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Discussion Starter · #1 ·
This last Winter I was busy installing new Lifepo4 batteries in my Saturn SC2. These replace the nine lead acid batteries that gave the car a range of 12 miles. Last weekend I drove the car 40.4 miles. I was thrilled to get that close to my estimate of 42 miles with the 36, 100 Ah cells that I worked so hard to install in the cold of Winter. Today, 05/17/2014, I went out for another test drive after installing a bank of large capacitors (131,600 uf, 200 volt, electrolytic ) Capacitors must be handled with extreme caution. They must be pre-charged slowly so, I mounted them in a chassis with a 100 amp breaker as a main switch and connected a pre-charge resistor across the breaker. I noticed that the car is more responsive and the motor is pulling fewer amps when I accelerate. Both performance and range have improved from the first test drive. The car has the AC-50 motor and a Curtis 3 phase controller with regenerative braking. The capacitors soak up enough energy to get the car rolling again after each stop. I can top 40.4 miles now, and I have had it up to 72 mph, but I still don't know what the maximum range is because I don't want to get stranded far from home. The top speed is still unknown because I don't like speeding tickets. What makes me really happy is that I can drive the 30 miles to work, plug in to a standard outlet, and after 6 hours, the car is fully charged and ready for the drive home. On the test drives, I have observed the reaction of other motorists. When people read the word "electric" on the back of my car, they often stay far back as if an explosion were imminent. Then there are other who are curious and want a better look. There are some who will ride my back bumper and then pass me in a no passing zone despite the fact that I am driving the speed limit. I don't want to park the car next to this last guy because he will slam his door into mine just to make his point clearer. I don't think any of us deserve that treatment because we are saving the world's petroleum just so guys like that can waste more of it. I am proud of the fact that I drive without burning a single drop of the stuff. And, like my bumper sticker says-"Electrons are cool- Plug it in."
 

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Discussion Starter · #4 ·
I have a total of 4, 200 volt caps. Two are 63000 uf and two are 2800 uf cans. They supply enough power to aid the Lifepo4 batteries for several seconds while the car is accelerating. The Lifepo4 batteries were not able to respond fast enough and the voltage was pulling down too low. Well worth $600.
 

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I am interested in caps, but it confess that I am ignorant in this area. I understand the principle of storing energy during off peak times and using it for peaks, but I don't know the math to prove them useful or not. What I have done, is I have read many threads on this site that discuss adding caps to the HVDC system. I do not recall ever reading a thread that indicates that the caps make any sense, so I am very interested in what you are doing.
 

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Discussion Starter · #6 ·
I am interested in caps, but it confess that I am ignorant in this area. I understand the principle of storing energy during off peak times and using it for peaks, but I don't know the math to prove them useful or not. What I have done, is I have read many threads on this site that discuss adding caps to the HVDC system. I do not recall ever reading a thread that indicates that the caps make any sense, so I am very interested in what you are doing.
Capacitors make sense because they can charge and discharge much faster than any battery. A really good car audio system needs a boost capacitor because a battery can not deliver enough power fast enough to produce the kind of bass that we all expect from ear splitting, high power, car audio. The same principle applies to an electric motor which needs both high voltage and high current to perform well. If you step hard on the accelerator the motor may lag because while high current is drawn, voltage will drop off sharply. A charged capacitor in parallel with the batteries will give the motor a huge boost, but only for seconds which is long enough to get up to speed. It is not simply a matter of how much total energy is stored in the car, it is a matter of how energy is used and whether energy is quickly stored or lost during regeneration. In the case of a stalled motor, the current draw will be huge and the battery will discharge rapidly even though the motor may never turn a single revolution before the battery dies. Motor start capacitors are usually quite small since their only purpose is to get the motor running. They have been in use for decades in general appliances so why should we not have them in electric cars?
 

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Capacitors have their place, but

You'd be better off to spend that money on more batteries. The energy of a cap is .5 * C * V^2. Compare this to the energy of your pack--it's in the noise. Plus once the voltage drops below the pack voltage they are no longer adding anything, so the energy contribution is even less. And caps must be charged up with energy from your pack--it doesn't create something for nothing. The discharge time is on the order of R * C and is going to be in the milliseconds, not seconds.

The caps on a 4-pole single phase AC motor are used to introduce a phase shift in one of the windings so the unbalanced torque will get it spinning. Not used for that purpose in a 3-phase AC motor.

Caps are used in amplifiers such as an audio system to provide filtering and reduce the ripple voltage, especially on a lead-acid battery system, to try to hold the buss voltage up--once again micro or milli seconds, not seconds.
 

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Discussion Starter · #8 ·
Re: Capacitors have their place, but

You'd be better off to spend that money on more batteries. The energy of a cap is .5 * C * V^2. Compare this to the energy of your pack--it's in the noise. Plus once the voltage drops below the pack voltage they are no longer adding anything, so the energy contribution is even less. And caps must be charged up with energy from your pack--it doesn't create something for nothing. The discharge time is on the order of R * C and is going to be in the milliseconds, not seconds.

The caps on a 4-pole single phase AC motor are used to introduce a phase shift in one of the windings so the unbalanced torque will get it spinning. Not used for that purpose in a 3-phase AC motor.

Caps are used in amplifiers such as an audio system to provide filtering and reduce the ripple voltage, especially on a lead-acid battery system, to try to hold the buss voltage up--once again micro or milli seconds, not seconds.
OK. T=RC when calculating time constants for RC networks where in voltage is not a consideration such as in a filter. Much more complex formulas are required to accurately determine the charge or discharge rate of a capacitor in a high power application. A cap charged to 1 volt will discharge well before the same cap charged to 200 volts. This is why the formula must include variable V. R or resistance is unknown because the motor controller is variable. However, if you charge up some big capacitors and use them to power a motor, you will find that the motor can run for seconds. The exact time can be measured without calculations of any kind. The acid test is to measure the performance of an electric car first with capacitors and then without. The capacitors get charged in the regenerative cycle as well as by the batteries. You can either capture that energy or loose it. Capacitors always help when startup loads are heavy and the results can be demonstrated.
 
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