Common caps for two escs?

[Jesse J]

Is there any reason to not do this:

Twin set up, common battery bank... I'm thinking of putting the caps in the parallel harness, as opposed to dedicated set of caps for each esc after the split.

Thoughts? I'm not imagining a problem, but would like to learn from somebody else's smoke if there is a reason to NOT.

Thanks
Jesse

[CraigP]

The only thing that comes to mind and the reason I wouldn’t do it is because you are doubling the ripple frequency. These frequencies are not synchronized, so they will dead beat on each other and there will be periods where the ripple current will be double, which is pushing the caps too far. We are already pushing capacitors well into the danger zone on FE boats. I would use two different banks, but share a common ground. So in essence you would be making a 3-leaded capacitor Bank.

[Jesse J]

Hmmm, would you mind posting a schematic? The ripple factor is the part I was concerned about.
I imagined the caps as shock absorbers damping out the ripple. But I can see how ripple from esc A could go also to esc B and get everybody cornfused.

[CraigP]

Yes, I’ll post a pdf showing how I would construct it. I use copper plates in my cap banks to keep resistance low and help to cancel the unwanted inductance in the conducting elements. I’m going to be building a dual motor hydro this year, and will use this exact method..

[CraigP]

Could you share with me how much current you expect to run to EACH ESC? BTW, the battery will be split into a 3-leaded configuration to match the cap Bank. The drawing will show that...

[Fluid]

Caps should be as close as practical to the ESC, so use separate banks for each ESC. For high frequency ripple, more, smaller caps will work better than a few big ones. That said, I’ve had great success on SAW setups using three to five 1000 mF caps, each one bridging the two power wires rather than all of them on a pigtail.

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[CraigP]

Here's a PDF of the Cap Bank I would use. As Fluid mentions, it is extremely important to get it as close to the ESC as possible. Using a common ground, you can use on the BEC outputs from one of the ESC's to power the electronics. If doing that, be sure to isolate the second ESC's output by removing the middle wire on the signal in at the ESC. A capacitor I'm having great results with is this from Mouser Electronics:

P/N 661-EGPD500E362MM40H, 3600uF, 50V Aluminum Electrolytic Capacitor. This capacitor has the highest ripple current rating and lowest ESR than any other cap I've found in this range. This is suitable up to 8S operation. FWI, caps with higher voltage rating always have lower ESR than a reduced voltage counterpart. Always apply a 20% de-rate to the voltage spec of a cap, never use it at the defined voltage rating! I hope this helps you...

[TRUCKPULL]

I always wire my ESC's together, so each motor see's the 2P setup.

Would I wire the caps the same as your pic or differently,

Right now I have 3 on the board and 5 on a pig tail on each ESC. 6-S 2P (Turnigy 180 - V2's)

Larry

[golfito]

I received help to calculate the amount of microfarads and this went for my T180 v2:

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25353585_1771890409779359_4807697131141977009_n.jpg

[Jesse J]

Here's a PDF of the Cap Bank I would use. As Fluid mentions, it is extremely important to get it as close to the ESC as possible. Using a common ground, you can use on the BEC outputs from one of the ESC's to power the electronics. If doing that, be sure to isolate the second ESC's output by removing the middle wire on the signal in at the ESC. A capacitor I'm having great results with is this from Mouser Electronics:

P/N 661-EGPD500E362MM40H, 3600uF, 50V Aluminum Electrolytic Capacitor. This capacitor has the highest ripple current rating and lowest ESR than any other cap I've found in this range. This is suitable up to 8S operation. FWI, caps with higher voltage rating always have lower ESR than a reduced voltage counterpart. Always apply a 20% de-rate to the voltage spec of a cap, never use it at the defined voltage rating! I hope this helps you...

Thanks Craig, I think I'll do something similar, but bring the batteries in series. I'd like them to get drawn down together. This may require separating the cap banks completely. I'll post a picture when I get it wired.

[CraigP]

Sounds like you’ll be using a separate external BEC and isolate both BEC outputs from the ESC’s? You need to keep your ground potentials close between the two ESC’s, or you’ll form unwanted ground loops thru the control side. Sounds like you know what you want, I’m sure it will all work great!

[Jesse J]

Ground loops? Only one I know about is the one when my rocket with pontoons did when I was a kid.
Yes external receiver pack, and all very short wires. I've pulled the red power wire out of both controllers.

[CraigP]

Hmmmm, confused Jesse, you’ve got 5,573 postings on an FE forum and this is the first time you’ve heard of ground loops? It’s only the #1 cause of erratic receiver behavior. I’m finding that hard to believe...

[golfito]

Excuse me,
would the loop be a junction node between the ground terminals of both sets of ESCs / motor / battery?
I understand that on my boat the ground wires that come out of the ESCs (RX) are the only ones that are attached. Also external BEC and both positive cables isolated in the esc. Use a "Y" harness where you eliminate the red wires.
I saw the drawing in pdf ... magnificent !. And I understood that I am missing the ground connection in both systems.

[CraigP]

A “ground loop” is not a purposeful wire. Rather, it is a unintended current flow thru some of the ground conductors. It is the biggest reason people experience gremlin like behavior in their boats. I’ll try and give an example. Suppose we wire the duel ESC’s as the OP suggests. It seems simple and straight forward. Wire each side independently, where one batt set feeds one Cap Bank and ESC and another similar pack feeds the other side. The grounds and hots are kept separated between the two systems. Now let’s assume this is running 4s on each side. Each ESC has a BEC output, but only one can be potentially used. The problem is WHICH battery is going to supply the power for the control electronics, either thru the BEC from one of the ESC’s or from an External BEC, fed from one of the batteries. The OP says no BEC, use a separate battery pack for the control electronics. That’s fine, but it doesn’t address the problem, which is the Y cable you mention.

You see, no matter how you connect this configuration, that Y cable is going to try and short the two grounds together. The problem is that the grounds EACH have their own ripple voltage, and the peaks and valleys are out of phase, or asynchronous to each other. When the ground voltages don’t match, then that Y cable is going to try and short this ripple voltage difference and guess what, the control electronics are going to lose the battle. It causes a large current flow in the ground of the Y cable, which can easily disrupt any part or all parts of the control electronics.

It is for that reason that my PDF shows the grounds of the two batteries closely tied to each other on the common ground plane of the Cap Bank. Now the ripple voltage will only appear different on the positive side of each battery. Now, you can tie the positives together too, and many do that so that they discharge evenly. But that desire comes at a cost. Tying the two positives will now force all caps to see the voltage/current ripple of BOTH ESC’s. They are out of phase to each other, but at times they will add up to huge values, and this is very hard on the caps. It’s just exactly like the manner huge rogue waves are developed at sea. Ocean waves as they get large, get asynchronous to each other. Every once in a while, two big waves add in height and you have a 100’ wave staring at you, from nowhere!

Same thing happens in an electrical system. So I put up with the small amount of uneven discharge. I alternate the packs every other run. But my caps run cool, and I don’t have huge waves of electrical power to contend with. My apologies to the OP, I’m sure he would prefer to see this posted elsewhere.