With a timer+governor setup (Hubin time + governing ESC, or a Renicle timer) you can expect that the peak power draw will be about 11 watts/ounce of plane weight (the Igor Burger timer demands more, but I don't know how much).  Current is just power divided by voltage, so:

(weight of plane in ounces)(11 watts/ounce) / ((number of cells)(3.7V/cell)) = expected maximum current draw in amps.

Or, for this battery pack, the weight of the plane in ounces times 0.75 will give the maximum current draw in amps.

Both the motor and the ESC should be rated at or higher than this number for maximum current draw.

Tell me how much the plane will weigh, and I'll give you a number in amps.

Thanks for the responses.  The final weight of the plane will be around 65 oz.

Peter Mick
AMA9581

Tim,

The KV on my E Flite 32 is 770.

William,

The specs for your Voltron listed in Stunt News indicates that you are using a Scorpion 3026-890 motor.  How does that compare to my 32?  I also noticed that you are using a 4S 3700 battery.  Would that work with my 32 or should I go to a different power plant so I can use a 4S battery? The 32 is still new in the box and I can return it if necessary.

Pete

Peter:

The Scorpion spins faster for the same voltage, which is why it'll work with four cells to the eFlight 32's five.  The Scorpion is rated for over 1000W continuous output: you'll have to check on the eFlight 32, but it's probably more than the average of 460 that I'm getting, or possibly even the peak of 720.

Keep in mind that as the cell count goes up, the cell capacity that you need goes down.  The amount of energy you need to draw from the pack is going to be pretty much the same for both motors, and the weight of a pack is largely dependent on how much energy it can store.  Whether there happens to be a sweet spot in the selection of cells that's available at any given moment that makes a 5S pack heavier or lighter than a 4S pack is pure chance.  The important thing to remember is that a 5S pack that can hold the same energy is not going to be 20% heavier than a 4S pack.

As an example, William's 3700mAh, 4S pack corresponds to a 2960mAh, 5S pack.  A quick check on the Hobby King site coughed up a 3700mAh, 4S pack that weighs 545g, and a 3000mAh, 5S pack that weighs 527g.  So for those two examples, you actually save weight by going to a 5S pack (don't take that as being a done-deal, all-the-time thing -- it's just a quick example).

Pete,
While I am not sure about the rule of thumb for electric as I am still learning myself, I recently complete an ARF SV-11 that came in at 65 ozs with a 4S 3300 battery with an Arrowind 2832 (820kV) spinning a 12x6 electric APC prop.  I am using a Phoenix Edge Lite 50 and have recorded a maximum of 38 amps during the few trim flights I have run so far.  Sounds like you are close with your current set up.
Regards,
John

Tims rough estimate is about the same as mine. You should be able to find a 5s pack of lower mah in approximately the same weight as a 4s pack because of the capacity difference. A quick search on the Hobby king site turns up 3 5s packs that should work including this 5s 3000 which weighs 379 grams which is less than my 4s 3700 35c batteries.. Sina used this battery http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8853  in his Tweener last year(2013) with a PHX 60 and a power 32 with great results. that plane is in the same size and weight range that you are planning to use. Your weight wont vary much eather way so pick a set up and use it. By the way I have run the phx 45 on planes up to 72 oz without any problems. That is on 4s setups only. I wont trust it with a 5s on that sized plane. For the record my 4s 3700 35c pack weighs 393 grams.

Hi Will, just a question on your last statement about the phx 45 on 5 cells. Surely 5s will have lower current than 4s for the same power, so would the 45 amp esc not work better in this case, or is it something to do with the voltage rating of the phx 45?? Thanks.

Keith R

Hi Keith,
I'm pretty sure it has to do with the voltage plus the Amps exceeding a certain number because I know that there are people that use the PHX 45 on 5s setups but it seems to me like most are lighter planes(around 55 oz or less) The 2 times I tried to run the PHX 45 on 5s on planes 65oz and over they failed so now I just tell people use a PHX 60 or similar and don't worry about it failing.

As far as I can see the only penalty for running an ESC that is rated somewhat higher than is needed is the extra weight but the penalty for using one beyond its spects can be smoke!

A switching amplifier (which is what the power stage of an ESC is) has losses that come from both the amount of current going through, and the voltage on the transistors.  "Losses" mean energy that's going to heat up the parts, instead of turning the motor.  So it's not surprising, particularly if the transistors are at the edge of their rated voltages.

Thank You Tim, I am not an engineer and didn't really know the "why" behind it I just knew that it happens and not to do it. Your explanation puts it in terms that are very clear to me.
William

Hi Will, just a question on your last statement about the phx 45 on 5 cells. Surely 5s will have lower current than 4s for the same power, so would the 45 amp esc not work better in this case, or is it something to do with the voltage rating of the phx 45?? Thanks.

Keith R

The Phoenix 45 (if we're talking the older plain Phoenix 45) is/was rated for 5 cells if the BEC wasn't used (3 cells if using the BEC).  This implies that the main limit to using more cells than normal is the amount of power the BEC has to eat when a 'normal' number of servos is used with a certain number of cells.  Our little timer doesn't pull any appreciable current, so people get away with 4 cells without overworking the BEC.  As for 5 cells, some investigation would be necessary.