Capacitor Banks

Molybdenum · Fri Jun 05, 2009 9:47 pm

I recently discovered the concept of capacitor banks on airsoft guns for the purpose of compensating for voltage drops at the back of the cycle. Having upgraded my gun to an m120, and thereby darasticly reducing the cyclic rate, I was looking for a cheap way to increase my ROF. I picked up 3 330 microfarad capacitors (3 cents apiece)and salvaged a 110 and 100 from old disposable cameras. I hooked them up in parallel, giving me a total of 1200microfarads. I plugged them into the gun, took the safety off, and pulled the trigger, only to experience a DROP in rate of fire. My handy dandy volt/ohm meter tells me 8.9 volts with everything hooked up, which seems odd because my battery is supposedly only an 8.4 v. Can anyone explain this to me, and what i need to do to get the desired result?

rp181 · Fri Jun 05, 2009 10:09 pm

voltage error is most likley your meter (is it a cheap one?)

The only thing I can thing of with the capacitor's not working is leakage. Try each individual capacitor seperate.

EDIT: did you make sure polarity was right?

Molybdenum · Fri Jun 05, 2009 10:20 pm

Polarity was right. I'm now finding that the capacitors take at least a few minutes to charge(Information from VOM). Unless I can make them charge faster, the capacitors are useless. Maybe if I had more of them though....

rp181 · Sat Jun 06, 2009 11:52 am

more would make it take longer to charge.

What power supply is charging these?

jimmy101 · Sat Jun 06, 2009 1:32 pm

Try a single 330 or 110 UF cap and see if that helps. Mutliple caps in series often don't behave as you would expect. Besides, ~1,000 uF is probably a lot more than you need.

dewey-1 · Sat Jun 06, 2009 1:58 pm

jimmy101 wrote:Try a single 330 or 110 UF cap and see if that helps. Mutliple caps in series often don't behave as you would expect. Besides, ~1,000 uF is probably a lot more than you need.

Whoops, typo error jimmy!

ramses · Sat Jun 06, 2009 2:07 pm

the capacitors should be in parallel with the battery. If the capacitors are standard 'lytics, they shouldn't take long to charge at all, and they hold only .042336 joules. you should look into a car stereo capacitor, they are multiple farads in size, but are physically large.

for more ROF, you could just buy a 9.6v battery, or solder another cell of the same capacity onto your existing battery.

dewey-1 · Sat Jun 06, 2009 2:52 pm

Molybdenum wrote: My handy dandy volt/ohm meter tells me 8.9 volts with everything hooked up, which seems odd because my battery is supposedly only an 8.4 v. Can anyone explain this to me, and what i need to do to get the desired result?

What type of battery pack?
7 cell ni-cad pack Ni-hydride pack?

8.9 volts is not that unrealistic in a "no load" condition.
That is basically 1.27 volts per cell. 1.2 volts per cell is the typical voltage at the rated mah rating load at initial use after being charged.

How old is the pack? How many times has it been charged?

It is possible that the internal resistance of a cell or several cells has increased.

If the pack can not charge some capacitors , assuming they are good and wired properly, it may be time for a new pack.

If you really want a good pack look into RC racing battery packs.
You get what you pay for.

When I was racing electric RC cars I would match the cells under 30 amp constant load. Sometimes it took 30 cells to get a 6 cell matched pack.

Molybdenum · Sat Jun 06, 2009 4:35 pm

dewey-1 wrote: What type of battery pack?
7 cell ni-cad pack Ni-hydride pack?

Its labeled a 8.4V 1100mAH Ni-MH, it has been charged / discharged about ten times, and is about a year old.

I replaced the little capacitors with one big one and found that the voltage drops on both semi and full where significantly less than without the capacitor. Despite this the ROF remains unchanged.

Pic of whole setup: I'm not going to bother making a housing tell I get it to work

Unless I'm mistaken, thats 14000microFarads

Sat Jun 06, 2009 5:00 pm

If your ROF remained the same after all changes, I'd say a mechanical change is needed to change the ROF. A gear change or shorten the stroke on the loader mechinisum...

Put up a picture of the system intact and I will point out some mods you can make, all I need is a look see.

Molybdenum · Sat Jun 06, 2009 5:52 pm

jrrdw: Thanks for the offer, but a mechanical modification would be much more expensive. Go to mechbox.com to find an animation of the mechanical portion if your still curious.

jimmy101 · Sun Jun 07, 2009 1:24 am

dewey-1 wrote:
jimmy101 wrote:Try a single 330 or 110 UF cap and see if that helps. Mutliple caps in series often don't behave as you would expect. Besides, ~1,000 uF is probably a lot more than you need.
Whoops, typo error jimmy!

Multiple caps in parallel ... hence the 1,000 uF.

Capacitor Banks

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