i have a general question how to calculate the power lifetime when powering externally.
The difficult part ist, that i'm using a step-down converter and so mAh is not the only given value.
My Arduino installation wastes 100 mAh at 5V. The stepdown converter need 6v to run, with an efficiency of 96%.
As Power-Soure i have a car-battery with 12V at 5Ah.
So normally i would say 5.000 mAh / 100 mAh = 50 Hrs. runtime.
But now the voltage-range is 6-12V. So the battery voltage can decrease to 6V and the the board is still running. How will that have impact to the runtime?
You didn't include the efficiency of the DC-DC converter in the calculation. It would help to know the capacity of the battery for the voltage range you intend to use. You may have to measure it yourself. The simplest solution may be to hook everything up and try it.
As Power source I have a car battery with 12V at 5Ah.
That seems unlikely; a quick check of a local battery retailers shows that a small car battery has a capacity of 35Ah and they only get bigger from there.
I assume you mean you have a small lead acid battery.
The voltage range is 6-12V. So the battery voltage can decrease to 6V and the the board is still running.
The battery voltage range is about 14V to about 11V. A fully charged 6 cell lead acid battery will have a terminal voltage of about 14V. When it is discharged it will have a terminal voltage of about 11V. Search on the internet for lead acid battery discharge characteristics or lead acid battery discharge curve. If you discharge it to 6V you will most likely damage it.
The stepdown converter need 6v to run, with an efficiency of 96%.
I'm not saying you're wrong, but 96% seems optimistic, please check.
Discharge time = battery capacity (Ah) / load current (A)
Taking the DC DC converter into account:
Load current (battery) at 12V = ((5/12) * load current at 5V) / converter efficiency (as a decimal, so 0.96 based on your figures) = 0.0434A
Taking the average battery voltage as 12V and battery capacity of 6Ah
6/0.0434 = 138 hours
Remember that any battery of any chemistry loses capacity over time, so a 6Ah battery will not have 6Ah capacity after 12 months of use. I suggest buying a battery with at least twice the capacity you want.
Do not leave a lead acid battery discharged for long, that damages them too.
Thanks for this informative feedback.
The battery I wanted to use is for small motor-cycles. I would like to take a smaller one than a bi car battery. But I see that it will have to less Ah.
So to understand it right - the voltage will decrease(14-11 V) while the battery gives its Ah.
But it should not go lower than 11 V to avoid to loose battery healthiness.
So I would need an other controller between battery and Step-down that turns off power wenn battery is to low?
My used step-down converter ist this one...
Sorry, given efficiency is 92%. not 96.
The battery I wanted to use is for small motor-cycles.
Vehicle batteries are not designed to be charged and discharged in the way you are planning. Mostly a vehicle battery is kept fully charged while the engine is running with the occasional heavy discharge for the starter motor or if the lights are left on. Your application involves charging the battery then completely discharging it. That will most likely damage a battery designed for a vehicle. You need one designed for deep discharge, such as are used in wheel chairs.
So to understand it right - the voltage will decrease(14-11 V) while the battery gives its Ah.
But it should not go lower than 11 V to avoid to loose battery healthiness.
So I would need an other controller between battery and Step-down that turns off power when battery is to low?
About right yes, but also consider that a lead acid battery must not be left discharged for long. You could use one of the analogue inputs to the Arduino, along with a potential divider to monitor the battery voltage. You need a circuit to disconnect the battery or warn you when it is close to full discharge. Alternatively, if you know how often you will be able to charge it make sure the capacity of the battery is high enough that it will never have time to completely discharge.
I'd avoid lead-acid if you can - yes they are cheap, but basically they are troublesome unless used as in a vehicle (regularly charged to 100%, seldom discharged below 90%)
batteryuniversity.com is the place to learn about battery chemistries. You'll need something able to handle a wide temperature range and have a large charge-cycle count.
You must protect your battery against over-discharge whatever the chemistry, that just ruins any rechargable
battery.
Batteries are difficult, basically, nothing is anything like ideal.
You'd probably get like 50 discharges out of a battery like that, then you can basically throw it away.
Lithium batteries would make much more sense. However calculating the capacity you get out of it is far from trivial.
For instance temperature is going to be a major factor. In the summer when it gets like 60°C in the sun you'll get the most capacity and the colder it gets the worse the capacity will be. At -20°C you may easily lose 1/3 of the capacity.
Besides that, there are some things you can do to reduce power consumption.
You need a deep cycle type of battery, those should be able to last many more discharges. This is the kind of lead/acid battery used in solar installations, or in RVs. Designed to supply a small current for a long time.
You mention 5,000 mAh - so why don't you get a LiPo powerbank? It's got all the battery management built in, including the boost converter for the required 5V output. They normally require 50-80 mA to remain on (depending on the model), your 100 mA should be enough to keep them on.
Expect to get some 80% of rated capacity (for a quality power bank; cheap made-in-China can be as little as half that).
