Conservation of charge. Balancing power and energy in MOEPED.
- From: "The Dougster" <DGoncz@xxxxxxx>
- Date: Sat, 2 Dec 2006 20:23:16 +0000 (UTC)
The Dougster (I) wrote:
As I search for such a meter for the MOEPED, I find things like:
http://www.rc-electronics-usa.com/ammeter-simulator.html
which may be of interest to educators here in this group for their
physics student's use. It simulates the operation of such a meter, the
"Watt's Up" brand. Battery resistance is 0.1 ohm default. For the
ultracapacitors in the MOEPED this is 0.012 ohm.
I am emailing makers of such integrating meters to determine
suitability for series/parallel ultracapacitor applications and will
report if I find a good match.
While it is clear that energy is conserved as it flows in and out of an
ultracapacitor pack, I am not sure that charge (Amp-hours) is conserved
when the pack is switched between two possible series/parallel modes
and I'd appreciate a post regarding this from one of you smart guys. I
am feeling a bit thick in the head right now and I have to get this
right, or the money for a meter will be wasted.
For my pack of 12 each 2700 F 2.5 V caps, there are six possible modes
and I will soon be using 12x1 (series x parallel) and 6x2 modes with a
switch I happen to have lying around. It's a DPDT knife switch and it's
been mounted on the MOPED's "dashboard" so that the knife throws
forward and back to indicate driving (high-voltage) and
accumulating/braking/regenerating (low-voltage) modes.
So let's say I switch to 6x2 mode and charge up with the battery charge
/ starter assist unit I have rated stored 15 A for 10 minutes, a
convenient time, a sixth of an hour. That's 15/6 Ah input or 2.25 Ah.
Let's say the integrating wattmeter reads 2.25 Ah. Let's say my pack is
charged to 15 V as it has been before, and that
1/2 C V^2 = 1/2 * ( 2700 x 2 / 6 F ) * 15^2 V^2 = 101,2500 J
That's about 75,000 ft-lbs IICC and for a 250 pound vehicle with
driver, represents a 300 foot climb at 100 % efficiency. About right
for a local 150 foot hill. But I digress...
I switch to 12x1 and drive up a hill with plenty of voltage, at least
at the start, until the charge just plain runs out. Energy is
conserved.
Is charge also conserved? The voltage is double. Doesn't charge have to
be conserved? No free lunch. Where do the electrons go if they are not
conserved?
I am just confused and I shouldn't be. This should be easy!
I guess if the integrating ammeter has a zero that can be set at every
mode switch, it'll be useful, and if not, I'll need to track energy,
not charge, buy knowing the pack state and following voltage changes.
That I can do, but I'd sure like a meter I didn't have to build from
scratch out of a DAQ and a laptop.
There's been no reply to email to makers of the Watt's Up meter. A few
meters on market, and rather a larger selection of Li-Po chargers I
came across, some with integrating meters and a precious few detecting
SxP configurations automatically.
I like the idea that you can pedal around for a while, then light a
cigarette with an electric lighter, or spot weld a stud to a bridge
piling with this setup, and while there is more energy in the advanced
batteries, the go-to place for hackers like me is More Power, and 4500
A at 30 VDC is, er, 135 KW, enough to run a rail gun, or a lot of other
"advantage" applications like winches, hoists, and jump-starting a
truck or tank motor. :) Around 200 HP.
Future design efforts to balance the power and energy capabilties in
small vehicle stores:
The divisors and divisor count of any number n are known almost
trivially. I am designing a PCB/battery holder module with a DPDT relay
and a DIP switch on each module to program each module for its place in
a pack such that any pack count with eight or fewer divisors can be
accomodated and controlled. Each module will hold either a D NiCd or a
D-sized ultracapacitor.
Once programmed by throwing the many DIP switches with *extreme care to
correct settings*, as a single missed setting could destroy the whole
pack, such a pack would adopt any of its up to eight modes under some
kind of multi-position "throttle" control. This is gonna take some
money to set up but may be more reproducible and useful to others than
my pack of the twelve large caps, which are really more for transit bus
and automobile use than small vehicle use.
The 144 D-sized NiMH cells in the Honda Insight are an example of such
a pack, in 144x1 mode, with no options for mode changes. They use
electronics to control energy flow. I am using swtiches. I think my
option costs more and is more efficient. We'll see. It's also much more
versatile, for everything from a laptop or a radio to a velmobile or
hybrid car.
How likely am I to need a rail gun? Not very.
What am I gonna do, deliver the morning paper with it? Whoosh!
I would like some help with the charge conservation issue, please.
Any ideas on writing a proposal for a grant, if you know a market for
the idea?
Doug
.
- References:
- Integrating Digital Volt/Amp/Watt Meters and MOEPED news
- From: The Dougster
- Integrating Digital Volt/Amp/Watt Meters and MOEPED news
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