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Mr.RC-Cam

µMetros: Battery Resistance Meter

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Extremely useful battery performance data can be determined by measuring the cell's internal impedance or resistance. This topic discusses a custom meter that does that with the ultimate in convenience.

First a little background. Most battery manufacturers report their cells' AC impedance. This test involves a 1KHz AC excitation. However, the DC resistance is often considered more useful. This measurement is also known as the dV/dI method, as specified by ANSI committee.

In a nutshell, the lower the resistance, the lower the internal losses, and the better the cell will operate with high current loads. For high performance R/C applications (e-flight), it is best if the cell resistances are as low as possible and carefully matched.

I've used the dV/dI method using a digital voltmeter, a digital current meter, and some test loads. It was not all that convenient for me, so I never really looked forward to doing the test. I often thought about building a dedicated test box, but I was never excited enough to gather up the parts to do it.

Then I read a very interesting application note that was published by a senior engineer at Linear Technologies (an IC manufacturer). His analog design offered very precise battery resistance measurements by using a slowly modulated dynamic load (MOSFET current sink). The published circuit is available online at http://www.edn.com/design/test-and-measurement/4323863/Dynamic-load-circuit-determines-a-battery-s-internal-resistance

The circuit was breadboarded and I found it worked great. It converts the battery resistance into a DC voltage, which can be measured by any good performing DVM. All the technical details are outlined in the link above.

I was impressed with the accuracy and found the circuit to be interesting enough to deserve more attention. In the end, I was able to build a fully self contained handheld meter that works like a champ. And of course it received a PIC microcontroller in the process. :) The super charged design is called µMetros (Greek for "little measurement")

Here is what the packaged meter looks like, as installed in a SERPAC H-67-AC plastic enclosure: umetros2.jpg

Edited by Mr.RC-Cam
Fixed broken link

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Continued...

The µMetros circuit board is powered by a 9V battery. The 2-line LCD readout offers some status info as well as the battery resistance value.

Resolution is 1 milliohm when the target cell/pack is less than 1 ohms total. Higher resistance measurements will be displayed at 2 milliohm resolution. Max resistance that can be measured is about 2 ohms.

Accuracy appears to be better than 2% (basically limited by the 1% tolerance resistors in the sense circuit and sample/hold amp). Reading a pack involves pressing a button. After about eight seconds, the displayed value stops accumulating and the button is released. The measurement is done. It is that easy.

The circuit board is shown here:

post-6-1118697859_thumb.jpg

Edited by Mr.RC-Cam

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Continued...

There is a surface mount MOSFET that is configured as a current sink. I originally thought that it would be wise to provide support for a tiny cooling fan. After adding the necessary software to control the fan, I determined that the metal heatsink was all that was needed, even with continuous use. So, the fan hardware and software was abandoned.

Besides some copper area on the PCB, a heatsink made from a piece of .060" aluminum was added. It is compress fitted against the MOSFET using two screws. A bit of heatsink grease is used too. Here is all it took to handle the heat from the dynamic load:

post-6-1118697836_thumb.jpg

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Lastly ...

When I have the time, *and* if there is any real interest, I will post the circuit and PIC12F684 hex files used in µMetros.

But keep in mind that the basic design, that is shown in the EDN link above, will work great too. The EDN schematic has one wiring error, as noted in the emailed comments on their site.

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G'day RC-CAM,

I would like to register an interest in the building of the uMetros.

Not sure what you can provide but the schematic and hex code is a great start.

Have you got any idea of the total component cost of this project please ?

Measuring a batteries internal resistance is probably not for everyone but for me it would take the guesswork out of battery performance on aging of the packs, we know they give poor performance with age this is the reason I believe.

Originally I thought we are just looking at the capacity of the cells dropping off but there was something else, internal resistance losses.

Regards, Bthree

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Have you got any idea of the total component cost of this project please ?

I was able to pick up the IC's as free samples from the vendors' web sites. I would estimate that µMetros' other electronics parts cost me under $35. The parts are sourced from digikey.com.

The ugliest expense for me was due to the custom PCB I used. I wanted something professional, so I exercised my charge card a bit. But, perfboard construction would have worked too.

but for me it would take the guesswork out of battery performance on aging of the packs, we know they give poor performance with age this is the reason I believe.

I totally agree. The meter will help sort through aging packs in no time at all.

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Hi Mr.RC-CAM ,

I , too, would be interested in this project.

I've got too many batteries that need profiling.

Thanks for the offer.

Cheers

Zlatko

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OK, sounds like a couple folks want to give this a go.

The project related documents will include the schematic, hex file, and bill-of-materials. This will allow an experienced electronic hobbyist to recreate this nifty meter. I will offer a simple warning: this is NOT a project for folks that don't have strong electronic skills.

I was up late last night getting the documentation in order. If I can survive another go at it tonight, then the tech info might be available as soon as tomorrow afternoon. If not, then it will be posted later in the week.

Please continue to watch this thread for updates and post your questions/comments here too (please do not email or PM me).

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Mr. RC-Cam,

I'm very interested in the µmetros battery testor but am not completely confident that my electronics experience is sufficient for its construction. I would feel able to "populate" a board with components from a kit. Is there any chance someone might commision you to build one of these? Or perhaps one of the readers would be willing to order parts for several and perhaps build several to savd shipping form Digikey.

I'll be watching:)

SciGuy

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I would feel able to "populate" a board with components from a kit.

Good news/Bad news: I have four extra PCB's and some components that I suppose I could sell. However, my opinion is that even if a nice bag of parts was offered, this project is not well suited for the typical hobbyist.

My years of publishing projects has taught me to be more critical of how "easy" something is to build. And this one is definately more difficult than most. Keep in mind that the documentation will be basic and so construction & testing will rely on the technical experience of the builder; the average electronic hobbyist will probably become very frustrated.

But regardless, I'll sort through the parts inventory I have and see if I can create a couple of semi-kits for uMetros.

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Hi RC-Cam, thats great progress. just love your schematics.

I would like to buy one of your PCB's and some components please, I have already ordered the three Linear Tech IC's and i guess they will arrive sometime.

My level of project assembly would be classed as very capable and would have no trouble assemblying/faultfinding the uMetros

My problem is sourcing components being in Aus and 500km from any capital city.

Things I cannot source so far.... besides the PCB

.HexFET IRLR024

.LCD 50448 with socket and header

.PIC 16F884

The rest I have in my workshop parts inventory

Regards,

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Some basic circuit notes:

(1) Although the EDN circuit is the basis behind the design, the LTC6943 IC was not used (the SMT package does not allow for easy perfboard construction). Instead, a Maxim MAX4619 is used in its place (nice DIP package). The 2.5Vref is the more common LM285. If you take the time to compare uMetros to the EDN schematic, you will see the other changes. But, the theory of operation is basically the same.

(2) Other than the LCD contrast pot, there are no adjustments. This is possible because of the 1% resistors that are spec'd in the indicated places. Do not use lower tolerance parts. Do not sub parts unless you really know what you are doing.

(3) Upon first use, the LCD probably wont show any text until you adjust the POT1 contrast pot. This is a relaxed adjustment that can be performed once the board is assembled and powered up. Immediately after uMetros is turned on, text is displayed on the LCD, so tweak away at any time for best pixel contrast. Once the pot is set it can be ignored.

(4) I do not recommend sockets on the analog IC's. However, the PIC should be installed on a quality machine pin socket. This will allow for firmware upgrades (if that should ever arise)

(5) TP1 is a convenient analog output that is scaled at 1 ohm = 1V. This can be used with a quality lab DVM for measurements that require more than 1mohm of resolution. If you do not need it then ignore it.

(6) DO NOT operate the board without a heatsink on Q1. Heat increases with increased DUT1 battery voltage.

(7) Before installing any IC's, check their power pin pads for correct voltage. All DIP IC's are powered by a regulated 5V source. Except, Pin-4 of U2 should have about -4V (negative) on it.

(8) The DUT1 battery test connections can be traditional 2-wire (Tare) or 4-wire (Kelvin force-sense). My PCB is setup for 2-wire use, which works well for me. However, my leads are very short (under 4") and heavy gauge (12AWG). I merely subtract the estimated lead/connector resistance (~35 mohms tare in my case) when I make a measurement.

(9) The meter is only as good as your measuring technique. The contact resistance of your probing will have a big impact. Keep the DUT1 leads short/heavy and ensure contact with the battery surfaces is solid. If you don't want to have any tare issues then you might want to use a carefully devised 4-wire Kelvin interface at DUT1 (but you are on your own in configuring this).

When you measure an assembled pack, the resulting value includes the resistance of the battery pack's connector, the internal and external wiring, as well as the sum of the cells resistance. This can come in handy in analyzing your flight packs. Of course single cells can be measured too.

(10) uMetros is NOT an ohmmeter. That is, you cannot merely connect a resistor to it and measure resistance. However, you can measure changes in resistance while a battery is measured. For example, a cool way to measure the resistance of a Rx's on/off switch, or a servo extension, is to first measure a good battery. Then, install the switch or extension in series with the battery lead and measure again. The milliohm resistance is the difference in the two measurements. You can also use this method to validate the assembled meter's performance (insert a 0.1 ohm 1% resistor in series with a battery and verify the resistance increased 0.100 ohm +/- 1%).

(11) To use the meter, you must press and hold the S1 switch until the accumulating resistance value stops changing. Once it is stable, or nearly so, you release the push switch. The displayed value will be held for you. It normally takes 8-10 seconds to perform a measurement. It can take longer. but I would suggest that the DUT1 battery is defective if it takes more than 15 seconds.

(12) The meter can measure up to 2.300 ohms or so. A defective DUT1 battery might be higher than this. When the value is out-of-range, the display will say "Error?". Generally speaking, our R/C cells are well under 50 mohms (0.050 ohms), so the out-of-range message is not expected unless the battery is defective.

(13) The DUT1 battery must be charged before testing. Its voltage must be at least 0.9V, but less than 13V. It must also be capable of sourcing 1A of current. It can be any chemistry: Alkaline, Carbon, Lead Acid, LipO, NiCD, NiMH, or a room full of potatos with copper nails in them.

(14) Operating current is about 5mA. The 9V transistor radio battery should last several dozen hours of constant use. If you turn the meter off after each use then the battery will probably last a couple years. If the 9V battery voltage is too low for use, the meter will display "Pwr Fail."

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UPDATE: All Sold

I have three printed circuit boards (PCB) and most parts for two of them. So, here is what I can do:

PCB with programmed PIC. $24 + shipping. Only one available.

PCB with PIC and all electronic parts except U2, U3, U4, LCD, PIC Socket: $45 + shipping. Only two sets available (UPDATE: NONE AVAILABLE). Note: the IC's can be obtained as free samples from the mfg; that is how I got mine.

Add 3% for PayPal. PM me through the forum "Inbox" link (don't use email) if you want one. I won't hold anything - first come first served.

Edited by Mr.RC-Cam

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Hi RC-CAM,

Parts arrived today from you thanks just have to wait for Dallas...

I seem to have lost the top and bottom view of the board component overlay, would they have disappeared when you updated the schematic or have I missed something.

Regards.

edit: not Dallas should have said Linear

Edited by Bthree

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Glad to hear the parts arrived. The board overlay drawings are in the special PCB assembly instructions I emailed directly to you a few days ago.

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Hi,

Just got mine too :)

Thanks!

Bthree, I just realised that you are a local too :) . I am in Melbourne and will be ordering some parts from DigiKey probably Monday. If you want, I can combine it with the stuff you need.

pm me if you are interested with your details or email me at

zed(at)techlink.com.au

Cheers

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I used the one with the 9V battery door (a nice touch). I doubt that the one with the molded compartment will work.

The board and battery fit in the enclosure, but just barely. Even though it does not have the molded compartment, it still has a nice little cover that slides off for the battery access. The PC board itself holds the battery in place (it is a snug fit).

The board was originally going to be used bare, so it was not designed for any specific case in mind. It would take a new PCB layout to allow a simple drop in install in the SERPAC enclosure, but the board will fit if you have a lot of patience. I created some extensions for the switches (used model building techniques) so that I could continue using the parts already used on the board. Chassis mounted switches would take less work.

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Hi MR.RC-CAM ,

I just finished soldering all the components from your kit.

I noticed that there were a few vacant spots on the PCB ( Q2, R1, U5 ). I couldn't find them in BOM or the scematic. Should I ignore them?

I also noticed R15 missing from the silk screen but there was a 10k SMD resistor on the bottom of the PCB which looks in about the right spot by the scematics. Is this okay?

Just have to order the other parts and finish it :)

Cheers

Zlatko

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Zlatko, did you get the special PCB assembly instructions I emailed you in June? It discusses the items mentioned in your questions.

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Hi Mr.RC-Cam,

Thanks for the reply.

My ISP had some "issues" with the mail server.

I've PMed you a note with a alternative email address.

Cheers

Zlatko

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Zlatko, I just resent the PCB instructions to you. Anyone else that has the custom PCB, but not the special instructions, then please contact me immediately. Please do not attempt to stuff the PCB until you have read the PCB specific instructions.

Early on, there was a lot of interest in the uMetros design, so someone has probably completed theirs. But, I have not heard from anyone that has built the project. If you have created one then please report on how it is working out for you.

I have one uMetros PCB/PIC set in my hands. If any of you are interested, then act soon. I do not plan on continuing the offer. Lastly, I might also have an assembled & tested uMetros for sale. But, it is not in an enclosure. It is my backup unit, so I'm not sure if I will sell it. I'm even less sure what I would charge for it. I'll have to think about this some more.

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