6 Farad Capacitor as scooter battery?

Gary · Post by **Gary** » Mon May 01, 2006 10:51 pm

I tried hooking up a 6 farad capacitor <http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... %3AIT&rd=1> to an old Zapworld.com scooter, but it doesn't seem to work at first crack.

The capacitor is 12-24 volt capacity, and has an automatic shutoff after 1 minute without any use. The directions say it will power up (light decorative lights, enable discharge of capacitor, and enable digital voltage display) when it senses a voltage drop of .5 amps. There is also a contact to turn the capacitor on when a third contact is connected to a battery presumably. There is automatic protection against reverse polarity also.

The scooter on the other hand, is normally in the off position. When you push the scooter to start, and hold the "on" button down, the motor engages.

On the first try, Sunday, with the capacitor replacing the old worn out battery, I couldn't get the motor to go. Also tried charging the capacitor with a battery charger for a moment till it lighted up, and then try to push and engage the scooter. It seemed for the briefest instant to try to go, then coasted. Further attempts then no good. Charger again, and brief pulse of motor, then dead.

Any ideas? Is this a futile attempt, or am I just going to have to do the grunt work and slowly analyze the circuts involved?

JPKNHTP · Post by **JPKNHTP** » Mon May 01, 2006 11:00 pm

-JPKNHTP

jwax · Post by **jwax** » Tue May 02, 2006 4:09 am

At 12 volts, you've got a measely 432 joules of energy stored.
A very little ride's worth.

Sambuchi · Post by **Sambuchi** » Tue May 02, 2006 4:56 am

I can think of lots of reasons why it my be doing this but how about telling a little more about the motor of the scooter that is acting as the "load".

How much power does the motor draw in watts?

P = V I

P = I^2R

you said (At 12 volts, you've got a measely 432 joules)

watts = joules/seconds

This could tell you why you are crapping out after a couple of seconds

Do you have another 6F cap? put it in parallel and see what happens..

Good Luck.

dyarker · Post by **dyarker** » Tue May 02, 2006 6:35 am

Lets see ...
watts = joules/seconds
1HP = 755 watts
1/4 to 1/2 HP to move an adult at scooter speed
432 Joules
Yeap, looks like 1 or 2 seconds of "go".

Capacitors don't even come close to sealed lead acid batteries for energy density by volume. Other battery types beat lead acid by weight. To do better than lead acid by volume you have to burn fuel.

Mind if I ask who gave you idea a capacitor would work? If you thought of it yourself, no problem, no you know why.

Gary · Post by **Gary** » Tue May 02, 2006 9:00 am

Now I know why. Never was that good at math and thought we'd wing it this time. Was hoping 6 Farads would at least be a sizable ride. 100 mfd used to seem like such a large capacitor, I was blinded by size and let hope take over. Motor is probably 200 or 300 watts. Eventually wanted to use it on an old 24v moped as a way to allow deep discharge lead batteries to give up power more slowly. Back to the old drawing board...

Have the balls to try a wild idea, but don't always have the joules to make it work... Will have to study the math more closely.

ps
The instructions mentioned ",5" amps as the trigger. Was assuming they were using the European convention of a comma for a decimal, even though the hardware was probably from China.

Chris Smith · Post by **Chris Smith** » Tue May 02, 2006 10:34 am

These caps arent designed for this. They are storage tanks capable of dumping all their load quite fast, for a fast response when needed.

jollyrgr · Post by **jollyrgr** » Tue May 02, 2006 4:58 pm

Battery for motor GOOD. Capacitor for motor BAD.

The energy density stored in a capacitor is not going to equal that of a battery (at least not with current capacitors). That cap would do great to store CMOS power, drive an audio amp during peak swings on sound levels, and would definately work out great for a bunch of LEDs for light output. You could, depending on the scooter motor, use the motor as a generator to recharge the caps by pushing the scooter. Then use the stored energy to power a bunch of LEDs.

Now to explain your scooter issue. How long did it take to charge your scooter's battery when it was new and working? Let's assume you had a 7 AH (Amp Hour) battery at 12V. If you put a 1 Amp charge on it this would take seven hours to charge the battery. Probably a little longer but let's make it simple for now. How long did it take to charge the capacitor to 12 volts with the power supply? Likely a few seconds. If you charged it with a battery, there would still be plenty of capacity in the battery after the cap was fully charged. So what does this mean? The capacitor can not hold the amount of energy available in the battery!

Sorry, current day capacitors will not have the capacity of a battery. A battery will never (using current design trends) have the capacity of a gasoline engine.

Chris Smith · Post by **Chris Smith** » Tue May 02, 2006 5:37 pm

Its physics and chemical speak, .....Dr Watson.

Caps have a low on resistance,.... meaning they can dump or absorb their load faster than any winding, power supply, circuit, or battery.

That is their main advantage and usage.

They are mostly a shock absorber, amongst many other usages.

jwax · Post by **jwax** » Tue May 02, 2006 6:55 pm

Well Gary, you had the jewels to try and make it work, the cap was just short on the joules!

Chris Smith · Post by **Chris Smith** » Tue May 02, 2006 7:32 pm

Message

MrAl · Post by **MrAl** » Wed May 03, 2006 2:18 pm

Hi there,

Well, you might get more pickup with a large cap like that
because the motor will start faster. How much faster depends
on the wiring ie how long the leads are from the battery to the
motor and the wire gauge.

T&L Publications

6 Farad Capacitor as scooter battery?

6 Farad Capacitor as scooter battery?

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