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December 01, 2020, 11:53:50 am

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Two Batteries into one rx.

Started by FlyinBrian, June 29, 2020, 13:13:26 pm

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FlyinBrian

To help with the CG on my Giant Aeromaster I need to use two 1500 mAh NiMH 5 cell batteries and am planning to use two switches into one Futaba Rx. I don't really want to mess about with diodes and from what I read many years ago there is no need.

The model has 5 x Hitec 625 heavy duty servos and a Futaba 3010 on throttle.
I also do not want to use Lipo batteries and regulators or overpriced power management boxes if possible.
Just two identical batteries, two switches and two leads into the Rx.

I am using a 2cell 2200 Lipo for the ignition on the MVVS 40cc petrol but only because I have one.

What does the team think - Where are the gotchas.
Basic Research is what I do - when I don't know what I'm doing!.

PDR

Is there a reason why you want to fit two 1500mAh batteries rather than one 3000mAh one?

What type of batteries are you using? (hopefully not AA size!)

As a general rule it's a bad idea to connect batteries in parallel without isolaion/management because any single cell failure will drag both packs down, and could draw enough current from the "good" pack to start a fire. So if it's capacity (or current) you're after it's better to have a bigger single battery. If you want increased reliability from having two batteries you must have either passive (diode) or active (power management unit) isolation - as a failure in either battery will drag the other one down as well. So without the isolation fitting two batteries makes it HALF as reliable, not twice.

If you don't want to fit an overpriced management unit you could always fit one that isn't overpriced.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

itsme

According to most of the LMA friends I have, the 'dragging a battery down' thing is a fallacy. It is fine to use two batteries without any diodes or nonsense. I prefer to use sub C cells, but often room dictates the use of AA cells, of which I have never had a failure. If the batteries you normally use have been reliable, then two batteries seems overkill.
I am a great advocate of two switches wired to the battery and Rx to give redundancy, as the switch is the weakest link, and failures are common, but a 5 cell NMH already has a bit of redundancy in that, if one cell goes down, the battery will function as a four cell adequately. Its your choice however, the ultimate being two switches, two batteries and two receivers. The dual switch thing will cost a bit of soldering, and a normal switch. I do that on all my IC models, even the Acrowot. Cheap for peace of mind.

FlyinBrian

Peter

Two 1500 pack will fit where needed but a 3000 will not.
Cells are actually 2/3 AA but I have tested the packs separately at 3A discharge and they stay above 5V until well over 1000Mah has been used so even with heavy use I should get a few flights out of them.

The two switches is just a convenience rather than make up a wiring loom.

Re Diodes and s/c - I can not remember ever having a NiMH (or NiCad ) cell go s/c and am not aware of anyone who has?????

I recall ( I think ) someone called "Red" who used to work for a cell manufacturer used to write about batteries in an online forum c1998 - 2000 and he reckoned s/c was a rare form of failure - occasionally dendrites? may cause a short inside a cell but they were very tiny and the current flow from regular use cleared them. They could build up to the point of damaging the cell if the pack was idle for a long time.

Basic Research is what I do - when I don't know what I'm doing!.

FlyinBrian

Quote from: itsme on June 29, 2020, 15:13:12 pmAccording to most of the LMA friends I have, the 'dragging a battery down' thing is a fallacy. It is fine to use two batteries without any diodes or nonsense. I prefer to use sub C cells, but often room dictates the use of AA cells, of which I have never had a failure. If the batteries you normally use have been reliable, then two batteries seems overkill.
I am a great advocate of two switches wired to the battery and Rx to give redundancy, as the switch is the weakest link, and failures are common, but a 5 cell NMH already has a bit of redundancy in that, if one cell goes down, the battery will function as a four cell adequately. Its your choice however, the ultimate being two switches, two batteries and two receivers. The dual switch thing will cost a bit of soldering, and a normal switch. I do that on all my IC models, even the Acrowot. Cheap for peace of mind.

Itsme, More or less my own thoughts.
Basic Research is what I do - when I don't know what I'm doing!.

JimG

Quote from: FlyinBrian on June 29, 2020, 18:06:23 pmI recall ( I think ) someone called "Red" who used to work for a cell manufacturer used to write about batteries in an online forum c1998 - 2000 and he reckoned s/c was a rare form of failure - occasionally dendrites? may cause a short inside a cell but they were very tiny and the current flow from regular use cleared them. They could build up to the point of damaging the cell if the pack was idle for a long time.


This was Red Scholefield on RC Universe and was a Manager for General Electric and finally Energiser. His information is still available as Red's Battery Clinic. http://www.hangtimes.com/redsbatteryclinic.html

Jim
Jim Gill
Dundee Model Aircraft Club

FrankS

More expensive than 2 regular mechanical switches, but cheaper than the power management boxes are the electronic switches which have dual battery input, i.e. https://www.esoaringgadgets.co.uk/index.php?main_page=product_info&cPath=46_108&products_id=108

FlyinBrian

Quote from: JimG on June 29, 2020, 20:56:27 pmThis was Red Scholefield on RC Universe and was a Manager for General Electric and finally Energiser. His information is still available as Red's Battery Clinic. http://www.hangtimes.com/redsbatteryclinic.html

Jim

Thats the fella, I thought the name was Schofield - wrong spelling so Google did not help.
Very interesting reading.

Thanks Jim
Basic Research is what I do - when I don't know what I'm doing!.

FlyinBrian

Quote from: FrankS on June 30, 2020, 08:07:11 amMore expensive than 2 regular mechanical switches, but cheaper than the power management boxes are the electronic switches which have dual battery input, i.e. https://www.esoaringgadgets.co.uk/index.php?main_page=product_info&cPath=46_108&products_id=108

I guess that is the Jeti version of Powerbox Sensor switch?
Basic Research is what I do - when I don't know what I'm doing!.

FrankS

Quote from: FlyinBrian on June 30, 2020, 09:41:33 amI guess that is the Jeti version of Powerbox Sensor switch?

Or the Multiplex one, they all work in a similar fashion, in that the switching is electronic.

paulinfrance

I keep mine simple, a 2400 mah tamiya pack, minus one element two plugs one for the radio and another for the ignition stop switch, Futaba radio and NO glitches !. :af
Mode 2 THE only way to fly

FlyinBrian

Quote from: FlyinBrian on June 29, 2020, 18:06:23 pmPeter

Two 1500 pack will fit where needed but a 3000 will not.
Cells are actually 2/3 AA ACTUALLY 2/3 A, not AA, but I have tested the packs separately at 3A discharge and they stay above 5V until well over 1000Mah has been used so even with heavy use I should get a few flights out of them.

The two switches is just a convenience rather than make up a wiring loom.

Re Diodes and s/c - I can not remember ever having a NiMH (or NiCad ) cell go s/c and am not aware of anyone who has?????

I recall ( I think ) someone called "Red" who used to work for a cell manufacturer used to write about batteries in an online forum c1998 - 2000 and he reckoned s/c was a rare form of failure - occasionally dendrites? may cause a short inside a cell but they were very tiny and the current flow from regular use cleared them. They could build up to the point of damaging the cell if the pack was idle for a long time.


Basic Research is what I do - when I don't know what I'm doing!.

itsme

I have had an individual cell go down in a pack, hence my preference for 6 volt 5 cells. Its a bit more redundancy. I am currently (!) running two of those 2/3 packs (could not buy a reasonably priced full pack) side by side in the Acrowot. Two batteries, two switches, no drama.

SMF

Likewise, I have done it for years and never had a problem. Don't forget that what you are doing is building a redundancy system which is supposed to get you to the end of the flight if something goes wrong. A quick check of each pack will show up any issues prior to the next flight if you really want to be safe.

FlyinBrian

Basic Research is what I do - when I don't know what I'm doing!.