Battery Pack 'Health' Monitor - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: November 07, 2015, 10:27:57 pm
content: I am beginning a project that is outside of my skill set at the moment. At the moment, the short term goals is to understand my basic parameters and study/experiment before pushing forward.The project is to integrate a battery health grading system in an existing power management system. By health, I mean a fairly coarse evaluation of the battery capacity and performance relative to a new one of the same type. The goal is not to build a super accurate lab-grade evaluation system, but rather identify battery packs that may be past their prime so that the end users can sort them accordingly.I have found some information on full discharge tests and battery capacity testing but they take a long time to fully cycle the battery. What I am after is a short test that can give me useful information about the battery. I was thinking that if I take a sample of 10 new battery packs and construct a well controlled series of tests I could use that information as the control data. When I take an unknown battery and do an abbreviated test, I could look at just that portion of the discharge curve and decide how close/far it is from the reference data. I am still trying to learn how the discharge curves vary over the lifetime of the cells. The chemistry is likely to cover some ground - NiMh, Li families, AGM. Voltages and currents ~12v - 30v up to about 20A. I will NOT have access to individual cells - only the final outputs. They will all be in sealed containers that cannot be disassembled and re-assembled.Specific information is definitely appreciated, although links and references to reliable resources that I can study is really what I am looking for at the moment. I don't mind reading, studying, and experimenting - just don't know where to start.What (if any) battery parameters are useful in determining where a battery is in it's life?Is ESR an indicator of battery health?Has this already been sorted out and put on a piece of silicon by somebody?

author: nctnico
author_rank: Super Contributor
timestamp: November 07, 2015, 11:04:12 pm
content: The last question: yes but can't tell more on this forum. TI has some fuel gauge chips which can keep track of a cells capacity and promise 1% accuracy but their firmware has to be tailored to the cells. You have to send some new cells to TI for characterisation and they'll provide firmware for the fuel gauge chip. There is lots of info on the web though about battery health monitoring using the complex impedance but how theory is converted to a working system is usually a closely guarded secret.

author: rx8pilot
author_rank: Super Contributor
timestamp: November 08, 2015, 12:11:55 am
content: By 'fuel gauge' does it guess the overall capacity of the battery? The critical problem is that my project needs to cover a range of batteries including systems that don't even exist right now. I was thinking if implementing a LUT for each battery that the use can upload.Essentially, I need a general purpose solution. I am willing to give up accuracy for flexibility - to a point of course.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 08, 2015, 01:42:27 am
content: The BQ34Z110 from TI is a Fuel Gauge-IC that can be used for Lithium and Lead Acid-Batteries and from what I read in the datasheet it can be programmed quite extensively to fit the requirements of the design.TI even supplies a Software with which the chips can be programmed.As far as I understand it, the chip measures the current and the voltage and calculates the remaining capacity by putting the change in voltage corresponding to changes in current into a secret formula.If the fuel-gauging doesn't have to be very accurate, I think it should be possible to implement a similar functionality in Software with a Microcontroller: Using a constant Current-Load that is switched on regularly and then measuring the voltage-drop, it should be possible to determine the "health" of the battery, while measuring the current and simply counting the amp-hours gives a rough idea about the remaining capacity.The accuracy of this solution is most likely determined by how well the batteries and the specific behavior of their chemistry are known.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 08, 2015, 05:14:50 am
content: Facing a similar design challenge ("just make it work with any external lithium battery we plug in!"), I've selected one of Maxim's ModelGauge series monitors for our next design. Importantly, the original ModelGauges don't use charge counters, only state-of-charge information (at least as I understand it), meaning they should be OK without recalibration if someone swaps out battery types.Unfortunately, being Maxim parts, they have also reminded me why I hate Maxim and only use them as a last resort. (Terriblesupply chain and distributor stock management.) If the rumory things are true and ADI does buy Maxim, I pray to the gods of all things electrical that ADI fires and replaces every last Maxim supply chain person. Maybe then I won't whine every time requirements point me in their direction.

author: KL27x
author_rank: Super Contributor
timestamp: November 08, 2015, 05:18:26 am
content: QuoteFor most rechargeable battery chemistries, yes. And this is where I'd go with it. I don't see what a fuel gauge IC would do for you, here.Fully charge the battery. Measure the float voltage. Then measure voltage under w/e is a "healthy load" for that battery. This will give you all you need to know, once you establish your baselines.The guy behind the counter at your local autoparts store has probably tested scores of car batteries with this method, using a manual tester that measures voltage while drawing current through a dummy power resistor load. One of my DMM's has dummy loads built-in for testing various batteries.

author: nctnico
author_rank: Super Contributor
timestamp: November 08, 2015, 07:25:41 pm
content: Quote from: KL27x on November 08, 2015, 05:18:26 amA fuel gauge chip does the same but also provides an estimate of the battery's capacity so it is easier to use one of those instead of trying to re-invent the wheel.

author: rx8pilot
author_rank: Super Contributor
timestamp: November 08, 2015, 09:45:52 pm
content: Quote from: nctnico on November 08, 2015, 07:25:41 pmI read up on the TI offerings to get an overview of the technology. They seem to be a lot of good tech packed on the chips, the downside is they seem to be designed only to be integrated into a specific configuration. Sealed and never touched again. I could be wrong, but that was my interpretation from the overview.In this application, any number of about 20-30 packs could be applied. Some have automatic ID via SMbus or a more generic I2C, others need to be manually identified. Either way, it needs to be changed constantly via a MCU with data about the batteries. Not sure if the TI options will be able to deal with this or not. Also all the analysis will be on the discharge cycle, it will never be charged in this circuit.May have to roll a custom solution, but will continue to look deeper into TI options.Quote from: exmadscientist on November 08, 2015, 05:14:50 amI know what you mean! Maxim is a pain in the ass. Wish they were easier to deal with from a supply chain perspective, I would use more of their parts. As it is, I have them as supplier of last resort.

author: Neilm
author_rank: Super Contributor
timestamp: November 09, 2015, 06:58:46 pm
content: Quote from: rx8pilot on November 08, 2015, 09:45:52 pmMany of the fuel gauges are similar. SoC monitoring varies depending on the battery technology. As the capacity of a battery decreases with the number of charge cycles it has been through fuel gauges generally need to know that information to provide accurate capacity estimates.I do remember a Maxim chip that was supposed to monitor the ESR of the battery, but when we tested it (some years ago) it was not very accurate for our application.

author: stmdude
author_rank: Frequent Contributor
timestamp: November 10, 2015, 07:38:31 am
content: While capacity is a good indication of overall battery health, it's not a definite answer, unfortunately.As a lithium battery gets "worn out" its internal resistance increases, but (in some cases) the capacity remains the same, as long as you discharge it slowly enough.So, a very quick way of determining the "health" of the battery would be to measure the IR, and compare it to a base-line of what-its-supposed-to-be.For a full health checkup, you'll need to:* Discharge completely, Charge it up, and check how many mW you put into it to see the total capacity of the cell(s)AND* Measure the internal resistance of either each cell individually, or the pack as a wholeAND* Check the resting voltage of the battery when charged above 80% (i.e, after the charging cycle has gone into CV)A deviation from baseline of any of those three will indicate some kind of damage/wear to the cell(s).Fortunately, checking the IR and resting voltage is simple and quick to do. The capacity-measurement will take at a minimum 1 hour to complete, so it's not the way to go if you have a lot of batteries to check.

author: rx8pilot
author_rank: Super Contributor
timestamp: November 10, 2015, 07:52:56 pm
content: Quote from: stmdude on November 10, 2015, 07:38:31 amThis seems like the right path in general. It would allow a reasonably useful checkup on a battery with a short test. The baseline data can be an average of a set of brand new batteries. Most of the time, the test needs to be quick and provide a relatively course guess. Other times, they could be fully cycled for a more accurate test.

author: KL27x
author_rank: Super Contributor
timestamp: November 10, 2015, 09:27:07 pm
content: Another measure you might care to do is something akin to self-discharge rate or internal leakage. If your batteries are going to be fully charged at some point in your testing, anyway, this might not take too much time. Hook the battery up to a DMM/current meter and a CV charger to see the current flow. Leave it connected until the number starts to bottom out.I say this as I have an abused 400mAh li ion cell on my lab supply, bottoming out at 19mA. Clearly damaged goods, and I didn't have to do anything other than charge it with a lab supply to put a number on it of 0.05C leakage. A good 400mAh cell would bottom out at 0mA.Same thing occurs with lead acid. A bad battery will suck some serious juice, indefinitely.

author: eas
author_rank: Frequent Contributor
timestamp: November 10, 2015, 09:44:42 pm
content: I started looking into this sort of thing for LiIon battery chemistries before getting derailed by learning more about electronics by repairing test equipment in order to improve my own testing capabilities.From the reading I'd done and a few battery folks I'd taled to, it seems that simple IR of LiIon batteries isn't as useful a test as it once was, at least not on its own. In fact, as I recall, some of the manufacturer datasheets even said as much, because they use various techniques to maintain IR over the normal life of the cell (and beyond). In the battery tests I did I year ago, it was pretty obvious that this was true. Cells with full capacity and cells with dramatically reduced capacity often had very similar IR.For quick checks, AC impedance can be useful (and seems to be what cell and pack manufactures use for binning/matching cells). I haven't gone as far into this as I'd have liked, but in essence, looking at the distortion of square wave AC can give a lot of hints about the cells health and its specific maladies (cathode/anode degradation and/or fouling, electrolyte fouling and/or depletion). Further, knowing that information can be useful taking measures to maximize the remaining lifespan. A lot of this info is proprietary, but I'm pretty sure that there is academic literature on it as well (I just haven't gotten around to digging it up).For more passive checks: coulomb counting for determining capacity, using the current measurements alongside voltage monitoring to calculate IR, looking at inflection points in the charge/discharge curves.Let's back up for a minute though: What's the goal? You said:QuoteIs this for proactive maintenance of an operating battery powered system(s)? What are the demands placed on the battery packs? At a high level, what constitutes proper function? Where is the threshold where proper function ends? What are the costs/risks of a false positive or negative? If this is an update or upgrade to an existing system used in the field, how are things handled now?As you've noted, a lot of the battery ICs I've seen are intended to be designed into the pack/system from the beginning.It sounds like this might be the sort of situation where getting the basic monitoring infrastructure in place and maybe starting with some crude pass/fail metrics and accumulating data and revising the pass/fail conditions accordingly could be the best way to go.I hope you'll provide us some updates on what you learn/try.