LiFe, BEC replacement - RC-Monster Forums

zeropointbug · 01:42 AM

I just went and bought a 850mah Hyperion G3 LiFe rx pack for $16 to replace my BEC (Pro BEC). I am going to use a rx pack because I don't trust the BEC anymore, at least for racing. I think these little things are great alternatives to a BEC for a couple reasons I can think of:
-it will always work (as long as it's charged), can never fail, no power interruptions
-higher voltage than a BEC, faster, more powerful servo operation

It doesn't weight that much either, just under 50g... so it wouldn't even be noticeable over a BEC equipped truck. It may even extend radio range because of less electrical noise in the system? BrianG?

I know Brian might be interested in one of these, no?

scarletboa · 08.14.2009, 03:47 AM

that looks like a great deal. i might just pick one of those up instead of a BEC if i ever need 1.

also, that adapter is the same as the hobbycity 3.5mm plug. maybe hyperion will help standardize the hobbycity 4mm plug as well.

BrianG · 08.14.2009, 09:55 AM

The only concern I would have is the voltage after a fresh charge. LiFe has a "peak" voltage of 3.6v/cell, which would be a little high (7.2v) for some receivers/servos. Granted, it does fall pretty quickly to the nonimal 3.3v/cell, but that initial hookup could conceivably be too high.

RC-Monster Mike · 08.14.2009, 10:46 AM

850mah isn't much capacity, either. :)

zeropointbug · 11:58 AM

Yeah, the higher initial voltage might be a concern, and the 850mah is kinda low, but that's just to keep the weight down. I have been using a 2s M1 cell pack for about a month now, I am getting quite a few runs out of it between charging, at least 10 I think, and nothing has gone wrong yet. :) So the capacity isn't that bad, if you don't mind recharging an extra little pack every 5 runs or so.

I got this batt just because the BEC shutdown on me at the 20 min mark in a 30 min main, cost me 2nd place, if not 1st. This obviously is not for everyone, but it's a good alternative to anyone who don't trust BEC's anymore... Brian?

Also, Hyperion says they work in all applications that a normal NiMH rx would, so you don't actually need high voltage servos, or higher quality units. I bought the 7955 as well, so I am sure it will give some decent twist with one of these. :)

BTW, thanks for posting Mike. :)

scarletboa · 08.14.2009, 02:21 PM

my nimh rx packs get up to 7.5V or more when fresh off the charger. i'm sure the hyperion pack will be fine fresh off the charger as well.

BrianG · 08.14.2009, 03:02 PM

7.5v seems a bit high. Yeah, I've seen NiMH voltages that high too, but only while it was being charged. An unloaded reading 5 minutes after the charge shows around 6.8-7v (1.35-1.4v/cell).

03:35 PM

Quote:

Originally Posted by zeropointbug

I bought the 7955 as well, so I am sure it will give some decent twist with one of these. :)

I've read that the 7955tg can handle 2s lipo's. I might get a g3 2s1100mah as rx pack myself. 5c charge rate...

edit:
http://www.rctech.net/forum/ask-hite...55-7965-a.html

Quote:

The 7955TG has been through extensive testing and is more than capable of handling 2 cell lipo unregulated. The specs at 7.4v are as follows:
torque= 383oz/in
speed= .12@60deg.

suicideneil · 08.14.2009, 04:38 PM

In other news, forecasted sales of BrianG's forthcoming BEC failsafe device were looking grim...

wallot · 08.14.2009, 05:03 PM

one thing is if servo can handle the voltage the other for how long :)

BrianG · 08.14.2009, 05:41 PM

Quote:

Originally Posted by suicideneil

In other news, forecasted sales of BrianG's forthcoming BEC failsafe device were looking grim...

lol, it was grim to begin with as it was not going to be a money-maker. I'm still gonna build some for myself. Aside from the BEC failure potential, I don't see any need to strap another battery in there somewhere.

zeropointbug · 10:22 PM

Oh for sure, most people wouldn't bother with a rx pack, especially for bashers, as a shutdown or hiccup is not going to cost you a top place (only parts if it crashes

).

I didn't know Brian was making a fail safe, is there a link to a thread? I must have missed it. But for me, a failsafe is not going to help the fact that the BEC just shut down, twice in 3 mins.

Unless... BrianG, could it have been the Receiver that reset? I suddenly had no steering servo no throttle, nothing, then brought it in, re-plugged it, did half a lap and it did it again. Put a different pack on it, and I finished the rest of the race without incident, but in 4th place, instead of 2nd.

What do you think?

Could the BEC have overheated? As it is packed tight into the left side Revo servo box with the receiver. It is the Pro BEC, S&T gave it to me when I fried my first Quark.

I just like the idea of running off a dedicated battery for communications. Plus the thing really is tiny, slightly larger than a standard servo.

florianz · 08.15.2009, 05:23 AM

the higher voltage in the beginning is no problem at all:

there are regulators for those lipo-rx packs, which keep the voltage at a constant 6v level. that's intersting especially for nitro guys. forget those nimh-packs, get a proper lipo/life pack, the regulator and enjoy the plus in power for the servos. and you only have to charge it evey now and then.

flo

zeropointbug · 08.15.2009, 02:19 PM

Yah, you can get regulators, but here we go again putting an electronic device in between pure DC source and the Rx. Besides, a Life pack has a flat enough discharge that you don't need one. I don't think lipo is a good idea for Rx, they shouldn't be fully charged all the time like LiFe packs. These LiFe's also have the same mah/gram, only lower voltage, which is think is better than 7.4v nom.(8+ volts for a good percentage of the discharge).

BrianG · 08.15.2009, 03:03 PM

zero: this is the failsafe thing I was building: http://rc-monster.com/forum/showthread.php?t=22819

And adding a regulator works when the voltage is high, but anywhere near LVC, the drop-out value (usually 1-1.5v, even with LDO types) limits its usefulness. When the 2s LiFe pack is peaked at 7.2v, a 1v drop-out is fine because you still get 6.2v. As you can see though, a drop-out even as low as 1v will limit the output much below that. At just 3v/cell, you can only get 5v out. Linear regulators are kind of limited when it comes to current and differential voltage. Neither can be too high or power dissipation is high as well. Switching regulators work, but you need to find one that is both a "buck" (which is what all our UBECs are) and "boost" depending on what voltage it is being fed.