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KS-16S electrical question (any wheel actually)


Wheeler Von Calamity
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I was wondering if there would be any benefit of doubling the capacity of the onboard capacitors of the KS-16S. Any knowledgeable people on here that know about electronics could chime in if there would be benefits or not by doing so. I have always been facinated by super capacitors and think it would be really cool to use a super capacitor instead of a battery on an EUC at the moment I think this is not possible or economically feasible, but how about some super capacitors on the output of the mosfets would that help in any way?

 

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14 minutes ago, esaj said:

Some people have asked me in private messages about replacing the caps. I've usually instructed them to either get the same caps as the originals, or some other with low ESR and at least the same capacitance as the originals. If your wheel seems to have become more weaker, a faulty capacitor can cause that (ie. the battery cannot dish out as much current as the motor is trying to pull if there's not enough capacitance or degraded capacitor there). You might get some more torque out of the motor with larger caps, but it's not guaranteed. Going way out and putting something like ten-fold higher capacitance there may or may not be feasible. I don't know if the capacitors play any other role than energy reservoirs, but doubt it. Still, if there's something like motor impedance compensation or such going on, changing the capacitance value a lot could affect things (good or bad), I don't know.

As always i cant hold up with Esaj technical knowledge :-)

Just some things to the caps...Yes, they only work as energie reservoir. Some people doubled their caps on the Ks18 in russia!

The only effect they reported was less voltage drop, but no better torque.

WHEN a caps is defect and not working anymore....you WILL know that, as your ks16 then works/reacts like swinging boat! Very weak pedals!

Sooner or later on the next real high current draw, you also will faceplant without working caps! (self experienced :-( )

@Wheeler Von Calamity

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4 hours ago, esaj said:

 

Now, increasing the capacitance of C1 & C2 (using bigger capacitors and/or more caps in parallel), there's larger "reservoir" feeding the motor (which is the only component in your wheel that draws large currents, the electronics in the mainboard don't need much). I tried making them bigger, which causes the current draw from the battery direction to go from 8-10A to 8.5-9.5A (so it averages around 9A in this case), but that was already with ten-fold increase in capacitance (2 x 10000uF). There are diminishing returns in making the capacitors bigger and bigger, so slapping a huge amount of caps and/or very large capacitance there isn't likely to be really useful after certain point.

Now, you can pretty much forget about super caps. Not only are they not economically feasible, but if you plan to replace the batteries entirely with them, you're in for a nasty surprise. The energy density of even the super capacitors is peanuts compared to lithium cells. If you'd get super capacitor bank capable of same voltages as the battery packs (which would be HUGE physically), they'd hold a small fraction of the charge of the lithium cells. You'd drain your bank just getting moving. Super caps are good for replacing batteries in devices which require very small amount of current (think something like remote controls, pocket calculators etc), or if size/weight is not an issue, you can draw very impressive amounts of current from large banks for very short periods (think spot-welding), but as a replacement for something that requires large charge capacity (especially when the device has to carry the banks), they're a no-go.

 

 

 

Great explanation thanks

as far as the super caps go i was thinking small package high capacitance to replace the original caps not to replace the main battery. But if you think there would be no benefit I guess there is no point. As far as arcing goes even now if I connect the battery pack and the caps are discharged the is one hell of a pop.

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4 hours ago, US69 said:

 

The only effect they reported was less voltage drop, but no better torque.

 

when you say less voltage drop do you mean in the circuit or at the battery itself?  I would think that less voltage drop at the battery would help with the health of the lithium battery. Especially considering that when going uphill I see a drastic drop in voltage reported in the app and then after the voltage stabilises 

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14 hours ago, esaj said:

As usual, I'm not an expert, but to my best knowledge there are (theoretical) benefits to adding more capacitance at the battery input of the mainboard (not after the mosfets in the half-bridges). The battery cells and the wiring going to mainboard have internal resistance which drops the voltage at high current draw, and the wiring also causes some parasitic inductance, that slows down the "change" in current (the old rule-of-thumb is "inductance opposes the change in current, capacitance opposes the change in voltage"). So when the motor pulls high current when the mosfets switch on, the resistance causes some voltage drop and the inductance in the wiring prevents a really fast rise in the current (it starts to charge up a magnetic field around the wires), and similarly, when the mosfets switch off, the inductance prevents fast drop in current (the magnetic field starts to collapse, which can also cause the voltage to raise when the current drops). Inductors have their uses (all switching-mode power supplies/DC-DC -converters are based on using an inductor to change the voltage), but in this case, it's not wanted. Likely the effect of the internal resistances of the cells and the wiring/connectors is more "meaningful" in the end vs. the inductance.

The capacitors in the mainboard are there to act as "energy reservoirs" (capacitors have a lot of other uses too, but I doubt their purpose here is other than that), that is, when the motor pulls a large current, the spike comes mostly from the capacitors, not from the batteries themselves, although of course some current comes from the batteries also. When the motor switches off (that's what it does, it turns "on and off" really fast at varying pulse-widths, and the controller changes which phases get energized etc. to turn the motor), the capacitors get charged from the batteries. In general, this keeps the current draw from the batteries more "steady", as shown by this (simplified) simulation:

l7me19g.png

V1 is the battery, 20S1P in this case, at 80V and with an internal resistance of 0.8 ohms (assuming there are 20 cells in series, each with 40 milliohms internal resistance -> 20 * 0.04 ohm = 0.8ohm). L1 is there to simulate some stray / parasitic inductance (100n is a low value, no idea how much there really is, not much but some). R_MEAS is just to have a component over which to measure things in simulations, you could think of it as the resistance of the wiring and the connectors (1 milliohm). C1 & C2 are the mainboard caps (I used 2 x 1000uF / 0.1ohm ESR for simulation). R_LOAD is "some load", like the motor, although a motor cannot be modeled as a single resistance, but for this example, it's sufficient. Finally, there's the U1 mosfet that's being turned on and off by V2 pulsing between 0V and 12V at 5kHz in this case (200µs period) with 50% duty cycle. 

In the above simulation, the graphs show the current flowing through R_MEAS (the green line) and through the R_LOAD (blue line). As you can see, the current draw from the battery is hovering around about 8-10A, never going to "full blast" (up to 20A over the load) or down to zero, where as the current over the load is going from 0A up to around 18A. Where does that "extra" current come over the load? It's from the capacitors C1 & C2. 

Now, increasing the capacitance of C1 & C2 (using bigger capacitors and/or more caps in parallel), there's larger "reservoir" feeding the motor (which is the only component in your wheel that draws large currents, the electronics in the mainboard don't need much). I tried making them bigger, which causes the current draw from the battery direction to go from 8-10A to 8.5-9.5A (so it averages around 9A in this case), but that was already with ten-fold increase in capacitance (2 x 10000uF). There are diminishing returns in making the capacitors bigger and bigger, so slapping a huge amount of caps and/or very large capacitance there isn't likely to be really useful after certain point.

Now, you can pretty much forget about super caps. Not only are they not economically feasible, but if you plan to replace the batteries entirely with them, you're in for a nasty surprise. The energy density of even the super capacitors is peanuts compared to lithium cells. If you'd get super capacitor bank capable of same voltages as the battery packs (which would be HUGE physically), they'd hold a small fraction of the charge of the lithium cells. You'd drain your bank just getting moving. Super caps are good for replacing batteries in devices which require very small amount of current (think something like remote controls, pocket calculators etc), or if size/weight is not an issue, you can draw very impressive amounts of current from large banks for very short periods (think spot-welding), but as a replacement for something that requires large charge capacity (especially when the device has to carry the banks), they're a no-go.

Some people have asked me in private messages about replacing the caps. I've usually instructed them to either get the same caps as the originals, or some other with low ESR and at least the same capacitance as the originals. If your wheel seems to have become more weaker, a faulty capacitor can cause that (ie. the battery cannot dish out as much current as the motor is trying to pull if there's not enough capacitance or degraded capacitor there). You might get some more torque out of the motor with larger caps, but it's not guaranteed. Going way out and putting something like ten-fold higher capacitance there may or may not be feasible. I don't know if the capacitors play any other role than energy reservoirs, but doubt it. Still, if there's something like motor impedance compensation or such going on, changing the capacitance value a lot could affect things (good or bad), I don't know.

If you plan to replace / add more capacitors in the board, check the datasheet of the original make & model. You'll want low ESR (how much is "low", depends on the situation of course, but likely in the hundreds of milliohms or less?) and higher voltage than what the battery can give out (also during regenerative braking, the motor may momentarily push the voltage to a higher value, so for 16S I'd go with 80V or higher voltage caps, 100V or more for 20S). For temperature, I'd go with 105C or higher rated caps.

Also, the more capacitance / less impedance you have there, the larger the current spike will be when you connect the batteries to the mainboard. You may want to consider anti-spark connectors to prevent them from sparking/melting when connecting the batteries to the mainboard.

 

 

Nothing to add... @esaj thanks for your technical explanations. Sure there is some suppositions but I'm quite agree even with that...

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