Wanted - an amp hour meter

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Post by coulomb » Fri, 18 Jan 2013, 19:40

jonescg wrote: Do you reckon it could plug straight into a digital panel meter?

No, designed for your fuel gauge only. I imagine you could adapt it, with some fiddling.
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Post by zeva » Fri, 18 Jan 2013, 19:44

Hi guys, the FGD is not really ideal for driving a digital gauge because its output is analogue (well, PWM), and the capacity adjustment also analogue (we could hard code that but usually there's an interplay between capacity adjustment and gauge scaling).

You could get a decent result using a (digital) voltmeter and hard coding a capacity. 100% SoC would be your aux voltage (13.5V?) and decrease linearly down to 0V = 0Ah / 0% SoC.

By chance the scale is actually pretty close volts : amp-hours!
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Post by jonescg » Fri, 18 Jan 2013, 21:02

Hi Ian, that could work out since I have a spare digital voltmeter. It wouldn't be super accurate since the auxiliary voltage (4s LiFePO4) will fluctuate depending on it's SOC and what other things are running, but good enough for a race bike.

I like your full blown instrument panel, but I really don't want to have to flick between screens while hammering down the back straight.
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Post by evric » Fri, 18 Jan 2013, 21:28

I'm confused... how can this be done with a simple panel meter? We are talking about measuring current against time.
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Post by jonescg » Fri, 18 Jan 2013, 21:34

The EVFG would still be used to measure current by time, but it's output could be sent through a digital panel meter as a fraction of full pack voltage. So it would start at 13.5 V and drop down to 0 V when empty.

I think that's how it would work, Ian?
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Post by zeva » Fri, 18 Jan 2013, 22:34

Yep, so still using the FGD to do coulomb counting, but its output can drive a voltmeter (turning it into a SoC meter, sort of). Fuel gauges in cars are basically reduced-scale voltmeters (in series with a varying resistance fuel tank sensor), the FGD gets tuned to match battery capacity with gauge scale.

You can also run an analogue voltmeter off the FGD, IMHO analogue gauges are easier to read at a glance and don't have problems with glare/washout from the sun, but up to you. Or we could get fancy and port the FGD code to a bigger micro then drive an LCD/OLED display digitally, but that'd be more work Image
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Post by jonescg » Sat, 19 Jan 2013, 03:58

At least an analogue meter can have the scale adjusted far more easily than a digital one - you just draw an extra zero over everything Image
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Post by Nevilleh » Sat, 19 Jan 2013, 20:28

Just came across this thread and, believe it or not, I started work on just such a thing yesterday! I have a spare bms master unit and I thought I'd make an AH meter out of it with a view to including in the next iteration of the master unit, but in the meantime its fairly easy to do as a stand-alone. The master bms unit comprises a small micro with a 2 line lcd and some other bits. My thinking is to use a Tamura Hall-effect sensor driving an op-amp to do some level shifting and scaling, then read the current via one of the adc inputs. Probably can do about 10 samples per second, add them all up to get amp-seconds or something and display current on one line and AHs used on the other.
Would that be the sort of thing you'd want?
Might take me a week or two to actually make one though.

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Post by jonescg » Sat, 19 Jan 2013, 21:40

Hi Neville

I believe such a device would be extremely popular. Just a simple 3 digit backlit LCD with a reset button, or at least a toggle between amps and amp hours. It could be a standard size, 72 x 36, like these ones: http://egomega.com/tester/new2018power/ ... 000Vdc.jpg

All it needs is a 12 V supply and two sense leads from the shunt (or from the Hall effect sensor). If you could work something out so it could be as ubiquitous as a digital voltmeter, that would be awesome. It shouldn't need access to full pack voltage I would think.

I'm willing to help fund any development of a one-off. BTW Justin of E-bikes.ca has offered to hack the CA so it can read 7 V from a divider, and report full 700 V on the screen. I will definitely use this for both the CRX and the bike, but it need not be located on the dash / instrument panel.
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Post by Nevilleh » Sun, 20 Jan 2013, 12:12

jonescg wrote:

I'm willing to help fund any development of a one-off. BTW Justin of E-bikes.ca has offered to hack the CA so it can read 7 V from a divider, and report full 700 V on the screen. I will definitely use this for both the CRX and the bike, but it need not be located on the dash / instrument panel.


I do this stuff for a hobby so my time is definitely not chargeable - else I'd owe myself a million dollars! - and the parts cost is very small, so don't worry about funding. Besides, if anyone gave me money to do this I'd have to get it finished!
I looked at adding voltage measurement and it would not be hard - I'd use a linear optocoupler to provide isolation. But, firts things first, make an ammeter, turn it into an amp-hour meter and go from there.

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Post by Nevilleh » Wed, 23 Jan 2013, 18:44

Well, I've made an ammeter/amp-hour meter by modifying an old bms control board.
It displays instantaneous amps (sampled 10 times per second) and amp-hours used.
It uses a PIC16F1826 micro, a 16 x 2 lcd, a 5V regulator, some R's and C's, an op-amp and a Tamura current sensor. My one works from 0 - 511 amps. jonescg wants one - what current range do you want?
Tamura L01Z***S05 range is what's needed at a cost of about $25. They have a big, rectangular hole in the middle for your cable and a 3-wire connection to the electronics - gnd, +5 and signal. The signal output goes from .5 to 4.5 v over the current range with 2.5 v as the zero point. I'm only interested in +ve current, so the op-amp level shifts and amplifies to give a 0 - 5 range for the 2.5 - 4.5 v input. The adc turns this into a number 0 - 1023 which is scaled to give the correct current.



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Post by jonescg » Thu, 24 Jan 2013, 02:30

Hi Neville!

Yes I would be interested in it. I will be pulling up to 400 A, so maybe make it 500 A (75 mV drop from a shunt). As long as it doesn't need access to full pack voltage all will be well.

Thanks man!

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Post by jonescg » Thu, 24 Jan 2013, 05:03

That looks really cool. Now all I need is for it to display amps and amp-hours on a single, three digit LCD display with a toggle button and a reset button.

Simplicity is my aim here. I don't need bluetooth, CAN-BUS or a smart-phone app to make it work, just another display with a single (at most, two) number display.
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Post by Nevilleh » Thu, 24 Jan 2013, 12:14

Andrew says he has turned off the image file scan (which has prevented me from uploading images) so here's a picture of my display, just to test that it now works:

Image

The features of this thing are:
Uses Tamura Hall-effect current sensor for 0 - 511 A range
Displays instantaneous motor current, sampled 10 times per second
Displays amp hours used since last reset
Reset by holding down reset button while powering on
Uses eeprom to keep data while power off

I will make this an open-source thing once I've done the pcb and sch revisions. Likely cost to make one is about $50.

Any ideas for other useful features? There is still 70% of the ROM available!
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Post by Tritium_James » Thu, 24 Jan 2013, 14:04

Nevilleh, is that showing instantaneous *motor* amps, or *battery* amps. Integrating (coulomb counting) your motor current won't give you the right answer!

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Post by Nevilleh » Thu, 24 Jan 2013, 14:12

You're quite right, of course. The sensor should be placed on the battery lead. I was looking at the lcd and wanted to pad it out a bit - just "Amps" looked a bit plain! - and I unthinkingly put the word "Motor" in just because it looked nice. I'll change it to "Batt".

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Post by jonescg » Thu, 24 Jan 2013, 16:30

Am I right in thinking this has nothing to do with the full battery voltage? One less set of HV wires / voltage divided wires to run.
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Post by jonescg » Thu, 24 Jan 2013, 16:34

Neville, is there a chance you could get it to display on a simple 3 digit LCD? I mean, if I'm going to have a full screen with all the fruit, I might as well stick with a Cycle Analyst. I probably will end up going with a CA anyway, but when the CRX is converted I will install the 700 V-modded CA in the car and put a simple Ah counter on the race bike.
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Post by Nevilleh » Thu, 24 Jan 2013, 16:37

No HV connection needed, its powered off 12v and hence isolated. Maybe a 3 or 4 digit lcd might do, I used the 16 x 2 because its there, but it might be cheaper to use a lesser thing.
Actually, the 3 1/2 digit things are usually 7 segment displays and you have to drive them with something. Also, they aren't a great deal cheaper than the 16 x 2 lcds, so there is probably little point in considering them. If size is what you are worried about, the 16 x 2 lcd is only about 25 x 70 mm.

I just had a look at the Cycle Analyst and it suggests a few features that might be worth adding. I was trying to avoid connecting to the HV battery, but if you want the voltage it will have to be done.
Mind you, the CA is $US155 plus $15 shipping and what I'm trying to do here is offer the technically able enthusiast a DIY gadget that will do most things at a much lower cost.
And I set out to make this thing with a view to incorporating it into the existing BMS master control unit, which is the next task.
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Post by jonescg » Thu, 24 Jan 2013, 23:21

OK so that 25 x 70 screen is pretty small. I like that.

In my circumstances, I don't want high voltages leading up to the instrument panel so if you were in incorporate battery voltage readings and integration, you would have to do it via a 0-5 V signal or something. As I said the CA does all of this and more, and Justin is currently hacking it for me so that I can run it off a 1:140 divider.

There is a small but solid camp of builders, especially on the motorcycle side of things who like to keep stuff simple. Over on Elmoto.net you will find guys who are content with an analogue ammeter, analogue voltmeter and a speedometer. However you still need to be able to count Coulombs so a simple meter would be the goods.
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Post by Nevilleh » Sat, 26 Jan 2013, 21:30

I don't really want any connections between the high voltage battery and the 12V circuits. This means using a linear opto-coupler and a small dc-dc converter, all of which adds to the cost. The CA has these connections and doesn't appear to be isolated. Might be OK for systems up to 48 V but not so good for any others. I'm also thinking a cheap voltmeter is easy to find, so why include it in this thing?


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Post by jonescg » Sat, 26 Jan 2013, 23:35

I don't want volts either Image I was just saying if you did want to incorporate volts into this unit, it would be troublesome, so probably best not to.

I can only run a voltmeter if I have a 1:100 or more divider inside the pack. I've ordered some big 14 W, 47k and 330R resistors for the job. Justin suggested I make a 0-5 V signal for volts and he can set the Vsense to read to within a volt or two.
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Post by Nevilleh » Sun, 27 Jan 2013, 01:46

You do realise that a divider attached directly to the battery still doesn't provide isolation, don't you? If you are powering the voltmeter from 12 V, it means the -ve battery terminal is now commoned up with the 12 V system -ve.

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Post by jonescg » Sun, 27 Jan 2013, 04:04

Sure it's not isolated, but it's a damn shade safer than having 700 V behind the instrument panel. I won't be grounding the low voltage system to the chassis, just running extra wire everywhere.

I rank the ideal invention as the most elegant, part simple, unbeatable creation which cannot be improved upon. The pedal powered bike fits the bill Image
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