# Capacitors for replacing batteries discussion



## ElectriCar (Jun 15, 2008)

This is an interesting subject and I've yet to see much discussion anywhere other than the thread on this forum of using caps at the controller to smooth out the sag from the controller pulses, not really measurable but there nonetheless. The sooner we kick it around the more ahead of the curve we'll be. 

Ever since reading of the promise of EEstor I've been hooked on the idea. Since I'm very familiar with capacitor use in power supplies and electronics, I know they charge about as fast as you can deliver the juice. If we're ever going to use all electric with no long distance assistance, we're going to have to go to something like capacitors. Who is going to sit at some roadside recharging station for an hour or two every 2-300 miles???

For performance applications, NO battery will compare to the discharge capacity of a capacitor! Those things can dump massive amounts of power instantaneously just as they can recharge super quickly.

I'm ready to start experimenting with ultracaps. Maxwell has some but they're rated at like 3.2V or such so they would have to be series ganged to reach operating voltages we use. So to replace one battery I'd have to have 3 of these for charging purposes. As of now I've seen prices of about $130 USD each for 3000F caps. With 3 in series I'd have a rating of 3000F at 9.6V at a cost of about $400. I'm disregarding prices for discussion purposes as I expect prices to come down as with everything. 

My question is, how many Farads do I need to replace the .8Kw in my battery adjusted for puekert effect? <B>That's the one thing I've struggled with, the formula to calculate how much capacity a given device can hold. The formula is a little difficult for me to decipher. I was a little up in smoke in algebra class...</B>


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## major (Apr 4, 2008)

ElectriCar said:


> My question is, how many Farads do I need


 
Some things were addressed here

http://www.diyelectriccar.com/forums/showthread.php/ultracapacitor-eye-candy-25994.html


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## Zemmo (Sep 13, 2007)

Another thing to know about capacitors in series is the capacitance drops by N number of caps in series. So in your example it takes 3 to get to the right voltage so the 3000 F caps would only be 1000F in series. So if you had a 120 volt pack that would take 40 of those, so 3000F / 40 is only 75 F at 120 volts. See how quickly the capacitance drops, it takes a lot of caps.


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## ElectriCar (Jun 15, 2008)

Zemmo said:


> Another thing to know about capacitors in series is the capacitance drops by N number of caps in series. So in your example it takes 3 to get to the right voltage so the 3000 F caps would only be 1000F in series. So if you had a 120 volt pack that would take 40 of those, so 3000F / 40 is only 75 F at 120 volts. See how quickly the capacitance drops, it takes a lot of caps.



I forgot about that Zemmo, you're exactly right. Capacitors in series are like resistors in parallel formula wise. I had electronics theory and all that stuff in 1981. That was in the last century so you'll have to forgive me for slipping!


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## Zemmo (Sep 13, 2007)

I happen to have another thread were I am investigating computer grade caps for battery improvements. So it happens to be recent for me. But I am looking at it for the fact of improving battery life and maybe a little performance (very little), not for replacing batteries.


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## ElectriCar (Jun 15, 2008)

deleted msg


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## Zemmo (Sep 13, 2007)

Thanks for the help. I actually got that fixed, I probably just never mentioned it. I got a cheap scooter on Craigs List and tore it apart for the parts. One of the parts happen to be an LED light flasher. It plugged right into my socket and it worked. I got the scooter for 5 bucks and used its parts all over!  But thanks again for the help.


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## ElectriCar (Jun 15, 2008)

major said:


> Some things were addressed here
> 
> http://www.diyelectriccar.com/forums/showthread.php/ultracapacitor-eye-candy-25994.html



So major, looking at your thread, technologic did some calculations and came up with some numbers saying that basically using ultracaps for battery replacement wasn't practical. I still don't understand the math. Obviously you do. 

Can you break it down a little more? This is a totally new application for capacitors for me. My algebra regarding capacitors was limited to capacitive reactance, Xc=1/(2*pi*fC). Never really dealt with joules etc. and 1F capacitors didn't exist that I know of in my school days!

I now run a 144V pack with 24 6V batteries. My battery is a 232Ah unit so it packs about 1.4Kw and after adjusting for peukert only about 800W. So what would I need and how would you calculate the answer to replace one battery. 

From what I've seen, a cap to hold the 8.2V my charger will dish out during the equalizing charge would require 3 2.7V caps. The cost I've seen is for a 3000F cap. Three in series would yield a 1000F 8.1V cap pack at a cost of $390 USD. How many kwh would that hold???

Now if my thinking is correct, when charging with a DC source, once the cap charged to the charging voltage it would halt charging as current flow would stop correct? Agree or disagree?


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## icebeater (Jan 21, 2009)

*considered batcaps?*

Have you considered Batcaps? This thread is probably a good place to rip apart this idea ....

"BATCAP© is simply a battery that can discharge just a s quickly as a capacitor."
12 V, 50AH (600WH)
Continuous Current: 1000A
3 second Current: 2000A
1500 cycles at 30 % (zero maintenance)
Weight: 34lbs (the only problem I see with these things)
"It also recharges faster than a conventional battery since the internal resistance is low."

I contacted Xstatic - they say that BatCaps work well in series - and would benefit from balancing like any other ev battery. They will ship bulk orders direct at $155 per Batcap 2000 (about $100 saving over retail!) with a minimum order around 22 batcap 2000's (I only need 10 - contact me if you're interested in a group buy.) They recommended Manzanita Micro for BMS & chargers etc so I plan to contact them next to get their opinion on these batteries. 

I'm thinking that using batcaps for peak power could mean that I can save $$$'s on high-energy LiFePo4 batteries for range extension. The two benefits of this approach are the ability to add (cheaper) low-AH LiFePo4 modules as I can afford them but with full performance out of the motor from day 1 ... and the truly high-power LiFePo4 batteries are a) expensive and b) often only rated at 1C to 3C continuous.

I also like the idea of a quick-charge option that could take me 18 miles (85% eff. X 70% DoD X 50AH X 120V / 200 WH/mile)


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## DIYguy (Sep 18, 2008)

Interesting. What is the Ah rating of the 8400 model? I assume the 30% is a D of D number? I wonder what a couple of these in series would do for a conventional battery pack.


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## icebeater (Jan 21, 2009)

DIYguy said:


> What is the Ah rating of the 8400 model?


The 12V 8400 should be around 210AH - it seems to have much better energy density than the 2000 but 10 of them would still weigh 700 lbs! I guess it could work for truck conversion. I'd like to see the controller that can use those 8400 peak amps (or 4200 continuous!) 



DIYguy said:


> I wonder what a couple of these in series would do for a conventional battery pack.


I'm no battery expert (and would appreciate a second opinion on this) but I think you'd need to run a full pack of these to get the full power out of them. You wouldn't put them in series with conventional batteries - but rather run the conventional battery pack in parallel with the batcap pack (which is also why I'm looking at the 2000's instead of the 8400's). With the traditional battery in parallel, low-power operation doesn't drain the batcaps as much but you have their full power available when you need it. Much like you'd connect a normal supercapacitor across a battery. It just sound like these batcaps are simpler to manage in series than a bank of supercaps are.


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## GTWCMT (Jan 22, 2009)

This is a good idea, I would like to chuck something in to ponder when you guys are thinking about this.

Could you have a bank of caps with a step up invertor or booster cap so that you had more volts for pulling away from a stop and limiting draw on the batteries?

Could the power system take it? with most controllers working in a limited power/current range.

I have seen a bank of supercaps that fed more caps to give the desired effective current, not sure how it would work.


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## rmay635703 (Oct 23, 2008)

Aren't Batcaps just a more complex lead acid battery?

I would guess you couldn't discharge more than 20% before you would destroy one?

Am I wrong?


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## Bowser330 (Jun 15, 2008)

rmay635703 said:


> Aren't Batcaps just a more complex lead acid battery?
> 
> I would guess you couldn't discharge more than 20% before you would destroy one?
> 
> Am I wrong?


http://www.batcap.net/2000.html

It says "1500 cycles at 30%" which i think we are translating to mean, "1500 cycles at 30% DoD" meaning you can draw 70% of the energy out of them without destroying them...which would be 35AH...

24 of them would cost 155 ea = $3720 
144V, two strings total 100AH, 70% DOD = 70AH usable (40miles?)
Total weight 810 lbs.


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## major (Apr 4, 2008)

ElectriCar said:


> I now run a 144V pack with 24 6V batteries. My battery is a 232Ah unit so it packs about 1.4Kw and after adjusting for peukert only about 800W.


Hi ElectriCar,

It appears that you multiply Ah times 6V to get 1.4kW (power). Why? I don't understand your math, or motivation for looking at caps. A 6V, 232Ah battery is certainly capable of much greater power than 800W. 



> So what would I need and how would you calculate the answer to replace one battery.


Well, first off, you cannot just replace one of the 24 batteries in the series string. You could replace all 24 batteries with a set of ultracaps of equal voltage (144). But you would have very short range. BYU did this in a drag car. They get a solid 1/4 mile range. They trade distance (range, energy) for power (acceleration and speed). Probably not what you're looking for.

Or, you can supplement the battery system with ultracaps. But you need to supply the power from the UCs (ultracaps) at system voltage (144). This either requires the UC bank to be rated at total battery voltage or have a voltage conditioner device in front of the UCs, a DC/DC buck/boost converter.



> From what I've seen, a cap to hold the 8.2V my charger will dish out during the equalizing charge would require 3 2.7V caps. The cost I've seen is for a 3000F cap. Three in series would yield a 1000F 8.1V cap pack at a cost of $390 USD. How many kwh would that hold???


Use energy = one half CV squared. Comes out to 32805 Joules. Or 0.009 kWh.



> Now if my thinking is correct, when charging with a DC source, once the cap charged to the charging voltage it would halt charging as current flow would stop correct? Agree or disagree


Yeah. If you have a voltage source of 2.7 volts across the cap, it will charge to 2.7 volts and current will go to zero.

Hope that answers some of your questions. I do not think UCs are reasonable to use in battery powered EVs unless you have some big reason for them, like regeneration. I suppose that just running a set of UCs in parallel with the battery pack will "stiffen" the battery, meaning that the sag under load will not be as great. This would be a way to smooth out the high current peaks on the batteries. But by the time you analyze that cost and weight, you'll probably be ahead just buying a better battery.

I used the UC in a hybrid vehicle. The UC was the only electric energy storage device. No batteries. It worked very well. In my opinion, better and less cost than any available battery.

Regards,

major

ps........Hey ElectriCar, check out http://www.metricmind.com/ac_honda/main2.htm Click on "Ultracapacitors Stack". Example of a battery/UC combo.


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