Audio/Battery Question

A batcap is quite literally the love child of a battery and a capacitor. Effectively, it ends up leaning far more heavily to capacitor than battery, as they can be made as power as 100 1-farad capacitors (You read that right), while still possessing enough capacity to run the vehicle and start it in the morning. The trade-off, of course, it that unlike a true capacitor, they cannot expel all their energy in one go, and unlike a battery, have pretty poor sustaining qualities (As I mentioned, the one I want is only a 20Amp-Hour reserve. Eighty is usually considered dangerously low, to put that into perspective). Audio guys love them for their ability to throw more juice out on demand than batteries. I love them for the fact they're approximately 1/4 the weight of a lead acid battery, and about 2/3 the weight of a gel cell battery. They're also splash-proof.

I know that device as a super or ultra capacitor. Of course, it may be a bit different, because the ones I was looking at were 20F and above, and could be drained right down. Also, charging time is limited only by thermal characteristics of the unit (In other words, very high current charge) yeilding charge times of less then 5 seconds from a source capable of it.

It's starting to be the solution to wasted brake energy in hybrid vehicles because they can absorb large amounts of charge in small amounts of time (Unlike a nicad/nmh, etc), and they are light as you're said (Weight is the enemy of efficiency in a vehicle).

I was looking at a bank to run my electric bike for 30 minutes at 6A, 30V and it was going to cost $2500. I think you'll run into the same cost prohibitive numbers if you look at the capacity you'd need.

Lets examine a starting situation:

 Let's say you want 500A for .5 seconds, then 150A for 2 seconds, and you need the voltage to stay above 12V. Let's assume you charged them to 14.5 volts from the car's system.

cap current = C*dv/dt (D = change in)

rearanging, dv = (dt*i)/c. For the 2 stages, you have to seperate two occurances:

 

Dv = (dt1*i1)/c+(dt2*i2)/c Where dt1 = .5sec, i1 = 500A, Dt2 = 2sec, i2 = 150

For DV = 2.5V For (14.5V-12V),

so 2.5V = 250/c+300/c, 2.5 = 550/c

c= 220F

500A and 150A may be pretty marginal. I'm not sure what the starter actually draws...

How much is 220F worth for the devices you are talking about?

 

A couple more things to think about...why not diode isolate the cap unit from the battery, and run the stereo off the caps, rather than try to start with them.

If you run 15A on the stereo, and you had a 200F bank, and you could accept running between 14.5V and 12V it would last:1200 MINUTES.

The only trick would be to control the charge current when you charge them back up. It will max your alt out until they are charged, because they look like a dead short to voltages above what they are charged at.

 

For example, if the 200F cap was at 12V, the alt would have to pump it max current (Let's say it has 30A extra to charge the caps) for 600 minutes. You could limit this (Which is what i would definately do) with a power resistor. For example, to limit the current charge to 20A, and a max diferential voltage of 4V (Let's say you ran them down to 10.5V) the resistor would be 1/4 ohm. The power is high though, 100W.

The diode would have to deal with this charging current as well, but that can be had no problem.

 

 

Chay

Check this out:

 

http://www.maxwell.com/ultracapacitors/products/medium-cell/ bcap0350.asp

 

Chay

http://www.tecategroup.com/store/index.php?main_page=product _info&cPath=26_62_66&products_id=112

 

Looks like they thought about automotive too. Cost = unknown. 100F box, the size of a big shoebox.

 

Chay

Oh, and David, the first post of this thread has a link in it. That will take you to the exact model I want, which also has a description of what they are and what they do.