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. In that case you will need to err on the side of caution (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.
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.
PWM at a nominally fixed current also simplifies the battery Ah side.
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.
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.
Maybe just use one battery to make the testing a bit quicker.
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.
I'd ignore any fancy nonlinear or temperature effects for the first analysis unless they prove problematic. 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.
You really want something that works well enough from an engineering point of view and fails safe by underestimating remaining capacity.
It is very much an empirical business - and one that most car makers have not yet mastered in their petrol/diesel range indicators.
Are electric cars any better at this I wonder? I suppose they have to be or we would see dead and dying electric cars stuck at the roadside.
A quick and dirty value for the discharge threshold is when the terminal voltage drops below some arbitrary limit ISTR 10.5 or something. 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.
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).
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.
Not sure I understand what you are trying to do but a MOSFET based PWM speed controller will give you something like >90% efficiency. This will give you a lot more battery life if you use low power most of the time.
Whereas a resistor will be wasting a lot of power dissipated as heat. Using a better more efficient speed controller is a far better option than managing the decline of the poor battery.
It's one thing for 'bm' to want to do a sloppy job if he was in the same situation as you but then he went ballistic just because you want to do it thoroughly. Why does he care so much?
No, nothing Dave ... I'm just still trying to be open to all ideas and options. ;-)
Quite.
I was hoping to display both, the actual voltage and the predicted cutoff voltage (at that instant current reading) just so that *I* can get a feel of how things are going (as my feeling on how things are going may be better in ITRW than any AI). ;-)
Ok
Well, if all things do tally then yes, it should. However, the specific gravity of the electrolyte is independent of the charge rate or terminal voltage, it's just an instantaneous reflection of the chemical charge state (so not much use ITRW when on the move without some auto-reading system (and a non gel battery) etc.
The fact that I have a degree in electrical engineering and a huge anount of experience in using lithoum ion batteries in model aircraft completely precludes me from offering what would instantly be shouted down as 'an opinion'.
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.
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.
I only intend using this in England Dave. ;-)
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. ;-)
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.
Quite, and something (else) I'm working on Dave. ;-)
Yes, that could be good.
Yup. ;-)
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.
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). ;-)
I really don't think it's a deep as that P, he is really just an attention starved troll. ;-(
He doesn't of course, he just likes the attention so will say anything to try to get some. Look how much OT stuff he posts here and how much weirdness it contains ... ? ;-(
Yeah, I was a 'sponsored' RC car driver so because of that I know all there is to know about Nicads and NiMh .... (not).
Yes, but if you had an EQ as big as your IQ you would be able to offer
*information* around my question wouldn't you?
No use whatsoever mate as it's a linear relationship and this isn't a linear problem.
As you know all there is to know about LiPo (as the chances are you
*wouldn't* be using Li-Ion in RC aircraft as they are too heavy) you would know that whilst Peukert's Law also apples to them in theory, you would also know it is pretty well cancelled out by the increase in efficiency found as the batteries heat up in use (proportionally to the current drawn).
So, what with you having a degree in electrical engineering, into using your (Linux) laptop as a typewriter (writing code for the likes of Gridwatch?) and like debating things so what's not to like? ;-)
So, cummon, have a go at being the solution, you know you want to. ;-)
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.
;-(
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).
Yup. Or use the 12V for 50% of the time ... etc.
Whilst it would ... I do need to use different speeds. ;-)
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.
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
Quite. ;-)
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.
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).
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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.
Agreed (with caveats as mentioned above etc).
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. ;-)
Well, you would like to think so and as few run on Lead Acid batteries these days (except mine ) 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! ;-)
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. ;-)
Agreed, 10.5V is often cited as the on-load 100% discharge point.
e.g.
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Agreed.
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).
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.
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.
Agreed, so I'm hoping to be able to do better than say 3 hours. ;-)
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. ;-)
Yeah, nice cop out mate! Just like the Linux questions I've challenged you with in the past when you initially portray you know what you are doing and pretend to offer help but when you are really tested you go very quiet ... ;-(
What do they say ...'All mouth and no trousers ...'? (Damn, now I have to get that image out of my head ...). ;-(
I think we have established that the relation between terminal voltage and current at a given state of discharge is a linear one.
If you want the *derived* quantity of remaining capacity *at a given current* for a given state of discharge this can simply be a look up table and is little or nothing to do with what the battery *has* been doing, depending only on discharge state, not how you got there. So you need no other measurements to estimate remaining capacity at a given current. Specifically it does not depend on how Peukert's law has applied during the use of the battery up to now, only on how you use it in future. You could automatically estimate remaining endurance (to a given future state of discharge) for the prevailing current at the time of measurement, but you still only need a look up table for capacity against current and your present state of discharge plus present current consumption.
You could use an approximation to Peukert's law to calculate capacity at a given current on the fly, but there is no advantage to doing so as the future behaviour is apparently not affected by how you have treated the battery except as represented by current discharge percentage. And, as the currents you are using are well within the range for which capacity is published interpolation would be easy. If the 'k' you can calculate is actually is constant you could program the Peurkert equation, but a look up table for capacity for each amp of current per battery up to ten might be even easier.
It depends how often you sample, the duration of the sample window and how many samples you send to each display reading.
Surely the cost of the meter is a mere drop in the river compared with the cost of boat and battery? The meter should be driven so that when the battery voltage is below 10V the meter is still on its zero mark, and full scale for 15V. You are driving it from the Arduino so it can be any old range e.g. 10mA fsd.
You could have a float sensor to vary the calculations depending on how low in the water the boat is, causing variations in drag.
Correct. You could sample every .5 seconds, put the results of 20 samples into an array and then display the average value (or something like that anyway). ;-)
Yes, sure.
In this case I was going to make a straight analogue meter Dave, both amps and volts mounted in a suitable box and to be (optionally) plugged in series with the load. If the voltage drop at max current affects the voltage too much I'll run a separate cable back to the battery directly.
But you are right, I could run it from the Arduino but I was hoping to keep this bit 'KISS'. ;-)
Now that would be a step too far mate (and way too complicated to compute). ;-)
Because there are times when a straight 'old fashioned' analogue panel meter can show (in a human sense) things better than a digital display (and why few car speedos are digital today AFAIK) I intend, as a parallel project, making a two panel meter box with an in-line plug a socket to match the outboard to battery connectors so that I can just put it in series with anything I'm playing with to give me a quick_n_dirty display of volts and amps. ;-)
The current can be read anywhere in the system but if there is any significant resistance in the lead between the battery and where I have this box, I can 'remote' the voltage mater supply / sense wires directly to the battery terminals to remove the influence of the voltage drop over the cables that will range from 0 at no current to maximum (that should still be pretty low in ideal circumstances) at full current.
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