On Thu, 21 Sep 2017 09:29:18 +0100, Martin Brown
wrote:

On 20/09/2017 10:09, T i m wrote:
On Wed, 20 Sep 2017 08:36:50 +0100, Martin Brown
wrote:

On 19/09/2017 19:02, T i m wrote:
Reference 'C' gives:
Current, Time to fully discharge:
21A, 1h (i.e. C/1)
11.9A, 2h (i.e. C/2)
5.40A, 5h (i.e. C/5)
3.10A, 10h (i.e. C/10)
1.60A, 20h (i.e. C/20)
0.36A, 100h (i.e. C/100)

snip

It's just knowing that there can be such a marked difference in
capacity as a function of the current drawn ... and my potential
maximum current (30A) *is* significant to this, even with 3 batteries
in parallel (~90Ah) there is a very good chance that some massaging of
the readings to predict a particular depth of discharge threshold
would be worthwhile.

A quick and dirty value for the discharge threshold is when the terminal
voltage drops below some arbitrary limit ISTR 10.5 or something.

Agreed, 10.5V is often cited as the on-load 100% discharge point.

e.g.
http://www.chbattery.com.img.800cdn....4153615343.jpg
http://www.coslightindia.in/images/2gel-graph-img1a.jpg

You
never want to take an individual cell below some critical value which is
temperature dependent or permanent damage will result. The weakest cell
in a stack of them always gives up the ghost first.

Agreed.

To a good approximation the terminal voltage ends up as

V = V0 - Ir

Where I is the current you are drawing and r is the internal resistance
of the battery (which itself is a weak function of temperature and I).

Ok.

In practice the difference may help to protect the battery if you just
use a fixed voltage cutoff value since if you are discharging it quickly
it will inevitably be hotter and potentially gassing slightly.

I don't think it would in this case Martin as that's what Lead Acid
and Peukert's law are all about (LA battery temperature doesn't rise
and so you get the non linear characteristics we see).

If I retained the original resistor based 'speed control', even I,
once I guesstimated what the capacity would be for the currents
typically seen at the 5 set speeds, could possibly write some code
that applied the variable.

Not sure I understand what you are trying to do but a MOSFET based PWM
speed controller will give you something like 90% efficiency.

Agreed. I was just saying that *even I* could probably come up with
some code to deal with the std resistor speed controller with it's set
steps.

This will
give you a lot more battery life if you use low power most of the time.

Understood. 15A was about the right speed (for the local rivers and
canals) and so with 3 of the 8GU1H batteries connected in parallel and
a capacity of near 28.5Ah (C/5) at that current, that should give me 3
x 28.5 = 85.5Ah and so a 15A draw to 50% capacity should be 42.75/15 =
2.85 hours.

Whereas a resistor will be wasting a lot of power dissipated as heat.

Agreed, so I'm hoping to be able to do better than say 3 hours. ;-)

Using a better more efficient speed controller is a far better option
than managing the decline of the poor battery.

As you can see I'm actually doing both and more (hydrodynamic fairing
on the tubular prop leg, tail spinner on the prop itself and possibly
some mods to the boat itself).

But, to be able to 'Manage what you can measure', you also need to
know how to make best use of those measurements. ;-)

Cheers, T i m