Hi
Good luck with your mod. I did a Pro with 2 batteries in top of each other myself. I removed the casing for both batteries and my main one with the "smart board" weighted in at 217 g and the "backpack" 213g, some padding added. The bare cells weight 60 grams each. I obtained 15 to 20 km range depending on conditions. But I don`t like to run the batteries all the way down. I would say "safe" is a 13-15 km flight with which I was a bit disappointed. I was hopping for a 18 to 20 km with 15-30% left on return. My total setup weight in at 925 g. The XT 60 connectors and cables add a bit of weight to.
The Mavic Pro has a design flaw which does not work well with added weight. The magnetic field generated by the back motors supplying cables may interfere with the GPS/compass unit located in the back much to close. As we can see with
Mavic 2, the unit was moved in the centre of the aircraft.
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Warning! Your modded Mavic generates higher magnetic fields! Danger of unespected ATTI mode anytime!
From my experience (if you want to go ahead) what I suggest:
-use only low noise propellers, not for less noise but they spin slower for the same given weight.
-do not fly the setup in sport mode unless you are in LOS.
-definitely do not tweak the Mavic for extra speed (tilt angle and vertical up speed).
-If you get "remote high interference" messages, it has noting to do with the remote but with the high current in the motor supplying cables. Reduce speeds!
-re-solder the supplying cables and get ferrite cores on them. This help with the concentration of the magnetic field. Ensure the 3 cables supplying the back motors are twisted around each other (this reduces magnetic interference)-plenty of references on the net about twisted cables. later supplied Mavics had the twisting done in the factory.
-compare (from dat logs) motors PWM and ESC temps of the standard mavic with your setup.
Dangers are PWM close to 100% and much hotter ESC temps than standard setup.
Other things to consider:
Your mavic is less aerodynamic and the batteries are warming less (cold weather flying warning).
Your mavic is more noisy.
If you fly out of range missions you are at risk of losing it due to the unit switching itself to ATTI mode when out of reach. This can happen randomly. Is no rule there but you can help by avoiding high mission speeds and very close to each other waypoints with high altitude differences and headings at 180 degrees from each other. Avoid high winds. If you have Phantom experience remember the Mavic is much more affected by wind.
Also ESC`s are heating more, the motors are stressed more.
I would say the safe weight for a Pro is under 800 G.
I did not made it myself (yet) but I suggest a battery made of six LG 18650 (3ooo mah each ) about 41 g each.
So you end up with up to 6000 mah per cell for a minimal weight increase, your battery should weight in under 300g. Best are Panasonics at 3400 mah but a bit heavier. if you need help with this have a look on the net or ask.
Do not buy any cheap chinese, no brand batteries who claim over 3400 mah!
Do not buy batteries with self protection!
But if range is really your thing why do not consider fixed wing (especially flying wing)?
A drone is a flying rock where a flying wing generates lift with all of its body with minimal drag.