cc bec question

[slimthelineman]

My question is what happens when input voltage approaches output voltage? Never done it this way but thinking of using it in my dx3s to keep the voltage below 6v (max rating for backlight el driver) and protect the radio when fully charged at 7.2v but wondering if I set it for like 5.5v when the pack starts to dump will the voltage output drop as well to trigger the low voltage alarm on the radio? Was thinking of using a regulator but I already have the cc bec sitting here and I like I can adjust the voltage outpu with the cc link. Any thoughts?

[BrianG]

Any regulator, linear or switching, has a "dropout voltage". This is the minimum input voltage required to get the rated output voltage, and can change a little as current increases.

From previous testing, the CC BEC needs around 2v higher input than the output. So, for 6v output, you should have at least 8v input. If the input drops, then the output drops too. The CC BEC I tested completely shuts down when the input gets to ~4.5v.

This is the results of the test I did a while back:

[slimthelineman]

Sweet chart mr. G thanks. Exactly what I was hoping to see, so I can set my cc bec for say 5.5v and set the lv alarm in the radio at like 5v so my alarm will go off well before my cells ever get low enough. Also with a very small load do you think the dropout voltage would be closer to the output voltage? Just wondering how well this setup will work with a life battery. Wish I would of thought of this the other day then I could have done it while the tx was apart and just used a lipo I already had. Oh well.

[scarletboa]

why don't u just use a small 3s lipo with the bec in your tx?

[slimthelineman]

Thought of that but not sure how I would know when I was aproaching the lvc of 9v for the 3s unless the cc bec has a lvc built in. Haven't been home to mess with it been on storm work for the last couple days bit I will mess with it tomorrow. Also the small 3s I have is too long I think to cram inside the tx where the four aa batts went. I'll post a pic or two of the whole setup when I'm done.

[JERRY2KONE]

Originally Posted by slimthelineman

Thought of that but not sure how I would know when I was aproaching the lvc of 9v for the 3s unless the cc bec has a lvc built in. Haven't been home to mess with it been on storm work for the last couple days bit I will mess with it tomorrow. Also the small 3s I have is too long I think to cram inside the tx where the four aa batts went. I'll post a pic or two of the whole setup when I'm done.

Brian G actually did a conversion thread on this a while back and once you open up the radio you can totally remove the 4 AA battery tray and place somthing else in its place if you wish. I am doing the same conversion on my DX3R but using a Hydromax NMH battery pack. I like the idea of using the CC BEC though to protect the radio, but not sure if you need it. Do a search in here for DX3R conversion and you might find that project that BG did. Good luck with that and let us know how it works out for ya.

[JERRY2KONE]

Here is his comments from that thread. I hope this helps to answer your questions.

I wasn't sure what the effects of running just 4.8v (4 X NiMH cells) was, so modded the radio tray to use 5 NiMH cells (6v) to match what 4 alkaline AAs would put out voltage-wise. The voltage of 5 freshly peaked NiMH cells is closer to 6.5v, but hasn't harmed my radio yet.

To use lipo, you could go a couple of routes:

1: Use 2s lipo with a linear voltage regulator. An LM7805 linear regulator would work fine, but only gives 5v. You can easily boost it with the addition of a couple of resistors to get 6v though. And since radio current draw is small, you shouldn't need a heatsink. I figure 400mA worst case current draw (judging by the runtime I get on 2500mAh NiMHs), which equates to 0.96w when the lipo is at 4.2v/cell, and 0.4w when the lipo is at 3/5v/cell. I wouldn't run the lipos down much beyond that because at low current draws, you have much less voltage "rebound" when you remove the load. And at that point, there isn't much runtime left anyhow.

2: Use whatever cell count lipo you want and hook that to a switching BEC. The higher voltage you go, the lower current draw from the lipos. If you use 4s lipo, current draw will be around 200mA if the radio is drawing 400mA @ 6v. The trouble with this setup is that if the BEC fails, it could send full battery voltage to your radio. Not good. So, I would use something like the CCBEC with a programmable output, set it to something like 7.2v, and then use a linear regulator to take it the rest of the way down to 6v. Power loss on the linear regulator will be minimal since the delta voltage is small, yet it provides protection if the BEC should fail. The worst that will happen if the BEC failed is that the linear reg would heat up and thermal, which should be a good indication that something is wrong. Despite the poor efficiency of linear regs, they are pretty bulletproof; you really have to try to break them.

In either case, you'll also need to add some sort of LVC, low voltage alarm, or small voltmeter panel (like this). Otherwise yoou run the risk of discharging the lipos too far.

So, I found it much easier to just stick with NiMHs in this case.

I run my DX3R on a 1S LiPo without any regulator or cutoff.
It continues to transmit until the voltage drops to 3.5V.

[JERRY2KONE]

Originally Posted by Ferret

I run my DX3R on a 1S LiPo without any regulator or cutoff.
It continues to transmit until the voltage drops to 3.5V.

Your doing good then so far. Keep in mind all it takes is one time of discharging too low and up in flames it goes. Just the nature of using Lipos without and LVC hooked up to it. Its a personal choice and one that I am not willing to risk on any of my radios.

Since the transmitter stops working at 3.5V the battery cannot be over discharged while driving.

However, I forgot the transmitter turned on for a few days and it did over discharge the battery. Twice. (Yes, I know it's nothing to brag about...)

The battery didn't it go up in flames or even puff and I was able to bring it back to life both times. I am still using it today.

Of-course I don't know how other batteries will react and I'm only reporting my own experience.

I believe that leaving a battery connected to anything (external cut off, regulator, BEC, etc.) will slowly discharge it. So I'm not sure there is a 100% safe solution.

Avner.

[BrianG]

Running a 1s lipo is probably the best lipo solution IMO. You really don't want to run a lipo down much past 3.5v anyway at such a low drain rate (less "bounce back" when load removed) so as long as you are still getting full output at LVC, then you should be good to go. I would make sure the Spektrum's voltage meter is accurate (by measuring with a meter and noting any difference) to ensure that what the display shows is really the state of the battery. I know on my Spektrum, the display reads around 0.3v lower than the actual battery voltage.

Running 2s lipo (or higher) with a BEC has been done (by MetalMan IIRC), the only downsides are 1) you don't know the true voltage state of the cells as they are masked by the BEC and 2) if the BEC fails, it could send full battery voltage possibly damaging the circuits. Although, since current drain is so low, it is less likely for the BEC to fry, but the chance is still there.

Another possible solution would be to use 2s LiFe and add a diode or two in series to bring down the voltage a little. 2s LiFe at peak (3.6v/cell) with a diode in series (dropping ~0.6v) would be 6.6v, which I know works fine. At LVC (3.0v/cell), the voltage would still be 5.3v. And this solution also lets you use the radio display to know the voltage so you can use the alarm.

I don't think overdischarging a Lipo at that low of a current draw will cause a Lipo to burst into flame; worst case is you have a dead cell. I don't think it would even puff since that is caused by overheating, and the radio doesn't draw enough current to do that. But like Ferret said, a BEC does draw current all the time, even when the radio is off, so you'd have to put it after the built-in switch, or add another switch.

[slimthelineman]

Interesting points Brian. I'm out and about today shopping, could you recomend a diode or resistor value and how many? Seems much less complex and fool proof than installing/cramming a ccbec in there with the life two cell pack. Trying to keep voltage to the radio about 6v for the sake of the backlight kit. It's at 6.6 right now but if I charge it it will be higher.

Ok did some surfing on the topic and it looks like a 1amp diode will give me the desired .5-1 volt drop. Off to radio shack! Will buy a few and do 2s2p just incase one of the diodes fails it won't pass the extra voltage straight to the tx. Thanks everyone for the help. Put some pics up later.

[BrianG]

Yeah, a 1A diode would be fine. IIRC, the Spektrum draws around 300mA. Diodes, being simple, rarely fail unless driven to over their current rating. Putting two in a 2p arrangement won't help if one decides to short (the shorted one will bypass the other paralleled one). You'd be better off getting a single 3A diode instead if you are worried about it, just be aware that the less current you draw, the less voltage drop there is. A 1A diode running at 300mA will drop around 0.82v (and dissipate ~0.25w), while a 3A diode running at the same current will drop closer to 0.7v (and dissipate around 0.2w). Whatever you get, I would get 2. That way, if one doesn't drop enough voltage for your liking, you can just add another one in series. A package of these (or similar) would be perfect: http://www.radioshack.com/product/in...ductId=2062578

Also be aware that the battery charging pigtail should be placed before the diode (between the diode and battery) or the charger won't be able to read the battery voltage, and even if it could, it would read ~0.7v lower than it really is. Also don't forget that the Spektrum display will read ~0.7v less than the actual battery voltage.

When you wire up the diode, make sure the cathode (the end with the white line) faces the radio circuit. If you hook it backwards, the diode won't blow or anything, but the radio won't work.

If you happen to have an assortment of suitable NPN transistors laying around (who doesn't), you could tie the collector and base together, and then use that connection as the anode and the emitter as the cathode.

[Pdelcast]

Originally Posted by BrianG

If you happen to have an assortment of suitable NPN transistors laying around (who doesn't), you could tie the collector and base together, and then use that connection as the anode and the emitter as the cathode.

Well, that's not really a good idea... Modern NPN power transistors won't take too much base current. Better to use two power schottky diodes.

[Pdelcast]

Also,

Brian, I disagree with your 2V dropout -- if you look at your chart there is practically no dropout. If there was 2V of dropout, a 7V input would yield a 5V output, and a 5V input would yield a 3V output. That's not the case.

The CC BEC uses a P-channel MOSFET for the power switch. The only dropout should be from the resistance of the MOSFET itself... So around 30 millivolts or so.