Lithium Receiver Pack Question

[PaulH]

Lately I have been thinking about converting over to lithium receiver packs and voltage regulators. I really don't have a good reason to do so other than being a techno-geek. :glasses2: My 4-cell 4.8v GP3300 packs work very well.

I noticed that most of the receiver packs available (Duralite, Fromeco, etc.) use a 7.4 (2s) Li-Ion pack and a linear regulator. I would think that a 11.1 (3s) or 14.8 (4s) pack with a switching regulator, like the KoolFlight UBEC, would be a better choice due to the higher efficiency and lower heat production. Is there any reason this isn't done more?

Also, I have an AstroFlight 109 charger. It's designed for Li-Poly packs. Is it possible to use it on Li-Ion packs that have protection modules? I'm thinking of using a UBEC with 3s Li-Ion pack like this: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2008 or a 4s Li-Ion pack like this: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2003 but I don't want to let the magic smoke out of anything. :)

Thanks in advance for your input.

[WillJames]

I run the Duralites becuase of hte build in Balancing Circuit and also the Charge Safe Circuit. Since the 109 detects the number of cells in teh pack, I would not use one with a pack without the charge safe circuitry unless it was out of the heli in a fire resistent area.

With the Duralites, I just charge and fly. Simple is good for me. I have been running them for 3+ years now and I am VERY satisfied. I take all the hype from other brands with a grain of salt, and just go with what I know works very well. I believe taht the Duralite system is the safest on the market. Good enough for me.

[playfair]

I have the 2000mAH 3S LiIon pack from battery space in my R50. Never had a problem, with 269 flights since last July on it. At $23 it's cheap! I saw the larger capacity you linked to, but didn't think it was needed for the extra $. I get 10 flights off that pack no probs.

BTW, if you have a voltwatch or similar 4.8v monitor, I have a retro for it which converts to a 3S indicator.

On my Evo 50 I tried the 4S 2000mAH pack because cool flight said the higher volts would make the UBEC even more efficient. The difference wasn't huge, about 40ma compared to a 3S (from 180mA/flight to 140 on 3S).

HOWEVER, on my 30th flight with this pack the "safety circuit" decided to shut down while in the air, leaving the throttle open for a pretty dramatic funky chicken. The killer is it "warned" me there was trouble because once in a while on the ground I would loose all power for no reason (servos at rest prior to starting). A quick off-on of the switch and it always came back, so I was in "diagnostic" mode. Since then, I pulled off the circuit board and one of the batteries, making it a straight shot Lithium and haven't had a problem since.

So, I'm not sure what to make out of these over current/balance circuits that are supposed to protect the pack at the expense of cutting power if "something" goes wrong!

To answer your charger question, yes you can use any LiPo charger at the appropriate cell count. I use a Triton.

[PaulH]

Will,

Thanks for the reply! I know the Duralite system has a strong following. I'm not knocking them at all. Experimentation is part of what makes this hobby fun for me, so I'm always on the look-out for new technology.

playfair,

Great information! I figure that $23 for the 3s Li-Ion pack and $30 for the UBEC gives me the equivalent of a 3700 mAh sub-C receiver pack. The battery pack would be around the same price either way. The only "additional" component is the UBEC, which is pretty inexpensive.

Please post your Voltwatch modifications to this site. That is something that all of us could benefit from.

Of course one nice thing about using lithium packs is that if ever did run out of battery packs in the course of a day, however unlikely that may be, I could grab a 3s Li-Poly from my Trex to run the receiver.

Now I just need to figure out how to shoe-horn some Li-Ion cells into my 9C and I'll be all set...

By the way, where in Rochy are you? I attended RIT from 93-98.

[DavidH]

So, I'm not sure what to make out of these over current/balance circuits that are supposed to protect the pack at the expense of cutting power if "something" goes wrong!

The safe charge/cell balancer circuits that are set up correctly will not cut the power to the reciever if something goes wrong. The circuits are for charging only. That is why the Duralite packs have two leads on them. One of them is the charge only lead. The other is the power to reciever only. From looking at the pictures the Peerless packs from Fromeco are set up the same way.

If you have a safe charge/cell balancer circuit and there is only one lead coming from the pack. When too much current is drawn thru that circuit, it is going to do what it is suppose to do. And that is shut the circuit off. I know Powerflight use to have a safe charge circuit in there packs when they first came out. They found out real quick you could not do everything thru just one lead. There circuit would shut down also when there was too much current demanded by the rx.

So it the receiver is powered thru the safe charge circuit. It is most likely going to shut down if there is too much current being demanded.

David

[PaulH]

What David said is right on the money. The over-current protection circuits that BatterySpace uses limit discharge to around 4 A. That means if for some reason you're pulling more current than that, or the total pack voltage is less than 7.2 V, it will shut down. It sounds almost like you had a cell dropping out or a short circuit if the safety was shutting it down.

[DavidH]

The over-current protection circuits that BatterySpace uses limit discharge to around 4 A.

Does this battery have two leads or just one lead?
If just one lead, what is going to keep the circuit from shutting down if the rx current needed goes above the limit setting?
Personally I would not use a battery that has a safety circuit and only one lead for the power. That is just me. But I really don't want the power shutting off if the model just happens to draw too much current thru it.

David

[WayneBrown]

I have a Duralite 4400 mAh pack and 5.3 regulator in one chopper, and the MAH 4400mAh pack and 5.3v regulator in the Gohbee.
The MAH pack has only one lead, and because I feel the same as DavidH, it's in the cheaper machine, not my Stratus.
It would not be that large of a stretch of the imaginiation to add the second discharge lead to the MAH pack, I'm just too damn lazy.. :roll:

[playfair]

Paul, I'm in Penfield- Small world!
If you want to reminisce the drive, our Heli FF is the end of August :)

Yes, the integrated safety circuit is on the output.
The idea is sound... Protecting the output prevents the "lithium bomb" from going off if harnessing shorted during a crash or the like. It also prevents the batteries from getting destroyed if you leave the receiver on, or had a "run away" model that took a while to find.

For now I'll leave the one in because it hasn't shown ANY signs of problems. BTW, the other one started giving trouble after a small crash. Each time it happened I would make some change, like checking the switch, connectors, etc.
(I'm just mentioning this so I don't look like a careless idiot for not heeding the warnings lol)

Remember that even at the max output of 5.4v @ 3A, the UBEC is only drawing at most 1.5A off a 3S battery. The Ubec would shut down long before drawing the 4 or 5A off the battery to cause the safety circuit to cut out. This is why a 2000mAH 3S pack will outperform a 4000mAH 4.8v at half the weight.

For the voltage monitor mod, you just need to add a 6.2v zener diode in series with the positive lead, with the "bar" on the diode facing the battery. This recalibrates it to show full green at >11.5 volts, and low red below 11v. Seems like a pretty narrow window, but I'll get a good 6 flights before the top green goes out, and I stop flying when the orange gets lit (checked after a flight).
I modify and sell a voltwatch for $20 shipped, or make a "kit" to do your own for $5. (I'm out of the female connectors to plug the existing voltwatch in, so it would need to be soldered)

[PaulH]

Playfair,

That's a seven to eight hour drive from the Boston area. I grew in Chatham, NY which was a four hour drive to Rochy. I think I have done enough long distance driving for one lifetime. :) Who knows though? My wife went to RIT/NTID and still has some friends in the area. We might be through at some point if both of us have a reason to go.

Just a thought for your voltage monitor: Since a lithium battery is usable down to about 3 V per cell, would it make more sense to calibrate the bottom red light for something closer to that? A nickel pack is discharged when you reach 1 V per cell (4 V for the pack). If you use a 5 V Zener diode, or better yet, a 5.2 V Zener diode for a little safety margin, the bottom red wouldn't come on until you get closer to 9.0 (or 9.2) Volts.

You're right about the safety circuit not being an issue with a switching regulator. If you assume 85% regulator efficiency and a 4.2 A cut-off in the safety circuit, you would have to pull over 7 A through the regulator before the batteries cut out.

[playfair]

Valid point offsetting the voltage with a lower zener, but the one time I started a flight with it in the orange I started loosing tail control during the flight. There were no LED's lit when I landed... It could have been the battery, as I was using an old Kokam 3S 1500mAH pack that was surely "unbalanced".

I'm sure a monitor will soon come out to cover lithium series cells. Ideally we'd want 12v full and 9 or 10 on empty, but I don't know how to change the "window size" of these off-the-shelf units without starting over.

[PaulH]

Check out the meter circuit at this link:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page11.htm#meter.gif

Since it uses a voltage comparitor, it wouldn't be hard to daisy-chain two of them together and make an eight-segment meter with whatever window size is desired.

[playfair]

That circuit is pretty simple, Paul. I saved it for a "rainy day".
definitely not building in quantity to sell for $5 though! LOL

If 8 Leds were used, when all were lit you'd be drawing 160ma, which is a lot for something you can't see most of the time. Surely some logic can be added to light only 1 at a time as the VoltWatch does.

Then again, you don't need to know your volts to the .1 mark- you could choose resistors such that the 4 LED's cover 2.0v or whatever you want.

[DavidH]

What is the problem with a good old fashioned loaded volt meter and checking the batteries before each flight? I would think the only way to see an onboard volt meter is hovering the model in front of the pilot. If a pilot is flying around, don't think the onboard meter is able to be read.

David

[PaulH]

I started looking at the differences in efficiency between switching and linear regulators. It was really quite interesting. Switching regulators are not "new", but in my opinion they are less mature than linear models. This results in a few benefits and a few drawbacks when compared to their linear cousins. I thought I'd detail it here for those who may never have looked at the math involved in choosing a regulator.

As the current flowing through a linear regulator is constant (Iout = Iin) calculating the efficiency of a linear regulator is easy: Vout / Vin. So those running 5.1 v out and 7.4 v in are seeing around 69%. The bad news is that the difference is disapated as heat. So assuming a 2 A load on the regulator, the power consumed by the load is 10.2 W leaving 4.6 W released as heat. However, the good news is that linear regulators usually can sink more current than a switching regulator due to the more robust design. This is why Fromeco can claim being able to supply 9 A @ 25% duty cycle. Of course the "gotcha" here is that when using Li-Ion cells, the discharge rating of the pack could be exceeded. As mentioned above, the Battery Space pre-made packs are limited to 4.2 A.

Oh yeah, the 25% duty cycle threw me a little. I suddenly realized that it's the same as KoolFlight saying that they can handle 3 A continuous and 5 A for two minutes. In a 10 minute flight, that works out to "5A @ 20% duty cycle". That's just the marketing department trying to make technical specifications sound interesting.

Now let's look at switching regulators. Good models have efficiency around 80%-85%. However, most have an 8v minimum input meaning a 3s lithium pack is the smallest that could be used. The efficiency calculation needs to be done in Watts instead of just Volts because of the step-down operation. For example, if the output is 5 V @ 2 A and the input is 11.1 V, the output is 10 W. Divide this by the efficiency rating on this (84% as an example) and you get 11.9 W. Divide this by the voltage of the pack and you get 1.07 A. Neat! What makes this even more interesting is that higher supply voltages reduce the supplied current even further, and depending on the regulator, can even make the efficiency go up!

I did some digging around and came across a switching regulator chip that I like: http://www.national.com/pf/LM/LM2678.html

The high points are:

- 84% efficient @ 5 V out
- 8 V to 40 V input (3s to 10s -- could be used on electric helis)
- Efficiency goes up with input voltage (6s pack = 87%)
- Adjustable version available
- 5 A continuous, 8 A peak.
- Very simple circuit (7 caps, one diode, one inductor)
- On/off pin that can be used like the Arizona pin flag setup.
- Switching frequency of 260 kHz shouldn't affect our radios.

I would like to get my hands on one of these to experiment with, but I'd welcome everyone else's input too. Maybe we can sort of "group design" this thing. Give me your feature wish list! :-)