Attach a load resistor of a known value and see how long it takes for the battery to go flat ?
If the voltage of the battery is stable for most of its life, then to know how much capacity is remaining, you would need to measure how much current\power the circuit is actually drawing from the battery over time, which is not easy at all.
Yes, I am aware of that. But there are too many lorawan sensors on the market that work with li-socl2. I had the idea to make an estimation by counting the working time, but I’m not sure how successful it would be.
Seems as good a way as any. Get a device, power it any way you like, measure it’s consumption for as long as you can stand, figure out the average sleep, active & tx consumption, add the three, et voila. So if you end up with 50uA/h per cycle, count the cycles and you can make the rash assumption how much battery may be left.
And it only measures how many coulombs have been used, which is only of value if a system was started from a known point, i.e with a battery of know capacity…
I think I’ve read that the voltage remains virtually constant to the end of the life of the cell. At this point the discharge curve becomes very steep. A 3% drop in voltage indicates that the cell is near end of life and should be replaced.
Take a look at recommendations by Qoitech. And even then take exceptional circumstances into account. For instance V3 sends downlink requests that might cause additional transmissions which at higher SFs take a lot of power.
Anecdotal, recently I modified firmware to allow for acknowledged uplinks once every X packets and forced things so the node would not get acknowledgments. Where the node used 20 micro watts in an hour before it now started retransmitting at increasing spreading factors resulting in it burning 50 milliwatt in one hour. That is a huge increase in consumption and a huge decrease in potential battery life.
Thank you. I know these. I know about battery optimization, but measuring li-socl2 is very difficult. Maybe I need to switch to LiFeS2 type batteries. It has high power capacity and a linear voltage graph.
I’m a bit confused here. I may be simple, but if I measure a couple of days worth of activity and I know what the different cycles are and I allow for the over excitement that is v3 MAC requests, surely Mr Spreadsheet will do the rest for me.
Maybe not down to the nearest month, but certainly down to a fraction of a year.
I mostly use the NRF Power Profiler Kit 2 as the interface is cross platform, concentrates on the job at hand and allows me to put a trigger pin in to the firmware to mark sections. The Microchip PowerDebugger is Windows only and takes a bit to setup, works well but is a little pricey. I’m not sure what they were thinking at STM but then I tend to think that about most of their stuff.
At worst, given the benefit of 10’s of thousands of uplinks, the average should give you a good idea when the lights will go out - particularly if you have 10 or more of these nodes, get half a dozen going as soon as the final PCB lands and then you’ll get some early stats in which again, using Mr Spreadsheet, can let you average stuff out from the uplinks, downlinks and shake-it-all-about-links to predict actual deployed device lifetimes.
If you need accuracy, then the $7 on one of those fancy battery / couloumb chips is a small investment.
However the simple answer to having a definite 5+ year life span is two batteries.
Yes, basically that route is not available for this chemistry, when your values drop you have almost no time left to replace anything.
However given your requirement of 5+ years without battery change measuring the voltage doesn’t make sense. Are you going to sell devices that last 5 years and after 3 years tell your customers they need to replace the batteries after all?
You need to measure battery usage and extrapolate based on that and add at least 80% capacity to account for differences in performance. And if your device firmware is field upgradable make sure to run a torture test on new versions before deploying it in the field to make sure you’re not suddenly using 10 times (or worse) as much energy.
Yes, you are absolutely right. I will use a 3.6V Li-SocL2 8500mAh battery in the first prototypes. Then I will calculate how much power the device consumes by taking out the power profile. I’ll keep the improvements posted here.
And easier technique, to get real world results, is to use the smallest LiPo you can find to power the node, say as low as 10mAhr. Its easy to measure the capacity and you can then just time how long the node lasts.
In the experiment described here;
A 155mAhr LiPo lasted 14 months. So with a 10mAhr LiPo, you would expect the battery to last about a month, which is not long to wait for an accurate figure for real world node power consumption, and no fancy equipment or electronics needed.
Your 8500mAhr battery would power the above node for around 60 years.