What BEC to use for MMP 1S version?
 

If you saw my thread not too long ago I was asking about the lowest operating voltage for the MMM and MMP since I want to run 2S LiFe packs in my TC3. After some consideration I ended up buying the 1S version MMP. Looks like an awesome controller, as usual, from Castle. Anyone use the 1S version yet?

My question is that I also would like to use the Castle BEC, but will that work with 2S LiFe? The low end of the range is 5V input, and I'd have my cutoff on the MMP set between 4.0-4.5V. I know I can't expect 6V from a 4V source (at least not without using one of those 1S boost modules which I'd like to avoid), but I really only need to power the Rx and a small low-end servo. Will the BEC turn off at less than 5V? Or just output a lower voltage? Am I missing something that could cause issues?

Any input is appreciated. Thanks!

EDIT: Also, since the internal BEC is removed from the 1S MMP do I still need to remove the red wire from the ESC to receiver?

I tested the CC BEC a while back. The bottom part of the results below is what you're interested in.

http://scriptasylum.com/forumpics/cc_bec_graph.gif

As you can see, I had it set for 6v. If the input is below 6v, the output tracks pretty close to ~0.3v lower than the input (looking at the higher current 2.5 ohm resistor load column). At around 4.5v, the BEC output shut off. I think you'll find that most step-down BECs will have similar results (if not worse). So, an LVC of anything below ~5v results in unpredictable behavior, and that's not including when the voltage dips for brief intervals on high current bursts.

To avoid all this, you could get Dimension Engineering's AnyVolt 3. It's quite expensive, but it will provide your set voltage no matter if the input is below or above your set BEC output. EDIT: Nevermind about this unit because it needs an input at least 5v.

So basically I can either set the LVC at 5V (which would rob me of some run time, would have to test to see how much) or get a 1S boost module? How about a Castle 1S boost module Patrick to go with the 1S MMP? :wink:

EDIT: I found this http://www.dimensionengineering.com/lvboost.htm but the max input is 5V

You can get the 5v dimension engineering park bec if you run 2s life.

The minimum input voltage on that BEC is listed as 6V. I thought it would behave the same or worse than the CC BEC once the voltage drops below 5-6V? I see you run 2s LiFe in your GenX-10, do you use this Park BEC with that and have no issues?

The genx doesn't need one with the 2s A123. pointless really.
If the voltage goes too low, the dm bec just passes the lower voltage through. I do however use the park bec in my nomadio rx. I use a 2s lithium ion pack and need 5V max for the RX. Been working great for a while now.

That's good to know. Wish the Castle BEC would do that.

Yes, the Park BEC does continue to output voltage, but according to the testing I did, the ParkBEC has a higher dropout voltage than the CCBEC:

http://scriptasylum.com/forumpics//park_bec.jpg

Brian the park bec that I use is the 5V version. Your test data shows the 6V version which is no good for 2s Life cells.

Why do you need a BEC at all? There should be several servos and receivers that work with 2s LiFe. Some even work with 2s LiPo directly.

Never thought of that, although it seems obvious now. The Rx is a Spektrum SR300 and the servo is a stock AE metal gear (for now). I don't care about the servo but don't want to fry the Rx. I figured the 2S LiFe pack fully charged would be too much for the Rx. Time to go check out the Spektrum manual.

EDIT: You are right Thomas, maybe won't need a BEC at all. Most Spektrum surface Rx's work between about 3.5-9V so that will be fine on 2S LiFe. The servo I have right now might burn up, but I can get a 7.2V capable one if I have to.

Higher voltage servos are the way forward for performance. I'm currently using a BEC Pro with my MMP in my Ve8, with the BEC output at 9.5V - lovely servo will do 12V, but Spektrum Rxs won't be happy with that. Transit times are at 0.03s-0.04s, and about 8kg/cm of torque.

I just ordered an AE servo (SHV1504MG) so I can use the 2S LiFe directly into the receiver. It's a cheap stock metal-gear servo like the other AE one I have in right now, but is made to handle 7.2V. It was the only cheap HV servo I could find, all others are $90-150 or even more. Might be willing to put a $100 servo into my truggy, but not my TC3 project.

A while back, I had an issue where my Koolflight BEC died and sent full 5s (IIRC) battery voltage to the servo and Spektrum receiver. When the servo blew, it pulled enough current to blow the PCB traces connecting the + and - pins in the receiver. But, the receiver itself survived fine. I soldered a bridge wire across the + pins and the - pins to replace the blown traces. I looked into it a bit more and it seems that the receiver has a small linear regulator between the pins and the receiver circuitry. And since the rx power draw is VERY small, it doesn't heat up. I don't know if it would be advisable to run higher voltage to the receiver all the time, but thought you'd like to know. BTW: this was on a SR3000 reciever. Not sure if the others have a similar circuit design.