## Boosting the peek current of a battery by super capacitors

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Hello everyone,

I am having one specific problem, and I can not think of a good solution.

I would like to boost the instant power of a 24V battery, by putting 10 super capacitors in series.
Super capacitor specifications are: 2.7V, 30F. Tolerance -20, +80%.

The problem is, since the tolerance for capacity can be from -20% to 80%, that somehow I must ensure that voltage on each capacitor must not exceed 2.5V.

The simplest solution I was thinking about is to put the zenner diode of 2.5V in parallel with each capacitor. But I can not find on the web zener diodes which would be reliable enough, because I would pick the one that can take at least 1Amp of continuous current, and about 5Amp peek.

The maximum current of a battery is about 7 Amps when short circuited.

Do you have some suggestion about some diodes I am not aware of, or maybe you have a totally new original idea for this problem.

Every help is appreciated.

Thanks,
Stypie

The guy solves it, but the maximum current is 300mA... Can I increase it by putting a stronger FET in this circuit?

I will probably use 12 instead of 10 capacitors, since the voltage of 24V battery can be 28V.

You've seen this solution at Dave's Circuits? Ultracapacitor Voltage Limiting Circuit

I've been considering something similar for controlling charging of a four LiFePO4 cell battery.

:)) I was just investigating it.
Very interesting aproach.

This operational amplifier basically serves as a comparator of voltage as I see it, and then it turnes on the FET if it's too high.

The only thing I need differently than this schematics is larger current. This is limited to 300mA.

For larger current of let's say 4 amps, I would need a stronger FET, and a smaller resistor which can dissapate more power, let's say 0.4 Ohm, 7W...

First of all, what battery type are you using? 12 volt lead acid batteries have an internal resistance of something on the order of 50 milliohms. A 24 volt battery consisting of two 12v batteries in series would double that, or about 100 milliohms. The specs I found online for supercapacitors indicated an ESR on the order of 30-100 milliohms each, meaning that your stack of 12 capacitors might have an order of magnitude less peak current capacity than the battery does. What's the peak current you need, and for how long a period of time does the peak have to last? Also, how tight a tolerance do you have on the battery voltage? There might be some high current R/C battery packs out there that could meet the needs without any extra circuitry.

Well, you got me wondering. Therefore I did more testing on the battery...

First of all I need to tell you a little bit about my system.
It builds upon pre-existing system which has two modes of operation. It can either work on AC rectified to 28V, or AC+Battery (then it can give more current).

My application normally need only 0.5Amps, but sometimes can draw peek current of 12Amps for less then 2 seconds, and in that case the voltage should not drop under 16V.

Here are the measured the internal resistances:
-Seams like the battery has internal resistance of about 1.1 ohm. Since it is older 3ah lead acid battery it is understandable. (measured voltage drop from 25.8->22.8 over 8.4ohm resistor).
-When working without battery the resistance of rectified power source is 5.7ohms (voltage drop from 28.8V->14.8V over 8.4 ohm resistor).

I would like to use those super-caps for several reasons:
1) It is obvious that new system will not work nicely on this power source.
2) Buying a newer/bigger battery is not suitable for me because I may not open the battery container. Also I can not guarantee how long will battery be good enough.
In contrast, super-caps can last for 10 years with no problem.
3) I would like it to be able to work without the battery to boost the rectified AC power..
4) This power booster should be as small as it could be.

Check out the boostcap modules at Maxwell Technologies. I think I have a Good Idea for an electric car acceleration booster using a 48V boostcap module and a Big Contactor in a 48V system like a golf cart. The boostcap module is connected in parallel to the battery with the contactor deenergized. When you step on the juice pedal, the contactor flips the wires on the boostcap module to put it in series on top of the battery. The module has a capacity of several KiloWatt-seconds... an extra ooomph to get up to speed. When you back off the pedal, the cap bank clacks back in parallel to recharge.

Imagecraft compiler user

Thanks for the info. I think these modules are too big & expensive for my application.

I basically need only 3F, on 30V...

ps. A good idea for red "turbo" button on a car :D

Can't you use some low-voltage high-F capacitor with boost circuit ? This eliminates the problem of your supply voltage falling down. I have here an Epcos 120F/2.5V supercap. Probably this solution will be smaller than using 10+ supercaps in series.

Stypie wrote:
Thanks for the info. I think these modules are too big & expensive for my application.

I basically need only 3F, on 30V...

And you expect this to be small? Good luck with that!

Small supercaps aren't designed for surge rating, sure they store a lot of energy but they're designed as battery backup for SRAM so the current you can draw back out of them is low.

You need a "farad" capacitor designed for automotive stereo use, as these are designed for high current. They're also not small, but such is life.

Stypie wrote:
My application normally need only 0.5Amps, but sometimes can draw peek current of 12Amps for less then 2 seconds, and in that case the voltage should not drop under 16V.

If you want to supply 12A with a voltage drop of 12V over 2 seconds, then you need at least 2 Farad. http://en.wikipedia.org/wiki/Farad

The closest I know are the capacitors used to improve bass response for car stereo amplifiers. http://www.sonicelectronix.com/cat_i6_capacitors.html

Putting two of those on series (with an equalizing circuit, as discussed before) should work and is reasonably simple to assemble.

Markus

The maxwell D-cell size boostcaps are 250 Farads, 2.7V, can source or sink 150 amps, and are about \$20 each from Tecate. Those big old boogers the car audio guys use are only a puny 1 Farad, and they cost lots more.

Imagecraft compiler user

I don't think the Maxwells will be cheaper, they are much bigger, though !

The OP needs 28V, which can be reached by two of the car capacitors for below \$100 total. He will need 12 of the 250F maxwell caps (\$250 total), not including the circuitry for equalizing them all.

In addition the car caps are packaged/tested to be used by joe user and quite robust, mechanically and electrically. Also, he can find the car caps in any local car radio shop too, maybe a bit cheaper (my link was just the first hit from Google)...

Markus

I was just asked to look at a design. The complaint was that it drew a heavy current spike when it triggered a solenoid to activate a gadget. Tinkering around, I found that the solenoid triggered quite happily if I connected it to a charged 1000uf capacitor. So I put a 1k resistor in series with a capacitor and a fet to trip the solenoid. Of course, if it had to trip more than once every 5 seconds, I'd have needed a smaller resistor and more current draw. In use, it trips once every few minutes.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut.

The Moment Of Learning: You took a peek at the design to see why the current reached such a high peak.

Imagecraft compiler user

markus_b wrote:
I don't think the Maxwells will be cheaper, they are much bigger, though !

The OP needs 28V, which can be reached by two of the car capacitors for below \$100 total. He will need 12 of the 250F maxwell caps (\$250 total), not including the circuitry for equalizing them all.

In addition the car caps are packaged/tested to be used by joe user and quite robust, mechanically and electrically. Also, he can find the car caps in any local car radio shop too, maybe a bit cheaper (my link was just the first hit from Google)...

OK, the Maxwell caps are cheaper per Farad. (Trying to salvage something out of the discussion)

Imagecraft compiler user

I find those supercaps impressive. Big caps are in the mF (milli) range, now these supercaps have five orders of magnitude higher capacity. I still thought supercaps have a couple of Farads, but not 250. At least I got some learning out of it :-), thanks !

A back of the napkin calculation gives the equivalent of 20mAh for a 250F supercap, not that bad for a capacitor.

Markus

Electronic Goldmine has 2600 Farad 2.5VDC Maxwell Boostcaps in various configurations.
A single boost cap is on sale until May 9th at US\$10 each.

bobgardner wrote:

OK, the Maxwell caps are cheaper per Farad. (Trying to salvage something out of the discussion)

salvaging some ego?

I hope you at least calculated the cost per farad based on the effective capacitance after adding them all in series to get the required voltage rating.

I'll admit to some confusion. Maxwell is offering a 125V module for a couple of thou. The Farads is smaller but the energy is way up there. I just can't remember when to use CV or .5CV^2. Its a couple of kilowatt-seconds, which is ?? hp-seconds, which would be great for a speedup occasionally.

Imagecraft compiler user