Hello guys,
I would be glad to hear experiences from guys who used a dual rx battery.

I'm going to use that approach, mainly for redundancy of battery and switch, but I've heard people who say that there are not advantages: if a cell become shortened, the other battery pack will be shut down by the bad one, with the result that you can imagine...

Is that true, or double battery is an effective way against switch and battery failure?




Any help would be greatly appreciated, and many thanks in advance guys!

If you were a [link=http://www.nsrca.org/]NCRCA[/link] member with access to the KFactor, you would have read your answer in the 2004-12 issue!

Lance Van Nostrand submitted:

"Here’s the circuit. It is easy to assemble. The key is the use of Shottky diodes on the output of the two battery systems. This type of diode is used in switching regulators where there needs to be extremely low voltage drop when they turn on. If either battery fails, the other will not drain into it. Each system is treated as a separate system and plugged into two different places in the receiver. This provides total redundancy. The 5 cell battery will be the “main� pack and do all the work. The 4 cell battery is the backup and only kicks in if the main pack fails, or if there is a big current surge and the system needs more juice."

Edit to add:
"Shottky diode: must handle current and voltage. It has a very low turn on voltage and a low maximum forward voltage drop. A good one is Digikey: STPS20L15D-ND"

Thank you Scott!!!

This circuit seems very good!

I have to deduct that simply plugging in two batts is not a good setup?

Go here and check the parallel operation link.

http://www.rcbatteryclinic.com/

GREAT INFOs!!!!!

Many thanks JVB!!!!!!!!!!!!!!!!!!

Doesn't the duralite 1900 pack have redundancy? 2 sets of two cells in parallel?

JVB,
from the description of the parallel operation link it is not clear to me if the system works ok even with a shorted cell.

Anyone can help on this regard?

Thank you.

There has been some debate about whether or not you can simply plug in a second battery and switch into a spare receiver port. My personal experience is that you can with no issues- and don't need any additional equipment so that the bad battery won't drain the good one. I have had personal experience with this when one battery went bad on a 35% Cap 232. I was not aware that one of the batteries was bad until I checked them before the next flight.

On my pattern plane I don't have a backup- perhaps I should. On my 35% planes I do have two batteries and switches going into a single receiver without any additional equipment.

Dave Michael

Here is some third party information about two batteries/one receiver including some discussion on the topic of do you need a diode or not. I'm no battery expert but this fellow is from the battery industry.

http://www.rcbatteryclinic.com/

click on "Parallel Operation = Reliability and Flight Time" on the left hand side.


Also, on the NO BS Batteries webpage (my understanding is this fellow used to work for SR Batteries before breaking out on his own), there is a frequently asked questions section. Here is the link with some good info and I have cut and pasted the specific recommendations he makes about not needing a diode.

http://www.hangtimes.com/rcbattery_faq.html

Here's the quote:

"Q: Ok, that’s pretty kewl.. but my instructor sez I need diode protection or the packs will cross-talk or try to charge each other.

A: Sorry.. your instructor has been gaffed by Under-Informed Magazine Columists or the Battery Mis-Information Committee of your local fields Wives Tale Tag Team. There is NO need for diodes or a 'backup' circuit board (something else that adds weight, complexity and another possible failure point) and in fact there's no flight-safety significant energy transfer between packs at different charge levels.

You can test this yourself.. just plug a discharged pack and a fully charged pack together in a Y-Harness and check the two packs the next day.. You’ll find the system will pass a load test if checked through the ‘Y’, and you’ll also find that less than 20% of the charged packs energy has been ‘used’ by the discharged pack when you separately cycle-test the two of them. Next, consider that your average flight is 10-12 minutes, not 24 hours! The mythical "Energy Transfer Between Packs" scenario is simply NOT a flight safety issue and checking both packs before flight with a loaded ESV will certainly 'pick up' a weak pack before you fly it anyway!"


Hope this is helpful to you.

Dave Michael

Stek79,

I am no expert, however, I don't believe that setup is designed to work with one pack shorted. He explains that shorted packs are very rare and a shorted pack should be caught with a voltage check before flying. The majority of failures are due to problems between the pack and the receiver (leads, switches, etc.) This method is very simple and provides redundancy for the most common causes of failure in the electrical circuit. I have never had nor have I ever known anyone with a pack that had a shorted cell. Perhaps people that like to reuse packs that have been in crashes would need to use a more rigorous battery backup setup.

JVB,

Sorry about reposting the rc battery clinic link- I didn't notice it before I posted.

Dave

Ok guys, now I understand more the issue.

The FAQ says that a 24 period of connection wont tear down the good pack, but I had the same understanding like you, i.e. that a shorted cell wont be tolerated, it seems. It is also true that those are extra rare failures...

I'll think about that, for now thanks again for all your info and opinions.

Guys,

Why not use the EDR-108 Pow'r Back'r from Electro Dynamics, which will isolate
a bad Battery. While both batteries are good it will automatically run from the
one with the most power reserve. You will have to use 6 Volt packs because the
Pow'r Back'r uses 0.9 Volts to operate. You're left with 5.1 volts operating current
for receiver and servos. Uses 2 separate switches and charching jacks.

Karl,

thank you for your suggestion. Personally, I don't like the idea to add other components between batteries and receivers... a switch is enough for me I've also thought about removing it...

Dave!!!

I haven't read carefully your post, here you say that the setup survived well with a bad battery. What do you mean by bad? Was it shorted?

Stek,

I was flying 5 cell nicads at the time and had just charged it at the field. Upon checking the voltage before taking off it showed good voltage- like 6+ volts to start.

When I checked the battery after the first flight and before the next flight it was reading a very low voltage- it was a 5 cell pack and was reading just a few volts- like 2-3volts if I remember correctly. Even so, with the other battery also connected to the receiver and turned on, I couldn't notice the bad battery when operating the servos.

Dave

I understand Dave, thanks for the info!

Do you remember if the battery was bad of simply discharged?

Battery was bad. In retrospect, I should have known it was bad before I flew. When I got to the field I checked before I flew and it was reading low- I assumed that I must have made a mistake in charging it at home so i fast charged it and checked it again before I went up and it gave me good voltage so I flew.

In fact, the battery was in the process of failing and it continued to do so while flying.

Dave

I've had pattern planes saved twice by running a backup battery. One time after a flight the main battery read only 1.3 volts, the other time the primary battery dislodged and completely disconnected.

I believe the information above about not needing a diode, however my setup is a bit different.

I use two battery packs run through two separate regulators each going independently into the RX. To save weight I have a primary and secondary battery. The secondary is as small and light as I can find and uses a 5.4 or 5.1 volt regulator. The primary battery is on a 5.8-volt regulator. What this does is cause all juice to be pulled from the primary battery UNLESS the primary battery drops below the voltage of the secondary regulator. This means that the backup battery never comes into play unless needed and therefore only has to be big enough to last one flight (this assumes you check the primary prior to each flight).

I previously use a 5-cell AAA 750 mah battery as my backup (to keep the weight down), but now that I've gone lithium here's my setup:

Primary: Lithium Ion 2000 mah (3.4 oz) on a 5.8-volt regulator/switch (1.? oz).
Backup: Lithium Poly 900 mah (0.4 oz) on a 5.1-volt regulator (1 oz.)

I've probably put 50 flights on the plane since last charging the backup battery and it's still at max charge. That's because no current is every pulled from the backup because the primary lithium Ion never drops that low. However, if my primary every goes out (again) I can fly for about an hour and a half on the backup! To me that's well worth the 1.4 oz penalty!!!

Another nice thing about this setup is that the regulators (Oddino's) won't allow ANY reverse flow in case one pack goes out. I've discussed this with Jim Oddino and tested it myself.

I've been using this method for 3 1/2 years.

Keith B

Keyth,
very nice idea running a lipoly as backup!



Thanks everybody for sharing your experiences, they are food for thought!

It seems to me that the 2 batteries plugged into 2 rx ports should be fine even if a cell shorts. If we say you are using 5 cell packs, then a 5 cell pack with a shorted cell will appear as a 4 cell pack (or just a discharged 5 cell pack). Then from the info above, we can see that even if operated for 24 hours, this would have no bad effects.

Therefore in the event of:

A) a cell going open circuit, the second battery would be 'disconnected'

and

B) the cell going short circuit, the second battery just appears to have a very low voltage, and the situation discussed above applies (ie. a very small amount of cross charging)

So I can't see any problems. I've used this setup before on large models, but haven't actually had a pack fail.

Thanks Peter,
I'll use 4 cells though. The same reasoning should apply I think?

Yes, the same reasoning applies with 4 cell packs. Works great, been doing the parallel pack setup with nothing else involved for years now. Other setups described work and have benefits too, but this is the simplest with the least stuff to fail.

Glad to hear that

Thanx NJ!

An interesting update.

I've spoken with Red Scholefield, the author of the rcbatteryclinic site. I've asked him the famous question about shorts and the dual batt & switch setup...

He told me that the setup is designed to work against OPEN circuits, i.e. a failure of switch & harness between the batteries and the receiver. It is not designed to work with shorts...

But here is the good news: he says that shorts RARELY occur in a charged cell, simply because a charged will "zap" itself, blowing away any short. That is the reason why he says that shorts can be detected before flying with a volt meter. The worst case is that a cell with a developing short will exhibit a greater self discharge, but that again can be detected with a meter, before every flight. Summarizing, he told me that we should not worry about shorts.

For more info, read the short section on his site, it is very informative.