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Old 03-10-2018, 05:20 PM
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Question
Back up battery

I want to use LiFe batteries in my aircraft, a 3000 Mah and a back up 1100 Mah .
My feeling is to separate the two and have the smaller battery isolated until needed. Two switches?
I have looked at several websites but cant seem to locate exactly what I'm looking for. Booma, MPI, Scorpion,etc.
Separate isolated batteries of different capacity but otherwise identical.
Also would like an LED to show it's on.
Anyone have a suggestion?
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Old 03-10-2018, 06:33 PM
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http://www.chiefaircraft.com/smt-batshare.html
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Old 03-10-2018, 07:50 PM
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Hi battery backups are switching devices that have an internal on off. the back up "turns on" if the primary input voltage falls below a specific point. what that also means there is no easy way to check the backup pack. as a result, one doesn't seem them being used too often.

a very common set up with two packs into a standard receiver is not to use a battery backup but run two packs of the same chemistry and capacity, and each pack has its own switch. Very easy to check both batteries before each flight and the receiver will draw from both packs.
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Old 03-11-2018, 06:26 AM
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Thanks guys. I looked at the Batshare.
Apparently with the Batshare, I can use

Two control switches (Booma with LED's)
My two LiFe batteries (3000 and 1100Mah)
The outputs can go to separate ports or be y together if port space is a problem
That would give access to both batteries via the charge ports on the switches for voltage checks and charging.
If you guys think my logic is flawed please let me know what you think.
Thanks again.
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Old 03-11-2018, 07:30 AM
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I think you're making it more complicated than you need to.
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Old 03-11-2018, 07:35 AM
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I think you're making it more complicated than you need to.
^^^^^^^
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Old 03-11-2018, 04:51 PM
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Yeah....2 alike bats.. 2 switches....done
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Old 03-11-2018, 08:00 PM
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Quote:
Originally Posted by FGNewbie View Post
Hi battery backups are switching devices that have an internal on off. the back up "turns on" if the primary input voltage falls below a specific point. what that also means there is no easy way to check the backup pack. as a result, one doesn't seem them being used too often.

a very common set up with two packs into a standard receiver is not to use a battery backup but run two packs of the same chemistry and capacity, and each pack has its own switch. Very easy to check both batteries before each flight and the receiver will draw from both packs.


By far the simplest and easiest to do.With some peace of mind.
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Old 03-11-2018, 09:08 PM
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Originally Posted by ponyboy082662 View Post
Yeah....2 alike bats.. 2 switches....done
+1^^^^^^^
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Old 03-13-2018, 04:24 PM
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I wrote to Booma RC asking about dual batteries, dual switches and isolators and received an informative reply.

I believe my initial fear was correct about needing /using something to isolate the batteries.
If I hear anything else I will post it.

These are questions and responses .

Let's say you have a simple redundant LiFe battery system in your aircraft (2x2500 Mah batteries and 2 std on/off switches ) Both plugged into individual ports on the receiver. Is it possible for one of those batteries to fail in such a manner that it, for lack of a better term "drags down" the good battery rendering the system voltage too low for good receiver/servo function? If yes, what type of failure? Also , if yes , that's the reason for an isolator!!?
Jim, batteries generally fail open circuit or one cell in the pack fails. In the case of a failed cell and depending on the configuration of the pack i.e. the series parallel arrangement of the cells the voltage of the pack with the bad cell may drop. Without battery isolation the good pack will become a crutch and try to provide voltage to the failed pack. This can cause a low voltage situation at the output of both packs and is the very reason why we run with an isolation circuit.

Second question, it's our assumption that all the power pins on the receiver are bussed together. That's why you can plug the battery into any port and the system will be powered up? I have read to "Y" in an additional (backup) battery into any servo lead and you will be ok but not to "Y" the batteries together.
Is this thinking incorrect? Does Ying the batteries together cause issues?

Y leading 2 batteries of the same type and capacity together creates a parallel cell arrangement and both batteries acts as one battery with the capacity of the two batteries added together. The down side of “Y” leading is that you are pulling current through one connector and JR/Futaba type connectors are only good for around 3amps continuous. Now this may not be a problem in smaller models but is definitely a potential issue in larger ones. Connecting two batteries to 2 input ports of the receiver will provide a parallel connection increasing the capacity and provide better current delivery but both batteries still act as if they were one large battery without backup redundancy.
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Old 03-13-2018, 04:32 PM
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I hate to argue with BoomaRC since I use and like their switches, but it has been proven time and again that if one pack fails in a dual parallel set up the "good" pack has only negligible transfer to the "bad" pack. There is simply no need to have any sort of "isolation" between the two packs.

Want to prove it to yourself? Take a fully charged and a fully discharged pack and connect them to the same RX. Go away for a couple of hours. You will see that there is virtually no "charging" of the drained pack by the full pack.

All that happens if one pack fails in the air is that the whole load of the plane is carried by the good pack, thus it will drain faster. But this is not because it is transferring anything to the dead pack.
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Old 03-13-2018, 05:17 PM
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Believe what you want (i was an electronics tech for 20 years on planes a lot more complex than ours)

Batteries don't try to charge each other. That story is bandied about by people that sell isolation switches
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Old 03-13-2018, 06:04 PM
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Isolation devices are what I call either a marketing solution or a very eloquent solution to a non-existing problem.

I think this "trying to charge" idea comes from seeing a greater drain on the remaining battery, which makes sense since it is now carrying the entire load of the plane And again, for anyone that does not want to believe me (or Andy) simply do the bench experiment I detailed above and prove it to your self.
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Old 03-14-2018, 07:21 AM
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The equalizing theory is a great idea but batteries aren't scuba tanks.
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Old 03-17-2018, 08:49 AM
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Another member sent me this link. I believe it to be the clearest and most comprehensive of all.
http://hangtimes.com/parallel_packs.html

According to the battery clinic it seems that both sides are correct. It all boils down to odds. A battery failure that would affect the system is very rare but it is possible. An isolator probably is more likely to fail that that rare "system affecting" battery failure.
Looks to me now that installing two batteries and two switches is the way to go.
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