JfromJAGs
Well-Known Member
- Joined
- Nov 17, 2005
- Messages
- 85
Depending on the application, battery current in general is limited by 2 factors, which shows in 3 different ways.
The 2 factors are:
a. internal resistance (IR)
b. capacity
The 2 factors show in 3 different ways:
1. Peak Current at maximum power output.
The theoretical peak current is: I_peak = 0.5 * Base Voltage / IR
This neglects that IR is build from electrical resistance of internal connections and the chemical ability to provide current, but it pretty much limits the power output for 3-5 seconds bursts - which is exactly SAW use. For SAWs, we are NOT limited by the capacity (stored energy) at all, we need only a small percentage of it.
For LiPo's this is represented by the "Burst C Rate". But although given in relation to the capacity, it has primarily nothing to do with it. Increasing the capacity by paralleling 2 identical cells mainly halfs the IR, that's why you can use double currents. Having better cells with half the IR will allow the same.
For prewarmed 5000mAh Enerland cells (Polyquest/Thunderpower/XCell/...) this 3-5s limit is known to be 300A+. I have measured them up to 160A for 10s and they just laught about that.
2. Average current based on Runtime and Capacity.
If you use the pack at moderate currents, then you can totally empty it - or better just empty it to a certain extend (leave 10-20% in to increase cycle life).
Then average current is I_avg = Capacity / Runtime.
Moderate means it is safe for the battery, either chemically, heat or whatsoever. For LiPo's this current is represented by "Continous C rate", which is right now between 20-25C (and soon 30C), or in other words: 3 min discharge time. But this already causes reduced cycle life (down to 10% compared to 1C discharge rate), so you may want to run the cells at only 15C, which means 4min minimum runtime.
This applies to enduro style racing (4min) or what NAVIGA does (5-8min runtime).
So for 3-4min runtime 5000mAh LiPo cells provide 75-100A continous.
=> Both applications (short bursts and safe continous load) are mainly safe because cells don't overheat.
i. Short bursts, because there is not enough heat build up in that short time
ii. Continous load, because the battery can dissipate enough heat to keep the temps ok.
This leads us to whatever happens between 5s and 4min runtime. The unsafe gap where US electric oval racing takes place.
3. Sustained current for a given runtime based on safe operation temperatur
As shown above, if you increase the current to a level above the safe continous load, the battery will overheat before it is empty. So to be as fast as possible, people need to race their packs close to or right at the temp limit - no space for picking up a leaf or any other trouble.
The reason why this worked somehow so far in the past was that motors and controllers limited people to an extend that the currents were kept safe for the batteries. Motors and controllers worked somehow as expensive fuses for the batteries.
But progress (that's bigger and bigger BL motors and newer and stronger BL controllers) allowed more current, so people are getting closer to the battery limits. Actually many ran their batteries over the limit in the last 2 years, so NiMH's lasted only very little cycles. That's why people complained.
Now, using LiPo batteries with 2x 5000mAh and lower IR allow higher 60s currents from the battery point of view. I found the 5000mAh Enerland cells can provide about 150A sustained for about 60s, which about empties them by half or less. 2 in parallel mean 300A are safe from the battery point of view.
So, motors and controllers are the limits again, expensive fuses. But how long? I know what to build and race right NOW. It's just not true to tell is will take decades, as the technology (BL) is available. It just needs stacking boards and bigger motors.
But I have to admit, that about 300A is close to the technology limit for the small cell count classes. With current BL technology it's about the limit to build a 7V/300A motor, as this will be a 1 turn 2 pole motor. The limits for the higher cells count classes are higher though.
Conclusion: the basic problem will always be the same, independent of the carried capacity: up to 3-4min runtime either batteries or motors/controllers will be the limits - so people will keep destroying the one or the other.
I see only 2 solutions:
1. Go up with runtimes to the safe operation limit of batteries and the rest of the equippment. That means at least 4min runtime. So the IMPBA ideas with a 2mile race distance makes some sence, it should be 3-4 miles though. Or even better, don't care about distance/laps, make it at least 4min.
Doing that, it does'nt matter to the batteries how much capacity you carry in total, it does'nt change the C rate. But 10,000mAh mean 150A average while 5000mAh mean "only" 75A (both 15C). 75A continous can be handled by today's off the shelf and rather cheap motor and controller, 150A don't - at the moment or it's expensive.
2. Use an electronic energy or power limiter as discussed by the F5 plane guys. By cutting down the capacity electronically it simulates cells with much higher C rates and can be made to be safe for motors and controlers (let's say a 75A average limit). The idea needs some more - what we call - "brain grease" though.
Finally:
With the highly reduced weight LiPo brings FE right into the range of nitro power and weight, for the first time in history. This is probably the future of both, nitro and electric. Choosing the right rules, it can also be both: affordable and fast&fun.
Joerg Mrkwitschka (Febr. 2007)
The 2 factors are:
a. internal resistance (IR)
b. capacity
The 2 factors show in 3 different ways:
1. Peak Current at maximum power output.
The theoretical peak current is: I_peak = 0.5 * Base Voltage / IR
This neglects that IR is build from electrical resistance of internal connections and the chemical ability to provide current, but it pretty much limits the power output for 3-5 seconds bursts - which is exactly SAW use. For SAWs, we are NOT limited by the capacity (stored energy) at all, we need only a small percentage of it.
For LiPo's this is represented by the "Burst C Rate". But although given in relation to the capacity, it has primarily nothing to do with it. Increasing the capacity by paralleling 2 identical cells mainly halfs the IR, that's why you can use double currents. Having better cells with half the IR will allow the same.
For prewarmed 5000mAh Enerland cells (Polyquest/Thunderpower/XCell/...) this 3-5s limit is known to be 300A+. I have measured them up to 160A for 10s and they just laught about that.
2. Average current based on Runtime and Capacity.
If you use the pack at moderate currents, then you can totally empty it - or better just empty it to a certain extend (leave 10-20% in to increase cycle life).
Then average current is I_avg = Capacity / Runtime.
Moderate means it is safe for the battery, either chemically, heat or whatsoever. For LiPo's this current is represented by "Continous C rate", which is right now between 20-25C (and soon 30C), or in other words: 3 min discharge time. But this already causes reduced cycle life (down to 10% compared to 1C discharge rate), so you may want to run the cells at only 15C, which means 4min minimum runtime.
This applies to enduro style racing (4min) or what NAVIGA does (5-8min runtime).
So for 3-4min runtime 5000mAh LiPo cells provide 75-100A continous.
=> Both applications (short bursts and safe continous load) are mainly safe because cells don't overheat.
i. Short bursts, because there is not enough heat build up in that short time
ii. Continous load, because the battery can dissipate enough heat to keep the temps ok.
This leads us to whatever happens between 5s and 4min runtime. The unsafe gap where US electric oval racing takes place.
3. Sustained current for a given runtime based on safe operation temperatur
As shown above, if you increase the current to a level above the safe continous load, the battery will overheat before it is empty. So to be as fast as possible, people need to race their packs close to or right at the temp limit - no space for picking up a leaf or any other trouble.
The reason why this worked somehow so far in the past was that motors and controllers limited people to an extend that the currents were kept safe for the batteries. Motors and controllers worked somehow as expensive fuses for the batteries.
But progress (that's bigger and bigger BL motors and newer and stronger BL controllers) allowed more current, so people are getting closer to the battery limits. Actually many ran their batteries over the limit in the last 2 years, so NiMH's lasted only very little cycles. That's why people complained.
Now, using LiPo batteries with 2x 5000mAh and lower IR allow higher 60s currents from the battery point of view. I found the 5000mAh Enerland cells can provide about 150A sustained for about 60s, which about empties them by half or less. 2 in parallel mean 300A are safe from the battery point of view.
So, motors and controllers are the limits again, expensive fuses. But how long? I know what to build and race right NOW. It's just not true to tell is will take decades, as the technology (BL) is available. It just needs stacking boards and bigger motors.
But I have to admit, that about 300A is close to the technology limit for the small cell count classes. With current BL technology it's about the limit to build a 7V/300A motor, as this will be a 1 turn 2 pole motor. The limits for the higher cells count classes are higher though.
Conclusion: the basic problem will always be the same, independent of the carried capacity: up to 3-4min runtime either batteries or motors/controllers will be the limits - so people will keep destroying the one or the other.
I see only 2 solutions:
1. Go up with runtimes to the safe operation limit of batteries and the rest of the equippment. That means at least 4min runtime. So the IMPBA ideas with a 2mile race distance makes some sence, it should be 3-4 miles though. Or even better, don't care about distance/laps, make it at least 4min.
Doing that, it does'nt matter to the batteries how much capacity you carry in total, it does'nt change the C rate. But 10,000mAh mean 150A average while 5000mAh mean "only" 75A (both 15C). 75A continous can be handled by today's off the shelf and rather cheap motor and controller, 150A don't - at the moment or it's expensive.
2. Use an electronic energy or power limiter as discussed by the F5 plane guys. By cutting down the capacity electronically it simulates cells with much higher C rates and can be made to be safe for motors and controlers (let's say a 75A average limit). The idea needs some more - what we call - "brain grease" though.
Finally:
With the highly reduced weight LiPo brings FE right into the range of nitro power and weight, for the first time in history. This is probably the future of both, nitro and electric. Choosing the right rules, it can also be both: affordable and fast&fun.
Joerg Mrkwitschka (Febr. 2007)
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