I am working on a project that it has to work at 2.5V-0.3mA, 12h a day during almost 2 years.
The probleme is that I don't understand something.
I did some calculation of the minimum capacity of the battery and I ended-up with some weird results.
Imagine we want a 9V battery.
We have 2.5V-0.3mA during 12h a day, it's 0.009Wh/day.
During 365 days*2 years, it's 730 days.
So finally my battery has to have a capacity of :
If I want an admissible discharge of :
- 50%, I have : 0.009*730/9/0.5 = 1.46Ah
- 80%, I have : 0.009*730/9/(1-0.8) = 3.65Ah
Wy does the battery has to have a higher capacity if I want to discharge my battery to 80% of its capacity than 50% ?
Does that mean if I take a 1.46Ah battery my system will stop to work at 50% ? Therefore at 4.5V ? I think I'm missing something...
Thank you !
The charge of a battery is not from 0V to xV, it depends on the chemistry of the battery, for example, a 3.7V LiIon battery will be 100% at 4.2V and 0% at 3.3V, so you assumption about being 50% of charge at 4.5V is not correct.
For your calculation you should just use:
This will give you around 730mAh, then you must calculate the efficiency of your DC DC converter for an input of 9V to an output of 2.5V.
Another thing to get in mind is that for such a great time the battery might self discharge, so you should know it's self discharging rate and add it into your Ah calculation.