6 min read

upgrading a toy quadcopter battery after rebuilding the frame

i have a small toy quadcopter. it was not bad, but the original plastic frame had too many unnecessary parts and the flight time was not very interesting.

so i decided to use it as a small hardware experiment.

i removed the original frame and other plastic parts, designed my own frame, and printed it. after putting the electronics, motors, propellers, and the original battery back onto the new frame, the total weight became:

47g

the original toy quadcopter was:

59g

so the custom frame version saved:

59g - 47g = 12g

for such a small drone, 12g is a lot.

after that the next obvious question was: if the drone is now lighter, can i use the saved weight for a bigger battery?

original battery

the original battery is marked as:

HPY 752035
3.7V
380mAh
1.41Wh

it is a 1s lipo battery. 3.7v is the nominal voltage, not the full charge voltage.

for a 1s lipo:

nominal voltage: 3.7V
full charge voltage: 4.20V

the test document for this battery lists it as a polymer lithium-ion battery with:

model: HPY 752035
rated capacity: 380mAh
rated energy: 1.406Wh
charge cut-off voltage: 4.2V
standard charging current: 76mA
max charging current: 1900mA
charge cut-off current: 7.6mA
max discharging current: 7600mA
connection: 1S1P

this is useful because it shows that the original battery is not just a random small lithium cell. it is a high-rate lipo made for a drone.

charging it from a laboratory power supply

before changing the battery, i wanted to understand how to charge the original one safely.

with a laboratory power supply, the correct mode is cc/cv:

cc = constant current
cv = constant voltage

the settings are:

voltage: 4.20V
current limit: chosen charging current

for a slow and safe charge, the standard current from the test document is:

76mA

but i first used:

200mA

for a 380mah battery this is:

200mA / 380mAh = 0.53C

so 200ma is not extreme. it is more than the standard current, but far below the listed maximum.

then i also thought about 500ma:

500mA / 380mAh = 1.3C

this is still below the documented max charge current, but i would not use it as my default with a lab power supply. the battery may support it, but fast charging is not the same as gentle charging.

my practical charging settings are:

4.20V
200mA current limit
stop when current drops to around 8-10mA

the important part is not to go above 4.20v.

with a lab power supply there is no smart lipo charger logic. it will not really "finish" the charge for me. i need to stop it when the voltage is at 4.20v and the current has fallen close to the cutoff current.

why the battery current rating is so high

the max charging current in the document looks very high:

1900mA

for a 380mah battery this is:

1900mA / 380mAh = 5C

and the max discharge current is even higher:

7600mA / 380mAh = 20C

this makes sense for a drone battery.

small quadcopters need short bursts of high current. motors do not behave like a small led or a microcontroller board. when the drone climbs, corrects position, or recovers from movement, the motors can pull several amps.

that is also why replacing the battery with an ordinary li-ion cell is a bad idea.

a normal li-ion cell may have the same voltage and even more capacity, but if it cannot provide enough current, the voltage will sag. the drone may become weak, reset the controller, trigger low battery mode early, or overheat the cell.

so for this experiment i only considered 1s lipo drone batteries.

weight after the custom frame

the rebuilt drone weighs:

47g with original battery

the original battery weighs:

9.60g

so the drone without battery is:

47g - 9.60g = 37.4g

this number is the base for all battery calculations.

custom frame without battery: 37.4g

now i can compare battery options.

option 1: 1600mah battery

the first tempting option was a much larger battery:

1600mAh
43g

the total weight would become:

37.4g + 43g = 80.4g

so the drone would be:

80.4g total

compared to the original toy quadcopter:

80.4g - 59g = 21.4g heavier

that is about:

21.4g / 59g = 36% heavier than stock

compared to my custom frame with the original battery:

80.4g - 47g = 33.4g heavier

that is about:

33.4g / 47g = 71% heavier

the battery capacity increase looks great:

1600mAh / 380mAh = 4.2x

but this does not mean the drone will fly 4.2 times longer.

the battery also becomes more than half of the whole aircraft weight:

43g / 80.4g = 53%

at this point the drone becomes almost a flying battery with motors attached.

it may still fly, but the motors will need much more throttle just to hover. the drone will probably become less responsive, and the motors may become hot much faster.

i would test this battery only carefully:

  1. mount it exactly in the center
  2. hover low over something soft
  3. fly for 20-30 seconds
  4. land and check motor temperature
  5. stop if the motors are hot or if hover needs too much throttle

if it hovers at 70-80% throttle, this setup is not good. it means there is not enough thrust reserve.

so the 1600mah battery is interesting, but too extreme for this drone.

option 2: 650mah battery

the second option is much more reasonable:

650mAh
18g

the total weight would become:

37.4g + 18g = 55.4g

this is a very different result.

compared to the original toy quadcopter:

59g - 55.4g = 3.6g lighter

so even with a bigger battery, the drone is still lighter than it was originally.

compared to my custom frame with the original battery:

55.4g - 47g = 8.4g heavier

this is a much smaller penalty.

the capacity increase is:

650mAh / 380mAh = 1.7x

this is not as impressive on paper as 1600mah, but it is much more balanced. the drone should still have a normal thrust-to-weight ratio, and the motors should not suffer as much.

this is exactly the kind of tradeoff that makes sense after rebuilding the frame: use the saved weight for a battery that is larger, but not absurdly larger.

expected flight time

the original battery has:

380mAh

the better replacement candidate has:

650mAh

that is:

1.7x more capacity

because the total drone weight with the 650mah battery is still lower than the original stock weight, the flight time increase may be close to the capacity increase.

if the original flight time was around 5 minutes, i would expect something like:

7-8 minutes

maybe a little more, depending on the motors, propellers, battery quality, and flight style.

with the 1600mah battery the stored energy is much higher, but the extra weight may destroy much of the benefit. the flight time may increase, but the drone will probably become heavy and inefficient.

final comparison

the numbers are:

original toy quadcopter:
59g total

custom printed frame with original battery:
47g total

custom frame without battery:
37.4g

original battery:
380mAh
9.60g

big battery option:
1600mAh
43g
80.4g total

balanced battery option:
650mAh
18g
55.4g total

the 1600mah option gives:

4.2x capacity
but 80.4g total weight

the 650mah option gives:

1.7x capacity
and 55.4g total weight

for this drone, the 650mah battery is the better engineering choice.

conclusion

after rebuilding the quadcopter with my own 3d-printed frame, i saved enough weight to use a better battery.

but the best battery is not the biggest one.

the 1600mah pack is too heavy for this size of drone. it may fly, but it will likely make the quadcopter slow, inefficient, and hard on the motors.

the 650mah pack looks like the sweet spot. it gives noticeably more energy while keeping the total weight below the original stock weight.

so the plan is:

use a 1s 3.7v lipo
keep full charge voltage at 4.20v
check connector polarity
avoid random low-current li-ion cells
use a drone/high-rate lipo
test motor temperature after short flights
choose 650mah instead of 1600mah

this is the nice part of small hardware projects. the final answer is not hidden in a datasheet only. it appears when the numbers, the weight scale, and the real object on the table all agree with each other.