Dual NIMH battery pack setup
 

Would like to set up a dual NIMH pack setup into a single receiver. I have plenty of spaces on the receiver to do this.
I am told the SAFEST way to do this is to have each battery pack with a different charge switch (total of 2 charge/receiver receptacles).
Plug on batterypack into BATT of reciever
Plug other into AUX or other open space
no other additional equip needed

If I do this I understand that the batteries will both drain during flying, resulting in more flying time, etc.
The only downside is you much charge each battery seperately through the individual charge switches.

Correct me where I am going wrong.

You're not going wrong. That's the way to do it.

You are correct. Get a charger that can charge two batts at a time. Or two single chargers

Be careful its recommended to isolate the battery grounds during charging using two chargers not sure about a charger that can charge two at a time. The way you have it wired it is possible for a shorted pack to short out the other pack as well. I use this from ElectroDynamics http://www.electrodynam.com/store/108.shtml

If you are so inclined you can Y the two batteries together for charging. The charger will see them as a single big battery, and they will both peak at the same time.

What are the advantages of this dual pack system, over a single pack with a larger capacity ?

Tom,
In case one fails, you have a backup battery. I have had NIMH batts go bad in flight, nice to have a backup with those batts. Since I changed to LiFe packs, haven't had that problem.

Switches can fail too. I worry more about switches than I do batteries.

I disconnect my leads to my smart board as I charge. Totally isolating my batts from the system.
the advantage is 2 complete indepentent power systems. I use a batt share which monitors the batts ans isolates a bad battery

Unless your switch is double poles the positive wire is usually the wire that is interrupted by throwing the switch leaving the grounds still connected resulting in a common ground.

From ElectroDynamics Web Site re charging two battery packs with a common ground.

Chargers may become confused by the "common ground" between the batteries, and the batteries may end up being improperly charged. The only safe way to charge those two batteries is to unplug one of them from the switch harness... or, use two separate, single-output AC chargers. What a hassle!

If one pack dies suddenly the other will keep things going.

However I am a firm believer in keeping things simple and always went for the one switch one battery option. But at the beginning of the season I would always cycle my NIMH packs a couple of times and check the capacity before flying. Doing this over the years I found 2 transmitter packs that were failing and 1 receiver pack that was almost dead yet read normal on a voltmeter. The small NiMH packs were replaced after 3 years. [ less than 400 mah. ]

Doesn't the failed battery drag down the good one ?

If a battery pack fails as an open circuit it will not pull down the other one. If one of them fails as a short it will. Usually the packs fail one cell at a time which make a short circuit scenario unlikely. Adding an isolator will reduce the overall reliability because of the additional components involved. You are better off charging the batteries disconnected from the switches. Dan.

If your switch has a seperate charging connector you can Y the two packs and charge them together. I can go over the science involved if necessary, but the major point I'm looking to make is that the two batteries will peak at the same time and perform just fine.

All that said, I think you are better off with dual LiFe packs for high performance applications, but the electrical principles are the same either way.

I don't think so. If one fails it will probably pull the other down with it. They should be isolated by diodes to keep on from pulling down the other.

A smart board or a bad share will isolate a bad battery. If you have a battery short out your going to be flying a smoking cinder anyway.

Awful lot of talk about battery failures. Well I’ve been in the hobby quite some time and watched battery technology move from non chargeable dry cells through NiCad’s to the lithium chemistries. It’s been the same for all this time. Those who invest in top quality batteries, radio equipment, chargers and who do their due diligence monitoring the same seem to have little to no problems.
Those that insist that the cheapest equipment is just as good as the quality brand name seem to suffer an inordinate amount of failures. Those who are caviller about care, maintenance and monitoring their equipment wonder why they have more than they’re fair share of failures.

I can honestly say I have not had one flight pack, receiver pack or transmitter pack fail on me in flight. By careful monitoring I have detected less than optimal battery conditions and swapped out the pack long before it had a chance to fail.

Years ago when we started indoor I decided to go the inexpensive route to have a separate system that I could just pick up and go. I purchased this stuff from the popular discount off shore supplier many here use. Never again, some of it didn’t even work out of the box and some batteries ended up nothing but paperweights after less than a dozen uses.

I would bet dollars to donuts most problems can be directly related to cheap equipment supplemented by lack of good common sense monitoring of the same.

In my opinion there is no substitute for quality equipment, educating yourself on how to get the most out of that equipment then developing a routine for care and maintenance of the same.

Dennis

Agreed, Propworm. I've heard of batteries failing as a short, but it's more of a heard of it sort of thing rather than knowing someone who has actually seen it happen. Sure, it's possible, but incredibly rare. What does sometimes happen is a single cell's resistance goes way up in a Nixx pack which will make your voltage go way down. That's where a dual setup will save you. Open circuit failures, which are basically mechanical failures either inside the pack or in the wiring are probably the most common which again, will be fixed with a dual setup. Go ahead and add dual switches to that to prevent the most common failure from costing you a plane. But by far the most common battery failure is old age when the battery simply can't deliver the capacity or amps that it should and a plane is lost because the voltage drops too low.

Took a while, but finally, valid correct information showed up - from one "Tom Crump". To revive a rechargeable battery, you simply apply a voltage higher than the battery's. Even if both packs have been freshly charged, then connected in parallel, if one has a lower voltage, the other one will attempt to charge it to its voltage.
Some individual cells do fail open, but rarely. More often they short, resulting in a 3 cell pack, assuming you started out with four. You won't know it has happened until you see the smoke or it is "I ain't got it" time.
Others followed Tom with similar valid information, do yourself a favor and pay attention. Slightly off the subject, but "Propworn" also makes a lot of sense and I can vouch for that from personal experience. What personal experience? Well, first R/C flight in 1953 with home made radio, and, starting with Orbit Electronics 1n 1970, working with and using a lot of batteries till now. EloyM

I've seen more switch failures than battery failures. Only saw one batt failure airborne 2 while charging.
seen a couple of switch failures.
Bottom line here is redundancy 2 batts 2 switches, is it fool proof no. A shorted battery will bring down the other. A batt share monitors the batteries an isolates one in case of a large amp draw compared to the other batt. If the batt shorts internally will you may have that flying cinder I was talking about

For what its worth jigeye The method you described in Post #1 is an effective method of increasing either redundancy and/or duration. Yes its not perfect however it will work. I have setup a number of cross country airplanes over the years where the purpose in doing so was/is duration and not necessarily redundancy this especially so back before there were such things as battery/share devices and there were only Nicd's of relatively small capacity.

I have never used a batt/share device or felt the need for one. Now on the other end of the spectrum I have done two airplanes that are strictly single mission airplanes, and that is arrested landings. Those two (a Corsair and a Hellcat) spend their whole life doing nothing but continuous circuits with an occasional successful trap so I choose the most reliable engines ever a pair of OS 1.08's. These two are the only airplanes that I did feel the need for glow heaters because of the continuous go arounds so I used the same set up as your description except the second battery bypassed the Rx and only powered the heaters. I have also used the same set up with a separate battery for electric brakes.

So using that setup for redundancy or duration is effective and practical. Don,t be scared off by stories of doom. Total redundancy is of course an impossibility, using dual batteries/switchs without a batt/share while it will not provide total redundancy it will add one more step in redundancy over a simple single battery setup.

John