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I'm not an engineer, I'm a software guy getting into hardware - so bear with my ignorance!

I'm in the process of building a system with many components (e.g. a computer, many sensors, a few motors, a servo, etc). These will all be powered by a bank of batteries (size and composition TBD).

I know the operating voltage and current of all the components of the system, and am looking to make an estimate regarding the power consumption of the entire system. The highest device voltage is 7V, the lowest is 3v. The battery bank will likely be a collection of smaller batteries rather than a single large battery.

If I have a battery bank capacity of say "x"Ah, how can I go about estimating how long the power supply will power my system?

Can I simply just sum the wattage of the individual components, divide by the hypothetical battery voltage and then use that result as the system current [amps]?, or is it more complicated then that (I'm assuming so!).

Thanks in advance!

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There is no simple answer.. Take a computer system with a 500W PSU. The sum of all parts may be capable of dissipating 1000W but utilization due to usage may only be 100W on the average. The best way is to plan for more capacity and then cost reduce later after testing it.

It is much more complicated than your suggestion. Current drawn depends on voltage, temperature . For sure you can estimate the nominal load current ought to be less than worst case load current. Unless you know something about the load curent vs voltage and determine what the Power Fail threshold when your system should shut down safely, otherwise you can let it stop in unreliable ways.

So you have no favourable measure of accuracy of predicting the operating time without experience or schematic and description to someone who understands.

We also have no idea what your design is and if you have any power fail circuit detection like the low voltage detection built into MOBO's

Sorry without more there is no simple answer to a software expert. It would be like a hardware guy telling howe many lines of unique code gets executed per minute. You could look at all the code and features and multiply by some magic utilization ratios. But the chance of it being accurate is inversely proportion to its complexity... Most like all you have to deal with is the power consumption of the top 3 items like the motors. But the the backup time is limited by the weakest link and we do not know if your power backup is balanced for each subsystem of batteries.

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  • \$\begingroup\$ Yeah I could tell from the lack of specificity that the question was going to be "too open". At this point I'm still on the drawing board, and attempting to establish an upper bound based on the components (load) I know that will be present. I'm basically looking for a way to measure/estimate so I can start reasoning about what capacity of battery bank I'll require, and how long that bank will take to charge from "some" source (i.e. solar). Thanks for your input! \$\endgroup\$
    – DJSunny
    Jul 17, 2012 at 4:32
  • \$\begingroup\$ Sounds like fun, I would use the best cellstack that matches your motor voltage and use external chargers. Solar is pretty much useless unless you have use it as a big sail due to size weight restrictions. and if you are designing a drone , it wont be very stealth. Just use compact Li-Ion cell packs and have spares. \$\endgroup\$
    – Tony Stewart EE75
    Jul 17, 2012 at 4:45
  • \$\begingroup\$ The project is actually a boat, and one with a hefty flat surface at that! So we actually have decent "square footage" for solar cells. The real challenge is to get a rough idea on the needed battery capacity for all the on board systems to have some comfortable run vs. charge time margin. \$\endgroup\$
    – DJSunny
    Jul 17, 2012 at 4:48
  • \$\begingroup\$ When I started in R&D in 1975, my 1st design was a 50mW VLF Nav receiver for an automated floating weather station with wind power and up link to GOES-1.. So ask the sub-system designers for their micro-power budgets and make the motors the only signicant load. unless this is an autonomous oil spill finding boat or SCADA system for other power hungry sensors. \$\endgroup\$
    – Tony Stewart EE75
    Jul 17, 2012 at 4:54
  • \$\begingroup\$ As to be expected, the motors while running will dwarf the power consumption of pretty much everything on board. There will be an on-board computer that runs continuously (probably a Raspberry Pi :D), and some sensors / camera, but nothing in comparison to two motors. Fortunately for us the motors need not always be running, only during transit between points of interest, and at intervals to maintain a "loiter" radius around some fixed point. \$\endgroup\$
    – DJSunny
    Jul 17, 2012 at 4:58
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Generally, there are 2 types of power in a system - Source Power and Load Power. Source power comes from your battery (or power supply) providing power to your system; Load Power is the power consume by load (devices).

The system current relies more on your load (devices) rather than the source; so, you can't sum the source wattage and divide the battery voltage to get the system current. Summing source wattage and divide it by the battery voltage will only get you the current that your source (battery) is capable of supplying, not the actual system current. The system current depends on the load (devices).

The easiest way (but not the most accurate way) of estimating the battery life of your system is summing the source wattage and divide it with the load wattage (sum of all the rated wattage of your device).

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  • \$\begingroup\$ In my post I mentioned summing the components wattage (load) but in your response you mentioned summing source wattage. Perhaps just a typo? \$\endgroup\$
    – DJSunny
    Jul 17, 2012 at 4:34

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