rrTV-PHOTO   New HD TV
HOME   rrTV-PHOTO   GALLERIES   MY GALLERY   HELP-FAQ
myHOME PM pmRR MEMBERS 687 ONLINE 19 EVENTS SEARCH REGISTER  START HERE
EDIT to Close 
2 pages [ <<    <    ( 1 )     2     NEXT    >> ]1943 viewsPOST REPLY
Ace Hobby . Esprit Model . Thunder Power RC

.
.
Radio - Servo - Gyro - Gov - Batt > Dual Batteries
 
 


Location:

Ive heard of people useing two batterys in planes, has anyone done this in helis? Is it wise? And how?
I've found the Raptor 60 to be tail heavy some and thought this might be a good idea to try, it would add some more weight to the nose and increase battery capacity.
04-07-2002 Over year old.
  WV   EDIT
 
 
GMPheli
Veteran
Location: W. Bridgewater, MA USA

It will work fine. Check out Reds battery web page. Go to rcuniverse.com. There is a link to Reds page at the top of the battery forum.

Alan Angus
04-08-2002 Over year old.
 
 
skyraider42
Senior Heliman
Location: Huntington,WV.

Dual Batteries

How do you hookup tow batteries Y harness or some other way
04-09-2002 Over year old.
 
 
Jared
Heliman
Location: Hickory NC

Mine are hooked up on separate switch harnesses. Each battery goes into its own switch harness, each switch harness goes into a separate channel on the rx. two chargers, two charge jacks, two switches to turn on, and ultimate reduncancy. The main reason that I did it was for flight time; with a 1000 mah pack I was getting 3 flights max, now with one 1000 and one 600 I can fly all day and haven't had to charge it yet.
04-09-2002 Over year old.
HOMEPAGE  
 
 
MikeDD
Senior Heliman
Location: Elkhart, IN

Bergen does this for some of his customer's twins. He uses the unit from Electrodynamics

Michiana R/C Choppers
04-09-2002 Over year old.
HOMEPAGE  
 
 
Julius
Heliman
Location: Tracy CA

Jared - Not a good idea

Initially, it seems a good idea to connect two batteries as you have for redundancy. What you have done is connected your two batteries in parallel through seperate switches, which is not completely bad. The mismatch in their capacities is where you could end up in trouble.

What happens is that as the 600mah pack expends its energy, the 1000mah pack will try to charge it - the 1000mah pack is now supplying energy to 1) rx, servo's, and gyro, AND 2) the 600mah battery.

The dual switch idea is good. You'd be better off using 2 packs of equal capacity. The problem still exists but to a lesser degree. This is because no two packs are exactly the same.

If your system is working for you, thats fine, just beware.

julius
04-10-2002 Over year old.
 
 
Jared
Heliman
Location: Hickory NC

Thanks Julius. I'm not trying to argue, but could you help me find info to back up your idea? I have found reliable info to the contrary, that says pack capacity doesn't matter, and I would certainly like to know that it does. I don't have much of an EE background, so I have to believe other folks.
04-10-2002 Over year old.
HOMEPAGE  
 
 
GMPheli
Veteran
Location: W. Bridgewater, MA USA

Nothing personal Julius, but I think someone gave you some bad info. Please check out Reds webpage, instructions are in the second post of this thread.


Alan
04-10-2002 Over year old.
 
 
RC-CAM
Senior Heliman
Location: USA

The mismatched (or matched) capacities are only an issue during the charge cycle. That is why two switch/charge harnesses are needed (allows separate charging). Other than that, dual NiCd or NiMH packs should not cause any grief during the discharge cycle.
04-10-2002 Over year old.
HOMEPAGE  
 
 
slant911
Veteran
Location: Las Vegas, NV. Hirobo, Magnum Fuels, MAH

If you want to do it the easy way contact these people at the following website.

www.emsjomar.com

They have what you need.
04-10-2002 Over year old.
HOMEPAGE  
 
 
Julius
Heliman
Location: Tracy CA

Jared and GMPheli:
Maybe I need to rephrase my reply a bit.
1) for redundancy, Jared has a good set-up.
2) for added flight time/capacity(1600mah), paralleling packs of differing capacity is OK, but not efficient. using equal mah packs is better
3) since we don't normally let our batteries get below 1.20 volts per cell, there shouldn't be a problem.
Now for the "why" of inefficiency:
The two packs of different capacity have different rates of discharge, the 1000mah will last longer than the 600mah pack by definition. The instant that a potential difference (difference in voltage) exists between the two packs as they discharge, current will flow (Ohm's Law) to establish equilibrium between the two - one discharging into the other. While installed into the heli, this is not readily apparent and not much of a concern. It is important to monitor the voltage of each pack seperately between flights using an ESV with a load of at about 500ma (I do this every flight).
The combined capacity of the two packs will be slightly less than 1600mah. From an engineering point of view, paralleling different capacity packs expends energy unnecessarily.

hope this make sense. oh, I studied EE at Univ. of Texas.

julius
04-12-2002 Over year old.
 
 
GMPheli
Veteran
Location: W. Bridgewater, MA USA

"The instant that a potential difference (difference in voltage) exists between the two packs as they discharge, current will flow (Ohm's Law)"

But a potential difference will not exist. As soon as the voltage of the 600 mil pack starts to get slightly below that of the 1000, more current will flow from the 1000 because of the higher potential. So the voltages will stay the same. One will not try to charge the other, except maybe right off of the charger if one pack happens to charge to a little higher voltage than the other. They will eventually equalize and remain that way. If you were to just connect the two batts, without any load, the difference of potential between the two will be very small, so not much current will flow between them and it will take a while till they equalize. When you connect both to a load, whichever pack has the higher voltage will flow more current than the other till they equalize. It will behave just like a 1600 mah pack. Please correct me if I am wrong, but I have been using dual batteries for about 4 years now with no negative results.

Alan Angus

P.S. I even tried connecting a 3 cell pack to a 4 cell pack to see what would happen if I had a dead cell and didn't catch it. The 4 cell pack charged the 3 cell pack at around 300 ma. So it would not take long to drain the 4 cell pack. This is still a better situation than trying to fly with one 4 cell pack with a dead cell.
04-12-2002 Over year old.
 
 
Julius
Heliman
Location: Tracy CA

Potential Difference

GMP,
Once the packs have reached equilibrium under load, what determines the rate of discharge of each pack? How is the equilibrium maintained?

Lets throw another component into the equation, internal resistance of the packs. Once two packs are paralled, so are their internal resistances. We don't have a choice in determining where or how the current will flow in a given circuit, Ohm's Law is the rule here.

" But a potential difference will not exist. As soon as the voltage of the 600 mil pack starts to get slightly below that of the 1000, more current will flow from the 1000 because of the higher potential"

You just contradicted yourself. I think I see where you are going with this, you have your thinking cap on and I applaud your efforts!

"One will not try to charge the other". One WILL maintain the other (equilibrium) until it no longer has the energy to do so . This point is below our limits of 1.2v per cell for safe flying.

Your experiment with the shorted cell in a parallel pack is proof of the value of redundancy. Parallel packs do work, but they work more efficiently if their capacities are equal. Having used this system for 4 year w/o a problem shows your diligence in maintaining your batteries.

julius
04-15-2002 Over year old.
 
 
RC-CAM
Senior Heliman
Location: USA

Why not see what really happens?

As mentioned, this dual battery application is thoroughly discussed at http://www.rcbatteryclinic.com/

I am willing to run an experiment to obtain some practical evidence to what should be expected. I can measure the individual mAH capacity of two packs of unequal capacity (800 and 1400mAH). I can then measure their capacity when used as a pair (via a "Y" cable). The results can be compared to see which personally held theory has the most merit.

I promise to be impartial and use care in my measurements. I will employ an Accu-Cycle system that has been calibrated. End of discharge will be to 1.05V per cell (industry accepted) and at {meter verified} 250mA constant current. All charging will be at C/10.

Sorry, but I am not prepared to formalize this with a statistically significant sampling. At best, I will repeat the experiment one time. Even so, would the results of my short experiment interest anyone?
04-15-2002 Over year old.
HOMEPAGE  
 
 
GMPheli
Veteran
Location: W. Bridgewater, MA USA

Sure Mr RC-Cam. I would love to see the results of your test. Julius, do you really think that the internal resistances of the packs are going to be that different? And that would only come into play when you are drawing high current, which we are not. We are not talking much difference of potential here. Let's see what Mr RC-Cams test results are. The proof is in the pudding!

Alan

P.S. This is straight from Reds page.

"The use of redundant parallel fight packs (packs may be of different capacity but MUST be of an equal number of cells) is an excellent way to increase the available flight time and significantly improve the reliability of the on power system. The simplest means is to run two complete wiring harness, switches and charge jacks from each pack and plug one into the normal battery port and the other into an extra channel on the receiver. No diodes or isolation is required (see below). This is simpler and more reliable than some of the complex battery backup systems being offered on the market. Whether you are using 4 or 5 cells is your option, remembering that a 5 cell pack will provide more power to the servos but at the same time discharge faster giving you less flight time.

Many pseudo battery "experts" put forth the argument that plugging two battery packs into the same receiver with out blocking diodes is NOT a good thing, claiming that his creates a host of problems and the two packs will end up fighting each other or "cross charging".

These concerns show a lack in the understanding of the charge and discharge potentials involved in Ni-Cd cells. One pack cannot charge the another (equal number of cells) as the discharge voltage of a pack can never be as high as the voltage required to charge the other pack. For the doubters here is an experiment: completely discharged one pack to 4.0 volts and then connected to a fully charged pack having an equal number of cells. There will be less than a 10% transfer of charge in a 24 hour period. Since shorts rarely occur in fully charged packs the risk of one pack "dumping" into one with a shorted cell are insignificant. A simple ESE preflight test would detect a pack with a shorted cell. "

04-15-2002 Over year old.
 
 
RC-CAM
Senior Heliman
Location: USA

Ok, I will give it a go. Please allow a few days for the results.
04-15-2002 Over year old.
HOMEPAGE  
 
 
RC-CAM
Senior Heliman
Location: USA

I have preliminary results. I'm still making measurements so there is more to come. This process takes awhile since pack cycling is S-L-O-W going.

My first measurements were with a 800mAH and 1400mAH pack. I cycled them several times to determine their baseline mAH capacities. The 800mAH was averaging approx 825mAH and the 1400mAH was averaging about 1100mAH. The 1400 pack is a retired fellow.

Although I know that the experiment was suppose to use two vastly different capacities (800 and 1100 seem a tad too similar) I was still curious about what the results would be if they were used as a pair.

They measured at about 2100mAH. That is slightly better than merely adding the two measurements up. This is somewhat expected, since as the relative discharge rate goes down the measured capacity tends to increase. There is also a margin of error of about 5%.

I am now in the process of base-lining a larger pack to be used with a much smaller pack. I will also perform a test where I will discharge the pair about halfway, then measure their individual remaining capacities to see how balanced the discharge was. This will take another week or so.
04-19-2002 Over year old.
HOMEPAGE  
 
 
Julius
Heliman
Location: Tracy CA

C O O L !

You just created energy.....
04-21-2002 Over year old.
 
 
RC-CAM
Senior Heliman
Location: USA

Comment deleted.
04-21-2002 Over year old.
HOMEPAGE  
 
 
RC-CAM
Senior Heliman
Location: USA

Test completed

I continued the tests using more appropriate discharge currents. These new values did not experience the inflated mAH values during the combined pack test. Two 4-cell NiMH packs were used. All measurements are accurate to about 5%.

Baseline measurements as follows:
Pack #1, 570mAH at 250mA discharge rate.
Pack #2, 1720mAh at 500mA discharge rate.

Combined packs:
Pack #1 and Pack #2 in parallel, 2280mAH at 500mA discharge rate.

A final test was performed to get a peek at the discharge load balance when two unequal packs are used together. I simply stopped the dual pack discharge at the mid-way point. Then I measured the individual packs to see the remaining mAH.

Modified Combined packs test (50-50 test):
1/2 discharge point (approx)= 1180mAH at 500mA rate.
Pack #1 remaining mAH= 220mAH at 250 mA rate.
Pack #2 remaining mAH= 790mAH at 500 mA rate.

I learned a valuable lesson here: performing dozens of discharge tests, over several days, is about as exciting as watching paint dry.
04-24-2002 Over year old.
HOMEPAGE  
 
 
2 pages [ <<    <    ( 1 )     2     NEXT    >> ]1943 viewsPOST REPLY
Mikado Modellhubschrauber . GrandRC . CanoMod

.
.
Radio - Servo - Gyro - Gov - Batt > Dual Batteries
 PRINT TOPIC Advertisers 

Subscribe to This Topic

Tuesday, December 2 - 4:36 am - Copyright © 2000 - 2008 runryder.com | email | link to rr | runryder needs cookie