milliamp is 1/1000 of an amp and an amp hour is the ability of a battery to produce a certain amperage for an hour. For example a 5,000mAh battery is a 5 amp hour battery. It is capable of producing 5 amps for one hour. To follow the the fuel tank analogy you can think of milliamps as gallons.
If your rig lasts one hour on this battery it is consuming 5,000mAh/60minutes mAh/minute (83.3mah/minute). This is like the mpg on a 1:1 car. MPG changes with driving style such as city or highway driving.
The C rating is how fast you can fill up and how fast you can empty your tank, aka battery. 1C stands for complete drain/fill up in 1 hour. In the above example your 5,000mAh battery is drained in one hour which means your average discharge is 1C. Sometimes it may be much higher, when you gun it, and other times it's much lower, idling.
In my research I have found that all brands exagurate about C rating because people want a "betterrerrer" battery. It's simply a marketing ploy that one company started then others had to follow to be able to compete. To read more about the phenomenon of a better number read this as it relates to price. Price-Quality Relationships by Steven M. Shugan
Now to get a bit more specific...
C is a multiplier of capacity. If you have a 5,000mah battery with 20 C it will discharge at 100 amps. A 2,500 mah at the same C rating will discharge at 50 amps. This is *CONSTANT* discharge. Peak discharge which lasts about 10 seconds is much more important when crawling and is about 2x of the batteries standard C rating.
To put it another way, a battery discharged at 30C will have a 2 minute life time. Discharge at 1C = 60 minutes, 2C = 30 minutes, ..., 30C = 2 minutes, ..., 60C = 1 minute, ..., 120C = 30 seconds. The math is 60(minutes)/C no matter the capacity.
In practical terms if you have a 30 minute drive time on a battery you have an average discharge of 2C. If you are driving a 5,000mah battery then you have an average discharge of 10 amps, 10,000 ma. If you have a 2,500mah battery then you have an average discharge of 5 amps, 5,0000 ma.
People have complained about the "punchiness" of a battery with a lower C rating. However their ESC can only handle 50 amps, for example, and the battery is more than capable of exceeding that. We are humans and are subjective. Many experiments have been done about this and we perceive better performance with a "better number." Refer to the link above.
If you really want to compare the quality and capability of batteries get the internal resistance from the manufacturer. The lower the better and that is the REAL measure of quality for a lithium based battery and is verifiable. Feel free to read about it here Battery Testing, Test Methods and Procedures
In summary if you have a 50amp ESC and a 5,000mah battery you can't use more than 10C.
Now, on to voltage... Voltage is the "pressure" at which the tank can empty and is the second half of how much power is delivered through the ESC to the motor. 5v at 10 amps is the same amount of power as 10v at 5 amps. Volts * Amps = Watts. However motors turn at a specific rate per volt. So this complicates the gas tank analogy so I'll try to use voltage as the octane rating. The higher octane gasoline has the higher the compression ratio the engine can be set achieving more power delivered to the drive train.
Back to motors turn rate. A 2000kv motor will rotate at 2,000 rpm per volt. So at 3 volts it will turn at 6,000 rpm. If there is more load, a heavier rig vs a lighter rig, then more amps (gallons) will be drawn draining the tank faster. Climbing a hill vs flat land, drag brake or binding in the drive train will increase the amount of work the motor needs to do which drains the battery faster.
Now you may be thinking that if I have a 7.4v battery how does the ESC work to make sure I have a wide range of control over the speed of my rig? The ESC sends power in on/off pulses to the motor achieving a lower average voltage. This happens about 8,000 times per second in a brushless system. Sending power one pulse every 8 opportunities will achieve 12.5% throttle. The motor will operate as if it's receiving 12.5% of 7.4v or 0.925v. At 0.925v a 2000kv motor will rotate at 1,850 rpm. Another example would be sending power every 80 pulses, the motor would turn at 1.25% of 7.4v or 0.0925v. At that voltage the motor would turn at 185rpm. The ESC does all the calculations to give you a very wide range of control over a motor with a constant rate of voltage from the battery. To follow the analogy the ESC changes the octane rating of the gasoline on the fly so the motor can operate at a higher or lower compression ratio to give you the speed/power/acceleration you need when you need it.
Now to get a bit more technical about the battery you may notice that lithium batteries have an S and P designation. S is the number of cells in series and P is the number of cells in parallel.
A cell is just a single lithium battery, think of a AA battery. A battery pack is multiple cells operating together, most TV remotes use two AA batteries. Those two cells becomes a pack.
By connecting cells in series each cell adds it's voltage to the next. A LiPo cell is 3.7v nominal (average) so a 2S LiPO is 3.7*2 or 7.4v. By connecting cells in parallel each cell adds its capacity, mAh, to the pack. So if we have a 2P LiPo it still is 3.7v but if each cell is 1,000mAh it creates a 2,000mAh pack.
Having 4 cells of 1,000mAh we can create 3 different types of packs...
1. A 1S4P pack, this would be a 3.7v 4,000mAh battery
2. A 2S2P pack, this would be a 7.4v 2,000mAh battery
3. A 4S1P pack, this would be a 14.8v 1,000mAh battery
As another example imagine we have four 9v batteries, the type that would go in a smoke detector, and we create three different packs as above. For the sake of the example they have 1,000 mAh capacities. The poles are denoted + for positive and – for negative. Poles in parenthesis are where the battery pack would connect to the ESC. The | denotes internal connection in the battery. S is a space that the site doesn't deal with very well at the beginning of a new line
Configuration 1: 1S4P (9v 4,000 mAh)
S+(-)
S| |
S+-
S| |
S+-
S| |
(+)-
Configuration 2: 2S2P (18v 2,000 mAh)
+(-)
| |
+-
|
-+
| |
-(+)
Configuration 3: 4S4P (36v 1,000 mAh)
+(-)
|
-+
S|
+-
|
-(+)
Each pack has exactly the same amount of power but is configured to provide that power differently. Remember V*A = Watts. 3.7*4 = 7.4*2 = 14.8v*1. By connecting in series you are increasing the octane of the gasoline, by connecting in parallel you are increasing the size of your tank. Not a perfect analogy and I'm open to other ideas!
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