1mA to 100A bidirectional current measurement in solar systems? - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: March 15, 2025, 01:39:22 am
content: None of the commercial coulomb counters used to calculate the state of charge (SOC) of LiFePO4 batteries have the range needed to cope with rapidly changing very high and very low input and output currents. Therefore over time the battery real SOC becomes less than the calculated SOC. As far as I'm aware the only fix is to periodically fully charge or fully discharge the battery to 'reset' the system end points, however fully charging or fully discharging a working system may not be possible.Is this an unsolvable problem, or is it that the cost is too high to be commercially viable?

author: indeterminate
author_rank: Regular Contributor
timestamp: March 15, 2025, 06:34:41 am
content: It is not a problem that needs solving.just bring the battery up to 100% charge and balance the cells while you are there and call it 100% soc.There are times in winter where we can go weeks without hitting 100 percent charge , if the soc is out a couple of present who cares.The cost of making things more accurate is just to high.

author: Smokey
author_rank: Super Contributor
timestamp: March 15, 2025, 06:36:09 am
content: Not an expert, but that sounds like a dual range sensing problem to me. Use two systems, both of which are always measuring the current. One optimized for small currents and another optimized for large currents. Then have a system that monitors both and uses the appropriate data at the appropriate times.

author: voltsandjolts
author_rank: Unknown
timestamp: March 15, 2025, 08:50:39 am
content: For high current sensing, these are about the best dynamic range you can get in one sensor;https://www.lem.com/en/fluxgate-current-sensorsBut they wouldn't meet your 1mA to 100A requirement. So, yeh, multi-sensor design.Or just consider whether it's really needed.

author: e100
author_rank: Frequent Contributor
timestamp: March 15, 2025, 09:26:49 am
content: Quote from: Smokey on March 15, 2025, 06:36:09 amThat something new to me. I found a bunch of automotive dual range sensors but the low range seems to stop at about 50 amps.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2025, 09:53:36 am
content: For really getting 2 ranges one needs to switch the shunt. This works OK for small currents like 1 or 10 A as upper limit, but not really well for the high currents. 1 mA to 100 A is a large dynamic range, like found in a 6 digit DMM - so not a cheap thing.Coulomb counting can be done with better accuracy, but one still has the problem of an offset and integrating over long times (like 1 year).The charge balancing can also not include battery internal leakage currents.For the Li-batteries one can still use the voltage and temperature to get a reasonable good value for the charge state. This may not work well all the time (e.g. just after fast charging or high loads), but for these phases the coulomb counting should work well. After a long time this will be more accurate than charge balancing. One could use both methods in parallel to check the voltage / charge curve and average over reading during charging and discharging.

author: Smokey
author_rank: Super Contributor
timestamp: March 15, 2025, 08:07:59 pm
content: Quote from: e100 on March 15, 2025, 09:26:49 amsorry, should have been more specific. I'm talking about two separate sensors, and then some processor to to read them and decide which to use based on rangeing.If you use hall sensors, like the ones with the hole, you can do multiple turns to increase the sensitivity. One turn for high range, multiple turns for low range. I'm not saying this one specifically, but like this:https://www.digikey.com/en/products/detail/tamura/L34S200D15T/11696449

author: Rick Law
author_rank: Super Contributor
timestamp: March 17, 2025, 08:07:53 pm
content: Quote from: e100 on March 15, 2025, 01:39:22 amQuestion often is, how much is it worth? and is it even useful?From 100A to 1ma, that is, 100,000mA to 1mA = 0.001%. This is the same range of measuring the weight of a Tesla Cyber Truck accurate to the ounce. (per Wikipedia, it weights 6603 to 6843lbs, 1 ounce is 0.000947% of 6603lbs.) Have the weight to the ounce could allow you to calculate Kw/Watt in this 0.001% precision range -- but a bag of potato chip in the car will change your result.Similarly, environmental condition (temperature) and self-discharge of your batteries will affect the SOC by likely > 1%There is no point in measuring one part with extreme precision while other parts have much higher natural variations.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 18, 2025, 11:56:57 am
content: I'm also having trouble thinking of a scenario in which such a huge dynamic range of current sensing would meaningfully affect your SoC/SoH estimation. Self-discharge is likely more significant than such small currents.