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Soltaro

Experience with Supercaps

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The initial impression from our local BlueNova guy was not very good. But, you can visually see that the batteries on sale now is a later revision, and apparently the one BN tested was an early "pre-release" model. So to be fair, some of the criticism might no longer apply.

There are some videos from some outfit called the Smart Energy Lab, where they apparently ran the battery for 6 months and liked it.

The AEVA guys had a long discussion and concluded that it's not a real "super capacitor" battery, most likely Lithium Titanate.

There are several Victron installs with these batteries, and I have yet to hear of a failure. So that's a good thing.

But conversely, there was one fire, though that one was blamed on a bad install (which it might well have been).

I seriously doubt the battery will last as long as the spec sheet claims. It's a Lithium chemistry battery. My personal opinion is that it's not worth it to pay a premium for these batteries, you will likely do just as well with another LiFePO4 battery.

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From what I've read, super-caps are perfect for 'bridging' between grid failure and generator start-up.

Here the battery/capacitor run-time is measured in minutes, not hours.

I imagine such a super-cap UPS will also be great when there are frequent short-duration hiccups - especially useful for loads that require large start-up currents.  The UPS acts like a large "AC capacitor."

 

 

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We had one 7.1kWh installed with a Victron Quattro 10kVA: running for a year in Cape Town. It does seem to have a high self consumption (around 100W on float), but no problems except extremely heavy and ungainly to move. Advisable to go for the 3.55kWh units.

Installed an additional one this year (so total of 14.2kWh capacity): cell balancing was selected off on new one (without me being aware). Cell imbalance caused the new battery to crash properly, and we got a replacement from NEE in two weeks: very friendly staff. No hassles except that our Eskom loadshedding and our clients geyser always seem to work in sync at night: so I've had the inverter cut out a few times at around 46V with only the one 7.1kWh battery after delivering 5kWh AC.

Haven't tested the new 296A discharge, or 10^6 cycles :) But it does store and release power quite predictably. I maintain a healthy skepticism until I can communicate via CAN on the Victron system, which it can't. However, no reason to dispute claims: bar "supercap" is likely a misnomer.

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I'm still on the fence here, I reckon we're a year or 2 away from fully realizing the real positives and negatives after running them in real life day to day scenarios.

Now we've had the hypercap thrown into this whole equation. 

The biggest and definite application I see(for now) is as a generator startup buffer, the cycle and lifetime claims none will be alive to confirm anyway lol.

Capacitors as I know them have definite shortcomings in terms of lifespan, that is my biggest ???

I like the new earth product, but I'm no scientist so I'm going on what I'm being told.

Interesting times for energy storage....

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Posted (edited)
10 hours ago, Soltaro said:

Capacitors as I know them have definite shortcomings in terms of lifespan, that is my biggest ???

There's a lot of confusion in this space. Ordinarily capacitors have an extremely long lifetimes. I mean, if we're talking about real capacitors. Ceramic, Tantalum, film... those will last a hundred years or more, and the watertank-look-alike electrolytics also last decades if you keep the temperature down. They can be charged and discharged millions of times.

But real capacitors have other drawbacks: They charge and discharge linearly. If you draw a chart of the voltage vs remaining capacity, it will be a straight line ending at zero. Now imagine your average renewable setup that needs at least 40V or so to run. If you build the pack so that it charges up to 60V (for argument sake), that mean you can use roughly a third of the capacity. The bottom two thirds of the capacitor's.. uhm... capacity is unusable. So of course there are ways to work around that, by using a boost converter to extract that capacity (look on youtube for examples of a "Joule thief", same principle), but 1) there are losses involved in this process, and 2) you still won't be able to get all of it out.

This is compounded by the second issue: The largest supercapacitors just aren't that big. I mean, don't get me wrong, they are big alright, and they pack a punch, but they are not battery-big. The largest supercap right now is made by Maxwell. The largest one is 3400 Farad. It stores 4.25Wh of energy when fully charged. You'll need 200 of them to get close to a kilowatt hour, and they have an energy density of 8.57Wh per kg, which means it will be much heavier than the equivalent traditional battery. For perspective, LiFePO4 batteries have densities of 90-160Wh/kg (at least ten times) and lead acid is 35-40Wh/kg (4-5 times), so imagine a large lead acid bank and then imagine making it 4 times heavier.

So this right there is why I (and many others) don't believe the kilowatt-labs "super capacitor" product is an actual supercapacitor. Bulky and heavy as it is... it is too light to be one. The discharge curve also doesn't show the linear behaviour one would expect, neither does it show a level behaviour one would expect if there was a boost converter involved.

So with that said... I'm not dismissing the use of actual supercapacitors for things they are good at, eg bridging the supply after a power failure until the generator starts up. These short-term high power applications where only a fraction of the charge (top third or so) is used is exactly what they are good at. What I am dismissing is the claims of so-called super-capacitor batteries. I don't think they are going to last a million cycles, and the reason I think that is not because I doubt supercapacitor technology... but because I am convinced these things aren't actual capacitors.

Edited by plonkster

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