Maxwell Ultra-Cap's/Lithium cells hybrid pack

[Serum]

But with the relative high internal resistance, they won't deliver too much current, but i am looking forward to the test results. Can you please post them if you have them?

Thanks!

[GriffinRU]

Originally Posted by Serum

But with the relative high internal resistance, they won't deliver too much current, but i am looking forward to the test results. Can you please post them if you have them?

Thanks!

I will, here simple simulation I did for 25A/75A transition circuit and graph
Over discharge batteries internal resistance goes up, so caps help will be more noticeable and with low-end batteries as well. At least in theory :)

Artur

[zeropointbug]

Serum, 62,000 Joules per second! LOL You must have done calc error, that battery pack can deliver up to 3000 Joules continuously (per second) @ ....3000 watt (apparently) 62,000 Joules per second would be around 3300 amps

An RC car would/could never use this much power, it's more instantaneous power pulses. For those that don't quite understand, the capacitors energy is directly proportional to the instantaneous voltage of the cap, not taking into account the very small voltage drop between the two terminals.

energy formula: 1/2 (CV) C - capacity in farads, V - voltage in volts

Any questions or comments welcomed

[Serum]

Sorry, my bad. it does 6100 joule. (6100 watts in bursts)

total amount of energy is 532800 joule.

I personally don't see the advantage of these cells, their internal resistance is too high, and with the right cells (read good quality) you wouldn't need them (which was my point) they would add nothing but weight.

Once they reached a lower internal resistance they will be more interesting in replacing batteries, with the ability of very quick charging.

[GriffinRU]

Originally Posted by Serum

Sorry, my bad. it does 6100 joule. (6100 watts in bursts)

total amount of energy is 532800 joule.

I personally don't see the advantage of these cells, their internal resistance is too high, and with the right cells (read good quality) you wouldn't need them (which was my point) they would add nothing but weight.

Once they reached a lower internal resistance they will be more interesting in replacing batteries, with the ability of very quick charging.

This cells will never replace batteries. They will boost batteries performance and replace them in applications with short-term power requirements.

Tens of mOhms is pretty low, not every battery can have it.

Self-discharge rate kill them as batteries
Charge-Discharge cycles rating put them ahead of batteries
Also wider operational temperature range...

Artur

[Serum]

But what is the use them in rc vehicles if you have got 6000 watts of peak power in your cells? (proper lipo cells, 5S 8000 mAh)

It doesn't need this boost, it will only add weight. You are going to test them with cheap cells, for that they can work miracles, i agree. but for good cells?

[zeropointbug]

Let's say the benfits are:

-More consistent power (very little voltage droop under burst loading) the controller would always be getting the same voltage...
--Longer runtime (capacity retention because of less strain)
-Safer, especially for LiPo
-Longer cycle life
-If power input leads shortened to esc, much better for the controller...


I can say this, BUT nothing will compare to real world testing.

[GriffinRU]

Originally Posted by Serum

But what is the use them in rc vehicles if you have got 6000 watts of peak power in your cells? (proper lipo cells, 5S 8000 mAh)

It doesn't need this boost, it will only add weight. You are going to test them with cheap cells, for that they can work miracles, i agree. but for good cells?

True,

But they can extend batteries life and help ESC.
Is it worth the added weight?

Artur

[Serum]

True,

But they can extend batteries life and help ESC.
Is it worth the added weight?

Artur

What do they weight then? I think it would be wiser to gain more cells (with the same weight) and choose a lower KV motor, to simple unload the controller/motor/leads from the high current and gain real runtime instead. (total available joule)

@zero;

i don't know what you think of this, or if you knew it at all; but a flighpower pack delivers 7V under a 100A load for their 2S 4900 mAh pack.. I don't think a more consistent cell is available at this time.. only dropping .4-.5V under a 700 watt load.

[GriffinRU]

Originally Posted by Serum

What do they weight then? I think it would be wiser to gain more cells (with the same weight) and choose a lower KV motor, to simple unload the controller/motor/leads from the high current and gain real runtime instead. (total available joule)

@zero;

i don't know what you think of this, or if you knew it at all; but a flighpower pack delivers 7V under a 100A load for their 2S 4900 mAh pack.. I don't think a more consistent cell is available at this time.. only dropping .4-.5V under a 700 watt load.

420g

It is ~5mOhms and at room temp...

Artur

[Serum]

420g is quite some stress for the motor/drivetrain/speedo to toss around.

I would prefer a lower kv motor, with a higher cellcount for sure! It will turn out more efficient as well. (and the added runtime is nice too)

If you ran 4S lipo, you can get an 8S setup, (gaining the same weight) double the runtime and roughly halve the current.

[zeropointbug]

the 140 Farad caps weight 29 grams each, you need 8 cells.

the 310 Farad caps weight 60 grams each, you need 8 cells.

Serum, putting more batts in the car is completely irrelevant, we are talking about a specific battery, compared...

Those Flightpower batts sound super sweet, however i will never buy a Lithium Polymer pack ever. They are too dangerous, can't be charged fast, and need special attention. And they cost alot more than A123 cells. Im sticking to A123 cells for now, until AltairNano comes to market with their Nanosafe battery.

[Serum]

You are talking about taking stress from the motor/speedo, and all of a sudden my sugestion is completely irrelevant? what is your point/goal??

I am suggesting something different, by adding cells, get the same weight and gain runtime, AND run more efficient.

[BrianG]

The capacitor idea is nice, providing the weight and size could be reduced. If the caps are the same size/weight (or more) as simply adding a parallel set of batteries, you'd most likely get more bang for the buck with the added batteries. This is why I said use something like a 2-5F cap; it still will offer substantial help with the high current pulses while remaining relatively small and lightweight. Even 1F will help a lot for 1,000 watt loads.

5 mOhm is actually not bad. That's only a 0.5v drop at 100A. Don't forget, that's just the voltage drop of the cap. The batteries will reduce that 0.5v drop to a lesser value because they are in parallel.

Also, if these caps are hooked in series, it add the voltage values, but halves the capacity. So, if you took two 2.5v 10F caps and hooked them in series, you'd get a single 5v 5F cap. You put 10 of those in series, and you get a 25v cap rated at "only" 0.5F.

I am absolutely sure using these is good for the batteries and provides a more stable voltage with substantially less sag at high current, but when used in an R/C, it's going to become a compromise of cap/battery performance vs total size and weight.

[zeropointbug]

ya, it's all a matter of energy density for the caps. a group of MIT scientists have made ultra caps with energy density of 60Wh/kg, NICE.

Better yet EStor has a capacitor with if im not mistaken, 120 Wh/kg? Don't quote me on that, i'll have to look it up again.

For my setup, 232 grams for a 38.75 Farad, 20 volts cap bank is pretty fair. i've emaile Maxwell Technologies for a price quote on the caps.