Considering Maxwell BoostCaps

[erth64net]

I'm considering the use of Maxwell Boostcap Ultracapacitors to capture braking/regen energy, and to assist with initial acceleration. The points not clear are:
- What's the maximum possible charging current when using Maxwell's balance boards?
- Is the derived energy density worth the cost and space required?

Used 2600 Farad (BCAP0010) models are readily available for ~$17/each. Which means a 60V setup would require 24 cells, costing ~$400 (for the cells alone).

With a 60V/100A EV setup, and after roughly accounting for transmission/conversion losses, and capacitor discharge curves, would it be safe to assume 5-10 seconds of acceleration boost?
- Each 2600 F cap has a specific energy density of roughly 2.2575 Wh @ 2.5V (525g/cell & energy density of 4.3Wh/kg).
- Unless I'm mistaken, a 60V serial pack would have roughly 54.18Wh of stored power (0.9A @ 60V).

When using 12 balance boards, is 3.6A @ 60V the maximum possible "charging" current on this pack?
- Maxwell's Boostcap Product Guide recommends balancing the cells.
- Maxwell makes an integration kit that utilizes cell mounted balance boards.
- Each Maxwell balance board can apparently shunt up to 300mA @ 2.5V, into an adjacent cell, 12 boards would be necessary for a 24 cell pack.

 

[major]

I'm considering the use of Maxwell Boostcap Ultracapacitors to capture braking/regen energy, and to assist with initial acceleration. The points not clear are:
- What's the maximum possible charging current when using Maxwell's balance boards?

Maximum charging current is way more than you could ever regenerate. It would be over 1000A.

- Is the derived energy density worth the cost and space required?

Probably not.

Used 2600 Farad (BCAP0010) models are readily available for ~$17/each. Which means a 60V setup would require 24 cells, costing ~$400 (for the cells alone). With a 60V/100A EV setup, and after roughly accounting for transmission/conversion losses, and capacitor discharge curves, would it be safe to assume 5-10 seconds of acceleration boost?

Sure, because you don't define boost or the base rate.

- Each 2600 F cap has a specific energy density of roughly 2.2575 Wh @ 2.5V (525g/cell & energy density of 4.3Wh/kg).

2.2575Wh is not specific energy or energy density but is the total energy per cell at 2.5V.

- Unless I'm mistaken, a 60V serial pack would have roughly 54.18Wh of stored power (0.9A @ 60V).

You are mistaken. 54.18Wh is energy, not power. 0.9A*60V = 54W which is power. So the energy in the caps could deliver 0.9A at 60V for an hour. Or 3240W for one minute, or 32kW for 6 seconds. 32kW at 60V would be 533A.

When using 12 balance boards, is 3.6A @ 60V the maximum possible "charging" current on this pack?
- Maxwell's Boostcap Product Guide recommends balancing the cells.
- Maxwell makes an integration kit that utilizes cell mounted balance boards.
- Each Maxwell balance board can apparently shunt up to 300mA @ 2.5V, into an adjacent cell, 12 boards would be necessary for a 24 cell pack.

The balance boards are an N-1 deal, so you need 23. And they do not support charge current. They just equalize cell voltage by bleeding higher voltage cells down to the level on lower voltage cells.

 

[erth64net]

- What's the maximum possible charging current when using Maxwell's balance boards?

Maximum charging current is way more than you could ever regenerate. It would be over 1000A.

1000A over circuit board traces? Highly doubtful...

Ultracaps are obviously capable of incredible charge/discharge rates, but that's not the only goal here. I've added emphasis above to help clarify.

Used 2600 Farad (BCAP0010) models are readily available for ~$17/each. Which means a 60V setup would require 24 cells, costing ~$400 (for the cells alone). With a 60V/100A EV setup, and after roughly accounting for transmission/conversion losses, and capacitor discharge curves, would it be safe to assume 5-10 seconds of acceleration boost?

Sure, because you don't define boost or the base rate.

Sorry if I was unclear about "...With a 60V/100A EV setup..." What I meant by this, is that the motor/controller combination is rated for no more than 100A at 60V.

- Each 2600 F cap has a specific energy density of roughly 2.2575 Wh @ 2.5V (525g/cell & energy density of 4.3Wh/kg).

2.2575Wh is not specific energy or energy density but is the total energy per cell at 2.5V.

I'm confused by your comments.

The originally linked spec sheet says (emphasis added):
"...
Specific Energy Density: 4.3 (Wh/kg) (2.5 V)
...
Weight: 525 g
...".

So if 4.3Wh * (525/1000) doesn't actually equal 2.2575Wh, I'd greatly appreciate help clarifying.

Further, when converting the spec sheet's "stored energy" of 8,125J to Wh, the result is 2.25694444Wh...which seems awfully close to my calculated energy density of 2.2575Wh/cell.

- Unless I'm mistaken, a 60V serial pack would have roughly 54.18Wh of stored power (0.9A @ 60V).

You are mistaken. 54.18Wh is energy, not power. 0.9A*60V = 54W which is power. So the energy in the caps could deliver 0.9A at 60V for an hour. Or 3240W for one minute, or 32kW for 6 seconds. 32kW at 60V would be 533A.

Thanks for catching that I misspoke; I should have said "...54.18Wh of stored energy..." Either way, upon review, I believe this is also where I introduced a math error. It seems simpler to finish this part using Joules (aka: watts/second), so I'll do that here (while also adding Wh notes as well)...

With 24 ultracapacitor cells, each having an energy density of 8,125J (2.2575Wh@2.5V):
- Multiplying 8,125J X 24 cells should mean we have 195,000J of stored energy available in the cap bank, correct?
- At 2.5V, 195,000J should yield 54.1666667Wh.
- At 60V 195,000J should yield 2.2569Wh, or 135 W/minute, or 8,124 W/second (aka: 60V @ 135A for one second).

When using 12 balance boards, is 3.6A @ 60V the maximum possible "charging" current on this pack?
- Maxwell's Boostcap Product Guide recommends balancing the cells.
- Maxwell makes an integration kit that utilizes cell mounted balance boards.
- Each Maxwell balance board can apparently shunt up to 300mA @ 2.5V, into an adjacent cell, 12 boards would be necessary for a 24 cell pack.

The balance boards are an N-1 deal, so you need 23. And they do not support charge current. They just equalize cell voltage by bleeding higher voltage cells down to the level on lower voltage cells.

Again, your reply is confusing:
- It's quite clear from looking at their integration manual, that Maxwell's balance boards are in-fact not an N-1 deal. Maybe they've integrated the function of two balance boards into one, but there's still just one board needed per two cells.
- What do you mean by "...they do not support charge current..."? With the balance boards in place, this seems counter to your earlier indication that "...maximum charging current is way more than you could ever regenerate...".

When shunting, these balance boards will need to pass some current along with voltage, and I'm trying to determine what my maximum charge current could be. For instance, can the boards be damaged by attempting to dump the approximate 50% of recoverable kinetic energy when regen braking a 1,500kg vehicle from 100km/h to 0km/h within 10 seconds?

I'm estimating recoverable energy using this formula:
((kg * .5) * (kmh/3.6)^2) * .5 = J

My values:
((1500kg*.5)*(100kmh/3.6)^2)*.5 = 289,352J of total braking energy to capture in one second to stop a 1,500kg vehicle traveling at 100km/h...or ~28,935J/second to capture for 10 seconds.

The caps bank could only hold 195,000J, so that's about 6.8 seconds of recoverable braking power. The rest could be friction brakes, or dissipated into a resistive load bank. My concern here is if we brake for 10 seconds, and are looking for someplace to dump 289,352J...when we only have, at best, 195,000J of capacity, then it's critical either capacity is increased, and that the Maxwell balance board capabilities are understood (also critical: charge-state monitoring and switching away from the cap bank before an overcharge!).

Upon each cap reaching capacity, does it look like the Maxwell balance boards (all-combined) would at best only shunt 216Wh?
- If so, that's 777,600 Joules/hour, 12,960 J/minute, or 216 J/sec...which could be a real problem if we're trying to dump nearly 29KJ/sec into the caps.

 

[fechter]

I think the real challenge is to get power in and out of the capacitor bank over a wide voltage range. They start out at zero volts and the voltage increases as you charge them, unlike a battery.

I'm sure it wouldn't be cost effective. You could get way more acceleration boost by putting the same money into more batteries.

 

[erth64net]

I think the real challenge is to get power in and out of the capacitor bank over a wide voltage range. They start out at zero volts and the voltage increases as you charge them, unlike a battery.

Yes, an ultracapacitor's linear charge/discharge voltage curve is a very valid concern. Although with an EV that can pull 100A @ 60V, and by establishing discharge goals at Vmax=60 and Vmin=45, over Duration=10secs. We should be able to apply a constant current calculation, and determine storage needs vs capabilities:
Farads = A*Duration/(Vmax-Vmin)

Which means:
100*10/(60-45)=66.67F @ 60V capacitance is consumed during a discharge to 45V.

To determine the number of 2.5V cells needed, you'd divide Vmax by the cell's V rating:
60/2.5=26.67 cells

My original estimate was between 5-10 seconds of boost, so with 24 caps in series, and not yet accounting for ESR or circuit losses...9 seconds of boost isn't a bad place to be. One could possibly leverage this to "undersize" their battery pack, with possibly lowered risk of exceeding C ratings.

Assuming there's no DC-DC convertor (with 20-30% losses) boosting the lower cap voltages upwards, then we're not only back to how fast one could charge these caps with "stock" cell mounted balance boards, your time to charge from 45V to 60V is also going to be distinctly less than the earlier 0-60V estimate. Which introduces further overcharge risks.

 

[major]

Yes, the balance boards are not N-1. They were when I used them a few years ago. They must have changed the design. But I do not believe the balance boards shunt charge current. The charge current does not travel through the circuit boards.



Here is a bank of boostcaps I did a few years ago. There are 156 cells and 155 balance boards. The opposite side looks similar with a bus bar and balance board on every other cap terminal.

And energy density typically is a measure of energy per unit volume, Joules per liter. Specific energy is typically a measure of energy per unit mass, Joules per gram. Neither term applies to energy per cell.

 

[major]

With 24 ultracapacitor cells, each having an energy density of 8,125J (2.2575Wh@2.5V):
- Multiplying 8,125J X 24 cells should mean we have 195,000J of stored energy available in the cap bank, correct?
- At 2.5V, 195,000J should yield 54.1666667Wh.
- At 60V 195,000J should yield 2.2569Wh, or 135 W/minute, or 8,124 W/second (aka: 60V @ 135A for one second).

O.K. There is no such thing as a W/minute or W/second. A Watt is a Joule/second. More commonly stated: 1 Joule = 1 Watt second or 1J = 1Ws.

When you have 24 cells charged to 2.5V each containing 2.26Wh there is a total of 54.2Wh no matter how you connect them. If they are connected in series, then the total voltage is 60V with 54.2Wh. 54.2Wh = 3250 Watt minutes or 195,000 Watt seconds. Which gave rise to my statement (with rounding errors):

So the energy in the caps could deliver 0.9A at 60V for an hour. Or 3240W for one minute, or 32kW for 6 seconds. 32kW at 60V would be 533A.

This was just a calculation to show the magnitude of energy, power and current for the time periods involved and does not address the details of efficiency and the fact that the terminal voltage of the capacitor must change for it to do work.

You started out this thread with this question:

- What's the maximum possible charging current when using Maxwell's balance boards?

I still say the balance board does not enter into the charging current. It will simply react to the cell voltage. A quote from the manual you referenced supports this:

This kit includes printed circuit boards (PCBs) with voltage management circuitry,….

Each individual cell voltage is monitored by the voltage management circuitry, which protects each cell from
entering an over voltage condition. If any cell does experience an over voltage condition, an LED will illuminate and the active circuitry will begin to discharge that cell. Once the cell is back within nominal operation voltage limits, the LED will extinguish.

 

[JackB]

I'm considering the use of Maxwell Boostcap Ultracapacitors to capture braking/regen energy, and to assist with initial acceleration.

The caps are relatively heavy so using them to accelerate means accelerating more weight.
This will not be a good tradeoff unless the added weight is not that significant to the total vehicle, which btw, can be heavy because of batteries.
What it really can help with is reducing the load on the batteries so they can last longer. 5-10C draw is not good for their lifetimes.

So given you have enough weight that caps don't add too much more, the next challenge is to fully utilize their voltage profile.
Having them in parallel with batteries just isn't good enough utilization. You need a good motor controller than can adjust
its output from the declining voltage of the caps.

In general, a hybrid approach is best used to combine two things with wider differences, e.g. poor low torque gas with high torque electric.
So in this case, combine caps with lead-acid batteries which have poor high C-rate performance,
so you get lower cost and better performance.

Most just accept with poor performance and/or wear out their batteries, that is the cheapest solution.

 

[rf]

Some amusing related videos ...

http://www.youtube.com/watch?v=GPJao1xLe7w

http://www.youtube.com/watch?v=z3x_kYq3mHM

 

[erth64net]

Some amusing related videos ...

http://www.youtube.com/watch?v=GPJao1xLe7w

http://www.youtube.com/watch?v=z3x_kYq3mHM

Yea, this idea is exactly what reignited my consideration of boostcaps; I'm already successfully using a couple of 6 cell 2600F packs on both my wife and my cars...although without any active balancing at the moment:
- Hers is a 2001 Prius, which technically doesn't need the cranking amps (as the gas engine starts off the traction pack; spun by one of the car's two motor/generators), but replacement costs of her "special" SLA (which recently sulfated to death) was higher than a "BoostPack". Toyota made some really silly decisions when it came to how the Prius 12V system is setup (a dying 12V battery is an extremely common issue). So a 5W solar panel + BoostPack has ended up working quite well.
- Mine is a 2006 Smart ForTwo, and...yep, replacement cost of these "special" vented batteries are higher in total cost than assembling a BoostPack from used caps.

Getting suitable balancing on those two packs is honestly one of two agendas behind starting this thread (the 2nd, being the more obvious; answering the question of cost-effectiveness in an EV and/or hybrid design).

 

[erth64net]

I'm considering the use of Maxwell Boostcap Ultracapacitors to capture braking/regen energy, and to assist with initial acceleration.

The caps are relatively heavy so using them to accelerate means accelerating more weight.
This will not be a good tradeoff unless the added weight is not that significant to the total vehicle, which btw, can be heavy because of batteries.

What it really can help with is reducing the load on the batteries so they can last longer. 5-10C draw is not good for their lifetimes.

Yes, these caps have a gravimetric energy density roughly 10 times less (edited: oops, left out "less") that of SLAs...but their charge/discharge characteristic are why I'm exploring their possible use, in the specific application of essentially improving an entire system's short-term "C" rating.

As for better quantifying cost vs quality...that's underway now

I'm thinking this entire setup is probably going to end up being too complicated, with too little returns - but I need to better understand the math behind this gut feeling before moving on.

 

[erth64net]

Here is a bank of boostcaps I did a few years ago. There are 156 cells and 155 balance boards. The opposite side looks similar with a bus bar and balance board on every other cap terminal.

What was the application?

 

[major]

Here is a bank of boostcaps I did a few years ago. There are 156 cells and 155 balance boards. The opposite side looks similar with a bus bar and balance board on every other cap terminal.

What was the application?

Commercial class 4 hybrid truck. No batteries. Energy recovery (regen) / launch assist. Approximately 1 MJ useable energy stored. Typically charged in 10 to 15 seconds.

 

[major]

Yes, these caps have a gravimetric energy density roughly 10 times that of SLAs...but their charge/discharge characteristic are why I'm exploring their possible use, in the specific application of essentially improving an entire system's short-term "C" rating.

I'm still having trouble with your use of terminology. In post #1 you gave the specific energy of Maxwell ultracapacitor as 4.3Wh/kg. I think that is about 10 times lower than Pb-Acid batteries which are about 40Wh/kg. The power density and specific power of the ultracapacitor is much greater than the Pb-Acid battery.

That 1 MegaJoule capacitor bank stored about the same amount of energy as a typical 12V car battery, but could deliver all of it in like 5-10 seconds and be completely recharged in 5 to 10 seconds. And do it efficiently, and repeatedly, for like a 1,000,000 times.

 

[erth64net]

I'm still having trouble with your use of terminology.

Please see: BCAP0010 spec sheet (also linked a few times earlier in this thread).

 

[fechter]

1 Megajoule is a lot of energy. Dare you to drop a wrench across the bus bar.

I'm not sure what Maxwell uses, but a crude version of a BMS circuit could work to balance the charge and prevent overvoltage on individual caps. Nice thing about caps is they don't need really precise balancing. A simple shunt circuit with a trigger to cut charge if any cap goes over voltage would be not that hard to do depending on the shunt current.

 

[erth64net]

Here is a bank of boostcaps I did a few years ago. There are 156 cells and 155 balance boards. The opposite side looks similar with a bus bar and balance board on every other cap terminal.

What was the application?

Commercial class 4 hybrid truck. No batteries. Energy recovery (regen) / launch assist. Approximately 1 MJ useable energy stored. Typically charged in 10 to 15 seconds.

Ok...that could make a nice launch assist - but if the truck was sans-batteries, what was your secondary power source?

 

[erth64net]

1 Megajoule is a lot of energy. Dare you to drop a wrench across the bus bar.

What wrench?

I'm not sure what Maxwell uses, but a crude version of a BMS circuit could work to balance the charge and prevent overvoltage on individual caps. Nice thing about caps is they don't need really precise balancing. A simple shunt circuit with a trigger to cut charge if any cap goes over voltage would be not that hard to do depending on the shunt current.

I was initially thinking something along the lines of a 1N5222B-TR zener diode (which can do 500mW), but then saw pictures of their BMS, and am still wondering why its any more complicated than a diode or two.

 

[major]

I'm still having trouble with your use of terminology.

Please see: BCAP0010 spec sheet (also linked a few times earlier in this thread).

That supports what I said. Thank you.

 

[major]

What was the application?

Commercial class 4 hybrid truck. No batteries. Energy recovery (regen) / launch assist. Approximately 1 MJ useable energy stored. Typically charged in 10 to 15 seconds.

Ok...that could make a nice launch assist - but if the truck was sans-batteries, what was your secondary power source?

Diesel fuel. It was a hybrid. Cummins diesel engine primary power and electric assist using ultracapacitor electric energy storage.

 

[erth64net]

Ok...that could make a nice launch assist - but if the truck was sans-batteries, what was your secondary power source?

Diesel fuel. It was a hybrid. Cummins diesel engine primary power and electric assist using ultracapacitor electric energy storage.

Well, now you've really piqued my intrest, and now I've bit...how did you combine power-plants?

Disclaimer: a few years ago, I built a gas/electric/human powered recumbent bike, which gets 246MPG when combining human/gas power for the longer trips (electric is typically reserved for in-city trips). Fuel economy was tested over multiple trips, spanning about 3 months apart, so the math holds up The whole assembly is currently sitting on a stand about 10ft from me; the "transmission" is being reworked a bit.

Aside from that - oustanding projects include evaluating a hydrostatic drivetrain (for true mechanical regen efficiencies of 70%+) for the Smart ForTwo, and another highway capable vehicle that combines a hydrostatic drivetrain + TDI powerplant (the Hydraulic Innovations guys are sadly gone now - which has left a "knowledge vacuum"). Obviously the fossil burners are OT here, but I couldn't resist

 

[major]

Ok...that could make a nice launch assist - but if the truck was sans-batteries, what was your secondary power source?

Diesel fuel. It was a hybrid. Cummins diesel engine primary power and electric assist using ultracapacitor electric energy storage.

Well, now you've really piqued my intrest, and now I've bit...how did you combine power-plants?

It was a post transmission coupled electric motor in a parallel hybrid. Electric motor through a gear reducer connected to prop shaft between the tranny and rear end. It was a step van, like a UPS or FedEx delivery truck. About 15,000 lb GVWR.

 

[liveforphysics]

You will add weight and packaging hassles and wiring and new failure modes to your vehicle for no realized advantages in return.

Playing this game always just ends up showing you how it very seldome makes sense to do anything but grow the battery if for some reason you think your regen levels will be so high you can extract more than the batteries safe burst charge current before the wheel starts breaking traction in regen. If this is for a rear motor, you will be surprised to see it's tough to even regen at 1-2kW, which most ebike batteries are fine with getting charged at for 5seconds while slowing from high speeds, can cause the tire to slow at a rate that breaks traction. The wheel never locks, as it can't generate a counter-torque unless the rotor has relative motion to the stator, but it does fishtail badly until you get back on throttle if you dial regen up too hard, and most normal decent sized ebike batteries can handle all the regen you can throw at them with no issues.