On Thu, 21 Sep 2017 09:26:15 +0100, Martin Brown
wrote:

snip

The problem with that Dave is that doesn't automatically compensate
for the change in capacity as the current changes?

OK. I think I understand now what you want to do is make a fuel gauge
for your rather larger than I imagined model boat.

Ah ha. ;-)

In that case you will
need to err on the side of caution

I've already factored that in Martin. The batteries are probably the
best there are out there and so will tolerate (some) abuse with no
real detriment (and partly why I bought them etc). So, if I aim for
say a maximum 50% DOD ... and overshoot a bit, it wouldn't be a big
issue.

(a bit like my car range indicator
which is incredibly pessimistic after a fuel fill showing a maximum
range that is typically 100 miles less than the distance actually
travelled on the last tank).

;-(

I suspect range or time remaining (at a
chosen speed/drive current) might really be what you should display.

True ... but whilst it would be nice, it may well be a step_too_far
when it comes to predicting it accurately and as you say, would only
be able to predict the time at the current speed (current).

Stop when you have used up half the capacity that 10A per
battery gives. Martin's formula times three gives 71Ah capacity at 30A, so
36Ah represents the half discharged point. At full speed you should then
have
36/30 = 1.2 hours before the battery runs out.

Ok.

The question is, how many hours from this point will you get at 15A?

Quite. ;-)

And that depends how you obtain your 15A. If it consists of equal
amounts of 0A and 30A at a frequency high enough to not be annoying and
low enough not to be too lossy you will go almost twice as far as if you
used a simple series resistor where half the power just ends up as heat.

Yup. Or use the 12V for 50% of the time ... etc.

PWM at a nominally fixed current also simplifies the battery Ah side.

Whilst it would ... I do need to use different speeds. ;-)

Expect a decent PWM power controller to be expensive and make provision
to switch back to a primitive all or nothing supply in case of failure.

Sorta ahead of you there Martin. I bought a couple of 30A capable PWM
speed controllers from China ... one is FWD only and the other FWD /
REV (using relays). I have also been looking at commercial speed
controllers or around those powers and they are more flexible and
*could* be controlled by say an Arduino micro controller.

snip

I suggest doing some empirical experiments with the boat itself and a
fully charged set of batteries to see how long it lasts run flat out, at
half power, and then half of run flat out followed by half power.

Again, once we know the currents at each of the 5 existing speed steps
(and we do, 7, 11, 15, 20 and 30A) it wouldn't be any issue to do that
on the bench ... and I have on all 3 batteries individually and at 5A
and got about 4 hours to what I thought was the 50% DOD voltage for
that current and 11.2V

Maybe just use one battery to make the testing a bit quicker.

Quite. ;-)

You need to make sure that if all else fails the panic indicator leaves
you with enough juice in the battery to limp home at a snails pace.

True ... or get the oars out. ;-) Again, once I have a reasonable
idea of the capacity / range / runtime (and I do, worse case at least)
the idea was to row out and outboard back .. or a mixture of them
both.

I'd ignore any fancy nonlinear or temperature effects for the first
analysis unless they prove problematic.

Well, whilst I agree re the temperature (although that would be fairly
easy to implement) I think the non-linearness of the capacity is
mostly what this is all about. eg, I could easy build an Arduino based
LVD (Low Voltage Disconnect (Alarm)) with what I have now (and have
one running in front of me displaying Volts Amps and Watts as we
speak) that triggers on the best case voltage (load at 7A) but then
that would trigger way too soon at 30A. (As the capacities at various
draw rates show).

http://www.mkbattery.com/images/8GU1H.pdf

The change in dynamic impedance
of the electric motor with speed will be more of an issue. The biggest
error by far will be that speed in the water against the current will
not translate to anything like as much forward progress as on dry land.

Quite ... and sorta how some of this started. ;-)

1) We already had a 60Ah cyclic battery that we have used on the
outboard to good effect but with it's very heavy duty copper cables
(the battery needs to go forward to get the balance right) weighs
around 20kg. Whilst I can lift it ok and the Mrs can just on
terra-firma, it's not so easy to lower such 3 ft down off the bank
into a boat bobbing about on the water.

2) I also wanted to extend the range, by a) increasing the battery
capacity and b) the drive efficiency. The battery bit got me with
100Ah worth of Li-Po batteries and whilst they were very light and
very efficient, required more equipment and were a bigger risk ITRW.
The drive efficiency would be improved by (initially at least),
putting the outboard on speed 5 (where the battery is connected
directly to the motor) and using an external PWM controller to do the
rest.

3) Because of the Li-Po risks, I went back to 3 x 31Ah Gel Lead Acid
cyclic batteries that could be transported individually and at only
11.5 kg's or so, much safer for all concerned.

snip

You really want something that works well enough from an engineering
point of view and fails safe by underestimating remaining capacity.

Agreed (with caveats as mentioned above etc).

It is very much an empirical business - and one that most car makers
have not yet mastered in their petrol/diesel range indicators.

Oh, completely understood Martin, it's just that with what I thought
what *might* be sufficient data for someone who could deal with such
things we could do a bit better than a finger in the air. ;-)

Are electric cars any better at this I wonder?

Well, you would like to think so and as few run on Lead Acid batteries
these days (except mine g) and with Li-Ion being pretty popular and
them having a fairy linear capacity across all current ranges ...
*and* some reasonable sensors, cpu and programmers at their disposal,
you would hope so! ;-)

I suppose they have to be
or we would see dead and dying electric cars stuck at the roadside.

And if you do that might be more likely because the nut behind the
wheel overestimated the range of the car or conditions changed (or
weren't predicted, like cold weather or hills) that lost them some
capacity etc.

I knew my EV only had a range of 20 miles and so knew that 10 miles
was my drive out limit and that I also had to consider the terrain and
even wind speed and direction as well. I would happier go 'out' uphill
knowing I could nurse it home easier downhill. ;-)

Cheers, T i m

p.s. When I designed, built and races an electric two wheeler I knew
my batteries had a reserve capacity of 25A for one hour (that is how
reserve capacity is measured, low long at 25A).

So, I kept an eye on my ammeter as I raced and if the out leg of the
course had a reasonably steady average of 25A and the same on the back
section, I could keep that up for the full 1hr 'race'. However, if
there was a hill (like at Cadwell Park), I would have to accept the
say 30A+ load on the uphill sections in the hope that I could save
energy by freewheeling down the downhill bits. In my head I had to
compute the overall drain (considering the reduced capacity at the
higher currents) and hope to cross the line with a *tiny* amount of
charge to spare.

If you ran out of charge before the end it meant you used it up too
quickly (so would have had lower battery capacity and increased
rolling and aerodynamic resistance) and if you had any charge left
over at the end it meant you could have gone faster. ;-)