Capacitors, can they help a lag problem?

You can try and and see if it will help, but if my calculations are correct it will make no noticeable difference.

Energy stored in capacitor = 1/2 * c * v^2
where c is capacitance in farads, and v is voltage, and energy is in joules (if I remember right).

1/2 * 1 * 24^2 = 288 joules, or 288 watts for one second. While this might seem like its at least a start, its really not very much, as this is down to 0 volts, and you will only want to discharge it maybe 5 volts. This means...

(1/2 * 1 * 24^2) - (1/2 * 1 * 19^2) = 107.5 joules per capacitor, so 215 joules down to 38 volts (for two in series). Carry this over 5 seconds and you have 43 watts to supplement your main pack.

Any idea how low the voltage is sagging?

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I think Andrew is right. A reasonable sized capacitor, like 1F, would give you enough juice for a few seconds before the voltage dropped to the point where it couldn't help anymore.

It sounds like the batteries might not be up to the discharge rate you're asking of them. Also check for loose wire connections, this is a very common cause of voltage sag.

If the batteries themselves are sagging, you may need to try different batteries.

How many amps are you pulling?

What would help is a bi-directional DC-DC converter between the caps and battery so the cap voltage can swing through a wider range. That could double the power to 86 watts over 5 seconds. Still not much.

Some ideas:

Switch a booster battery (can even have a bad cell) in the circuit once you get up to speed. This must bypass the controller.
Heat the batteries via. fast charging/electric blanket. Fast charging can be done easily by using light dimmer and rectifier (haven't tested doing this myself though).
Do run, and measure voltage of each battery individually. Find bad one and add small amount of water or replace.
Do few more cycles. If your batts are new they will be sluggish.
Add Powercheqs as people have noted an improvement in performance here.
Up the wire size
Is motor controller cutting back due to heating? Check motor volts/amps and battery volts/amps and compare.
Pre-cool the motor (s) to lower resistance
Find any points of high resistance in circuit, such as bad/corroded/dirty/pitted contactor
Check motor brush condition and spring condition. Replace or break in brushes. Replace springs if needed. Clean commutator, and machine if necessary.
Inspect motor wiring. Smell for burning or look for damage.

Sorry the list got so long!

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Question: I'm planning on doing a 72v motorcycle conversion. I've been giving thought to adding three 80 farad capacitors running @ 24V each. Does anyone think this could hepl, particularly with intial takeoff and rapid acceleration? Or is it just a waste of money. Any thoughts would be great. The batteries I'm planning on using are B&B 50amp hour batteries.

just a note:

When you connect capacitors in series the capacitance will drop. In your case 3 - 80 farad capacitors will yield 26.667 farads.

Here is a great site which shows some important facts about DC voltage.

DC Theory

ILBCNU
V is for Voltage
Moderator Captain

Jazzyrigged,
Thats a load of capacity! How much do the capacitors cost? How much do they weigh?

I'll need to know the DC resistance of the capacitors. Can you link a spec sheet for them?

To give some kind of idea, without taking resistance into account, than with three caps in series, they can provide the following amount of power down to 55.8v (how low the batteries will sag to at 450 amps assuming .006 ohms DC resistance):

Energy stored in capacitor = 1/2 * c * v^2
where c is capacitance in farads, and v is voltage, and energy is in joules

(1/2 * 80 * 24^2) - (1/2 * 80 * 18.6^2) = 9201.6 per cap * 3 caps = 27604.8 joules. This means 27604.8 watt seconds. Carry this over 6 seconds and it is 4600.8 watts. The batteries will be putting out somewhere around 25110 watts at 450 amps. This means that the batteries in parallel with the caps could put out 29711 watts for 6 seconds without taking resistance into account.

For comparison, if you spent a bit more for some better batteries such as Hawker Odyssey PC1200 than you could get 26932 watts out of them with no supplement capacitors at 450 amps.

Also for comparison (since you would probably spending a lot for those caps) you could go to a 96v system which will yield a lot more power out of the motor and batteries. At 96v the above hawker Odyssey batteries would peak 35910 watts at 450 amps.

Additionally, since your ultimate goal is more power (or so it seems), you could throw on an extra etek or perm and use a higher amp controller. LogiSystems sells some high amp controllers, but I don't know how long they will last before they will blow up.

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Avatar taken from http://www.electricmotorbike.org/
Anyone got one they might want to sell?
My KZ750 Project: here E.T.A. 1 mo