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Old 2008.01.10, 02:38 PM   #16
lsarccc
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Quote:
Originally Posted by mk2kompressor
damn no bowing smiley
how are you working out the internal resistance?
Hmmm - I haven't finished thinking that through properly...

The minimum is that you let the cell rest, measure a voltage - hit it with a big discharge current. Take the dischage away and immediately start measuring the voltage. The voltage drop is due to the internal resistance and for me indication enough of the punch of the cell - punchier cells have higher voltage and hold that voltage better under load. The exact value of IR is not necessary as long as all cells are subjected to the exact same load?

I think you can make a calculation of internal resistance value, but I think voltage loaded and unloaded is enough for our purposes? I'd never compare an IR I'd measured and calculated against anyone elses number - it's going to be meaningless.

Then we measure recovery by sampling the rate of rise of voltage after you remover the load. Fast is good!


Still more questions than answers
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Old 2008.01.10, 02:41 PM   #17
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@Wolf...

overloaded! - I need to sit with a soldering iron and smell some lead fumes.

Thanks for the comments - considered replies will follow might take the weekend to fully take it in
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Old 2008.01.10, 02:44 PM   #18
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im going to hook up an E.S.R meter to a pack of the duracells,hopefully tomorrow(used for testing capacitors whilst still mounted)it goes down to .01ohm so hopefully its low enough to see if my mah ratings match to the internal voltages(ive put them in packs of 4 based on the end mah after break in)
the results will be on my spec lists for the next meet

i was hoping you could have a look into adding something like an esr meter into the charger,there are a few circuit diagrams floating about if you google it
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Old 2008.01.10, 03:07 PM   #19
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I thought this was simple Kirchoffs - if you know the load resistance, the current and the voltage you can calculate the missing resistance which is in the cell?

If the current was stable I could see how to do this easily - but I'm switching the load and I'm not sure of the validity of anything...



What is your thinking Matt? That I'm wrong to assert that all that matters is Voltage or that IR values can be accurately measured easily so why not do it.


I claim ignorance on this - I have loads of reading to do

More detail please mate
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Old 2008.01.10, 03:46 PM   #20
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oh no im not saying yours isnt the way
i was discussing what i learned in germany the other day with my uncle(tv repairman and tinkerer for years)he said i should find the internal resistance very acurately using his esr meter,it fires a signal through the battery to determine IR regardless of the v in the cell,as we are talking miliohms and the battery varies in voltage whilst under load it looks like an easier way
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Old 2008.01.10, 04:55 PM   #21
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You seem to be very precise in the way you go about these things - I'm sometimes tempted to be slack and cut the odd corner to get there quicker... But a good team in the forming methinks.

I have had a small box of Mini-Z stuff arrive today and 4 small scales that do upto 100g in 0.01g increments.





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Old 2008.01.10, 05:33 PM   #22
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ooooooohh i like boxes of goodies,esp ones with 4 micro scales in them

i cant wait to see how all this turns out
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Old 2008.01.11, 08:50 PM   #23
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Smile Building the telemetry bug

So I'll use the 12F675 in DIP format for this. All we're trying to do is get a trace of current draw going round a track so this will be a little 'ghetto'.

Incidentally Microchip do the rfPIC12F675F chip which is a surface mount 12F675 with a nice RF TX built in, also it's available with it's counterpart RX as a free sample from their site. I ordered 3 pairs today. Using this will allow us to make a telemetry bug a little more worthy of going into a Mini-Z!

This bug is quite easy to do and I'm sure it could be done as a PICAXE so this should be a very accessible way to play with telemetry.

The first 3 way socket links to the power and ground pins and MCLR. (blimey my soldering doesn't bear close scrutiny )


Then the Programming and Data and external trigger pins - all these are multi functional and could be connected to sensors to measure thins like voltage and rpm.


This plug will be the one we use to measure the current - to make it easy I have power and the link to the pin that we will use to do the analog to digital conversion in one plug block.


The el cheapo 433Mhz transmitter just tacked on top. connected to power and the pin I'll use to output the signal. The fourth unconnected connection sticking up from the TX is the aerial lead - I'll tack on a bit of wire for an aerial later. This TX and it's matching RX are easily available, about £10 as a pair from Maplin but I guess you can get them cheaper elsewhere.


So far just over 2g, but I have to add the aerial, a link to tie the MCLR pin to Vdd that I forgot about and perhaps I'll add a betterpower plug to take power from the car.


A test fit with the iWaver Enzo shell suggests the bug will fit here. Whether this is a good place to avoid intereference will need to be trial and error, you'd think putting it next to the car crystal is a bad idea. I've never had an issue in the past - I guess there is a lot of signal separation between 433Mhz and 27Mhz . The blue thing is a ferrite ring that I will dremel a slot in for a hall effect sensor - the positive power lead from the batteries will be extended and wrapped around the ring. The three pins of the sensor will be connected to the plug in the third pic above. The coil will fit here.



More doing and less talking/thinking today Hopefully I'll get around to seeing if the chip will take a program and also get to measuring the pwm freq of the MR02 tomorrow.

Hmm 2am in UK... goodnight all

Last edited by lsarccc; 2008.01.12 at 04:31 PM.
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Old 2008.01.12, 10:18 AM   #24
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Matt - I'm trying to learn more on the ESR meter. Very interesting, I still don't really get it. Be good to see some more info about use on batteries looks like a better/proper way to measure IR. I am becoming more convinced that the biggest problem we face is that not many applications care about IR - if cameras were affected by IR we'd be inundated with data.

I have found a reference to the patent that Tekin (makers of escs and chargers for 1/10th electric) took out for power capacitors:
Tekins Patent for using power capacitors with escs
Quite a bit of info there, power capacitors came and went as a fad at the end of the last millennium (in 1/10th and where I race at least), but the info in the pdf is interesting I think.


We're looking for a 1:1 car for our youngest daughter and other such major distractions happening all over at the moment. I find the odd moments to research on the net whenever I'm in a free wi-fi area, gotta love the ipod touch! I'm too tight to pay for 3G phone access...

At home for a few hours so I'll get cracking on seeing if I killed that pic chip with all the soldering I did on it last night...
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Old 2008.01.12, 04:30 PM   #25
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Smile Preliminary tests show the bug works great!

Well I ended up with more time on my hands than I expected

I finished the bug, including tying the MCLR pin to Vdd with a 100K surface mount resistor. Added an aerial (I carefully calculated a 1/4 wave antenna length then just ignored that and cut a semi-random length of wire based on what looks right!). I've also added a power lead separate to the power used by the programming dongle.



I made a dongle to connect between my USB in circuit pic programmer and the sockets on the bug. The thing in the bottom right of picture is my programmer, I chose that one because it means I can more easily reprogram trackside from my laptop. So the next thing was to try and program the pic...


Success! The 433Mhz RX/TX pair works very simply, whatever the state of the signal pin on the TX is transmitted to the signal pin on the RX. In the picture the Board in the middle has the RX board with the output signal pin connected to an LED and the oscilloscope. The TX bug is on the rightmost board and programmed to send a fixed width pulse at fixed intervals. The oscilloscope is connected to the RX signal pin and displays a clean square trace of the signal being sent by the TX. This has proved the easiest way of testing the telemetry link, you can set up a clear signal pattern and set the oscilloscope on a long time base then jiggle the TX and subject it to revving the motor up jiggling the servo and testing distances. If you can't maintain a reasonably clear and square trace on the oscilloscope then you have an issue that you must resolve before you go to try and send a real more complex signal. Once I'm happy with all of the connections I'll pot the whole thing in epoxy which will make it very robust.



Here the TX is about 10meters away in line of sight and as you'd expect the trace is fine - my aerial length can't be too bad!



Here it is sat on my MR02 ready for an interference test on the bench, this revealed some issues and I think various capacitors will be employed to clean the trace up. In the worst case I buy a sub miniature LiPo cell and use a dedicated power supply - usually I'd expect to be able to solve these issues without resolving to that.



Well I didn't get round to measuring the drive frequency of the MR02. But I did start the discharger itself on a prototyping board. This is the 18F4331 chip with power and a programming interface ready for me to start developing the code further.


Pretty successful day I reckon - settling the TX down so it transmits cleanly whilst it's in a car razzing round a track will be the next challenge.

Last edited by lsarccc; 2008.01.12 at 04:36 PM.
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Old 2008.01.12, 04:43 PM   #26
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Arthur, just superb work my friend. Matt and I have had some discussion on this and we're trying to keep up with you, we really are

Anxiously waiting on the next installment!

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Old 2008.01.12, 06:14 PM   #27
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Thanks Jonny - it'll be me trying to keep up where it matters (on the track) at the Hurda challenge!

I'm racing 1/10th early tomorrow morning so I'd better at least look at my car, depending on how that turns out I'll start work on the forum...
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Old 2008.01.12, 06:27 PM   #28
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top work arthur,cant wait to see it for real,it looks small enough
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Old 2008.01.13, 10:48 AM   #29
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Thanks Matt, the bugs have developed over a few years. These three gives an idea of the things I've tried. The central one hints at how small we could go with better pcb designing and making ability using surface mount version of the 12F675.


Quote:
Originally Posted by Wolf a few days ago
- I agree with the scope and assumptions.
- One of the reasons why I do like constant discharge rates is; you can compare matched cells between companies. Many matchers has many accronyms. VRS, VDS, XXMAX, bla bla. They all mean same thing Virtual Race Smulation and also indicate something that you can not compare the results of brand X to brand Y. That's why I always sticked to constant ampers. I know, this is not a technical reason but still a valid hobbiest excuse. May be constant ampers would be included as an option.

- Infact variable discharge may be even better considering AAA. The reason we see low constant discharge rates on manufacturer infos for AAAs is they not designed for it. You can easily draw 35-40A from a sub-C for several minutes but you can not even draw 3-4A for several seconds from a AAA

- Yes. We would be interested in a 2.4 telemetry. Infact we have access to those modules and test boards but have no time to work on it to fit our needs. Just send me an email.

- Meanwhile we already have the software you needed. I've written it a year ago for C1. It reads from RS232 according to our protocol and stores detailed data in a mdb and then graphs it. You can compare the graphs of both charge and discharge.
I agree with your pros and cons of fixed vs variable discharge rate. After some thought I realise that it is the hobbyist side of me wanting to make something with a pic chip driving this effort more than a deep technical belief that this will reveal some major new piece of info.

Maybe you think my telemetry approach is a bit amature now ? Well I think that this approach is surprisingly accessible and feels more like a hobby and less like a maths exam - I hope some people will be interested in doing stuff like this. Once you've built a bug like this you can easily attach various peripherals like rpm sensors, accelerometers and use rare earth magnets and hall effect sensors to easily detect the changing positions of chassis elements. A great hobby on it's own! I'll drop you an email later in the week about telemetry and your database schema you use.

Ah a big Sunday dinner with the family
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Old 2008.01.14, 01:16 AM   #30
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In fact being a hobbiest approach is the best thing we can have. PIC drived discharge mechanism is not only hobbiest approach but also manufacturers product. We are announcing our microprocessor controlled individual discharger this week. Hope people will like it and don't consider it as a lab rat device

There is wording;" Geniousity is being able to divide big complex problems in to small easy parts". That's why I think your approach is very affective and result oriented.

Meanwhile we were discussing with Ozgur yesterday (other partner of TGR who is electronics engineer), we thought using a seperate PCB + Motor with a load, with the same frequency would also give you the data you need. Ofcourse it is not extensible as yours but might be sufficient and trivial.

Waiting for your email about telemetry.
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