Stiffening capacitors useless for ebikes?

[vanilla ice]

Soo, I've got these 15v 1/8 farad capacitors collecting dust in the garage. In a past life they were used for a car stereo system. Can I put them to use again? If so, how do I wire them up? I understand the draw is very high, basically like a short, which can damage batteries. Any way around this?

[fechter]

The controller usually has a real wimpy version of these built in. They would go across the battery terminals of the controller. You would need to put several in series to have enough voltage rating for the batteries you are using.

There is an extremely high current when the connection is made to the batteries. This won't hurt the batteries, but could blow any fuses in line or kinda burn up the contacts where you make the connection.

The way around this is to precharge the capacitors with a resistor before making the connection. I use a precharging circuit on my scooter and a large relay to make the connection.

I don't know how much improvement it would make. Probably not much. In theory, they should help improve the efficiency of the controller at partial throttle and possibly reduce the peak of any voltage spikes in the system. It would make more of a difference in a very high power system, just like for car stereos.

[The7]

I've got these 15v 1/8 farad capacitors

Wow. 1/8F = 125000 uF
A very large capacitor.

[vanilla ice]

I think I have like six of these left. Yes they are quite large for caps, but surprisingly light weight. Even in my car stereo days I was frugal (cheap), so I bought 8 electronic surplus 1/8 farad caps and wired them in parallel rather than use one of the commercially available car audio 1 farad caps.

The resistor/relay thing is a bit over my head. Could you go in to more detail on this? Or would I be ok just pre-charging each with a 12v battery charger or something? How fast do capacitors discharge? Will they discharge down to nothing over night when I turn off my bike?

Even if they don't do much I may as well give them a new lease on life. They are so light, and I *AM* at partial throttle most of the time... I have not bumped up the voltage yet, so I can do 2s2p wiring on the caps for now. Mebbe 3s2p when I try for 36!

[fechter]

The resistor/relay thing is a bit over my head. Could you go in to more detail on this? Or would I be ok just pre-charging each with a 12v battery charger or something? How fast do capacitors discharge? Will they discharge down to nothing over night when I turn off my bike?

The capacitors will probably discharge to near zero overnight.

Below is a basic precharging setup. With those big caps, it might take 30 seconds or so to charge up. The value of the resistor is not really critical.

It would be interesting to see if you can notice any difference in performance.

[vanilla ice]

Ok I think I get what it does. For storage with no battery drain- I'm guessing I would need an additional switch between the resistor and battery?

Appreciate the diagram and help.

[fechter]

Yes, for long term storage, you'd want to completely disconnect the battery.
The drain from the controller is mostly leakage current in the capacitors which is very low, so you could leave it connected for like a week with no problem.

[bobmcree]

a 1 farad capacitor will discharge 1 amp for 1 second so a 1/8 will do 1/8 of that, and you will need to put several 15v caps in series to get enough voltage, so that will cut the capacitance by the number in series. let's say you put 4 in series to stand off 60v, the least you would want to use on a 48v system, you would end up with 33,000 uf. this is enough to discharge 10 amps for about .003 seconds. you will not see any significant reduction in voltage drop when you hit the throttle, except for the first few milliseconds. the caps just cannot store enough energy to run the motor for any significant length of time.

the caps in the controller are necessary to maintain a stable dc voltage for the fets which switch very quickly, and they are kept charged by a low impedance path to the battery. a precharge resistor will cut down on the inrush of current that occurs when you connect the battery, and will mostly eliminate the spark you see, which will save wear and tear on a power switch if you have one in the battery wire. in the controllers most of us use the caps are kept charged all the time and the switch controls a transistor circuit that provides power to the controller logic circuits.

supercapacitors that have 10 farads or so would provide some real output current, but only for seconds at a time. they are just not a good substitute for a better battery.

in a car stereo the big capacitor helps provide the peak current for transients that require much more than the base line power, so the headlights will not dim on the bass notes, but these peak demands last much less than a second. most people these days use at least a 1 farad 20v cap, and one of those caps is bigger than a beer can. even a sixpack of them would make little difference in performance on a typical ebike.

the big caps on a car stereo power line also serve to protect the battery from the high peak current, which can shorten battery life by eating chunks out of the plates. the cap lets the current come out of the battery more slowly so the chemical reaction can be maintained at a safer level

[Dalecv]

This post made me go review some electronic theory.

I always knew to buy capacitors rated at or above the needed voltage but didn't recall that putting capacitors in series to match the voltage would decrease the capacitance.

[vanilla ice]

So I guess the answer to the thread title is "Why yes, yes they are useless." As I thought, but wanted to check first. Here they are any how, managed to find four in the garage tonight.

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[xyster]

So I guess the answer to the thread title is "no." As I thought, but wanted to check first. Here they are any how, managed to find four in the garage tonight.

Wow. Be careful not to drink from the wrong one!

[Dalecv]

Well the blue can would probably taste like a 9 volt battery for about .03 seconds and then you would speak with an Australian accent and pedal your electric bike at over 120 rpm.

[fechter]

The capacitors built into some controllers are inadequate, so an external one would help reduce heating and losses in the capacitors. But those are sort of overkill for that application. They look cool, though.
Funny, I think I have some that look exactly like that.

[The7]

Do you mean these used on my mono block MOSFET amp?