It is not a problem to create a known current in the order of 10 Amps either by using the existing charger on the vehicle or an external power supply. That produces a 1mV drop for a 0.1 Ohms resistance. Since the measurement does not need to be extremely accurate, you just want to verify that all connections are in the 0.1 mOhm range and none are more than double that, you can suffice with pretty simple instrumentation that allows you to distinguish between 1 and 2 mV, while you send that approx 10 Amps through the connection, either by actually charging or with external means. You just need to take care to always probe on the same spot on the battery terminal (preferably as close as possible to the cell themselves, so the Alu or copper contact would be good if you can reach it, the alu nut around that contact would be the next best thing. Read up on 4-point resistance measurement (Kelvin sensing) if you are not familiar with how the sensing points will influence the measurement: http://www.allaboutcircuits.com/vol_1/chpt_8/9.html
Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.info Skype: cor_van_de_water Tel: +1 408 383 7626 -----Original Message----- From: EV [mailto:[email protected]] On Behalf Of Michael Ross via EV Sent: Tuesday, August 19, 2014 11:09 AM To: Peri Hartman; Electric Vehicle Discussion List Subject: Re: [EVDL] CALB bolt terminals getting hot - I think you are correct, measuring the exact situation is always better (more accurate) than simulating and scaling it to make the measurement easier. The question is, can your meter accurately measure 100microOhms well? If it has a good rating (read the spec sheet), then you can read the resistance directly and apply the accuracy they claim. If you can create a particular current and accurately measure that, then you can measure the voltage drop accurately and calculate an accurate resistance. But you have to ask the same questions about your ability to measure current and voltage - read the spec sheet to know for certain. I have a Fluke 117 that I carry around. Range/Resolution: 600.0 Ω / 0.1 Ω Range/Resolution: 6.000 kΩ / 0.001 kΩ Range/Resolution: 60.00 kΩ / 0.01 kΩ Range/Resolution: 600.0 kΩ / 0.1 kΩ Range/Resolution: 6.000 MΩ / 0.001 MΩ Accuracy: 0.9 % + 1 Range/Resolution: 40.00 MΩ / 0.01 MΩ Accuracy: 5 % + 2 This has only a resolution of 0.1Ohm in the smallest range - clearly not good enough for measuring 100uOhm directly. I am sure it would round off to zero. In the DC millivolt function it will measure: Range: 600.0 mV Resolution: 0.1 mV Accuracy: ± ([% of reading] + [counts]): 0.5% + 2 This is better, because at 300A and 0.0001Ohms the drop will be 30mV and resolution is 0.1mV. Accuracy is ±.005x30 +2 = ± 2.15mV out of 30. However, you need to make 300A steadily while you read the drop. I am not sure how you guy would do that at home. I can imagine having some data acquisition equipment and taking current an drop data over some period of time. I have a Fluke 289 at work - lets see how its accuracy looks: *it has a 50mV range and accuracy of 0.025%* which if I am reading the specs right means 30mV ± (.00025x30) = 30mV±0.075mV That is not bad at all, but I can't use my little Harry Homeowner meter to check battery terminal resistance. The 289 costs about four times as much to buy. Hope. I got this all right. Mike On Tue, Aug 19, 2014 at 12:25 PM, Peri Hartman via EV <[email protected]> wrote: > Question on how to measure resistance. If you use an ohm meter to > measure resistance, I presume it is using a 1mA or smaller current. In > electronics applications, that's probably fine. But in a situation where > typical current is in the hundreds of amps, will taking a measurement that > way be reliable? > > It seems that for low current, you will be measuring more-or-less the best > path through the circuit - in this case from terminal to cable. That is, > there may be small areas with good metal contact and lots of areas with > lesser contact or none. With high current, won't those good contact areas > become overloaded and their resistance increase? > > If so, then I presume a better way to measure resistance under high > current would be exactly that: apply high current and measure the voltage > drop. > > Anyone care to validate or invalidate this? > > Peri > > > ------ Original Message ------ > From: "Cor van de Water via EV" <[email protected]> > To: "Electric Vehicle Discussion List" <[email protected]> > Sent: 19-Aug-14 9:15:36 AM > Subject: Re: [EVDL] CALB bolt terminals getting hot - > > Martin, >> Delivering 300A at 120V is 36,000 Watts, so losing 350W in the wiring >> and terminal connections means only 1% loss in the grand scheme of >> things. >> A single traffic light turning red in front of you so you have to stop >> and again accelerate up to speed is likely a bigger difference in the >> efficiency of a trip... >> >> Cor van de Water >> Chief Scientist >> Proxim Wireless Corporation http://www.proxim.com >> Email: [email protected] Private: http://www.cvandewater.info >> Skype: cor_van_de_water Tel: +1 408 383 7626 >> >> >> -----Original Message----- >> From: EV [mailto:[email protected]] On Behalf Of Lee Hart via EV >> Sent: Tuesday, August 19, 2014 9:08 AM >> To: Martin WINLOW; Electric Vehicle Discussion List >> Subject: Re: [EVDL] CALB bolt terminals getting hot - >> >> Cor van de Water via EV wrote: >> >>> Depends on your current. Typically I would say in the order of >>>> magnitude of 0.1 mOhm (milliOhm) because a 300A current will then >>>> give 30mV drop, which produces 300A x 0.03V = 9 Watt of power loss >>>> as heat. >>>> >>> >> Martin WINLOW via EV wrote: >> >>> At EVERY connection? In a 120V lithium pack that 's 38 x 9 = >>> nearly 350W of heat! >>> >> >> No; Cor has it right. And, 0.1 milliohms is a *good* connection. You >> will discover that it is damnably difficult to make a connection that >> good with aluminum terminals! >> >> -- >> You never change things by fighting the existing reality. To change >> something, build a new model that makes the existing model obsolete. >> -- R. Buckminster Fuller >> -- >> Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm >> _______________________________________________ >> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub >> http://lists.evdl.org/listinfo.cgi/ev-evdl.org >> For EV drag racing discussion, please use NEDRA >> (http://groups.yahoo.com/group/NEDRA) >> >> _______________________________________________ >> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub >> http://lists.evdl.org/listinfo.cgi/ev-evdl.org >> For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/ >> group/NEDRA) >> >> >> >> > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/ > group/NEDRA) > > -- Put this question to yourself: should I use everyone else to attain happiness, or should I help others gain happiness? *Dalai Lama * Tell me what it is you plan to do With your one wild and precious life? Mary Oliver, "The summer day." To invent, you need a good imagination and a pile of junk. Thomas A. Edison <http://www.brainyquote.com/quotes/quotes/t/thomasaed125362.html> A public-opinion poll is no substitute for thought. *Warren Buffet* Michael E. Ross (919) 550-2430 Land (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google Phone (919) 631-1451 Cell (919) 513-0418 Desk [email protected] <[email protected]> -------------- next part -------------- An HTML attachment was scrubbed... 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