Jay Summet via EV wrote:
Hurm....you'd need to limit upper voltage of each "cell" to prevent
harming the capacitors. I assume preventing full discharge is good to
prevent them from being reverse charged...but since supercaps can go
down to 0 volts with no harm, it is more likely that a full pack low
voltage cutoff would be able to work for that well enough, as you
probably won't be taking the full pack down anywhere near zero, so there
is a lot of safety factor there.
Rebel Rebel via EV wrote:
I've found super caps rated at 2.7v 3000F... RC car lasts about 40 minutes
on AAs 6v but with the Caps for about 50 min at 7v.
I'm surprised they last 50 minutes. Four 3000F capacitors in series is
750F (capacitors in series divide). 50 minutes = 3000 seconds. With a
continuous discharge current, I=C dv/dt, so 750F x 7v/3000 = 1.75 amps.
Is that all the current your R/C car uses?
If it is 1.75 amps, then your AA cells must have delivered around 1.75a
x 40/60 = 1.17 amphours. That's pretty good for AA cells.
Obviously the voltage is different and that makes a difference
but to me in theory it has potential.
Yes, it does. A team converted a GM EV1 into a drag racer with
supercapacitors, and it had very respectable performance for the short
times it took to make a 1/4 mile run.
But it's very difficult to get the kind of range out of supercapacitors
that we routinely get out of batteries. The supercapacitors are also
much larger and more expensive.
> I currently have 44 caps and want to use it with a Prestolite
> mtc-4001 motor
44 of these 3000F cells is going to give you a range measured in feet
rather than miles. But you can always switch to lithium batteries.
I know i'll need to have a pretty custom BMS.
The usual BMS for supercaps is fairly simple; just a resistor or zener
diode across each cell. With the resistor, a constant-voltage
current-limited charge will settle out with the same voltage across
every cell. The charging current just needs to be low enough so a
low-capacity cell won't peak early and go over-voltage.
The zener allows a faster charging current, and provides some protection
against reverse-voltage (which happens when you fully discharge a string
with different-capacitance cells). But zeners "leak" more current when
the voltage is near their threshold; so the pack self-discharges sooner.
Lee
--
Whether we or our politicians know it or not, Nature is party to all
our deals and decisions, and she has more votes, a longer memory,
and a sterner sense of justice than we do. -- Wendell Berry
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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