Receiver switches.

[Engine Doctor]

2 hours ago, Nigel R said:

Don't, unless you have a failover device for handling two batteries.

 

Parallel connection of batteries gives you (essentially) one single battery with a higher chance of some failure occuring.

 

 

 

Hi Nigel Two batteries and switches but with diodes to stop any discharge/backflow of current should one battery fail is a good and simple  back up providing model has A the room and B the capacity to carry the added weight . It costs pennies to modify the switch harnesses with schokty diodes.  I now  always fitted this to bigger expensive models ever since I had a cell go down in a new pack destroying an expensive model some years ago . Battery checked out OK for voltage but collapsed completely with any load applied .

Edited August 5 by Engine Doctor

[Andy Stephenson]

I agree on the diode approach because it may happen that a fully charged battery is switched into a fully discharged battery through the receiver bus. Without diodes, a high current would flow from the charged battery to the flat one which may damage the switch, the wiring or the connectors which are not rated for this level of current. There is a penalty in terms of voltage drop through a diode but the Schottky type has only a small drop of 0.1 to 0.3V . If this is a problem, then adding a cell to the pack would more than compensate.

[Cuban8]

52 minutes ago, Solly said:

And yet toggle switches have been used extensively in full size aviation for many years. But not slide switches.

I think you'll find that devices designed for use in full size aviation either civil or military (MIL) are a far cry from cheapo stuff that manufacturers get away with in consumer items.

[Cuban8]

30 minutes ago, Engine Doctor said:

Hi Nigel Two batteries and switches but with diodes to stop any discharge/backflow of current should one battery fail is a good and simple  back up providing model has A the room and B the capacity to carry the added weight . It costs pennies to modify the switch harnesses with schokty diodes.  I now  always fitted this to bigger expensive models ever since I had a cell go down in a new pack destroying an expensive model some years ago . Battery checked out OK for voltage but collapsed completely with any load applied .

Four cell or five cell pack ED?

[Martin Harris - Moderator]

I’ve long subscribed to the two battery two switch harness set up as the best redundancy solution for my larger models. In my view, simplicity is the key to reliability. 
 

Yes, it doubles the chance of one battery losing connection to the receiver but it drastically reduces the chances of both failing on a single flight.  I always do a pre-flight redundancy/individual voltage check and receiver voltage telemetry (on all my receivers) will indicate a cell failure on one pack during flight reducing the effective input voltage. 
 

Additional protection can be provided by using 5 cell Nixx packs, or my preference, 2S LiFe cells, with a silicon diode in series with one leg of each connection to the receiver, which protects against battery failure and reduces the voltage by a little over 1/2 a volt. 
 

I’m unaware of a “special” failure mode on NiMHs but the restriction on making parallel packs is due to the inadvisability of charging them in parallel. 

[Chris Walby]

11 minutes ago, Simon Clark said:

I disagree! I have been fitting two batteries with two switches for years now and there is absolutely no problem (providing that both batteries are nominally the same). Purely out of interest, I have tested using one fully charged battery and another half charged with no adverse effects, although I would definitely NOT do this in practice. I have also tested using a new battery in parallel with an old battery with a failed cell and definitely no problem here either. The vast majority of failure modes with batteries / switches are some kind of bad connection / high cell resistance. All of these failure modes are mitigated by parallel connection. 

 

Each to their own, but I have been doing this for years now on every model and have never had any problems at all. Before that, I lost one plane due to a bad switch (slide type before anyone asks) and since, I have seen a few planes crash due to bad switch / battery that I am certain would have been saved by using two switches / batteries!

 

Simon

I agree with Simon.

the point he makes is that the vast majority of failures are either open circuit of one cell or poor connection somewhere in circuit (switch or connectors).

 

In practical terms the combination of battery internal resistance and difference between partially charged batteries voltages + resistance of various wires means that there won't be a massive transfer of charge.

[Paul Marsh]

I use these on larger models,  though there is a single cell version as well which is a contactless switch and fails to on if it fails - only thing there is a small current draw so need to unplug the batteries if not using for a while.

[Solly]

1 hour ago, Cuban8 said:

I think you'll find that devices designed for use in full size aviation either civil or military (MIL) are a far cry from cheapo stuff that manufacturers get away with in consumer items.

Too true. However aerospace and military grade toggle switches are freely available from many sources, mostly in the USA, costing from about £25 upwards. I used one myself for some years on a scale model without problem. 

[Nigel R]

1 hour ago, Cuban8 said:

I think you'll find that devices designed for use in full size aviation either civil or military (MIL) are a far cry from cheapo stuff that manufacturers get away with in consumer items.

 

They are indeed - they also tend to be used in a way such that failure rates are analysed and appropriate mitigation for failures are put in place.

 

1 hour ago, Martin Harris - Moderator said:

I’m unaware of a “special” failure mode on NiMHs but the restriction on making parallel packs is due to the inadvisability of charging them in parallel. 

 

There is a positive feedback scenario - past peak voltages after charge - one pack can discharge into a second and then push the second further into the condition.

 

That said - join them together with a couple of diodes, perfectly viable and simple method of failsafe (caveat, diode is correctly rated).

 

 

 

[Martin Harris - Moderator]

I’d like to know a bit more on the scenario above - you would assume it would be a self regulating position if the higher one discharged into the lower - why does this not simply raise the lower pack’s voltage until they equalise?  Is there some complication relating to individual cells perhaps or a quirk in the chemistry of NiMHs?

[john stones 1 - Moderator]

I must just be a Lucky modeller, Always used the switches sold by JR, Futaba, et al and in a lot of years never had a failure, nor a battery in flight or a radio failure. I don't go for lights on my switches, gizmos to tell me this that or tother, just keep it simple and well looked after, kept in a dry storage room.

[Phil Green]

42 minutes ago, Nigel R said:

(caveat, diode is correctly rated).

Suggested 2nd caveat: diode is supported on a PCB or stripboard - simply soldering them inline under a tube of heatshrink leaves them vulnerable to physical failure - they're not intended to have their legs under any tension, or being subject to repeated bending...   🙂

 

If you're concerned about voltage drop, use two ideal diodes instead 🙂

 

Re paralleling, generally any chemistry that uses a cc/cv charge can be directly paralleled cell-to-cell, Lipo, LiFePo4, lead-acid, etc but NiMh, NiCd etc aren't cc/cv  🙂

 

 

 

Edited August 5 by Phil Green

[Martin Harris - Moderator]

I’ve come across faulty switches during maintenance checks and had a single cell die in flight - early days of telemetry so a beeping alerted me to a problem which became obvious after a rapid precautionary landing. 

[Martin Harris - Moderator]

2 minutes ago, Phil Green said:

Suggested 2nd caveat: diode is supported on a PCB or stripboard - simply soldering them inline under a tube of heatshrink leaves them vulnerable to physical failure - they're not intended to have their legs under any tension, or being subject to repeated bending...   🙂

Oh - not something I’ve considered but I solder mine into one of the wires between the switch and the receiver so I don’t think there’s any bending or tension involved. Much different if soldered to the switch directly though. 

[payneib]

I've got two twin switch/twin battery set ups, both using PowerBox switches:

 

One is the dual regulator/dual switch/dual output built in unit, the other is two separate single units, feeding a Rx with two dedicated inputs. In one set up, the switch/regulator does the work of which supply (and feed) to use, in the other the Rx is the intelligent part. 

 

Unfortunately it's difficult to fit either in to anything "Acrowotish" sized and smaller, where I'll run the eTronix push button switch off a separate regulator (fitting the regulator between the battery and the switch to allow as much physical distance between the regulator and Rx as possible for RF reasons).

[Cuban8]

1 hour ago, john stones 1 - Moderator said:

I must just be a Lucky modeller, Always used the switches sold by JR, Futaba, et al and in a lot of years never had a failure, nor a battery in flight or a radio failure. I don't go for lights on my switches, gizmos to tell me this that or tother, just keep it simple and well looked after, kept in a dry storage room.

I don't think the genuine items manufactured by the likes of Futaba and JR etc ever gave bother - certainly never had any cause for concern with their products over many years. The worry is the explosion of cheap 'compatible' items that have flooded the market in recent years. Some are OK but many are of very inferior quality that many modellers just won't notice. For some, a switch is a switch so how different can one be from another? Well, the answer is a hell of a lot of difference as I found with the 'heavy duty switch' that I obtained which can only be described IMHO as not fit for purpose and potentially dangerous.

Edited August 5 by Cuban8

[john stones 1 - Moderator]

20 minutes ago, Cuban8 said:

I don't think the genuine items manufactured by the likes of Futaba and JR etc ever gave bother - certainly never had any cause for concern with their products over many years. The worry is the explosion of cheap 'compatible' items that have flooded the market in recent years. Some are OK but many are of very inferior quality that many modellers just won't notice. For some, a switch is a switch so how different can one be from another? Well, the answer is a hell of a lot of difference as I found with the 'heavy duty switch' that I obtained which can only be described IMHO as not fit for purpose and potentially dangerous.

 

Yes C8, just read your first post, switches are harder to come by now, agree with you, I am lucky enough to have bought a few spares when times were simpler.

Toggle switches ? Some very well respected names on some supplied by quite a few respected outlets, plenty dual ones aimed at petrol flyers, not vibration free models they're used in. Lecky threads tend to get techy very quickly with what's best.

[Nigel R]

2 hours ago, Phil Green said:

use two ideal diodes instead

 

Or use an ideal voltage source, no need for two packs that way 😄

[Nigel R]

 

2 hours ago, Martin Harris - Moderator said:

I’d like to know a bit more on the scenario above - you would assume it would be a self regulating position if the higher one discharged into the lower - why does this not simply raise the lower pack’s voltage until they equalise?  Is there some complication relating to individual cells perhaps or a quirk in the chemistry of NiMHs?

 

When you overcharge a nicad or nimh the voltage actually starts going down (think "peak detect")

Overcharge further - even more voltage drop,

Bigger voltage drop - more current will be pushed in.

Rinse, repeat.

Will that last forever? Probably not, ultimately pressure and temperature would cause a conclusion.

I don't imagine a 4 cell AA pack contains enough energy to be able to make pyrotechnics occur on a second pack, but it certainly would not do the second pack any good.

 

Lithiums as you state, will just equalise.

[RottenRow]

I’m not sure anyone was suggesting charging NiMH packs in parallel, just using them with two switches to both share the load and provide a degree of redundancy.

 

With two switches, the positive terminals of each pack will be isolated from each other when the switches are in their off positions, this position also connects the third (charging) connection into the circuit, allowing one pack to be charged at a time (or both together, with two chargers, as long as they don’t mind having their negative outputs connected together).

 

Brian.

[Simon Clark]

Yes, a Nimh cell voltage when on charge will drop once full charge is reached but this is only a few millivolts (less on a Nimh cell compared to a NiCd) and will still be higher than a cell that is not on charge due mainly to the effect of internal resistances. If you take one pack that is at the absolute peak voltage and connect to a second that has just been slightly overcharged then some current will indeed flow from the first cell to the slightly overcharged one. However, by nature of the internal resistances of both packs, the voltage on the first pack will immediately fall and the voltage on the second pack will immediately rise and so the current will be reduced significantly. Even if this small current did cause the second battery pack open circuit voltage to fall a bit by nature of a small overcharge, the first battery voltage will also fall as a result of it becoming discharged and this effect will be much greater. So, yes, there may be a small amount of overcharging happening initially and you could consider this to be a temporary 'positive feedback' situation, there will be a very rapid roll-off and the first pack voltage will reduce to a point where no current flows and it is essentially self regulating ('negative feedback', if you prefer). I would argue that 'positive feedback' only occurs when the rate of change of the second battery pack voltage is greater than the rate of change of the first battery pack voltage and I don't believe that this will ever be the case (just look at the typical discharge curve of a Nimh cell).

 

Diodes (Shottky types) are a good addition that will allow totally mismatched batteries to be used together but, in my opinion, not required if you have matched batteries packs.

 

Simon

 

 

[Martin Harris - Moderator]

That sounds far more logical than the disastrous runaway situation suggested.  My preference for silicon diodes is based on their forward voltage drop which I feel can be kinder for less voltage tolerant servos.  Silicon diodes are noted as reliable components - I don’t know whether Schottky diodes have a similar reputation?

[Engine Doctor]

8 hours ago, Cuban8 said:

Four cell or five cell pack ED?

With a diode I use a 5 cell pack as diode does drop voltage a little 

[Andy Stephenson]

3 hours ago, Martin Harris - Moderator said:

That sounds far more logical than the disastrous runaway situation suggested.  My preference for silicon diodes is based on their forward voltage drop which I feel can be kinder for less voltage tolerant servos.  Silicon diodes are noted as reliable components - I don’t know whether Schottky diodes have a similar reputation?

There's nothing wrong with using Schottky type diodes they are as reliable as any other simple semiconductor and they don't dissipate as much power in the form of heat.

[Don Fry]

4 hours ago, john stones 1 - Moderator said:

Lecky threads tend to get techy very quickly with what's best.

No argument there. All we are trying to do is deliver a few volts, via reliable plugs, switches. I never quite understand why its complicated.

Cheap old tech cells, dump. Old servos  wot wont support modern cells ditto. Cheap switch, ditto.

If they won’t last forever, reckon a change cycle ( yer, I hear, new stuff fails in the start of life).

Hang all fakers by their products, hanging used to be nasty, the drop to break necks was kind.  The original used to be leisurely, a slow lift off the ground. Again, simple, no tech, delivers the desired result.