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Aug 07, 2015, 04:19 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Careful!

current increase with one or two cells added, simple table


  • Scary examples
  • Two simple tables
  • Explanation/theory

Motorcurrent at full throttle is proportional to battery-voltage², Kv³ (in rpm/volt), prop-diameter⁴ and -pitch¹. Controller does not come into play. All this is the same for brushless and brushed motors.

Scary examples
Just to give you some idea.
  • Going from 2s to 3s will increase current by factor (3/2)² = 2.3, current will double.
  • Doubling voltage will give an increase in current by factor of 2² = 4, current will quadruple.
  • An increase in Kv by factor 1.2 will give an increase in current by factor 1.2³ = 1.7, 70% more.
  • Doubling Kv will give an increase in current by factor 2³ = 8.
Two simple tables
Listing the extra current when adding one extra cell. Without the e-blurb and geek speak.
Calculation for two extra cells is simple too, multiply the consecutive factors, or sum the respective percentages. E.g. going from 3 to 5 cells, current wants to increase by factor 1.8 × 1.6 = 2.8 (= 5/3)²).

Note that the extra current depends on the number of cells you start off with.

Always measure your current, even when using an e-flight calculator: to calculate is good, to check is better.
Code:
Increase in current
cells factor extra
1 → 2  4.0   300% 
2 → 3  2.3   130%
3 → 4  1.8    80%
4 → 5  1.6    60%
5 → 6  1.4    40%
6 → 7  1.4    36%
Same table sideways
Code:
Increase in current
cells 1  →  2  →  3  →  4  →  5  →  6  →  7
factor  4.0   2.3   1.8   1.6   1.4   1.4
extra   300%  130%  80%   60%   40%   36%
The factors/percentages in the tables are worst case, and are never reached. Because higher voltage drop in battery/wiring/controller/motor is ignored. Ken Myers did some comparative measurements for the Ampeer Newsletter:
www.theampeer.org/ampeer/ampnov15/ampnov15.htm#ADD


Explanation/theory
Motors do not behave like a simple resistor (where current = voltage / resistance). Motor-current is proportional to voltage squared and proportional to Kv cubed . This is worst case, because battery voltage will decrease/sag a bit due to higher current and voltage losses in the motor will be higher too.

Why current is proportional to voltage squared, and proportional to Kv cubed, motors want to keep rpm the same, no matter the load:
www.wattflyer.com/forums/showthread.php?p=591935#post591935

Sticky: e-flight calculators (compilation)

Don't throttle down if current is too high, it will not reduce motorcurrent. And life is even harder on ESC/capacitors at part throttle than at wide-open-throtlle, no PWM switching/chopping at WOT. Here's why ...
Changes in setup (and lousy Kv specifications!) can have surprisingly considerable/huge effects.
E.g. doubling voltage will four(2²)fold current, doubling Kv will eight(2³)fold current, and doubling prop diameter will sixteen(2⁴)fold current.
Even a small 10% change/difference in Kv will already lead to a 30% difference in current.

General rules:
Motorcurrent is proportional to pitch¹, voltage², Kv³ and diameter⁴.
Power-drawn is proportional to pitch¹, voltage³, Kv³ and diameter⁴.

Therefore always measure current in a new/changed setup. If/when current gets too high for motor and/or controller and/or battery, you have to prop down to get the current down again: less pitch and/or smaller diameter.

Vriendelijke groeten Ron
Last edited by Ron van Sommeren; Sep 25, 2019 at 12:58 PM.
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Dec 06, 2015, 02:01 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Quote:
Originally Posted by Ron van Sommeren
... This is worst case, because battery voltage will decrease/sag a bit due to higher current and voltage losses in the motor will be higher too. ...
The above percentages are worst case, and are never reached.
Ken Myers did some measurements for the Ampeer Newsletter:
www.theampeer.org/ampeer/ampnov15/ampnov15.htm#ADD

Vriendelijke groeten Ron
Last edited by Ron van Sommeren; Mar 09, 2016 at 06:55 PM.
Dec 07, 2015, 01:44 AM
Registered User
manuel v's Avatar
In practice I see these numbers more appropriate when the same voltage readings are taken per cell.


Note. Reading the article by Ken Myers, I see that my board is very similar numbers, the largest discrepancy of 2-3 cells.

Excel, provides some very useful convert-ions in our area.
Last edited by manuel v; Dec 07, 2015 at 01:59 AM.
Dec 10, 2015, 09:43 AM
WAA-08 THANK FRANK!
JimNM's Avatar
Excellent presentation - thank you. Bookmarked for future use... why type when I can cut/ paste -eh?
Jan 11, 2016, 04:44 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
It is a good practice to have some headroom built in ...
Lucien Miller (Scorpion USA) about derating motors, controllers, and batteries, electronics in general:Vriendelijke groeten Ron
Jan 11, 2016, 09:33 PM
Registered User
vollrathd's Avatar

Hacker A60-5S powered by 3S to 12S LiPos


Attached is a screen dump from www.motocalc.com of the Hacker motor, powered by a 12,800 MilliAmpere Hour battery pack. The oversized battery pack takes some of the battery issue out of the equation. To simplify, the same sized 18X10 prop is used.

Note that the current increases rapidly with more cells, while the watts input skyrockets. Going from an 8S to a 10S Battery pack nearly doubles the watt level, from 1500 Watts to 2800 Watts. This is why a wattmeter is so important on electric power. As Ron is pointing out in his thread, what might seem to be a minor change in battery cell count, motor KV, prop size can make a very significant differece in the power input of the motor.

Try this with an inefficient motor with high winding resistance, and you'll get smoke quickly with this sort of test.

I've got one of the Hacker A60-5S motors, running on a 10S2P A123 battery pack, with a 19X12 APC-E wide blade prop. Maximum current is 78 Amps. The motor temperature is only slightly above ambient after a flight.
Last edited by vollrathd; Jan 11, 2016 at 09:40 PM.
Jul 23, 2017, 12:08 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Added several examples to opening post:

Just to give you some idea
  • Going from 2s to 3s will increase current by factor (3/2)² = 2.2, current will double.
  • Doubling voltage will give an increase in current by factor of 2² = 4.
  • An increase in Kv by factor 1.2 will give an increase in current by factor 1.7.
  • Doubling Kv will give an increase in current by factor 2³ = 8.
Vriendelijke groeten Ron

1,679
Last edited by Ron van Sommeren; Jul 23, 2017 at 12:13 PM.
Aug 25, 2018, 10:22 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Quote:
Originally Posted by JimNM
... why type when I can cut/ paste -eh?
You and me, both lazy
I love the Firefox Clippings add-on, use it a lot.

Removed from OP
Two dead links, too bad really, especially the CastleCreations link, that was a very good explanation as well.

Added to OP

Changes in setup (and lousy Kv specifications!) can have surprisingly considerable/huge effects.
E.g. doubling voltage will four(2²)fold current, doubling Kv will eight(2³)fold current, and doubling prop diameter will sixteen(2⁴)fold current.
Even a small 10% change/difference in Kv will already lead to a 30% difference in current.

General rules:
Motorcurrent is proportional to pitch¹, voltage², Kv³ and diameter⁴.
Power-drawn is proportional to pitch¹, voltage³, Kv³ and diameter⁴.

Therefore always measure current in a new/changed setup

Prettig weekend Ron
2,987
Last edited by Ron van Sommeren; Aug 25, 2018 at 10:48 AM.


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