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Parallel Supercapacitors with LFP bank

Denis said:
Yes sump is small, I prefer to have the water evacuated before it becomes even with the weeping tile.

These are all 16 v 500 farad banks with balancing circuit on each two capacitors,
4 bank parallel minus 2 capacitor.
My batteries top voltage is 56.8 v so I needed at least 21 capacitor for that voltage.
But because the balance circuit is good for two, I've put 22.
The capacitance is 3000/22 = 136.3636 farad
Click to expand...

Oh, OK, came with balancing circuit. 16V, probably for use in 12V car stereo systems.
But now that you have several of these in series you might need balancing between the banks as well.

136F ... I had the right answer, except I was off by a factor of 10. That happens sometimes, when you use a slide rule.
7.8 kW for 10 seconds - wow!
 
Got most of the gear for my latest standalone system a couple of days ago.

The maxwell supercaps are second hand units, the 48V is how they come (complete with cell management). The 16V is one third of a 48V pack with it’s own cell manager.

I’ve seen in practice the caps supplying a large surge current, and not receiving a corresponding large surge current from the parallel battery bank. When i set up the system i will see for myself and report back here.

The theory is that the high load causes a voltage drop in the battery and the caps, but the caps with the lower IR will supply most of the current. When the load reduces, the battery and the caps are at the same voltage, so no current flow between them, the charge controller supplied current will be the only current back into the system.
 

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My concern with straight parallel between the battery bank and the capacitor bank is what will happen after a longer high current surge, such as accelerating a car.

If a longer duration surge manages to pull down the caps just 0.5 volts, the internal resistance of the battery and cables could allow this dip, but the cells really did not discharge much due to the flat discharge curve of the LFP batteries. So when the surge ends, the caps are still down 0.5 volts, but the battery bank's state of charge is not. The battery will be trying to charge the caps back up through their internal resistance and any wire resistance between the battery and the capacitor. In your experiment, can you have an amp meter between the capacitor bank and the battery bank?

Here is my theory of what might happen...
A short 200 amp surge will show after the caps feeding the load. The battery may only see the surge hit maybe 30 amps during that big surge. But then after the surge event is over, the batteries stay pushing 30 amps into the caps and slowly tapering off as the caps charge back up to the battery resting voltage. The less resistance in the circuit, the higher the battery current will be, but for a shorter amount of time. To really see what is going on with short pulses, it may require a fast sample rate data log of the current traces at the battery output and the capacitor output.

My concern is if the caps are pulled down too much, the current required to charge them back up could be damaging to the LFP cells. This is why I proposed using a switching charge controller to limit the battery current into the caps in my EV design. Just having some intentional resistance will save the cells, but if we are talking very high currents and capacities, this becomes very inefficient. Even that 0.5 volt drop could be a lot of watts turning into heat. A switching converter can easily be 95% efficient.
 
KISS

Resistor and diode in parallel connecting supercap to battery.
A 1V drop across diode in a 48V system is 98% efficient.
A 1V drop across a resistor (lightbulb maybe?) is what, 98% efficient?
Not sure on the second one, because I know I^2 R is power dissipated, and 2x R cuts power dissipation in half ... oh, operates twice as long charging.

Of course the LFP cell will still see the dropped voltage and high current during acceleration, but only for those few seconds. As soon as your drag race is over, voltage pops back up and the lightbulb glows until recharge is complete.
 
I am using this system for powering a 2.5kw water pump, and 3kw RC aircon. Surge currents for these last only a few seconds..

I have no experience using the supercaps in parallel for sustained high surge current.

I will be AC charging this system quite often, so the supercap will be useful also for removing the AC charge ripple.
 
toms said:
I will be AC charging this system quite often, so the supercap will be useful also for removing the AC charge ripple.
Click to expand...

I'm not sure how effective supercaps are at 60 Hz. Do you have data on frequency response or inductance?
Could be electrolytics work better. Although, even if supercaps only present 10% or just 1% of their capacitance at that frequency (impedance is 10x or 100x higher than pure capacitance suggests), could still be beneficial.

I found some curves, but not ESL in a data sheet.
Curve page 6 shows linear decrease in impedance with frequency, expected of a capacitor, only to about 5 Hz:


The capacitor will draw AC current and heat up as it tried to smooth the 60Hz ripple.
The data sheet I found has an amps rms spec. Looks to me like 0.6% of voltage spec would be max ripple, so your system needs enough capacitor to spread out current draw and limit ripple to that amount.

Supercaps are meant to supply current for seconds. In electronics design we use a hierarchy of caps with different value and frequency response. The electrolytic caps in your inverter probably take care of most 60 Hz ripple.
 
With my XW-Pro, I was rather shocked when I saw the amount of ripple current both charging and discharging the battery bank. Since it is a true PFC corrected system, the current is a 120 hz full wave rectified sine wave. If you look at the current on a scope, you have the first half of a sine wave, and then the second half of the sine wave is positive again, and this just keeps repeating. So when it is charging at 30 amps, it is actually going from almost 0 to about 42 amps. This is filtered a little by the capacitor bank, but not near as much as I expected. I am going to look into adding an LC or Pi filter to try and smooth it a bit at my battery bank. I used to have a few of the power filters we used on Xenon lap supplies, the ripple from even a 3 phase bridge would chew up the bulbs if the filter caps failed. The series inductor was designed around the 3 phase 360 Hz ripple frequency though, so I would need about triple the inductance. The caps also had to be sized for the 150 volt open circuit before lamp ignition, so they were only about 12,000 uF and still 12 inches tall and 3.5 inches around. I do have 2 of them here. I tried just paralleling one of them at the battery bank, but with just #12 wire to the cap, it made no difference to the ripple at the batteries. Seeing this ripple current, I totally understand why the current reading in the BMS jumps around a bit, it is just taking a sample off the shunt without enough of a filter.
 
GXMnow said:
The series inductor was designed around the 3 phase 360 Hz ripple frequency though, so I would need about triple the inductance.
Click to expand...

Single or split phase delivers power that ripples with the sine wave, but 3-phase ideally has no ripple at all.
If fed through diodes to charge caps, of course power factor would be bad.
A good PFC should draw current in proportion to voltage from each of the 3 phases, and deliver steady DC current that is the sum of the power drawn from three phases. Just the reverse function of a grid-tied inverter.

A PV inverter feeding single phase AC has to draw ripple current from the caps. A 3-phase inverter should impose no 60 Hz ripple current at all on the caps, only the ripple from its higher frequency switching. Inductors would carry the piecewise linear or stairstep current synthesizing each sine wave, but the three phases together sum to no sine wave at 60 Hz, just steady-state current.

"current reading in the BMS jumps around a bit, it is just taking a sample off the shunt without enough of a filter."
I would filter the sense lines; that is low current so doesn't need large filter components.
 
3phase does have far less ripple, but there is still some. Without the caps, the xenon bulbs would only live about 1/10 life and the electrodes looked like they were beaten with a mace. Just 5% ripple was enough to shorten their life. And the ripple is up at 360 hz. Like I said, the inductor was only about 50 turns of #3 solid copper on an E I core, so it is not enough to filter the 120 hz ripple I am seeing.

I am not in a big hurry to open up the BMS and try to find the shunt sense line to add a filter, but yes, that would not need large components like it will take to filter the battery current line.
 
I have a naive question. If you need supercapacitor properties could a lithium titanate battery pack do? They cycle more than lifepo so certainly could deal with surge and ripple, have a pretty high C rate / low internal resistance and about the same voltage as super caps with higher energy density. Are there caps that discharge that much more quickly, and do you really need that much speed for a load?

I’m not aware of cheap super caps - could someone post a link?
 
I think the price of Maxwell Ultracaps has skyrocketed since Musk/Tesla bought them out. It might be due to their getting new business direction and not making caps for general market so all there is left is what is in the existing inventory pipeline.

I have tried a few of the Chinese supercaps and found them to be pretty poor. Very high internal resistance compared to Maxwell Ultracaps.
 
LTO cells would be great, in ten years time when i’m looking for my next battery they will be on my list if they haven’t shown significant failures before then.

In the meantime they aren’t cost effective.

Problem with batteries is that to cost them properly you need to wait until the end of their life - most simulations rapidly age them.

I’m still waiting for the first pack i have been involved with to get below 80% capacity - and that one is just over 10 years old.

All i can do is look at packs that have degraded, and see what is being done differently to the ones that are being used without degradation.

I can show you the scope traces of reduced AC ripple, and lowered surge currents- but you will likely have to wait more than 10 years for a result on increased cell longevity.
 
That is the point I see here - for normal use, say below 5C - it’s possible to find cells that do just fine and life expectancy isn’t an issue unless the cells are hot. But it would only take a small pack of LTO to serve as a super cap. It’s too late to do the math but I might figure out what it takes to deal with ripple or gigantic inrush, or slamming the brakes on a scooter, etc
 
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