Collaborative Design Challange - I/V curves for batteries - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2024, 04:12:09 am
content: Batteries, either alkaline or rechargeable, have a specific I/V curve under load. The goal is to plot this I/V curve for AA/AAA cells, in the hope that this will be relevant for the battery life. DIY style, not restricted by made for manufacturing rules. Every idea is appreciated, anything goes.Don't know how much fun (or if this is possible at all), would it be to design something together.For those thinking this is an attempt to get a design for free, I have SCPI controlled instruments that can be put to plot I/V, so it is not the device itself that I am after, but the attempt to a collaborative design, that's under test. If you want a different design theme, just say it and we can do that instead, the I/V load battery tracer was just a suggestion.Anybody interest for such a collaborative design prototype?

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2024, 04:22:41 am
content: I mean, it's really not complicated, and many people have done it using simple apparatus available in any home lab.But, it's also impossible to do, in a way. There are at least four parameters: current, voltage, temperature and time. At any one instant, you can make a measurement. But while you make that measurement, the battery is discharging and getting warmer due to internal heating. So when you try to make the next measurement, the parameters are not the same any more.The best way people approximate this is to start with a fully charged battery and the lowest current, and measure the voltage. Then let the battery rest for a while to recover and re-stabilize. Then increase the current and measure the voltage again. Repeat the process in steps, working up to the highest current. This gives a good practical approximation to the curve.You can do it manually, and you can also do it automatically using programmable instruments and laboratory automation.At the end of the day, I'm really not sure what your question is?

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2024, 04:32:18 am
content: Then let's limit to AAA/AA load I/V curves only.I've tested a few recheargeables the other day and noticed that by loading them with something in the range of 0-1A might give a good indication about battery charge, and maybe about battery age, too (in the sense that very old/very wear out cells will be distinguishable from their I/V curve). So I thought maybe this topic might be of interest.Anyway, the theme is not important. I was curious about the possibility of a collective design, no matter the theme.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 10, 2024, 06:08:38 am
content: Are you thinking of something like this?https://lygte-info.dk/review/batteries2012/CommonAAcomparator.phphttps://lygte-info.dk/info/Batteries2012Info%20UK.html

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2024, 09:45:52 am
content: Yes, but not V over time evolution, rather V at different load currents, something like this plot:except in my observations the different AAA or AA plots were very different, not overlapping like in this example. Could even distinguish the charging state by looking at which current the voltage goes under 1V or so.That will not be an absolute and precise measurement, but a good enough indicator to avoid those rechargeable cells that self-discharge in storage while waiting to be used.

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2024, 09:56:29 am
content: I/V is not really relevant, since current is determined by the load anyway, and some loads can draw full current down to ~0.8V/cell for instance. What is relevant is OCV and ESR at any given state of charge, at a fixed temperature. Using that you can determine the maximum power that you can extract from the cell at that point in time, and calculate the energy wasted as heat inside the cell. This will give you a proxy to calculating life at that discharge power, though it will not be perfect as all primary batteries have chemical variations that mean they cannot discharge quite as much at higher power levels.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2024, 10:04:03 am
content: Quote from: tom66 on October 10, 2024, 09:56:29 amYes, agree it's not accurate, but in practice, when you have a bowl full of batteries and you have to choose a charged one from there, a voltage-current curve might be a useful indicator.Anyway, if the design theme is not good, say another one, the battery I/V curve was just an example.

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2024, 10:08:36 am
content: You can accurately measure the remaining life of an alkaline battery with a multimeter and a small load, say 10-22 ohms. The voltage shown on your meter will be proportional to the remaining cell life.It's not perfect but it will get you within 10%.For other types of cell like Lithium-Iron AA (e.g. Energizer L91) the problem is much more difficult since these cells are 'full' at 1.83V and 'flat' at 1.70V under no load and even under light load show little variation in voltage. But as soon as you put them under any appreciable load for more than a few minutes their cell voltage falls off a cliff.

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2024, 02:17:46 pm
content: Quote from: RoGeorge on October 10, 2024, 10:04:03 amAn interesting point from a recent Technology Connections video is that a bowl full of batteries is (hopefully) an unusual situation. The ideal situation is that alkaline batteries remain fresh in their package until they are needed. They are then transferred to the device that needs them and used until they run out, then when they are depleted they are discarded.