Replace your car battery with capacitors!

[redpoint5]

It appears Maxwell has released a new supercap!



While attempting to find some literature on a 430f 16v supercap on Ebay, I discovered a 3400f 2.85v capacitor on Mouser for $80. That's a heck of a lot of money. The 3000f supercaps only cost $64. But wait! The 3400f caps are rated to 2.85v instead of 2.7v like all of the other caps Maxwell sells. If you could get away with wiring 5 in series instead of they typical 6, then you wouldn't have to buy as many, and the capacitance would increase.

5x of the 3400f caps comes to $400 , but 6x of the 3000f comes to $384. You might be thinking that $16 more for a measly 13% increase isn't worth it, but the actual increase is 36%! The total series capacitance of 6x 3000f capacitors is 500f. The total capacitance of 5x 3400f capacitors is 680f. On top of all that, the 3400f cap has the same dimensions as the 3000f unit.

Here is the risk; 5x 2.85v only gives 14.25v. My alternators all put out 14.4v, so I would be over-volting each capacitor by 0.03v. Perhaps that is close enough to not cause problems, but if not, it would be an expensive science lesson.

I'm considering shelling out the dough to see if I can eliminate the battery altogether on my truck.

[Enki]

I'd still do 6, or at least set up a balancing system that will bleed the extra voltage off (diodes would be best).

Also, I have the strangest nerd boner right now.

Edit: Keep in mind that your over voltage figures are assuming that each cell charges equally; this will never be the case, unless you have some seriously high current capable balancing electronics.

[redpoint5]

Originally Posted by Enki

I'd still do 6, or at least set up a balancing system that will bleed the extra voltage off (diodes would be best).

Also, I have the strangest nerd boner right now.

Edit: Keep in mind that your over voltage figures are assuming that each cell charges equally; this will never be the case, unless you have some seriously high current capable balancing electronics.

Balancing would certainly be necessary, and I would probably resort to LED/diodes to do the work. I still need to look at the balance boards offered on Ebay. Perhaps they are more efficient, but I don't know if they would be good for 2.85v.

The other things I'm thinking is that the caps would only be over-volted when the car is driven, meaning that the majority of the time when the vehicle is sitting, the voltage would be trimmed to safer limits. Perhaps a very small over-volt for short duration is acceptable?

It would be nice if Maxwell had some 2.85v rated 350f caps to test on before shooting for the moon.

[Enki]

In my mind one of the draws to using supercaps is the life expectancy; sadly, this goes down considerably when charged to full (and especially more). I honestly think you'd be better off burning that extra voltage off as heat, or having the alt rebuilt to only put out 14 volts flat, or having an adjustable regulator put in to allow you to tune it.

[redpoint5]

Originally Posted by Enki

In my mind one of the draws to using supercaps is the life expectancy; sadly, this goes down considerably when charged to full (and especially more). I honestly think you'd be better off burning that extra voltage off as heat, or having the alt rebuilt to only put out 14 volts flat, or having an adjustable regulator put in to allow you to tune it.

I have been wondering how difficult/expensive it would be to modify the alternator to output 14v. Perhaps it would be as easy as installing a power diode in series with the alternator.

[Enki]

In my Mazda, the voltage output of the alternator is adjustable; on a cold start, it will hit upwards of 14.2 volts to flash charge the battery, then holds steady around 13.8. I'd imagine you can either have it rebuilt to use an external regulator or buy a kit and do it yourself, somehow.

[redpoint5]

^^Lucky^^

Is that a Skyactiv motor?

I have a ray of hope of getting this to work, and survive for a long time. From the datasheet:

Absolute Maximum Voltage 3.0 V

The leakage current seems to be much higher though.

[Enki]

No, I have the speed 3 (MPS 3 overseas). The regulator might be computer controlled, or have a delay in it; not quite sure.

By the time I get around building the bank for my project car, they may have 4k farad caps out, or the carbon nanotube capacitors with chemical battery like densities might be available.

As a side note, I don't recall if I said this in another post, but since the charge rate on these things usually follows a curve that drops off as the voltage increases (across the cap) then a viable balancing solution might just be to run a bunch of them in parallel.

For example, if you have 3 banks of 5 caps, the charging rate across each row of 3 caps should balance out themselves based on the differing properties/charging rates of each cap; they should also leak down to a similar level, and probably a bit more quickly. One thing I noticed in my testing was that after the caps leaked down and settled, when recharged to full voltage, the leakdown was nowhere near what it was on the first charge up from 0 volts.

This is something I'll probably have to test, but it's worth looking at IMO. Maybe I'll pick up 2-3 banks of the 350s to play with.

[ECONORAM]

Originally Posted by redpoint5

I have been wondering how difficult/expensive it would be to modify the alternator to output 14v. Perhaps it would be as easy as installing a power diode in series with the alternator.

I am guessing this would only work within a certain RPM range? I say this because after installing an under drive crank pulley, my alternator's output is about a volt lower (just by reading the AeroForce gauge) at idle. Could the voltage output be adjusted higher? Or could one of these capacitor banks hold the voltage up at a stoplight...long enough until the engine went off idle?

[oldtamiyaphile]

Originally Posted by ECONORAM

I am guessing this would only work within a certain RPM range? I say this because after installing an under drive crank pulley, my alternator's output is about a volt lower (just by reading the AeroForce gauge) at idle. Could the voltage output be adjusted higher? Or could one of these capacitor banks hold the voltage up at a stoplight...long enough until the engine went off idle?

I suspect your alt is just spinning too slow to make more than 13v, the regulator's not limiting it's output, there just isn't enough of it.

The supercap won't make a difference either, I just tested a 350uf bank on my Jeep, the total charge capacity is around 0.2Ah, your battery is probably around 100Ah.