Accumulator cable as shunt for current sensing - Page 1
Forum section: electronics / projects

author: volpx
author_rank: Newbie
timestamp: June 04, 2021, 09:42:58 am
content: Hi I'm a member of a team of electric racing cars in a students league and need some help for .We have an Lipo accumulator(battery) capable of a nominal 400V fused at 110A (but we would never exceed 50A under normal operations) for which we are using a hall sensor with 200A range (I think this one but id doesn't matterhttps://www.lem.com/sites/default/files/products_datasheets/htfs_200_800-p.pdf). Anyway an hall sensor is great for the cruise consumption but it's accuracy is not great for smaller currents in the order of the <100mA while in idle.So as a joke yesterday I proposed to use the fuse as a shunt resistor and from that one a colleague doubled proposing to use one of the stretches of cable itself as a shunt resistor.Then I started searching if it could be done and here are the data:Current range of measurement: +-300mA (Higher current we would likely use the hall one)Cable material: copper rho=1.7e-8 Ohm*mCable section: 25mm2Cable length: ~10cmCable nominal resistance: R=rho*L/A=6.8e-5 OhmVoltage @1A: Vs=R*I=60uVThe cable would be crimped on the two ends forever (so it is stable) by inserting the sensing lead onto the accumulator exit end and the sensing lead + the ground lead for the measuring circuits and stuff onto the actual battery end:

author: coromonadalix
author_rank: Super Contributor
timestamp: June 04, 2021, 10:16:13 am
content: for a 0 to 50 amps you have Allegro ACS 712 series of hall effect current measurementhttps://www.allegromicro.com/en/products/sense/current-sensor-ics/zero-to-fifty-amp-integrated-conductor-sensor-ics/acs712You have tons of "How To" on the web, easy to interface and codeI think you over complicate things a little bit, the BMS adc could be used for all the current range .... 10 bit precision is 1024 values / steps but you seem to have a need for a low current monitoring and a higher one too ??You can't get a full resolution or sensitivity say 0-50 amps and hoping you'll get very precise in the low ranges, unless you compromise somewhere or you have some kind of current / range switching ?? = added electronic(s) in your design(s)Or something like a DC current clamp meter ?? with a range switch ??"Wire fuses" exists, but i would not take a wire to do current measurements, i would use a dedicated shunt, the wire may heat creating properties changes, could be small but ...You have shunts with very low voltages loss 50 - 75 millivolts or you have already made ACS 712 pcb's that can go to 30 amps with ease ... you have other hall effect models who can go 3 5 10 20 30 50 75 100 and 200 amps ... with their integrated shunt or an external oneFor current transducers, you have to build all the logic / interface around them, not necessarily more precise than a hall effect ? a waste of time and efforts, simpler solutions exists.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 04, 2021, 01:46:44 pm
content: The TSC2011 has a +- 200 uV offset guarantee, and offset drift of maximum 5 uV/C. With a 60 microohm shunt, you'd have a +- 3.5 A initial error to trim out, and 86 mA/C of thermal drift just from the amplifier. Detecting a 300 mA change in current under real world conditions could be tricky. It also has pretty low gain, so offset contribution from the second stage amplifier and the ADC in the microcontroller could also add on top of this, with its own temperature dependency.The TSC211 looks significantly better. It does not solve all the issues with trying to reliably measure differential voltages in the tens of microvolts though. Even thermoelectric voltages can be a big issue here, copper-nickel junctions will generate a voltage similar to the current you're trying to detect with just a couple of degrees C of temperature difference. You will have a better chance of getting an accurate reading if you have a bit more resistance to work with, at less than 100 amps the dissipation wouldn't be too difficult to deal with if you increase your resistance by a bit, in case that can be done practically. A real current shunt would be a good option here, as they generally have much better temperature stability than copper wire when it comes to the resistance value.If you use a fuse as the sensing element, the temperature coefficient would likely add more error, as the wire element temperature changes significantly with current.

author: langwadt
author_rank: Super Contributor
timestamp: June 04, 2021, 02:02:34 pm
content: don't forget that the resistance of copper changes almost 0.4% per degree

author: volpx
author_rank: Newbie
timestamp: June 04, 2021, 05:08:57 pm
content: Thank you for the answer! That shunt wire sensing is not mandatory for us, I was only considering to add it as an experiment with super minimal changes in the design since directly using the cable is practically free + 2 euros of components. Plus having the current flow through the pcb of the BMS is excluded so that chip is not for us.Using the 10bit adc for 50A at scale was excluded because we wanted to monitor the idle mA (50A/1024=48mA) plus we would lose another chunk wanting to measure the charging current...The range switching would have been achieved probably by limiting the output voltage of the current sense amplifier or letting it go to saturation and in that case we would use the hall sensor that we already have in place.Thank you for your time! Was mainly curious of how stupid of an idea it was ahahEdit: ops sorry, Yes I asked also for completely different approaches, In that case just ignore my comment above, I know it's not super ideal how I'm doing this.

author: volpx
author_rank: Newbie
timestamp: June 04, 2021, 05:48:12 pm
content: Yes I know the fuse was too much of a stretch ahah.Uhh I secretly hoped I could have just ignored any seebeck effect. To account for that I would need a thermistor on the cable and one near the amplifier right? But if we use copper cables everywhere and have only a nickel plated connector and tin junctions on the pcb near the amplifier and thermistor (hence at the same temperature) we shouldn't have any seebeck current, right? Otherwise (never done Seeback calculations so it may be wrong) at let's say 10K temperature difference between pcb and cable, nickel (worst case) has a coefficient S=-15uV/K and I would measure Vseebeck=S*T=150uV?? But again I'm not sure since the "hot junction" is copper everywhere.Also I don't understand that 200uV offset guaranteed you cite from the datasheet, what's that? I can (and would) calibrate the ADC clearly.Thank you!

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 05, 2021, 01:04:50 am
content: Perhaps change to a sensor that's just a little above 50A full scale (and a max continuous current high enough for your application) and then use voltage drop across the cable to detect an abnormally high current?

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 09:21:02 am
content: This is tricky. You don't have 10 bits of ADC, where it really matters like this discard the last 2 bits. The first home battery system I tried had an ATMega2560 in it and made errors in measurement when trying to balance it's output against the grid connection to bring it to 0. I worked out that given the 100A range of measurement required both ways so only 9 bits absolute and the likely 2lsb error the amount of error I calculated was exactly the amount I was seeing.There are lots of options. Use a higher resolution ADC external to the µC. This will give you readings down to a lower current but do look carefully at any fixed errors that will be in the system as the smaller the measurement the more any fixed errors which you may be able to compensate for and other errors creep in. A current transformer may not be good at low percentages of it's full scale range.You are probably best off with a proper shunt resistor like this:https://uk.rs-online.com/web/c/automation-control-gear/process-control/shunts/?applied-dimensions=4294568185,4294571950,4294563921,4294584685,4294571182,4294567012,4293440566and use a switchable gain amplifier. You can get some ok ones and some nice and that are specific for current measurements, not as cheap as op-amps but generally better specs on things like offset voltages that will stuff you up on high gains and small currents but they are still not a free lunch as they do have offset voltages that do go up with the gain selected.You could use multiple amplifiers and chose to measure the output of the first one that is not in saturation but this will need some software and time as you would have to measure each amplifier output and then work your way up from the most sensitive until you find a reading that is under the threshold you have determined to be saturation.

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 05, 2021, 12:20:21 pm
content: Don't use copper cable as a shunt, you are wasting your time. A real shunt is cheap enough