Greetings fellow electro-heads! First-time member/poster here, so I'm hopeful for a warm welcome! =)

I am currently designing a fun personal-vehicle project involving an electric catamaran/quad amphibious vehicle, and I am reaching out for some advice about the powertrain possibilities for a water-jet propulsion system in each of the two hulls.

I would like to use a series of LiFePO4 3.2VDC, 280Ah prismatic batteries (in series blocks of 12VDC to the most efficient voltage (e.g. 12, 24, 48, 72VDC?)) in combination with supercapacitors on protection boards to balance the load, absorb any load amperage spikes, provide some quick charging capacity, and generally enhance the lifetime/durability of the system.

I'm looking for advice specifically about the potential configuration of the power system and the optimization thereof:
1) Take a "Green-Cap" 2.7VDC, 500F Supercapacitor for example--could one of these (paired with a PCB voltage protection board) be connected directly to a LiFePO4 3.2VDC, 280Ah prismatic battery (i.e. one on each battery in series), or do the voltages have to match exactly? If so, would that be the ideal configuration for supplementing the batteries with supercapacitors evenly distributed vs. a separate bank of SCs at the head of the battery "train" acting in parallel (i.e. does it matter the distribution of supercapacitors?)?
2) Ideally (and if budget permits) I was envisioning four 12VDC LiFePO4 battery cells in series and, in conjunction with, a 16.2VDC 48-supercapacitor bank (6x8 array of 2.7VDC, 500F cells with PCBs) in each of the two roughly 15-ft-long catamaran hulls (double water-tight). Would a 2,700W solar array serving as the roof be sufficient to supplement the batteries with intermittent motor use throughout the daylight hours?
3) In your opinion, what are the ideal motor specs that I should utilize for this application?
4) Any other helpful suggestions or criticisms? =)

Thank you in advance for your time and consideration!

~TMac~

 

Welcome to AAC!

Take a "Green-Cap" 2.7VDC, 500F Supercapacitor for example--could one of these (paired with a PCB voltage protection board) be connected directly to a LiFePO4 3.2VDC, 280Ah prismatic battery

No. You'd destroy the cap, since 3.2V > 2.7V.

I was envisioning four 12VDC LiFePO4 battery cells in series and, in conjunction with, a 16.2VDC 48-supercapacitor bank (6x8 array of 2.7VDC, 500F cells with PCBs)

Four "12V" batteries (actually each battery may be only 3 cells in series) in series = 48V. But 48V is > 16.2V, so that also would destroy the caps.

 

Welcome to AAC.

Have you decided to add capacitors because you know the LiFePO4 cells can’t produce sufficient peak current?

If not, I fear you may be engaged in premature optimization, a common mistake even among experienced engineers. Assuming. this is the case (you don’t have numbers yet) consider that you are adding a great deal of complexity where it may not be required.

Very often as designs mature they become more simple, not more complicated. If you get a head start on this a by building from the simple up, you can save a lot of money and time. Before designing something so complex iut is a very good idea to build simple prototypes to test your assumptions.

In any case, even if the capacitors are a good idea, the complexity of including them shound not be underestimated. You’ll need to consider how to manage the very high current, and in particular, the very high current demand of the depleted capacitor when you attach a power supply to charge the system.

If you’ve already done this groundwork, and understand the implications of ythe capacitors high current output and near dead short nature when not charged, please don’t take this wrong—I am basing this advice on what seems to be the case taking into account your questions.

One more point: it seems to be the case that you are choosing components, then designing the circuits This is upside down. You should have a clear list of requirements for the circuits, then choose components. This is one way to avoid the putative solution turning into the problem. Seek to solve the problem as defined by what the system is going to do not by whitch parts you naïvely chose before fully characterizing the problem.

Attributes like cost, runtime, top speed, component lifetime, environmental compatibility, ease of construction, and others must be balanced so a plan fits what you can reasonably expect before starting. You will, of course, be surprised by things you haven’t thought of—but when von Moltke said “no plan survives first contact with the enemy“ he certainly wasn’t counseling not making one. He was saying you needed to be flexible and adjust. If you have already nailed down major components, how can you?

 

Supercapacitors, used in conjunction with LIFePO4, are expected to make a big impact in EV power trains in the coming years but they aren't there yet. They are good for high-current specific power demand, but poor at long-term energy delivery. Thus they complement batteries. But their use in this way is complex and isn't just a case of tying them together with bits of wire! AFAIK only one commercially available EV uses the technology today and that's a Lambo! The energy management systems to look at and understand the instantaneous EV demand and route power from either battery or SC as appropriate are non-trivial and, as yet, not supported by tested and trusted off-the-shelf technology.

 

Welcome to AAC!

No. You'd destroy the cap, since 3.2V > 2.7V.

Thank you for the welcome and response! So if I were to consider caps, they would need to match the voltage of the batteries is what you're saying?

 

To summarize, you design the system to the actual requirements, not what you think the requirements might be.

But if you don't even know that you can't apply more voltage to a capacitor than its rating, I am not optimistic that you will succeed in building a workable design.

 

IMHO you should forget the super capacitors and start with someting simpler like a collection of marine deep cycle batteries. This will allow you to get yor feet wet, so to speak, without excess complexity.

 

So if I were to consider caps, they would need to match the voltage of the batteries is what you're saying?

Depending on what you mean by "match", that's not what I'm saying. Simply putting the caps in parallel with a battery would require the battery voltage to be no more than the rated maximum voltage of the cap.
If you had caps in series then the series string would need charge and discharge management circuitry to ensure no one cap ever received more than its rated maximum voltage.
As others have said, you should set out the requirements of your system. For starters, is your catamaran going to be raced, or is it merely for trolling at 1mph? The power requirements would be completely different.

Edit:
Doing the math, one 2.7V 500F supercap can store about 1.8kWs of energy, of which about half might be usefully recovered before the cap voltage dropped too far for any DC-to-DC converter to work efficiently.
Assuming a 1kW motor, that should be enough energy to power the motor for about 1 second. So 48 caps would power the motor for 48 seconds. Is that going to be sufficient for your needs, given that we don't know the intended purpose of the caps?

 

Welcome to AAC.

Thank you! I appreciate you taking the time to reply!

Have you decided to add capacitors because you know the LiFePO4 cells can’t produce sufficient peak current?

I don't necessarily "know" that the LiFePO4 cells can't produce sufficient peak current, but I thought that the capacitors would alleviate some strain on the batteries (mainly power from dead stop while accelerating) and help them last longer, acting as a "buffer" of sorts.

If not, I fear you may be engaged in premature optimization, a common mistake even among experienced engineers. Assuming. this is the case (you don’t have numbers yet) consider that you are adding a great deal of complexity where it may not be required.

Very often as designs mature they become more simple, not more complicated. If you get a head start on this a by building from the simple up, you can save a lot of money and time. Before designing something so complex iut is a very good idea to build simple prototypes to test your assumptions.

I am totally open to the possibility that it could be overly complex; however, if the concept of using supercapacitors would be beneficial for the system then I am interested to learn how the circuit should look and be implemented correctly.

In any case, even if the capacitors are a good idea, the complexity of including them shound not be underestimated. You’ll need to consider how to manage the very high current, and in particular, the very high current demand of the depleted capacitor when you attach a power supply to charge the system.

I do understand that capacitors charge and discharge differently than batteries, and could plot the differences on a P vs t graph, but I am curious as to whether or not there would be a benefit to adding the capacitors (if done so correctly). Also, with the capacitors being in the circuit with the batteries, as the charge in the capacitors is depleted via the motor load, would they be pulling charge from the batteries simultaneously? As for a charging power supply, I would probably integrate an onboard charging unit to ensure proper charging from shore power.

One more point: it seems to be the case that you are choosing components, then designing the circuits This is upside down. You should have a clear list of requirements for the circuits, then choose components. This is one way to avoid the putative solution turning into the problem. Seek to solve the problem as defined by what the system is going to do not by whitch parts you naïvely chose before fully characterizing the problem.

Fair enough--I chose some components just to use for example and for the sake of this discussion, and drew a quick 3D sketch for visual reference--however, my approach to the system as a whole does consist of the attributes that you listed (i.e. "...cost, runtime, top speed,..." etc.). I was attempting to ask the experts here for advice on the specific topic of how (and if) adding capacitors would work in conjunction with batteries for a powertrain, and what that should look like. Understanding this would help me make those design decisions.

I have seen people use capacitors to replace a car battery, and in conjunction with a small lithium battery to supplement and allow for additional starting attempts, but I don't have the personal experience to know how well that would work. If it would take special charge control circuitry (i.e. having to add a computer), then I will probably scrap the capacitors and just use batteries in a simple circuit, especially since I'm not going to be racing this craft or anything with high demand.

 

Supercapacitors, used in conjunction with LIFePO4, are expected to make a big impact in EV power trains in the coming years but they aren't there yet. They are good for high-current specific power demand, but poor at long-term energy delivery. Thus they complement batteries. But their use in this way is complex and isn't just a case of tying them together with bits of wire! AFAIK only one commercially available EV uses the technology today and that's a Lambo! The energy management systems to look at and understand the instantaneous EV demand and route power from either battery or SC as appropriate are non-trivial and, as yet, not supported by tested and trusted off-the-shelf technology.

Thanks for the reply! I was afraid of this...I have designed and built simple circuits before (and have even taken a couple college intro courses in electrical engineering), but the folks I've talked to previously about the battery/SC complimentary system made it seem like it was more simple in nature and didn't require complex circuitry. If that is not the case, however, then I will probably just stick with batteries.

 

Depending on what you mean by "match", that's not what I'm saying. Simply putting the caps in parallel with a battery would require the battery voltage to be no more than the rated maximum voltage of the cap.
If you had caps in series then the series string would need charge and discharge management circuitry to ensure no one cap ever received more than its rated maximum voltage.

Understood, thanks. Seeing as how adding caps would require charge/discharge management circuitry, it makes sense, but probably not for my application.

As others have said, you should set out the requirements of your system. For starters, is your catamaran going to be raced, or is it merely for trolling at 1mph? The power requirements would be completely different.

The catamaran is not going to be raced, so I would not really need a capacitor charge for quick power at peak demand. I was only considering them to complement the batteries.

Edit:
Doing the math, one 2.7V 500F supercap can store about 1.8kWs of energy, of which about half might be usefully recovered before the cap voltage dropped too far for any DC-to-DC converter to work efficiently.
Assuming a 1kW motor, that should be enough energy to power the motor for about 1 second. So 48 caps would power the motor for 48 seconds. Is that going to be sufficient for your needs, given that we don't know the intended purpose of the caps?

The caps themselves in this scenario would obviously not be sufficient when considering overall runtime. I've just seen other scenarios where the capacitors act to "flatten" the discharge curve of the battery by minimizing the spikes in power demand. If that is not how it works, however, then I will steer away from capacitors because I wouldn't otherwise have a need for a large/quick release of power.

 

Thanks for the reply! I was afraid of this...I have designed and built simple circuits before (and have even taken a couple college intro courses in electrical engineering), but the folks I've talked to previously about the battery/SC complimentary system made it seem like it was more simple in nature and didn't require complex circuitry. If that is not the case, however, then I will probably just stick with batteries.

Just putting the Caps across the batteries achieves relatively little with good Lithium cells as the internal resistance is very similar. A 280Ah LiFePO4 cell can easily handle 5C discharge - that's a 1000A! I seriously doubt your water jet motor will ever pull that sort of current, let alone the controller supporting it. The busbars etc to handle that sort of current are substantial and a supercapacitor with that sort of capability is going to be a substantial unit costing 1000s - its not going to be a bunch of PCB-lead caps wired together.

Are you buying an off-the-shelf propulsion unit or planning to build one from scratch? Do you have any idea of the motor parameters for this requirement?

 

Just putting the Caps across the batteries achieves relatively little with good Lithium cells as the internal resistance is very similar. A 280Ah LiFePO4 cell can easily handle 5C discharge - that's a 1000A! I seriously doubt your water jet motor will ever pull that sort of current, let alone the controller supporting it. The busbars etc to handle that sort of current are substantial and a supercapacitor with that sort of capability is going to be a substantial unit costing 1000s - its not going to be a bunch of PCB-lead caps wired together.

Thanks again for your patience with answering my questions! That explains a lot and is helpful. Would the caps

Are you buying an off-the-shelf propulsion unit or planning to build one from scratch? Do you have any idea of the motor parameters for this requirement?

The propulsion unit for each jet-drive system will be a combination of off-the-shelf items assembled together:
1) 1.5kW to 2.0kW 48VDC motor (or somewhere in that neighborhood), with max RPM around 4K to 5K I think. I am open to suggestions here, but the idea would be to have enough speed and torque to propel the craft up to about 20-30 knots max.
https://www.amazon.com/Mophorn-Elec...489508&sprefix=kw+48v+dc+motor,aps,183&sr=8-3

2) I thought about fabricating the jet-drive, but I found these from China for about $800-$900 each, so I may go with these. They are made for jet-skis obviously, but I'm planning to run them at a relatively lower RPM.
https://www.alibaba.com/product-det...473.html?spm=a2700.details.0.0.53456cd5KUvlXA

3) I need a third motor to propel the vehicle on 26" MTB wheels (common rear axle with sprocket), which I imagine would be similar to the motors above. I've figured out a mechanism to make the "landing gear" retractable for amphibious capability.

 

Thanks again for your patience with answering my questions! That explains a lot and is helpful. Would the caps


The propulsion unit for each jet-drive system will be a combination of off-the-shelf items assembled together:
1) 1.5kW to 2.0kW 48VDC motor (or somewhere in that neighborhood), with max RPM around 4K to 5K I think. I am open to suggestions here, but the idea would be to have enough speed and torque to propel the craft up to about 20-30 knots max.
https://www.amazon.com/Mophorn-Electric-Brushless-Controller-Motorcycle/dp/B07KF8M5W6/ref=sr_1_3?crid=3LL8QKUTS9DUS&keywords=kw+48V+DC+motor&qid=1664489508&sprefix=kw+48v+dc+motor,aps,183&sr=8-3

2) I thought about fabricating the jet-drive, but I found these from China for about $800-$900 each, so I may go with these. They are made for jet-skis obviously, but I'm planning to run them at a relatively lower RPM.
https://www.alibaba.com/product-det...473.html?spm=a2700.details.0.0.53456cd5KUvlXA

3) I need a third motor to propel the vehicle on 26" MTB wheels (common rear axle with sprocket), which I imagine would be similar to the motors above. I've figured out a mechanism to make the "landing gear" retractable for amphibious capability.

Just putting the Caps across the batteries achieves relatively little with good Lithium cells as the internal resistance is very similar. A 280Ah LiFePO4 cell can easily handle 5C discharge - that's a 1000A! I seriously doubt your water jet motor will ever pull that sort of current, let alone the controller supporting it. The busbars etc to handle that sort of current are substantial and a supercapacitor with that sort of capability is going to be a substantial unit costing 1000s - its not going to be a bunch of PCB-lead caps wired together.

Are you buying an off-the-shelf propulsion unit or planning to build one from scratch? Do you have any idea of the motor parameters for this requirement?

Hi Irving, would you be available to help me design a circuit for this system using LiFePO4 batteries, BLDC electric motors with controllers (2x for each of the jet drives, and 1x for the wheel/land drive), and solar charging system with BMS? The main area I need help with is the selection of the motors/controllers and the optimum voltage at which I should run everything. If not, that's ok, I'm just putting it out there! Thanks!

 

Hi Irving, would you be available to help me design a circuit for this system using LiFePO4 batteries, BLDC electric motors with controllers (2x for each of the jet drives, and 1x for the wheel/land drive), and solar charging system with BMS? The main area I need help with is the selection of the motors/controllers and the optimum voltage at which I should run everything. If not, that's ok, I'm just putting it out there! Thanks!

Happy to provide input. You need the specs for the water jet units, some info about your catamaran hull and, of course, your expectations on performance. Only then can we see what's reasonably achievable and what motors will deliver the goods...

I can tell you now solar charging is probably going to be disappointing...

 

Happy to provide input. You need the specs for the water jet units, some info about your catamaran hull and, of course, your expectations on performance. Only then can we see what's reasonably achievable and what motors will deliver the goods...

I can tell you now solar charging is probably going to be disappointing...

HI Irving, thank you for taking interest! I have attached the spec's for the water jet drives, and I will describe the hull design below. The performance expectations are such that I would like a relatively low max cruising speed, as I am not planning on racing or charging into large waves (mainly fresh/brackish water).

The twin hull design is currently drawn at 15'-0"L x 1'-6"W x 2'-0"D for each hull, having a "tall and narrow" version of the classic "V" hull cross section, with a sharp bough and a flat back, slightly tapered again towards the rear (almost like a throwing knife, but not that exaggerated), so that it's able to displace the water more like a vertical, symmetrical hydrofoil. The overall width from port-outside-hull to starboard-outside-hull cannot exceed 96" (8ft) to comply with FL vehicle regulations.

As for the solar recharging, I am fully aware that it will not produce enough charge to maintain charge capacity indefinitely....however, I figured it couldn't hurt to keep a potential "positive" charge of about 2,700W continuous on a good day with qty. 9x of at least 300W solar panels each and a charge controller with BMS. I envision a short drive on land to a water entry point, switch to jet drive on water for a day-long fishing or leisure trip, charging with solar throughout (mostly while stopped at each waypoint), and returning to origin with enough battery charge to get me back.

Despite the solar aspect, I would like to have enough battery capacity to run the motors for 6 to 8 hours if possible at a modest water cruising speed (at least 10 to 15 knots) and an on-land speed of about 45-50MPH max.

 

Do you have a drawing/CAD filefor the hull?

 

Do you have a drawing/CAD filefor the hull?

I am currently working on a SketchUp model, which you may find here: https://3dwarehouse.sketchup.com/mo...6693/CarCat-Amphibious-Solar-EV-v10-TMcKenzie

The model is incomplete and not fully-detailed. It is for private collaboration ONLY, and is not intended to be shared for commercial use. Please do not copy or share without permission (general disclaimer--don't take it personally lol). The hull dimensions are not set in stone, as they are represented as single-dimension line drawings in the model.

 

I am currently working on a SketchUp model, which you may find here: https://3dwarehouse.sketchup.com/mo...6693/CarCat-Amphibious-Solar-EV-v10-TMcKenzie

The model is incomplete and not fully-detailed. It is for private collaboration ONLY, and is not intended to be shared for commercial use. Please do not copy or share without permission (general disclaimer--don't take it personally lol). The hull dimensions are not set in stone, as they are represented as single-dimension line drawings in the model.

Hmmmm.. 3DWarehouse says it can't find it... can you export as a STEP file or some other 3D format?

 

Hmmmm.. 3DWarehouse says it can't find it... can you export as a STEP file or some other 3D format?

I think it’s because I made it private. I can send you the .skp file (~34MB). Is there a way to direct msg or email?