For a electric newbie, its always confusing to understand the difference between volts and Amphs in a Lipo Battery Pack and how to relate that to motors. I have put together some questions. Pls let me know your thoughts.
Longer flying time: Can longer flying time be acheived by increasing the voltage or amphs? or both? or neither? :)
Parallel / Serial What does it mean to connect two batteries in serial versus parallel. What will be the final volt and amphs in serial mode and parallel mode? What is the end result of connecting them in serial and parallel mode?
If the battery power (Does power means volts or amphs?) is more than what is required, will it toast the motor / speed contoller or receiver?
Hey Red! Do you have a Really Basic FAQ for the newbies?
1:
There's three things you need to worry about: voltage, current, and capacity (amp-hours). Often newbies confuse the Ah rating of a battery with the current capability -- they're two different things.
Voltage is just that -- the volts that the battery supplies.
Capacity is what they put in big letters on the pack. It's a measure of how much electrons you can pull out of the pack before it's discharged, and for batteries it's expressed as amp-hours (1 amp for 1 hour, or 1/10 amp for 10 hours, etc).
Current capability is the amount of amps (_not_ amp-hours) that the battery is capable of delivering without damage to the pack. You'll often see this expressed as xC, so a 5C pack can deliver 5x it's amp-hour rating (presumably for 1/5 of an hour, but practically less), a
20C pack can deliver 20x, etc.
2:
That's series and parallel -- you've been working on too many computers.
Connecting batteries in series increases the voltage that they deliver
-- 1.2V per cell for NiCd and NiMH, 3.7V per cell for LiPo. So you decide what voltage you want and start stringing batteries together.
Connecting batteries in parallel increases the amount of current that they can deliver and the amp-hour rating of the pack. BUT! For charging you can't just connect two multi-cell packs in parallel; you should either charge separately or connect individual cells in parallel then connect those paralleled packs in series. Furthermore, you should only do this with cells that are as identical as you can get them -- doing this with dissimilar cells is a Bad Thing.
3:
The motor & speed controller will (within limits) take as much current as it can get from the battery; if it takes more than the battery, wires, or speed controller can handle then yes, you'll let the smoke out of those delicate electronics. Some speed controllers will limit the current, but it's been a long time since I've shopped for one.
If you can get it, a speed controller that lets you set a current limit to what the battery is capable of would be a good thing -- generally a motor will suck more current when it is stalled, and sometimes it's hard to remember to flick the throttle down when you crash. LiPo's, in particular, don't like to deliver too much current.
The higher the energy the longer the flying time, all else correctly adjusted -- energy being voltage * capacity, more or less.
For the same number of cells, certainly a 2100mAh pack will fly longer than a 1500mAh pack (LiPo, NiCd or any other cell chemistry).
Unless you're just pounding a poor set of NiCd cells into the ground, as long as you get a pack that can supply current to the motor you'll be in the air for a satisfactory flight. Before I remodeled the airplane's nose with dirt, my brushed speed 400 had a 600mAh NiCd pack. With throttle management it flow longer than I wanted to (now it sits in a box, awaiting some glue).
| For the same number of cells, certainly a 2100mAh pack will fly longer | than a 1500mAh pack (LiPo, NiCd or any other cell chemistry).
Common sense tells you that this is true, and in most cases it is -- but not in all.
The capacity and cell count/voltage of a pack are not the only variables. There's also internal resistance and weight, and the discharge curve (which varies from chemistry to chemistry and to a lesser degree even between batteries of the same chemistries.)
Putting in a pack with the same number of cells but higher capacity cells will usually increase weight, which will require a higher average throttle setting to stay airborne. If the battery weighs too much more, the plane may not be able to fly at all (because you'll need 110% throttle, for example), and even if it's not quite that bad, you may find that the battery is sufficient to keep the plane flying when it's 100% charged but not once it drops below 70% (or whatever.) In those extreme cases, a larger pack will cause shorter flights than a smaller pack.
On the bright side, as a general rule of thumb (and there are many exceptions), larger capacity packs have lower internal resistances, so along with your extra weight you usually get marginally better current flows with all else being equal. But as I said, there are many exceptions to this, so it's best to look at the actual specifications of your batteries rather than relying on this.
Tools like Motocalc take into account most if not all of these variables and can help you work out appropriate power plant components for your plane. It's certainly a lot cheaper than just buying stuff and hoping it works.
Paul , to put it very simply , think of the MAH rating ie: 1000 mah ,
1500 mah and so on as the SIZE of a gas tank.
Voltage being the fuel pump pressure or speed at which it flows from the tank to engine.
Amps being the volume or diameter of the fuel line allowing more or less fuel to pass through.
It can. A simple formula to figure flight time.
amp hour rating x 60 / amps = flight time.
For example , 1800 mah = 1.8 amp hours. 1.8 x 60 = 108 amp minutes. Divide this by the amps that your motor will draw . lets say 8 amps.... will give you 13.5 minutes flight time drawing a constant 8 amps. Since you don't always fly at full throttle , or I should say most don't , you will probably get approx 20 minutes flight time , maybe more.
Wattage is the main number to look at. This would be akin to horsepower in a gas engine. To arrive at the wattage:
volts x amps = watts.
a little rule of thumb
35-50 watts per pound of aircraft will keep it in the air. No aerobatics.
75-100 watts per pound will around you to do mild aerobatics such as loops , stall turns etc
124-150 watts per pound will allow 3D flying...hovering , torque rolls unlimited vertical.
The above figures are approximate . Your mileage may vary. I know someone will probably attack these figures and elaborate on them but I'm just trying to explain it in simpler terms.
Example 11.1 volt 1800 mah pack
Paralell : connect battery 1 positive terminal to battery 2 positive terminal , connect battery 1 negative to battery 2 negative. Now connect your charger , speed control etc to the postive and negative terminals of battery number 1 or 2. This results in a 11.1 volt 3600 mah pack. Same voltage but a bigger 'tank'
Series : connect battery 1 positive terminal to battery 2 negative terminal. Now connect your charger , speed control etc to the negative terminal of battery 1 and the positive terminal of battery 2. This results in 22.2 volt 1800 mah battery. More voltage but the same size 'tank'
Power is referred to as watts which is volts x amps.
It depends on how much more and which you increase. Normally , increasing the voltage is the critical part . Increasing voltage = increasing the amps. This is what fries a motor. The motor will be rated as to how many amps it will handle. Going too much beyond this will toast the motor or speed control or both. Just make sure you stay within the mfrs specs and you'll be okay. As you gain more experience you will learn what you can and cannot get by with. Electricalc or Motocalc is a great piece of software for computing all the numbers for you. Here's the link for motocalc. You can download a trial 30 day version. Would highly recommend as it will give you a much better understanding of all this 'stuff'
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I hope this helps as I'm not so good at explaining things on the keyboard.
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