Dual RX battery system

[NightSwan]

Does anyone out there have a link/schematic of a dual RX battery system? (e.g. the back-up battery is switched in in case of failure of the primary RX battery)
I know, two diodes would work, but I dont like the voltage-drop and power dissipation.

Hans

[GordonTarling]

Hans. you don't actually NEED diodes or any other device between batteries and receiver. Simply plug both batteries into receiver and it'll work fine and be just as reliable - see Red Scholefield's treatise on the subject here - http://www.rcbatteryclinic.com/parallel.html

[NightSwan]

Gordon, I heard about that and (mostly) agree.
My only points of concern :

- if one battery mysteryously fails and decides to become a short (or low impedance, low voltage source) it will drain the other battery. IMHO this is just the same as one big battery pack with high capacity

- I would like the second pack only as a backup, preferably with a light that lights when the primary battery is drained, so I know when to recharge my pack.

I'm pretty good at designing my own circuits, but mainly am wondering how the electronics decides to switch packs, and how it is avoided that the electronics keep switching between packs (if one pack is switched off, the load on it drops, its voltage increases and may rise above the voltage of the other pack, triggering another switch etc..)

I could ofcourse solve this problem with a microcontroller, some sort of latch/flipflop that only switches one way etc..
Also, I'm wondering how to switch without momentarily loss of power (relays are not a real option) and without connecting the packs in parallel momentarily. Big cap maybe?

Only a diode in the backup pack maybe? And detect if current is flowing there, triggering a light on?

Got to think a bit more about it.. Hints still welcome :D

Hans

[Gary Warner]

I'm guilty of using PIC processors to solve most of my electronic problems. It's something like cutting butter with a chain saw. So, that having been said...

I'd take a PIC16C17 (has A/D inputs - others PIC's are available too with A/D inputs) and send each batteries voltage to an A/D input. Then I'd have two output ports drive two MOSFET's to switch (control) which battery was in use. I'd have a setup button on one port and one or two LED's using other ports. When the system turned on, battery #1 would be selected "on" (if it's voltage was over 5 volts). When the voltage falls below 5 volts, the second pack would be switched "on", while the first would be switched "off". This would happen very fast and most likely not cause a receiving problem. It also removes the possibility of one shorted pack killing the other. When you land, you can note the LED lights and tell if the batteries were switched in flight. I'd have the light flashing if the second pack ever fell below 5 volts. Switching back to a known dead battery wouldn't do much good, so the flashing light would be enough to tell me IT'S TIME TO CHARGE! Also, with the setup switch, I could select which battery to start off with (not just battery #1).

Like I said, cutting butter with a chain saw :D .

[NightSwan]

Gary, that is an option. I'm still thinking opamps or something, but
the PIC option also has some advantages.. I'm thinking 16F676 (flash, A/D) or 12F675 (flash, 8-pin, A/D)

Lots of new goodies from microchip! :D

BTW, if you ever need a (Cheap, good) C-compiler :
http://www.picant.com
I'm very pleased, especially with the price!! (under $50,- IIRC)

Hans

[GordonTarling]

Hans - suggest you re-read Red's treatise - Nicads won't normally fail short-circuit whilst in use, only when not being used.

If you really want a GOOD backup device, I suggest you take a look at the offerings from Modellbau Deutsch - I am installing their Powerbox 40 in my 1/4 scale Tiger Moth, NOT because I don't believe what Red Scholfield writes, but because I want to use a battery and a Universal BEC unit together. Their 'Powerbox 12' should be adequate for most electric models and is only EUR 50.

Sorry, but this doesn't much help you toward making your own unit!

Sorry - here's the link - http://www.modellbau-deutsch.com/

[AndyOne]

Connecting two batteries in parallel is not a route to creating a back-up system all it gives you is more capacity when they are both working correctly. If one battery fails into short circuit (agreed fairly rare) it will pull the one that is in parallel with it down as well.
Diodes are a good way to go as it is simple and therefore reliable (the prime purpose of the exercise) but it may be worth using one more cell per pack to overcome the 0.6V drop you get with standard silicon diodes. Schottky diodes have considerably lower drop, as low as 0.28V so you might use these without extra cells.
Both packs will, of course, discharge unless one pack maintains a voltage, which is always higher than the other one.

I personally wouldn't trust a valuable model to a software-controlled device, at lease one I programmed myself.

Andy.

[NightSwan]

Well, sometimes I trust my own software more than bought software ;)

Been talking with some collegues here at Philips, and we have found a nice solution with only two MOSfets anda nand-flipflop.
Dunnow if I'll build it yet ;)
Gordon, thanks for the link!

Hans

[GordonTarling]

Andy - Whilst Nicad and NiMH cells CAN fail short-circuit, this is only likely to happen when the pack is not being used. Now think about what happens if a single CELL in a pack were to short out internally - OK, that cell will get hot, BUT the voltage of that whole pack will only fall by the voltage of ONE cell and you'll still have 3.6V to power the system. Another pack which is effectively in parallel with it will only get 'dragged down' to 3.6V and no lower. Most systems will continue to operate quite well, if a little more slowly, on 3.6 Volts. In all my years (I won't tell you HOW many!) of modelling and specialising in electric flight, I have never had one cell fail due to an internal short.

The whole point of Red's treatise is that the MOST likely mode of failure in the system is NOT that of a cell shorting internally. Switches and connectors/connections are far more likely to fail in service, particularly if subject to vibration and his system goes a long way toward eliminating that type of failure.

Hans - only two Mosfets and a flip-flop? Sounds like a nice way to do it, please let us know if it works OK.

[NightSwan]

If I ever get to building it, I'll let you guys know :D

Hans

[Ernie]

Hans - suggest you re-read Red's treatise - Nicads won't normally fail short-circuit whilst in use, only when not being used.

If you really want a GOOD backup device, I suggest you take a look at the offerings from Modellbau Deutsch - I am installing their Powerbox 40 in my 1/4 scale Tiger Moth, NOT because I don't believe what Red Scholfield writes, but because I want to use a battery and a Universal BEC unit together. Their 'Powerbox 12' should be adequate for most electric models and is only EUR 50.

Sorry, but this doesn't much help you toward making your own unit!

Sorry - here's the link - http://www.modellbau-deutsch.com/

Gordon, does this work? I want to do the same (battery and ubec) but the manual says only to use Nicd or nimh

[Thomas B]

Dual Rx batteries simply plugged into the RX have been happily and successfully powering MANY large scale and jet models for many years. Good battery hygiene is always prudent. of course, even if you are using two of them.

Doing this, without any extra electronics, makes your model vanishingly unlikely to be lost due to on board battery issues, so you will then have to proceed to worry about getting shot down by another TX, other radio interference, radio/servo/connector/switch failure, midair collisions, pilot error, random chance and bad luck...:)

[GordonTarling]

Ernie - sorry, I missed your recent post! Yes, it works fine and has been doing so for three flying seasons now. Just ensure that you use the 5V UBEC and a 4 cell receiver pack or the 6V UBEC and a 5 cell receiver pack (with correct Powerbox, of course!)

[Ernie]

Thanks Gordon,
I'll order one from your site
one more question, since there is no gasservo, did you connect one other servo directly to the receiver as mentioned in the manual ?

[EloyM]

Simple battery backer: www.uoguelph.ca/-antooon/gadgets/backup.htm
Regardless of what the rich and famous in RC claim re parallel batteries, it is in conflict with what the real experts at Sanyo, Panasonic, etc say on the subject. (Copies available!) They all strongly recommend against it, due to the discharge from the good to the bad mentioned, going to far as to say that the heavy current can cause wires melting and fires. They also state that the greatest failure mode is shorted cells.
Why are so many parallel installations successful? I think it is because the batteries themselves, if they are of high quality to begin with, seldom fail shorted or otherwise which does not prove the value of that thinking.
For buyers, a variety of battery backers is available from Cermark at acceptable prices.