Questions/curiosities bout Super Capacitors....

[jjchvl]

I hope that no one thinks Im arguing with them. I am gratefully digesting and learning from all the input. Any questions i ask are genuine and every answer I get much appreciated.

 

[toms]

In practice with a supercap in parallel with LiFePO4 there is no need for a current limiter. You can set up the system so that the charge controller charges the supercap before the battery contactor is closed.

During rapid load application the majority of the current comes from the supercap as the voltage drops. Because the voltage of the LiFePO4 and supercap are still the same they both recharge at the current limit of the charger - there is no current from supercap to battery after initial connection.

 

[jjchvl]

In practice with a supercap in parallel with LiFePO4 there is no need for a current limiter. You can set up the system so that the charge controller charges the supercap before the battery contactor is closed.

During rapid load application the majority of the current comes from the supercap as the voltage drops. Because the voltage of the LiFePO4 and supercap are still the same they both recharge at the current limit of the charger - there is no current from supercap to battery after initial connection

What about a supercap connected to a bank of AGM's? Does the number of AGM's connected in parallel to form the bank make any type of difference? For example, Im thinking that replenishing a suddenly depleted supercap is going to be more taxing for a single 100ah AGM battery than for four 100ah AGM batteries connected in parallel because the total current required to charge the supercap will come from four batteries instead of just one. This sounds logical to me but Im not an electrician so I fear that I may have missed some important concept.

Anyway, I think my question boils down to this: is a capacitor recharge less demanding on a battery bank than the start up load that the capacitor is supposed to mitigate? If so, then why would a current limiter be needed in either type of battery system?

 

[toms]

There is no capacitor recharge from the battery bank. Under load surge condition the voltage of the battery and capacitor drop together, but much more current comes from the capacitor. When the load is removed the two are still at the same voltage - no current flow. The recharge comes from the charger.

For some reason most people think the supercap discharges into the battery but that is not the case.

 

[jjchvl]

There is no capacitor recharge from the battery bank. Under load surge condition the voltage of the battery and capacitor drop together, but much more current comes from the capacitor. When the load is removed the two are still at the same voltage - no current flow. The recharge comes from the charger.

For some reason most people think the supercap discharges into the battery but that is not the case.

Thanks. I wondered about the "backwards into the battery" thing. It never made much sense to me since internal resistances of battery vs capacitor pretty much dictate a one way possible flow. And a capacitor cant have more voltage than the battery its connected to. But like you said, a lot of folk seem to insinuate that such "backflow" is a risk.

Thanks for the reply. Im learning a lot here and via other net research.

 

[Bob B]

There is no capacitor recharge from the battery bank. Under load surge condition the voltage of the battery and capacitor drop together, but much more current comes from the capacitor. When the load is removed the two are still at the same voltage - no current flow. The recharge comes from the charger.

For some reason most people think the supercap discharges into the battery but that is not the case.

So .... if there is no charger in the circuit ... or there is a solar charger and it's dark ... and load is reduced or completely removed .... the battery and capacitor voltage levels come back up .... what recharged the capacitor?

 

[jjchvl]

So .... if there is no charger in the circuit ... or there is a solar charger and it's dark ... and load is reduced or completely removed .... the battery and capacitor voltage levels come back up .... what recharged the capacitor?

My understanding is, in the scenario that you suggested, that even as current is being pulled from the capacitor, the capacitor is pulling from the battery bank as the capacitor attempts to maintain equilibrium with the battery bank. I think the only time there might be an issue is if you connect an "empty" capacitor to a battery bank. The capacitor will attempt to equalize with the battery bank VERY quickly. Maybe too quickly depending upon the battery bank youre connecting the capacitor to. Thats why I was asking if connecting a Supercap to a single 100ah battery or four 100ah batteries connected in parallel makes a difference. I suppose the best option is to always pre-charge a supercap before connecting it to a battery bank.

The first time I connected my Supercap to my AGM battery bank, I had pre-charged it a couple of days before and it was down to 8v. My 900ah AGM battery bank, made up of seven AGM batteries in parallel was at 13v. I had also installed a high amp, DC on/off switch(canablized from an old jumper pack) on the negative cable of my capacitor bank. Keeping this switch off during connection prevented sparking issues. Once I connected the Supercap to my AGM battery bank, I monitored the voltage of my battery bank and turned the capacitor switch on. I saw a VERY slight drop in voltage from my AGM battery bank despite the Supercap being nearly depleted upon connection. I suspect this is because the supercap was equalizing from a 900ah AGM battery bank and not a single 100ah AGM battery. Had I connected a near empty super capacitor to a single 100ah AGM battery, I suspect that I would have seen the voltage of that battery drop quickly and significantly the instant that I connected the capacitor to it.

Im no expert. Not even close! Im just now learning enough about this stuff to be dangerous so take that into consideration as you read on. I believe a lot of how capacitors and batteries interact with each other is based much on the comparative internal resistances of each. Like water, current will flow most readily into areas where there is little resistance to its flow. Capacitors have less internal resistance than batteries so hooking up an "empty" capacitor to a battery will cause current to want to flow from the battery, into the capacitor as quickly as it can get there until the volume of the battery and capacitor equalize. The voltage of a capacitor is entirely dependent upon the voltage of the battery that its connected to. In a set up where a capacitor is constantly connected to a battery bank in parallel, there should never be a time when the capacitor gets "emptied out" since the instant a load is applied to the system, current from the batteries will begin "pouring" into the capacitor trying to keep the capacitor and batteries equalized. I wrote all that to reason it out for myself as much as to try and explain it to others so hopefully it makes at least some sense. And if Ive got anything wrong PLEASE correct me! Its one of the ways I learn.

 

[Bob B]

My understanding is, in the scenario that you suggested, that even as current is being pulled from the capacitor, the capacitor is pulling from the battery bank as the capacitor attempts to maintain equilibrium with the battery bank. I think the only time there might be an issue is if you connect an "empty" capacitor to a battery bank. The capacitor will attempt to equalize with the battery bank VERY quickly. Maybe too quickly depending upon the battery bank youre connecting the capacitor to. Thats why I was asking if connecting a Supercap to a single 100ah battery or four 100ah batteries connected in parallel makes a difference. I suppose the best option is to always pre-charge a supercap before connecting it to a battery bank.

The first time I connected my Supercap to my AGM battery bank, I had pre-charged it a couple of days before and it was down to 8v. My 900ah AGM battery bank, made up of seven AGM batteries in parallel was at 13v. I had also installed a high amp, DC on/off switch(canablized from an old jumper pack) on the negative cable of my capacitor bank. Keeping this switch off during connection prevented sparking issues. Once I connected the Supercap to my AGM battery bank, I monitored the voltage of my battery bank and turned the capacitor switch on. I saw a VERY slight drop in voltage from my AGM battery bank despite the Supercap being nearly depleted upon connection. I suspect this is because the supercap was equalizing from a 900ah AGM battery bank and not a single 100ah AGM battery. Had I connected a near empty super capacitor to a single 100ah AGM battery, I suspect that I would have seen the voltage of that battery drop quickly and significantly the instant that I connected the capacitor to it.

Im no expert. Not even close! Im just now learning enough about this stuff to be dangerous so take that into consideration as you read on. I believe a lot of how capacitors and batteries interact with each other is based much on the comparative internal resistances of each. Like water, current will flow most readily into areas where there is little resistance to its flow. Capacitors have less internal resistance than batteries so hooking up an "empty" capacitor to a battery will cause current to want to flow from the battery, into the capacitor as quickly as it can get there until the volume of the battery and capacitor equalize. The voltage of a capacitor is entirely dependent upon the voltage of the battery that its connected to. In a set up where a capacitor is constantly connected to a battery bank in parallel, there should never be a time when the capacitor gets "emptied out" since the instant a load is applied to the system, current from the batteries will begin "pouring" into the capacitor trying to keep the capacitor and batteries equalized. I wrote all that to reason it out for myself as much as to try and explain it to others so hopefully it makes at least some sense. And if Ive got anything wrong PLEASE correct me! Its one of the ways I learn.

In my opinion .... You got all that right.

 

[jjchvl]

So .... if there is no charger in the circuit ... or there is a solar charger and it's dark ... and load is reduced or completely removed .... the battery and capacitor voltage levels come back up .... what recharged the capacitor?

Anyway, I think thats what toms meant when he said that "there is no capacitor recharge from the battery bank". That once connected and equalized with the battery bank, a super capacitor simply maintains at whatever the battery bank is at. Once connected, a capacitor is never actually depleted, even when the system is under load, so wouldnt need to be "charged". Does that make sense?

 

[jjchvl]

In my opinion .... You got all that right

Thank you so much for reading that long mess and giving me feedback. I appreciate it very much!

 

[Bob B]

Anyway, I think thats what toms meant when he said that "there is no capacitor recharge from the battery bank". That once connected and equalized with the battery bank, a super capacitor simply maintains at whatever the battery bank is at. Once connected, a capacitor is never actually depleted, even when the system is under load, so wouldnt need to be "charged". Does that make sense?

It is not completely depleted ... but charge will be removed from it if a sudden load pulls down the battery voltage. When the load is removed and the battery voltage goes back up, the battery will replace the charge to the capacitor in the scenario I posted.

 

[jjchvl]

It is not completely depleted ... but charge will be removed from it if a sudden load pulls down the battery voltage. When the load is removed and the battery voltage goes back up, the battery will replace the charge to the capacitor in the scenario I posted.

So technically speaking, the batteries actually do "recharge" a capacitor that they are connected to. Considering that they are separate types of storage devices and one must stay "topped off" using current from the other-AKA re-charging.

 

[jjchvl]

It is not completely depleted ... but charge will be removed from it if a sudden load pulls down the battery voltage. When the load is removed and the battery voltage goes back up, the battery will replace the charge to the capacitor in the scenario I posted.

I find all of this kind of stuff incredibly interesting. Im having a great time learning about it but feel like Im way behind at my age. I wish Id gotten more into the field of electricity a long time ago.

 

[Bob B]

So technically speaking, the batteries actually do "recharge" a capacitor that they are connected to. Considering that they are separate types of storage devices and one must stay "topped off" using current from the other-AKA re-charging.

That would be my opinion.

 

[Warpspeed]

One thing about supercaps to realize is that the stored charge is proportional to voltage squared.
In order to remove charge from the supercap to do some useful work, the voltage MUST fall.

Basically there is some finite charge on the supercap at full battery voltage.
Connect a huge sudden load to both, and the voltage across both supercap and battery will fall.
What is sourced by the battery, and what is sourced by the supercap, depends mostly on the internal resistance of the battery, and the value of stored charge on the supercap. The internal resistance of the supercap will be incredibly low and can probably be discounted.

While its true that an infinitely lage supercap will have negligible voltage dip, that would also cost an infinite amount of money.
With practical sized supercaps, the dip will be smaller, maybe much smaller, but it will not go completely away.

What all this means is that if you have a relatively weak (small) battery, there may be very large dips in voltage during high surge loads, and a supercap can help a great deal with for example, the infamous light flicker problem.

If you have a huge lithiuum battery that is large enough to easily supply peak surge loads, with only a very small voltage dip, adding a supercap is going to do very little to improve things. The voltage HAS to dip before the supercap will supply any current at all. The worse the dip, the more a supercap will earn its keep.

Now the situation arises, if you have a significant voltage dip problem, should the money be spent on adding supercaps, or on upgrading the battery by adding extra cells?
Its not always a clear cut decision.
Both batteries and supercaps are expensive, so its well worth thinking the whole thing right through.

But a supercap will only help paper over the voltage dip problem and give no worthwhile increase in total stored battery amp hours.
A significantly larger battery is going store many extra amp hours, and at mid winter for example, that might be money much better spent than on supercaps, especially if the bigger battery also cures the voltage dip problem.

Personally I would rather spend the money on a larger and better battery, than on supercaps.
But I am sure others will disagree strongly with that.

 

[TorC]

Looks right to me, Warpspeed. The place I think the supercap would be helpful is by the inverter after a long wire to the inverter that has drop under high surge. And yet, even there, you'd almost certainly be better off upgrading the wire or moving the inverter closer rather than adding the supercaps, if I'm any judge.

If you have the supercaps lying around because you bought them for something else and didn't use them, or you find it interesting to play with, by all means install them if there's a hint of issue they might help solve. Otherwise, from my reading, it's probably pretty rare it's financially worthwhile.

 

[toms]

So .... if there is no charger in the circuit ... or there is a solar charger and it's dark ... and load is reduced or completely removed .... the battery and capacitor voltage levels come back up .... what recharged the capacitor?

That’s the thing, there is negligible voltage drop with a supercap in parallel - it doesn’t sag and return like a LiFePO4 by itself.

 

[toms]

One thing about supercaps to realize is that the stored charge is proportional to voltage squared.
In order to remove charge from the supercap to do some useful work, the voltage MUST fall.

Basically there is some finite charge on the supercap at full battery voltage.
Connect a huge sudden load to both, and the voltage across both supercap and battery will fall.
What is sourced by the battery, and what is sourced by the supercap, depends mostly on the internal resistance of the battery, and the value of stored charge on the supercap. The internal resistance of the supercap will be incredibly low and can probably be discounted.

While its true that an infinitely lage supercap will have negligible voltage dip, that would also cost an infinite amount of money.
With practical sized supercaps, the dip will be smaller, maybe much smaller, but it will not go completely away.

What all this means is that if you have a relatively weak (small) battery, there may be very large dips in voltage during high surge loads, and a supercap can help a great deal with for example, the infamous light flicker problem.

If you have a huge lithiuum battery that is large enough to easily supply peak surge loads, with only a very small voltage dip, adding a supercap is going to do very little to improve things. The voltage HAS to dip before the supercap will supply any current at all. The worse the dip, the more a supercap will earn its keep.

Now the situation arises, if you have a significant voltage dip problem, should the money be spent on adding supercaps, or on upgrading the battery by adding extra cells?
Its not always a clear cut decision.
Both batteries and supercaps are expensive, so its well worth thinking the whole thing right through.

But a supercap will only help paper over the voltage dip problem and give no worthwhile increase in total stored battery amp hours.
A significantly larger battery is going store many extra amp hours, and at mid winter for example, that might be money much better spent than on supercaps, especially if the bigger battery also cures the voltage dip problem.

Personally I would rather spend the money on a larger and better battery, than on supercaps.
But I am sure others will disagree strongly with that.

The main reason i use a supercap in parallel with my LiFePO4 is to allow a relatively small battery to power a large inductive load while still always remaining below 0.5C discharge.

Effectively i halved the battery requirement.

Every off-grid system i install has two independent systems, i find that it is most cost effective to have one system to power all large inductive loads (smaller battery // supercap), and one larger battery to power critical loads.

Things like irrigation pumps / RC aircon / heat pump HWS / workshop equipment etc are generally non-critical, and that battery can reach inverter LVD with no issues, allowing the most effective use of capacity.

I certainly don’t advocate using a supercap just for kicks - they can be useful though, and for some reason their actual behaviour is often misunderstood.

 

[jjchvl]

One thing about supercaps to realize is that the stored charge is proportional to voltage squared.
In order to remove charge from the supercap to do some useful work, the voltage MUST fall.

Basically there is some finite charge on the supercap at full battery voltage.
Connect a huge sudden load to both, and the voltage across both supercap and battery will fall.
What is sourced by the battery, and what is sourced by the supercap, depends mostly on the internal resistance of the battery, and the value of stored charge on the supercap. The internal resistance of the supercap will be incredibly low and can probably be discounted.

While its true that an infinitely lage supercap will have negligible voltage dip, that would also cost an infinite amount of money.
With practical sized supercaps, the dip will be smaller, maybe much smaller, but it will not go completely away.

What all this means is that if you have a relatively weak (small) battery, there may be very large dips in voltage during high surge loads, and a supercap can help a great deal with for example, the infamous light flicker problem.

If you have a huge lithiuum battery that is large enough to easily supply peak surge loads, with only a very small voltage dip, adding a supercap is going to do very little to improve things. The voltage HAS to dip before the supercap will supply any current at all. The worse the dip, the more a supercap will earn its keep.

Now the situation arises, if you have a significant voltage dip problem, should the money be spent on adding supercaps, or on upgrading the battery by adding extra cells?
Its not always a clear cut decision.
Both batteries and supercaps are expensive, so its well worth thinking the whole thing right through.

But a supercap will only help paper over the voltage dip problem and give no worthwhile increase in total stored battery amp hours.
A significantly larger battery is going store many extra amp hours, and at mid winter for example, that might be money much better spent than on supercaps, especially if the bigger battery also cures the voltage dip problem.

Personally I would rather spend the money on a larger and better battery, than on supercaps.
But I am sure others will disagree strongly with that.

Thank you for taking the time to write all of that out. Parts of it helped me make a "little" more sense out of what Ive seen others say in another thread related to supercaps. It also gives me at least one other direction to take in my Supercap research.

Im a full time RV'r living in a 38ft Motorhome that Ive got parked inside a 44x16 foot garage. Ive got 2760w of solar panels on the roof of the garage, a 1000ah LifePo primary storage bank and 900ah's of AGMs as "backup" storage. My biggest electrical draws are a 700w microwave, a medium sized, very energy efficient compressor type fridge, and the RV's original propane hot water heater that Ive converted to propane/electric using an electric conversion kit. The fridge cycling on is practically un-noticeable whether Im drawing from the lithium battery bank or the AGM and then pulls about 9 or 10 DC amps as it continues to run. The microwave causes the voltage of both battery banks to dip as it continues to run over a couple of minutes-but the AGM's more significantly so, and the hot water heater pulls about 4 amps at 120v so probably a steady 40amps from my batteries via the inverter while actively heating water. The water heater effects the voltage of both my battery banks as well but again, more so in the AGM bank and not to a point of low voltage disfunction for either bank. Im talking after dark for all of this by the way. Not during daylight charging hours. I also have a workshop filled with power tools (table saw, lathe, routers, etc.) which I rarely use these days. On the occasions that I do go out and use them, my system doesnt seem to have much trouble powering the tools up individually. Having just two arms and two hands, I dont normally use more than one power tool at a time anyway, so that works out well.

So after analyzing my "current" needs (see what I did there??) and judging from what you explained in your post, I dont need to use a super capacitor with either of the two battery banks Ive got connected to my system. To be honest, I never thought my system "needed" a super capacitor in the first place. I got into this Supercap kick after cleaning up my garage and rediscovering a supercap Id made a few years back while in the midst of my first and only other supercap kick. That got me looking into supercaps again and I stumbled upon a couple of articles written by people who had connected supercaps to the battery banks of their solar charging systems. Continuing my search for more information on the topic quickly led me back to this forum, hoping to take advantage of all you folk who are generous enough to give away such knowledge for free! Please keep in mind that I didnt have to know very much about supercaps to "make" a supercapacitor bank. A poorly trained chimp can solder together a bank of supercaps after watching a YouTube tutorial on the subject.

So anyway, I connected my homemade supercap bank to my AGM battery bank simply because I had a supercap bank just lying around already and figured connecting it up to the AGMs would do no harm. Unless someone tells me that the supercap could do harm by staying attached to my AGM's, Ill probably leave it there.

In the meantime, I hope to learn more about the different "sizes/capacities/ratings" of capacitors in relation to their potential for stored energy goes-something you alluded to in the first line of your post. For example, I understand that any size capacitor can be safely connected to a battery as long as the capacitor has a voltage capacity that is greater than that of the battery. So a 48v capacitor can be connected to a 12v battery but the capacitors "charge" will be limited to the voltage of the battery. What I dont understand yet is how to translate the various specs of a capacitor-such as its max voltage, farad rating and so on-into how much "energy/charge" it has the potential to store.

Take my homemade Supercap bank, for example: Its constructed from six 2.7v, 500f supercaps. I know without really knowing why that this translates into a 16.2v, 83f supercap bank(I understand how voltage adds up in a series connection but know nothing about farads). I soldered balancing disks to each of the six "cells" which limits the voltage of the entire bank to 15v. This is pretty much all that I know about the technicalities of how much energy can be stored by a particular supercap. I want to know much more about the how and why's. I dont expect to learn it all here but am very grateful to anyone who offers me any insight into the topic.

Yup. Another way too long post. Please forgive. i get carried away while typing sometimes....

 

[Bob B]

That’s the thing, there is negligible voltage drop with a supercap in parallel - it doesn’t sag and return like a LiFePO4 by itself.

That's only true for a very short load spike .... It will if the load is extended.

Kinda seems like you totally changed your opinion from your previous post.

There is no capacitor recharge from the battery bank. Under load surge condition the voltage of the battery and capacitor drop together, but much more current comes from the capacitor. When the load is removed the two are still at the same voltage - no current flow. The recharge comes from the charger.

For some reason most people think the supercap discharges into the battery but that is not the case.