ORIGINAL: pontebuda
-Bvd, two 4000 mah 18.5 30c.Batts This set up is used on several of my big birds. No problems till now.Everything in this bird 72'' Yak 54 is new. Thanks for you quick return i appreciate it. Bob
Bob, see your quote above. 4000 mAh is capacity not voltage. 18.5 is voltage, and is the nominal voltage (3.7 volts per cell) of a 5S pack. (18.5 / 3.7 = 5)
Fully charged a 5S pack is 21 volts (4.2 x 5 = 21) A fully charged 4S pack is 16.8 volts (4 x 4.2 = 16.8) Which neither matches your info, so I took it as a nominal reading of a 5S pack.
If you parallel wire the two packs together you double the capacity (8000 mAh), but voltage stays the same (18.5 volts nominal or 21 volts fully charged).
If you series wire the packs, capacity stays the same (4000 mAh), but voltage doubles (37.0 volts nominal or 42 volts fully charged)
Since you are using a 8S rated motor, I seriously doubted you were running it on 5S, and took it as you wired the packs in series to get a 10S rating.
If so you have a 10S4000 mAh 30C pack set up.
Post a link to the exact packs you are using. Also if you can a photo of your connection between the two packs, that way I/we can be sure what you have and can better inform you.