The title of the post is "DC converter for gas generator" I was wondering if it would be feasible to get a smaller bank and boost it. What the supercapacitor bank is powering is irrelevant. I gave the voltage the generator put out and asked is 24v was feasible instead of 48v. All the info was in the OP

No, all the info wasn’t in the initial post. The current requirement specifically wasn’t posted until post #15. That was what was frustrating! I asked for the output/load current requirement first in post #4.

 

Just a dumb question of mine. What are kids being taught today in science/electronics? I ask because they seem to think that by adding the words"super" or "ultra" to the word capacitor, it changes what a capacitor is and how it works? Unless I'm wrong, don't "super caps" still live by the same rules, the time constant?

 

Just a dumb question of mine. What are kids being taught today in science/electronics? I ask because they seem to think that by adding the words"super" or "ultra" to the word capacitor, it changes what a capacitor is and how it works? Unless I'm wrong, don't "super caps" still live by the same rules, the time constant?

Super or Ultra capacitors are EDLC capacitors and are completely different from electrolytic capacitors.

A capacitor bank does not behave like a battery. A battery gives a reasonably constant voltage for a large part of its discharge cycle. The output voltage of a capacitor bank varies greatly depending on it's state of charge. So a 48 volt capacitor bank would be giving out 33.9 volts (48/root 2) when it's charge is 50%. If you try to supply it with 48 volts in that state you would require an infinite current. You would require a charging circuit that would provide a constant output power until the output voltage reached 48 volts. This power would be the maximum output of the generator. The motor driver circuit would also need to give a variable output voltage (Depending on the required speed.) for a wide range of input voltages. (Depending on the state of charge of the capacitor bank.)

Les.

The generator will be keeping the Supercapacitor bank at a constant 48v, the discharge curve of supercapacitors is irrelevant

OK, no 24 volt is not feasible in this case. Why do I say that? I want to power a motor, a 1.0 HP DC variable speed permanent magnet motor. At 90 VDC this motor will typically have a FLA (Full Load Amp rating of about 10 Amps and the reality is my motor under load delivering rated HP including losses between heat, friction and other losses will require 1 KW. So now what becomes easier since 1.0 KW at 48 volts is equal to 1,000 / 48 = 20.83 amps or if I half the voltage 1,000 / 24 = 41.66 amps. The idea being it's easier to make the higher voltage lower current control than a lower voltage higher current control. Additionally the same is true of the motor. Now, based on the information given if I had the choice since I have a 48 VDC generator rated for a max current of 31 amps I would go with a 48 volt system all around. I see no reason to half the voltage and double the current to get the same amount of work done.

How you choose to go about this is entirely up to you. I am merely saying how I would consider doing it based on what I know to be available. I would also be looking at weight between the engine, generator, motor and any capacitor modules to figure out if this approach is even practical. Those capacitor modules are not cheap, even the refurbished ones out there.

Ron

I figured as much. The reason I wanted to try a 24v supercap bank was because 48v banks are super expensive.

You might want to amend your thread title to reflect that?
There are loads of DC-to-DC converters advertised online. Have you checked to see if one is available which meets your input and output requirements? If it does, then buying one would almost certainly be cheaper and easier than building one.

I definitely could have been more specific. I couldn't find any off the shelf DC converters that could handle 40A that we're extremely expensive or Chinese junk

 

Super or Ultra capacitors are EDLC capacitors and are completely different from electrolytic capacitors.

They still have a charge/discharge time constant.
www.mouser.com › pdfdocs › MurataEDLCtechnical_guide_e_rev

 

OK, late to enter this conversation. Here's what I'm getting: You have a bike with a gas motor. You want to add a DC electric motor to it. You want to power it from a super capacitor. You want to keep that cap charged up by a generator also on the bike. You DO realize the load you're going to put on the motor don't you ? ? ? The motor will have to drive the bike and the generator. The generator will have to charge the caps. That charging will be a parasitic drain on the gas motor's ability to move the bike. You want to use an electric motor to assist moving the bike. That motor will draw power from the caps. Those caps will have to be recharged on the fly - and they don't instantaneously charge, just to make you aware of that small factoid. Given the losses due to inefficiencies and all the extra weight - I predict you won't be happy with the outcome.

Sure, there are gas/electric hybrid bikes out there. A TON of engineering has gone into them to make them feasible. Can you do the same? I'm sure you can. But your approach with caps is not likely going to be successful. Charging a battery under normal riding is easier to do, and its power density is better than that of a cap. Plus the size is more compact. GREATLY smaller for the same energy density of battery versus super capacitors.

Yes, they're doing it. Hybrids. But with batteries. Caps have not yet evolved to the same level as batteries. Until that day batteries will reign supreme as an electro-motive power source over caps.

Now, to answer your question "How to regulate the charge?" There are plenty of circuits and controllers out there to do the job. Small and light weight AND capable of the same power requirements you're thinking of. Unfortunately it's not anything I've ever attempted in my very limited skills and hobby. But the answer to your question is out there. The questions I have are "Why caps? Why not batteries?" Lithium batteries are plentiful, small and have significant energy density. That's what powers Tesla's cars, other hybrid cars (and bikes), my laptop and a whole lot of other things. Caps are good for an INSTANT burst. But by "Instant" I mean for a very short period of time - especially if you're drawing off massive amounts of stored energy. And has already been said, as their voltage is consumed they drop off steadily. Batteries go a long time before reaching their energy curve, where they begin to show signs of growing weak on charge.

 

OK, late to enter this conversation. Here's what I'm getting: You have a bike with a gas motor. You want to add a DC electric motor to it. You want to power it from a super capacitor. You want to keep that cap charged up by a generator also on the bike. You DO realize the load you're going to put on the motor don't you ? ? ? The motor will have to drive the bike and the generator. The generator will have to charge the caps. That charging will be a parasitic drain on the gas motor's ability to move the bike. You want to use an electric motor to assist moving the bike. That motor will draw power from the caps. Those caps will have to be recharged on the fly - and they don't instantaneously charge, just to make you aware of that small factoid. Given the losses due to inefficiencies and all the extra weight - I predict you won't be happy with the outcome.

Sure, there are gas/electric hybrid bikes out there. A TON of engineering has gone into them to make them feasible. Can you do the same? I'm sure you can. But your approach with caps is not likely going to be successful. Charging a battery under normal riding is easier to do, and its power density is better than that of a cap. Plus the size is more compact. GREATLY smaller for the same energy density of battery versus super capacitors.

Yes, they're doing it. Hybrids. But with batteries. Caps have not yet evolved to the same level as batteries. Until that day batteries will reign supreme as an electro-motive power source over caps.

Now, to answer your question "How to regulate the charge?" There are plenty of circuits and controllers out there to do the job. Small and light weight AND capable of the same power requirements you're thinking of. Unfortunately it's not anything I've ever attempted in my very limited skills and hobby. But the answer to your question is out there. The questions I have are "Why caps? Why not batteries?" Lithium batteries are plentiful, small and have significant energy density. That's what powers Tesla's cars, other hybrid cars (and bikes), my laptop and a whole lot of other things. Caps are good for an INSTANT burst. But by "Instant" I mean for a very short period of time - especially if you're drawing off massive amounts of stored energy. And has already been said, as their voltage is consumed they drop off steadily. Batteries go a long time before reaching their energy curve, where they begin to show signs of growing weak on charge.

You are describing a parallel hybrid. I am designing a series hybrid. The gas motor only serves as a generator, it does not provide traction. The traction will be provided by a 48v hub motor that is powered by the supercapacitor bank. Also supercapacitors have a higher power density than lithium batteries. You are thinking energy density, which I do not need as the generator will keep the supercap bank charged.

 

I figured as much. The reason I wanted to try a 24v supercap bank was because 48v banks are super expensive.

Sometimes in the world of design it seems life is all about trade offs. So we look at method A and then we look at method B and both methods will get us where we want to be. Then comes the annoying cost factor so we crunch numbers. Then life goes on.

Ron

 

I'm wondering if just a capacitor bank will even help you that much here. When you floor it, the capacitor voltage will quickly drop below the 48v generator output, and it will stop providing any extra current. You will get a very short boost, and use only a small fraction of the total energy stored in the capacitor. An ideal controller for supercapacitor power would need to have a buck/boost convertor, to allow the capacitor voltage to be run down to maybe half, while still giving the full motor voltage. You might also want that for recharging the capacitors, to avoid resistive losses. This would be a totally different controller from what battery electrics use.

 

You are describing a parallel hybrid. I am designing a series hybrid. The gas motor only serves as a generator, it does not provide traction. The traction will be provided by a 48v hub motor that is powered by the supercapacitor bank. Also supercapacitors have a higher power density than lithium batteries. You are thinking energy density, which I do not need as the generator will keep the supercap bank charged.

I'm wondering if just a capacitor bank will even help you that much here. When you floor it, the capacitor voltage will quickly drop below the 48v generator output, and it will stop providing any extra current. You will get a very short boost, and use only a small fraction of the total energy stored in the capacitor. An ideal controller for supercapacitor power would need to have a buck/boost convertor, to allow the capacitor voltage to be run down to maybe half, while still giving the full motor voltage. You might also want that for recharging the capacitors, to avoid resistive losses. This would be a totally different controller from what battery electrics use.

The motor controller is indeed designed for use with a lithium battery, so it will cut off power if the voltage goes under 40-43v.

The supercacitor bank is 54 watt hours. So 54wh*60min= 3240 watt "minutes". Divide that by 4 considering I can only use 1/4 of the packs rated voltage with out a buck converter. 3240w/4= 810 watt " minutes"

I should be ok, but what do you think from those calculations?

 

I should be ok, but what do you think from those calculations?

I get about 169 Farads from those numbers. If you start at 48V and your cutoff is 43V, that's about 38,448 J, or 641 Watt-minutes. For a 40V cutoff it's 991 Watt-minutes, and your estimate is right in the middle of those. What are the specs of your hub motor?

 

I should be ok, but what do you think from those calculations?

That you didn't read the link I gave you.

 

OK, yes, I had in mind a parallel system. However, with a series system - - -

I'm wondering if just a capacitor bank will even help you that much here. When you floor it, the capacitor voltage will quickly drop below the 48v generator output, and it will stop providing any extra current. You will get a very short boost, and use only a small fraction of the total energy stored in the capacitor. An ideal controller for supercapacitor power would need to have a buck/boost convertor, to allow the capacitor voltage to be run down to maybe half, while still giving the full motor voltage. You might also want that for recharging the capacitors, to avoid resistive losses. This would be a totally different controller from what battery electrics use.

The amperage versus the acceleration versus the power drop off - - - I think you'll be disappointed with that approach. A better approach would be to have the gas motor power the generator to power the motor. Either a set RPM governed when the load gets heavy or a linear throttle to provide more power when needed. That way you avoid the weight and bulk size of the caps. You also eliminate a potential failure point. One cap go bad and you're going to have a pretty hot fire under your seat. Or wherever you put them.

Just my uneducated opinion. If others disagree with me - that's OK. I can learn something new. I'm not THAT old a dog.

 

I get about 169 Farads from those numbers. If you start at 48V and your cutoff is 43V, that's about 38,448 J, or 641 Watt-minutes. For a 40V cutoff it's 991 Watt-minutes, and your estimate is right in the middle of those. What are the specs of your hub motor?

48v 1500w continuous
31A continuous
50A peak

 

48v 1500w continuous
31A continuous
50A peak

OK, your generator puts out 31A too, so you're just looking for that extra 19A boost. If my calculation of 169 F was correct, then at 19A, your capacitor bank voltage will drop 1 V every 8.9 seconds. With the engine off and delivering all 50A, the capacitors would drop 1 V every 3.4 seconds. I don't know the series resistance of your generator, so I'm not sure how it will react to the dropping voltage. Will your engine use a governor?

 

I just now was directed towards this thread, and I have read my way from the start to here. So your intention is to have a motorbike with electric drive of some capability and a serious capacitor bank to drive it, instead of simply controlling the generator output to have a really good speed control. OR perhaps the goal is a really high powered drag-style bike. I accidentally did a run on one like that years ago. We had just put in the engine and connected it up, and I did not realize that there was no throttle return spring. So starting it out with a quick blip of the throttle and it was a block long wheel stand and only after the wheel came down was I able to shut the engine down. My point being that you may get that much takeoff power, but it may not drive very well. And an adequate super capacitor bank will cost a WHOLE LOT.

So do you have the drive motor selected yet? Really, picking the drive motor and it's capabilities will allow us to see what mus go in between the generator and the motor. The 1500 watts can provide a nice ride, and a battery could give you a decent startup power.