DANGER using BEC + Lipo for RX BATTS

[genexis]

I found this out by a very costing way.
As we know Gas RC you need a receiver pack to power the servos, and quite a few of us are using Lipo polymers, connecting into a BEC to step down the voltage. In this way, we hope that we do not need to charge the batteries as much as using NiCd or NiMH.

As for my, I'm using a 2S Lipo, connecting into a BEC whose minimun input is 5V, output also 5V. Apparantly, my voltage checker didn't indicate that my Lipos are low because the minimum voltage of the lipos when discharged ranges from 2.5V - 2.7V. Thus, the voltage output from the BEC is also 5V. In the end, the lipos just gave up half way in flight and cut the power to the receiver halfway in flight.

when i brought it home and tested the electronics, all i need was some vigourous movements of the cyclics, the BEC will just shutdown. The more i tried, the faster it took for the BEC to shutdown. Just last night, i charged the batteries again, first time since i crashed, and i tried again this morning. No shutdowns

To summarize, the battery voltage checker if connected to the receiver will not be a good indication, or even a bad indication of how much power the battery has left. Unless the checker is connected directly to the batter.

P/S: Please correct me if I am wrong about this, because i believe that these are many things to electronics which i am not aware of.

[ez zai]

Hi, most electronic speed controls (ESCs) use battery eliminator circuitry (BEC).Although that separate battery pack for the receiver, they do however make the radio systems more glitch-prone. Just my thought. Ezzy.

[X34]

Genexis,

I am sorry to hear about your mishap.

Using Lipo and bec for flight pack is a good solution, it provide a constant 5V for the electronics. But that doesn't mean u should not charge/maintain your lipo.

The flight electronics will require more current in flight than sitting idle on the ground.

THe battery indicators are just indicative, it should be placed in a way that u can see it from the ground while flying. Otherwise, it serve no purpose.

Finally, if your 2s lipo is at 7.4 volts u should recharge it. Lipo voltage at rest of 7.4 volts is about 5% capacity left.


Hope it clears some doubts.

[X34]

Hi, most electronic speed controls (ESCs) use battery eliminator circuitry (BEC).Although that separate battery pack for the receiver, they do however make the radio systems more glitch-prone. Just my thought. Ezzy.

Good observation.

All ESC create some kind of noise that causes glitching. Thats Y hacker/schulze/kontronik esc comes with the "opto" option. which uses an optical link for the rec. connector so that it doesn't send noise to the rec. to confuse it.

I am sure other major brand esc without bec will be available with the opto link too.

[genexis]

Genexis,

I am sorry to hear about your mishap.

Using Lipo and bec for flight pack is a good solution, it provide a constant 5V for the electronics. But that doesn't mean u should not charge/maintain your lipo.

The flight electronics will require more current in flight than sitting idle on the ground.

THe battery indicators are just indicative, it should be placed in a way that u can see it from the ground while flying. Otherwise, it serve no purpose.

Finally, if your 2s lipo is at 7.4 volts u should recharge it. Lipo voltage at rest of 7.4 volts is about 5% capacity left.


Hope it clears some doubts.

But if my voltage is always at 7.4 volts, won't the indicators just simply show that the pack is fully charged ?

[X34]

But if my voltage is always at 7.4 volts, won't the indicators just simply show that the pack is fully charged ?

Battery indicator plugged into rx socket - the battery indicator is not showing you battery condition, it is showing you the bec condition.

Unless u get a 9v battery indicator install between the battery and bec than it would show u the battery condition.

[genexis]

Ya, this was what i was trying to say :P So, battery indicator when using a uBEC is not accurate. Therefore, becareful

[HZM]

Seriously, this thing never cross my mind b4 .... I am glad u manage to narrow down the problem.

Thanks

Mike

[Super-Hornet]

This what I know... I maybe wrong:

1 - A full charger LiPoly should be 4.2V per cell. Therefore, 2 Cell is 8.4V.
2 - All BEC or all Voltage regulator require voltage different between the o/p and i/p by some value. Take example, LM780x series require voltage different of at least 2V. Anything less than that, the Voltage Regulator is not working/not turn on. In other words, to produce a 5V supply, a LM7805 require i/p voltage of at least 7V in order to work. For switch mode regulator like uBEC, if I'm not wrong, the Off state situation is might equal to open circuit... that is no o/p.

For your case, if your battery is 7.4V, and if when battery reaches 7V, or less, the BEC might not in a ON state because the voltage differences between i/p and o/p is too low. It is better to use BEC with 3Cell instead.

Another situation of using BEC. If direct connect batter to RX, the point of failure will be connect to RX. (Most of us uses only one Servo like connector to connect battery to RX). When u use BEC, u actually have 2 point of failure instead. One is the connector (contact problem) and the other will be the BEC burn out/shutdown.

One possible safetly precaution that I use previously is that connect RX battery to 2 or more servo like connector instead. The 2 connector is connected to RX instead. Therefore, if one connector having contact problem, u still got another one working.

Super-Hornet

[genexis]

What is o/p and i/p ?

[Super-Hornet]

o/p = short form of Output
i/p = Input.

In this case, we are refering to:
i/p = Supply voltage... or your Li-Poly
o/p = Output voltage of BEC... your 5V supply.

Super-Hornet

[genexis]

Guess what SH, you are right. Min. is 7v for the BEC. TIme for a 3S, who has 3S for sale ...