Getting closer to figuring out my battery requirements

Question

I have been doing research for at least a week and am still a bit confused, but think I am getting to the point where I am close.

I need to power 5V 3-5A. Some days this power source will hook up to my Beaglebone (3A), others to my Arndale Octa Board (5A) (I believe I can supply 5A to the Beaglebone without issues). I need a very light battery, that will last quite awhile hopefully.

So after reading a lot about batteries I think I need either a 2S or 3S LiPo battery. I need to know if a UBEC (Ultimate Battery Elimination Circuit) can be more than a voltage regulator, instead work as a buck converter. I think if its just a voltage regulator I need to stay at the 2S, but if I can use a buck converter the 3S should have longer battery life?

So here's my question, and you can put links or part #'s or just the specs I need to keep an eye out for. What LiPo battery, items I need to safely charge it, and then what buck/ubec I need for my power requirements (5V 3-5A).

Answer 1

You might try http://www.adafruit.com/products/353 Two of these in series will take a volume of ~ 140 mm x 54 mm x 18 mm, and will give you 7.4 volts at 6.6 A-hr. Assuming an 85% efficient converter, this will give you about 1.6 hours for a load of 5 volts and 5 amps. And, if you're a little flexible on your dimensions, you can fit 4 cells in ~ 140 mm x 110 mm x 18 mm, with a total endurance of ~ 3 hours. I don't know if weight is a problem.

With this form factor, theoretically you would then have room for a 140 mm x 100 mm pc board for your regulator, with about 10 mm thickness available for components. The realities of packaging, of course, will cut into this.

Answer 2

I think you may still be a little confused

(1) The battery produces an output at some sort of voltage. (2) If the voltage is greater than what your board can handle, that's bad. (3) Therefore you want to make sure the battery has a rated voltage (not amperage) that is within the board's operating parameters. battery voltage <= the number you find for the board voltage to be safe. (4) The battery's rated amperage will not damage your hardware. (5) You can think of the battery rated amperage as how hard the device it's connected to can suck current. Your boards have some current-suckage requirement. If you're battery can't handle the current suckage-requirement, the board may not be able to function, but likely is safe/OK for the board.

Battery voltage too high (for board's rating) = potential damage to board

Battery voltage too low = board may not function

Battery voltage rating = "can the board handle your battery"

Battery amp rating too high = board will be fine

Battery amp rating too low = board may not function

Battery amp rating = "can the battery hand your board"

Anker carries some pretty beefy batteries (something insane like 20000mAh) that operate @ standard voltages using USB.

Last - are you sure the Octa requires a 5A draw? That's seems pretty hefty. Their site seems to indicate <3A @ a quick glance.

Last pt 2 - it looks like both Octa + Beagle can run on a 5V power supply. Both boards probably have some for of voltage-reg built-in too. Not sure why you think you need a voltage regulator, unless I've misunderstood your question.

Answer 3

This is a question that no one seems to answer directly and clearly. It has to do with watt-hours, which is never mentioned in these answers. Is the voltage differential between the LiPO cells and the power output at a lower voltage just wasted, or is it useful energy that translates into useful power?

Example - I can use a 2S 7.4V 1600 mAh LiPO cell with a UBEC to power my radio and servos. Let's say I can fly safely for 40 minutes with this setup. The battery (in theory) contains 11.4 watt-hours of energy, which lasts for about 40 minutes.

Now, what if I use a 3S 11.1v 1600 mAh battery with the same UBEC and radio setup? Does my flying time increase by maybe 50% because I have gone from 2S to 3S, which has 17.76 watt-hours, or 50% more power than the 2S LiPO battery?

If the answer is that energy is not simply wasted (as would be the case if a resistor was used to lower voltage), an increase the number of cells with the same mAh rating will translate into increased power output at the same amperage and voltage. One can then state that if you increase the number of LiPO cells of the same capacity, your power output will increase accordingly. If you double the number of cells, you will (closely) double the length of time that the UBEC output is available (at the same amperage and voltage).

Likewise, if you replace a 5V 2000 mAh Nicad battery powering a receiver and servos with a 7.4V 2000 mAh LiPO battery and a UBEC, you will increase your flight time significantly, maybe by 40% or a little more.