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Using Super Caps after a DC-DC to supply inrush current on inverter

What you say is true, the problem is that a supercap can only source current by allowing the voltage to fall.
And that drop off in voltage can be pretty fast, it only works if the expected current surge is relatively short in duration.

A battery on the other hand, will have an initial drop in voltage under load, then the voltage remains relatively constant for an extended period of time. Supercaps are much better suited to very short surges, fractions of a second stuff.

And two batteries will not share the surge equally, because the resistance of the long cable limits the available current from the far battery.
Current will split in inverse proportion to the series resistance of each battery. The battery at the inverter end is going to get hit with the greatest part of a high surge, even if its only a relatively small battery

Starting motors up under full load can involve surges lasting several seconds, and that is the problem being discussed here.
Either solution could be made to work, if the supercap or battery is made sufficiently large.
Its just that for a two second long surge, a battery would work out to be a far less costly solution.
 
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It is not ,why doesn't the inverter have this built in. It would only work once for the first thing plugged in.

Not sure I understand this statement. Once the caps take up the initial load, they would just charge right back up gain.
 
So i didn't understand your quoted section either until i went back and re-read the context. It was in response to the idea of building NTCs into the inverter, not capacitors.

NTCs can never be at the 'supply' end because they have to be sized in proportion to the load to serve as 'inrush current limiters' and a large inverter doesn't know whether the load the next second is going to be 1 watt or 5000 watts, so a fixed 'hardware approach' like NTC's can't be implemented at the supply end but CAN be implemented at the load end if the load is a fairly fixed, predictable thing.

At the supply end (the inverter) there ARE a bunch of large capacitors built in, but current limiting of the output is done with high speed switching of the output, not through any physical 'surge suppressing' devices that i know. But i am not an EE and don't know all that much about how inverters function. But i have seen the following relevant video, so that's something...
 
I use a 125F capacitor in my 48V LiFePO4 and it supplies surge current for long enough to cover my water pumps and A/C compressor start up.

I use a 500F capacitor in my 12V starter batteries and that supplies surge current for long enough to start a truck / tractor.

Until I see a secondary LiFePO4 battery that matches the performance of what I currently use I’ll stick with what I know. Theory and practice don’t always align, especially over long term conditions

I’ve lost track of how many LiFePO4 systems I’ve seen driven to an early grave by using parameters that “work amazingly well” for a few years.
 
I use a 125F capacitor in my 48V LiFePO4 and it supplies surge current for long enough to cover my water pumps and A/C compressor start up.
Could you give some more details about this 125F capacitor system you have please? Did you build it yourself out of supercaps or is it a 48V capable module you purchased? Can you measure or notice a difference in the startup of your pumps and A/C compressor with it?
 
Could you give some more details about this 125F capacitor system you have please? Did you build it yourself out of supercaps or is it a 48V capable module you purchased? Can you measure or notice a difference in the startup of your pumps and A/C compressor with it?
Maxwell supercap, 16V 500F x 4 in parallel for 64V 125F.
Start-up current for pumps is lower than 1/2 on the LiFePO4 bank.
 

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Maxwell supercap, 16V 500F x 4 in parallel for 64V 125F.
Start-up current for pumps is lower than 1/2 on the LiFePO4 bank.
Thanks, That's awesome. I think you mean they are in series not in parallel right? So that's over $1k worth of supercaps?
 
Alright...I think this is going to be tested as an experiment anyway! :ROFLMAO::ROFLMAO:

I got a huge deal on amazon fro 5x 2.7V 500F super capacitors for just 27 euro's! (dollar is about the same right now) So I could not resist and ordered two sets for 10 capacitors in total! Today they arrived and man....these are HUGE!!! :eek::eek:

I'll take a picture of them later next to something recognizable to compare the size. A victron MPPT for example? ;)

I did order the protection boards a few months ago already...since they were kind of cheap but just noticed an issue (for me). They only allow 1A of current for either charging, discharging or both. Not really sure yet. But that seems to me like a HUGE limitation for these big super caps that can deliver much more amps. So I am researching other ways to keep the voltage "per cell" below 2.6v to protect them and still allowing 20A or something for the discharge current.
I am thinking a battery BMS "might" work if I can program the voltages correctly. Meaning 0.0V as the "low-voltage" and 2.6v as the max. I have seen a video on youtube recently where they build a DIY BMS kind of circuit, but it was a lot of work! And per cell the circuit was WAY bigger than the pre-build options.

Just to be clear: I know using super caps for the described purpose of this thread is basically pointless. I just want to try anyway and learn from it. And if I remember correctly the main argument against it was price. Which is now less because of that deal I got.
 
After much delay I finally tested the fridge with various NTC thermistors. The results are pretty underwhelming and I don't think using a thermistor to reduce the inrush current of fridge compressors is that great of an idea, but I wanted to report what I found nonetheless. Again I'll mention that to do this test you need to have a scope probe that doesn't have an earth grounded shield or even better use a specialty high voltage probe, so don't try this at home unless you know what you're doing.

Here is a snapshot of my test setup. I'm using a pair of 1ohm power resistors in parallel to make a half ohm shunt on the neutral wire. I measure the voltage across the shunt to and use this to calculate the current. Then I soldered different combinations of thermistors to add to 2, 4, 6 and 8ohms in series with the half ohm shunt. Then I cycled the fridge many times and tested the peak current for each run. I used SCK202R0 and SCK204R0 for the thermistors. These can be purchased on ebay for $3.

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Results: Adding a large 8ohm NTC thermistor reduced the peak current from 7.8A to under 6A. But that lower current surely reduces the starting torque of the compressor so it might lead to the compressor failing to start in some conditions. I waited about a minute or two between runs to let the refrigerant equalize and the only time the fridge rotor locked up was when I didn't wait long enough in between runs. Even with 8.5 ohms of extra resistance, it started reliably in my 45F shed/lab.

Overall I was hoping for a more dramatic reduction in current. If you have a situation where your inverter can almost power your fridge or it runs it most of the time but it sometimes overloads the inverter, then an NTC may be able to help remedy that. But in general an NTC won't dramatically reduce the startup current of the compressor..

I found that the total length of time that the current stayed high (above 1A) didn't depend too much on the resistance, it depended more on how many times I ran the test since the PTC that heats up and removes the start coil was staying warm from the previous test and turning off sooner. So without waiting an hour between runs or something, I don't have an easy way to measure that.

So there you have it. Hope that's interesting to some people.

I am not sure if it would work in your situation, but there are some devices called "soft start" that help with this locked rotor current problem. They are widely used on RV air conditioners.

Not 100% sure, but I think that they way that they work is to temporarily add a capacitance during start up that shifts the current sine wave relative to the voltage sine wave.

At least on RV air conditioners, the locked rotor current reduction is pretty dramatic.
 
Just wanted to say I've had a supercap bank on my inverter for ages and I think it's necessary for a 12v system.
I dont even have a long run, just a bit under 1 meter of 25mm2 or 4awg copper to the pack.
After installing the bank I could run my pressure washer off the inverter no problem, or pretty much any motor up to about 1.5hp.
the bank is a small one too, 83F at 12v, made out of 6 500F caps with a balance board.
 
500F Maxwell supercaps have about 2.5 to 3 milliohms of ESR each. Chinese clones are usually worse.

Six in series is about 15 milliohms to 18 milliohms, not including PCB and connecting cable contribution.

3000F Maxwell supercaps with large threaded terminals have about 0.15 to 0.20 milliohms ESR. That makes a big difference in their benefit.
 
500F Maxwell supercaps have about 2.5 to 3 milliohms of ESR each. Chinese clones are usually worse.

Six in series is about 15 milliohms to 18 milliohms, not including PCB and connecting cable contribution.

3000F Maxwell supercaps with large threaded terminals have about 0.15 to 0.20 milliohms ESR. That makes a big difference in their benefit.
Good Info! To put some example numbers to this, assume you have a 12V 1200W inverter during a max power surge from an AC compressor starting up. That surge might peak at 15A 120V. Which is 15A*120V=1800W. To attempt to supply that load, the inverter is drawing a lot of current from it's 12V terminals: 1800W/12V=150A! The current causes a voltage drop between the battery and the inverter. Lets assume you used 5 feet of 4ga copper wire between the battery and the inverter. You can look up resistance per foot of wire and 4ga copper is 0.25ohms per 1000ft or 0.25milliohms per foot. We need to account for the round trip resistance of the wire through the positive and negative wires, so that's 10ft or a total of 2.5milliohms of resistance in the wire. At 150A that will cause a voltage drop of 0.0025ohm*150A=0.375V. If you had used 10ga wire the voltage drop would be 1.5V. Also notice that milliohms of resistance in your crimp connections, terminals, fuses, and even your BMS can really add up and be significant here. You can probe around with a voltmeter during a high load on the inverter and measure the drop across different components on the route between the battery and the inverter. You can calculate the total resistance by comparing the voltage at the battery terminals to the voltage across the inverter terminals and then divide this by the current consumed by the inverter at that time.

Supercaps can help by providing an additional source of current to the inverter for these very short peaks. It's important to place them as close as possible to the inverter and to use large gauge wire and solid connections to minimize any voltage drop. In addition, as @RCinFLA pointed out, the caps themselves have internal resistance that is accounted for as Equivalent Series Resistance (ESR). So a bank of six 500F Maxwell supercaps might have a voltage drop of 18mohm*150A=2.7V while a bank built with 3000F Maxwell supercaps might be 1.2mohm*150A=0.18V. So it's important to know the ESR of your supercaps.

In general you can take your measurement of the series resistance of your system from the first paragraph and if the expected resistance of the supercaps is similar or less than that, then the supercaps would be effective. However, if the resistance of the supercaps is much larger than the existing resistance in your system, it's not going to have much effect. Also consider how upgrading your wiring, fuses, or connections might be more cost effective. I just wanted to work out the math for people who might not have as solid of a grasp on this. Hopefully that helps.
 
I installed an old Maxwell 350F X 6 array (58 Farads total) on my old Jeep about 4 years ago. This is a super cap in parallel with the lead acid battery in an automotive application. The battery is a mid-grade 42 month battery, manufactured April of 2016. As of this post, that battery is 88 months old! It is still strong. Adding the super cap in parallel allows the low ESR super caps to absorb the starter current and initial alternator charge surge.

Batteries have a lot of internal resistance, super caps have virtually none in comparison. Electricity will flow the path of least resistance. I know I'm touting how super caps can help a battery last longer, and this thread is about keeping surges to a tolerable level, but I think the punch line is the same.
 
An auto 12v lead-acid battery in reasonable condition will have an initial starting impedance of 5 to 8 milliohms.

A single 350 Farad Maxwell Super Cap has about 2-3 milliohms of ESR. Six in series is 12-18 milliohms. The simple wire leads are their weakness.

You have to go with the 3000 Farad stud terminal super caps to get the ESR down to about 0.15 milliohms per supercap.
 
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As of last night, the Super-Capacitors are up and running preliminarily, and I already have some amazing observations. This pack is x24 of Nesscap 3500F 2.7V super capacitors. Total capacity works out to 145 Farads at a maximum of 64.8V, and that's enough safety margin for my 62.4V FLA Equalizations. The capacitor bank is connected to the PDP DC busses with 2/0 cable, and through a dedicated Schneider 250A DC breaker. The cables are as short as I could keep them, 5 feet each.

I wanted to run these open-air for a bit to observe them under high inverter load. For over an hour today the inverters were sustained under an AC load of 11.5kW, 12kW DC which works out to about 200 Amps on the DC bus, 58 Volts at the time.
- The capacitor temperatures went up only 1.5 degrees Fahrenheit over ambient, measure with infrared thermometer.
- So much current was pulsing through the DC cables that they repelled each other and vibrated. It is entirely DC ripple causing this alternating current flow.
- Using an inductive clamp meter set to AC, I measured 28A (AC) to the capacitor bank, and 23A (AC) to the battery bank.
- When the Central Air Compressor starts up (on an EasyStart 368) it immediately adds 5kW of AC load to the inverters. If DC is topped off and PV is limited, voltage drop on the DC bus is minimized so much that you can watch the kW from the batteries slowly come up to meet the inverter input. The missing kW source between the inverters and the battery is the Super Capacitors supplementing both the surge and the load.
- It took TWO HOURS to pre-charge these things to a point where they were within a few volts of of the DC bus. (I did not want a 200$ Schneider 250A breaker being welded into uselessness.) I used a very temporary contraption of a 120V 1100 watt hair dryer as a resistor, and that works out to about 13 ohms which at 52 volts is only about 4 amps or 200 watts. This is hilariously inadequate, so I ordered a 5ohm 500 watt heat-sinked resistor to use for pre-charging and dis-charging the Caps. Its rating will be momentarily exceeded during initial inrush at 540 watts, but that will quickly drop as the capacitor bank voltage is pulled up through the resistor. In fact, the capacitors only need to be brought up to 50V of delta between them and the DC bus across the resistor for the 500W spec of the resistor to be unexceeded. A center-off SPDT switch will be used to switch the resistor between charge and discharge. It would be "safest" to have a meter across the resistor too.
- Both XW Pros sound different. They're either quieter or the frequency has changed. Something is different. I do have a fairly decent dB meter as well as a reference calibrated microphone, so I may attempt to measure and capture this once I have a pre-charge resistor circuit properly in place.

Anybody dabbling in these things - ABSOLUTELY NEVER connect a super capacitor bank to a DC bus without first pre-charging through a resistor to equalize the voltage! These capacitors are happy to effortlessly pass 1500 AMPS! I am not brave (or ignorant) enough to have tried it, but I assure you there is plenty of potential to earn yourself distinguished esteem in the "up in smoke" sub-forum.

I have far less battery capacity than I actually need for the system size; in fact the panels can pull the batteries from 55% to 100% before even 11AM. Even Schneider recommends 400Ah of FLA minimum per XW Pro, and I've got 2 of them on only 440Ah of battery. Needless to say, I don't have enough battery to run more than 5kW of AC load without substantial voltage drop, so I have the central air conditioning condensor wired to be interrupted by the auxiliary relay in the master XW Pro when battery voltage falls below 49V. That is the largest load than can be shed and it's an easy one to manage. This should only be a condition met if the grid is down, it's night, and I'm inverting from battery. I would never want the central air running on battery alone anyway, so this is completely fine. The capacitors in this system are making an undersized battery bank respond to load changes and surges like a much larger bank. Pretty neat.

I'll probably post something more formal once I polish up the mess.

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