mgm 120 bec

just got my mgm 120 today and put it in my rustler. everything went fine with the 3s lipo packs, took awhile to get used to the sudden reverse it has but overall i like it. thought i would put a 4s pack in it and do a speed run and everything went down hill after that.
i programmed it for 4s lipo, put them in and drove it around the shop for a minute. then i noticed my fans were running way to fast so i disconnected it. those fans smoke up fast! obviously, the bec failed me, or is it not designed to work with 4s lipo? the instructions only say 12 cells, which are not exactly equal but close.
so my question, is it not capable to use the bec funtion with 4s lipo? or is there a problem with mine? thanks

[MetalMan]

The BEC on the MGM controllers can handle 4s Lipo. Where you by chance using it in a monster truck? Because the BEC circuit can only handle so many watts, because it converts the wasted energy into heat. The equation to figure out the watts that need to be turned into heat is:
(input voltage - output voltage) x amps pulled
So in your case: (~14.8v-5/6v) x amps that the servo pulls. Let's say that the servo pulls 1 amp. This gives you 9-10watts of heat. That's quite a bit for something that can't displace the heat. And if you are using a heavy monster truck, a servo in that can easily pull 3amps. So, up to 30watts! Sinking 30watts requires quite a large heatsink, and will also require quite a good fan!

no monster truck, just a rustler with two of the little novak fans on the motor and esc. so much voltage went to them that they just smoked up. im going to work with it today and see if i can figure out the problem. thanks for the reply metalman.

okay, so, after some r and d, dont put 4s lipo on your mgm 120 if you have a cooling fan wired to your reciever. after a wasted fm reciever, fans, and a few curse words, i found out that my bec no longer worked after plugging in all that power.
could have read the instructions a little more but instructions are for wimps. haha. luckily, the controller still works flawlessly. just wanted to let people know my mistake so they wont make it again. cheers.

[MetalMan]

I don't think that from just one situation that the MGM can be ruled out from use with 4s Lipo. Especially because when I ran my MGM 120amp, I ran 4s Lipo. I ran it in my Rustler with a Feigao 12s. I was also using a pretty powerful servo, which puts more strain on the BEC chip.

your right metalman, it still works great with 4s lipo, i guess i should have explained it better. everything works fine on the controller but the bec on it is no longer working. if i plug a 3s lipo into it, im getting over 9v out of the reciever. my only guess is that when i put 4s lipo on it, with the two novak cooling fans, it was too much of a load and fried the bec.
all i did was remove the positive lead from the controller plug and run a reciever pack. works like a champ.