On Thu, 21 Sep 2017 08:54:26 +0100, "Dave W"
wrote:


"Dave W" wrote in message
news

snip

Some further thoughts:

Ok. (and thanks for such Dave). ;-)

The display of Vo should stay fairly constant as the current is ramped up
and down.
If it doesn't, the assumed value for Rt can be adjusted until it does.

So Vo is our calculated cutoff voltage Dave?

I suspect the motor current will be very noisy, making any digital display
very
erratic and difficult to read.

All that would be 'damped' by sampling Dave.

Better to use a mechanical meter, with a big
capacitor across it to smooth out wobbles.

I already have such a solution in some monitoring kit I've made up
(moving coil meter and suitable shunt) but that's no good for any
logging of course.

Or perhaps use software to create
a
running average.

That's the idea. See, once I have it 'electronically' I can do loads
of things with it (like logging to an SD car to provide a 'Battery
Life' record.

A meter would be easy to read in bright sunlight.

I only intend using this in England Dave. ;-)

The meter could have a big mark at 12.19V, with perhaps everything below
11.5V
marked in red, as that's where your discharge graph starts to drop like a
stone.

Sure, analogue meters are great for showing stuff (and why I still use
them) and during the initial runs of anything I come up with may well
include some analogue versions of both Volts and Amps, just to provide
a 'simple' comparison. The problem is a moving coil voltmeter is a lot
more expensive than an electronic one (or even an electronic
wattmeter!) and it really only serves as an indication that there is
around 12V there (because you are only interested in the range 10 -
15). I did think of using a 10V zener and a 5V meter as at least then
you would have an expanded range. ;-)

If you are using a simple rheostat to adjust the motor current, that's a big
waste of energy in the form of heat.

Agreed. It's actually what Minn Kota (the people who make the
outboard) call 'Speed coils' that are just resistors in the underwater
unit somewhere and selected via a multi-way switch in the outboard and
controlled by the tiller / twistgrip.

Better to have an efficient
switched-mode
power converter to convert the battery voltage into motor voltage.

Quite, and something (else) I'm working on Dave. ;-)

Ideally this would have current feedback to compensate for the resistance of
the motor, in effect raising the voltage as more current is taken.

Yes, that could be good.

The
control
knob could then be calibrated directly in propeller speed.

Yup. ;-)

The feedback could be further modified if you know the relationship between
boat
speed and propeller speed, and the knob could be calibrated in boat speed.

Whilst that would vary between boats (and there are 5 we may use it
on) it might be of more use than just some arbitrary values.

This
would be helpful for estimating journey time.

As would the current etc.

Cheers, T i m

p.s. Part of the whole 'electric outboard' experiment is to build a
test tank (I now have kindly donated by Mr Lamb of this very group)
and again, use an Arduino micro controller and voltage, current, load
(strain) and water-speed measurement, initially make comparisons
between the efficiency of resistor V PWM speed control and later, prop
/ leg hydrodynamic improvements (or not). ;-)