My quest to replace conventional lead-acid car batteries with supercapacitors was inspired by
this thread. I have started my project and will share all of my findings with as much detail as possible, and hopefully get some answers to lingering questions.
Questions:
1. How do the energy losses of charging/discharging a supercapacitor compare to lead-acid batteries or LiFePo4 batteries?
Lead-acid batteries are approximately 50% efficient considering the charge/discharge losses. The losses are not linear, and increase as the battery nears full capacity. This is particularly unfortunate as lead-acid batteries need to be maintained at full capacity to avoid sulfation and damage.
LiFePo4 batteries are 90% efficient in the charge/discharge cycle.
Super capacitors are 90-99% efficient at storing and releasing a charge. The superior efficiency is obtained because a capacitor is merely storing a static charge, whereas the lead-acid battery relies on a chemical reaction. Efficiency is reduced at high frequency pulsing of charge/discharge, and at higher currents. A steady, low current draw will approach 99% efficiency.
2. How does the self-discharge of a supercapacitor compare to the self-discharge of lead-acid or LiFePo4 batteries?
There is some self-discharge, although it's difficult to quantify. The majority of the discharge occurs towards peak voltage. The effect is more pronounced due to the lower charge capacity and the corresponding drop in voltage. Where a battery may self-discharge and loose overall capacity but maintain voltage, a capacitor looses capacity by loosing voltage.
3. What is the typical minimum voltage that a modern vehicle needs to maintain functions such as radio presets, clock, etc and still be able to start?
The motorcycle requires 10v on my 400 farad capacitor bank to start. It requires 8.5v to run all computer functions and display. Memory is lost at voltages below 6v.
4. How many amps can be drawn and for what duration can be tolerated by the relatively thin terminals of the supercapacitors?
Unknown. Resistance is a function of material, diameter, and length. The very short copper leads of the capacitor can likely handle hundreds of amps without melting. So far, I've only gone up to 200 amps an 12 AWG wire, but I plan to push them harder.
5. What is the expected operational lifetime of a supercapacitor battery?
Likely the life of the vehicle, assuming a balance circuit is used and under-hood temps do not get too hot. Life is a function of voltage and heat multiplied by the amount of time the capacitor is held under those conditions. Maximum operating temperature is 150F (65C)
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To give one example, a 15% reduction in rated capacitance and a 40% increase in rated resistance may occur for an ultracapacitor held at 2.5 V after 88,000 hrs at 25 o C. The plots, along with the fact that the influence of temperature has a doubling effect for every 10 o C, can be used to predict the expected performance change for a variety of conditions.
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6. What is the optimal balance circuit for automotive applications?
Wiring a single diode with a 0.5v drop in series with a single LED with a 1.8v drop is ideal to trim voltage. This is an example of passive balancing. There are also active balance circuits that compares the voltage of 2 capacitors and then drains a little of the voltage in the higher voltage cap into the lower voltage one until they are in balance.
7. In the long run, can I save money by using a capacitor/LiFePo4 combo instead of buying lead-acid batteries every 7 years or so?
I'll let you know in 7 years. There doesn't seem to be much info available regarding LiFePo4 batteries in automotive use. One concern is that charging them in sub-freezing temperatures can cause permanent damage.