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  #11  
Old March 17th 17, 09:09 PM posted to rec.bicycles.tech
Joerg[_2_]
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On 2017-03-16 15:07, Jeff Liebermann wrote:
On Thu, 16 Mar 2017 10:20:43 -0700, Joerg
wrote:

There is no need for 27.5W but there is for 8-10W. That should be
reasonably possible, else the dynamo would have smoked out during their
testing.


If all you're powering are head and tail lights, 10 watts should be
sufficient. However, if you're powering other devices (GPS,
smartphone, rear view camera, electric shifter, fog horn, etc), then
more power is better.


I also have to power an MP3 player. No smart phone since I won't likely
buy one until they can function as good as a GPS devise even while off-grid.

Still, 10W suffices because I can ratchet down or turn off the light on
many trails and MUP. It's mostly needed on roads which I try to avoid
where I can.


http://www.ebay.com/itm/122373782338
(Yet another project that I'll never finish).

That is a neat little instrument.

Yep. I don't have one yet, but it's a real temptation to install one
on all my various unmetered power supplies and battery packs. However,
there's a catch. The common ground is positive (+), not negative.
You can see that in the schematic:
https://img.alicdn.com/imgextra/i3/121163002/TB2NoBogpXXXXahXpXXXXXXXXXX_!!121163002.jpg
where the + leads of both the "DC in" and the load are connected
together. The only ways I could make it work in a negative ground
system was either an isolated power source, isolated load, separate
isolated power supply to run the meter, or a DC-DC inverter.


That would be a show stopper in most applications. Got to have high-side
current sensing.


Impediment, not show stopper. What most users have done after turning
the + wire into an impromptu fuse, is to install a small isolated DC
to DC inverter. The problem is that the current measuring part of the
circuit needs to have some source of power to run the devices used. If
one part of that power source happens to be grounded to either the +
or - input of the voltage measuring part, it won't work. So, the
vendor recommends either floating the input or output grounds, using
independent volt and amp meters, or using an isolated power supply to
power the conglomerated meter. I know that one can get such DC to DC
inverters quite cheaply, but I haven't bothered to search for them.
I'll post something if I find one as I think it might be useful for
your battery pack and dynamo.


Digikey has lots of those. But when such extra measures are needed this
all gets old. Why can't they do high-side sensing? It ain't rocket science.


From a dynamo it's still feasible after things have
been turned to DC because the hub versions usually have two ground-free
terminals. Bottle dynamos unfortunately not. However, when doing MPPT
there already is a micro controller which can then also perform the job
of energy metering.


Yep. I've setup some cheap MPPT solar charge controllers (from eBay).
The better one's have extensive monitoring capabilities, usually on an
LCD or OLED display. If you're counting and budgeting coulombs, it's
a good way to do it.

I would not need that though, maybe just an
indicator of whether the dynamo is able to feed enough for a given light
setting.


Easy enough. The power output of the dynamo is directly proportional
to the RPM and unaffected by the load.



No really. Keep in mind the various losses.


... The load might change
radically, but the dynamo will only deliver XX watts at XX RPM. You
could just measure the RPM, build a lookup table in some kind of
NVRAM, and display the output power.


All I need is a sensitive voltmeter. That tells me whether my battery
juice is being consumed or juice is being added. I have seen waterproof
LED meters with two digits after the decimal point for less than $5.
This is what I am going to add some day. It will also let me see the
charge state before heading out on either bike.


Or, you could get fancy, measure the load voltage and current, and
calculate the power needed to run the lights or whatever. Then
compare the input power available, with the average output power
needed. If there's not enough input power available, you need to shed
some load, or pedal harder.


Nah, too fancy :-)


Even that is expendable if you have a voltmeter riding along
with the Li-Ion battery. In my case I'd just have to make sure it won't
get close to 7V where the electronics will eventually shut things off.


If you do go over, you might want to shed the load slowly or you might
lurch forward as pedaling suddenly becomes easier.


With a 10-15W load?


Nothing beats trying it on the bench, using a variable speed drill or
similar. I haven't observed a sharp knee effect on any of them so far.
Which probably explains the large number of bulbs I blew out in the 80's
when I had to get somewhere fast.


Yeah, I should do that. However, as I previously mumbled, I'm
overloaded, busy, lazy, and uninspired right now.


Same here. The MTB still needs to be fixed, taxes to be done, then there
is work, and right now I am busy with a much higher priority. I brewed a
Belgian Tripel this morning and this afternoon a Superior Strong Ale is
in the process.

--
Regards, Joerg

http://www.analogconsultants.com/
 




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