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View Full Version : Jeti 70 Opto on 20 nimh cells?


kwilliby
12-13-2005, 06:40 PM
My friend is setting up a new plane. He will be running an AXI 4120-18, 20 nimh cells and pulling about 45 amps WOT. He currently has a Jeti 70 Opto which is rated for 70 amps and 16 nimh cells. He has bench tested this setup with 20 cells and it runs fine. Motor and ESC are barely warm after multiple runs.

My concern is that the Jeti 70 Opto is only rated for 16 nimh cells. And he is using 20 cells.

Will the ESC handle 20 cells since he is only pulling 45 amps? Or is the voltage still a possible problem?

I'd hate to see his plane crash because of the ESC failing.

Thanks.

Matt Kirsch
12-14-2005, 02:46 PM
The voltage is a problem, or at least it will be when he starts doing more than short runs on the bench. He needs a 77 Opto to handle 20 cells.

kwilliby
12-14-2005, 04:05 PM
The voltage is a problem, or at least it will be when he starts doing more than short runs on the bench. He needs a 77 Opto to handle 20 cells.

That's what I told him. Since it ran fine on the bench he thinks it will be fine. I'll try one more time to convince him otherwise. This plane will be too beautiful to risk a mistake like this.

It is actually a scratch built replica of the Flying Wing from long ago. It will have two of these AXI motor's and weigh about 20 lbs. He has done an outstanding scale build on this plane.

Thanks for your input!

kwilliby
12-15-2005, 06:16 AM
The voltage is a problem, or at least it will be when he starts doing more than short runs on the bench. He needs a 77 Opto to handle 20 cells.

the one thing that confuses me on the Jeti 70 Opto. The specs say up to 16 nimh and 6S Lipo. using nominal voltage specs that would be 16 nimh at 1.2v = 19.2v and 6s lipoly x 3.7v = 22.2v.

5s lipoly x 3.7v = 18.5v. Seems the rating of 16 nimh and 5s lipoly would have been 'safer'.

i think my friend figures that if the esc can handle 22.2v on a 6s lipoly that he can get by with 20 nimh cells. i.e. 20 x 1.2v = 24v. Only 1.8v beyond the 6s lipoly rating.

oh, well. i think he's pushing the limits but it's his plane. all I can do is give him my input as well as yours.

thanks again.

r_kopka
12-15-2005, 02:48 PM
A similar ESC (also 16NC) uses MOSFETs which are rated to 30Vmax. So I think it's just a value the manufacturerer sets to be on the safe side. It may work with a higer voltage but you loose some of the safety.

There are also effects with voltage spikes on the battery wires if they are too long etc. which can lead to higher voltage at the MOSFETs. This may be another reason not to go over the manufacturers specs.

There are even ESCs (Kontronik e.g.) which measure the voltage and don't work if the voltage is higher than the maximum defined.

RK