I went to the Duracell website to see if I could find numbers on D cells. Duracell batteries will last for at least 5 years on the shelf. I'm sure Energizers will, too, but I happened to go to Duracell's website.
If you discharge a Duracell D battery down to 1.1V, with a constant 0.25W power draw, it is more than a 10 amp-hour cell. So two D's, discharged down to a combined series Voltage of 2.2 V at a power rate of 0.5W would be more than 10 Amp-hours also. At very low discharge rates, such as you are contemplating, the cell will last much longer. And if you allow it to go down to 1V, you will get even more life out of it.
1000 discharges on your 17 mAh battery would be sort of like 17000 mAH, or
17 amp-hours. In reality, of course, this model of 1000 full charge discharge cycles is totally bogus. The device probably can't support anywhere near that many full discharges, and you won't be discharging it very deeply anyway. Or so you say. But I don't know how to assess the life of the battery at the shallow discharge rates you are talking about.
Do keep in mind that if you discharge it very deeply even once, it will basically be toast.
It would help if you had some idea of total backup time you expect to need over the 5 or more years you want this thing to work. Can you estimate a percentage availability over the 5 years?
Anyway, it seems quite possible that two non-rechargeable D cells will last just as long or longer than your 17 mAh cell, and will probably be simpler to use. I don't know whether your design can tolerate the weight/size of them, though.
Oh, yes. Duracell batteries will last 5 years on the shelf. I'm not sure how storage temperature affects shelf life, but at room temperature, you should get 5 years from your Duracell batteries. (and probably Energizers, as well).
Incidentally, high temperatures greatly accelerate the aging of Lithium primary cells. I wouldn't be surprised if it does the same thing to rechargeable cells, too.
That sounds like a good solution. If you can find a dual schottky in one package, then they will be thermally closer together than most mechanical methods if seperate packages. Possible exception: if both are TO-220 then a thin insulator and mounted back-to-back may give better thermal coupling between them. Either way, if they track, then the thermal variations will roughly cancel out in practice.
Sanity Check. Over the decades I've seen engineers go to great lengths to fix problems that don't exist. Might be a good time to re-examine your priorities. I'm to lazy to reread the whole thread, but as I recall, you don't have to log while the power is off. So... How often does the power go off? How long does it stay off? What are the consequences of losing a data sample? If you do nothing, how does the probability of losing a sample compare to the probability of system failure due to other casues? What additional failure modes are created by the "fix"? Does the reduced reliability of the system create more problems than it solves? Is cost no object? If it's a manned mission to the moon, you should behave differently than if it's logging temperature in your back yard.
What if you sensed power failure and copied RAM to Flash? That solves the write cycle limitation and long term storage problems. How much energy does that take? Multiply that by how many times it might happen. How does that affect your mAH requirements over the life of the device? Could you manage it with a simple capacitor? If you put your energy reservoir at the input of the regulator, you get much longer holdup per farad...and you don't need any diodes or fets or external RAM.
Sometimes it's more productive to be clever than to try to be perfect. mike
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