Receiver switches.

[Nigel R]

Far more problematic than the above scenario, is that cells can fail to a dead short. Two cells in a vanilla parallel arrangement have no protection against that.

 

[Martin Harris - Moderator]

This is where telemetry is so useful.  One cell shorted in a 4S 2P arrangement should allow control but report a low voltage with a sensibly set alarm threshold - worst case scenario is a 2S 2P LiFe but you’re starting from a higher voltage so still within limits for most receivers/servos. I have had this happen on a single 4S 700mAh nicd pack so I speak from actual experience. 
 

If you’re talking about a wiring short after the pack, it’s (a) unlikely and (b) probably going to melt any standard receiver pack wire very quickly, acting as a fuse!  If it happens on a single pack set-up, it’s disastrous anyway!

[FlyinFlynn]

The 125V as stated in the specs is most probably there for our American cousins. It makes no difference to the switch contact if it is 125v or 5v, it is the current that strikes the arc.

 Whilst it is true there is no, or very little, regulation on the speed of operation of a slide switch the switch does at least wipe the contact during operation, something a lever switch does not do. 

It would be quite easy for an instantaneous current in excess of 1 amp to flow if say, something like a retract servo was stalled due to a bent leg door fouling the wing.

 

Once the arc has been struck you can kiss goodbye to your silver coating on the contact and once that happens all bets are off.

 

'A bit down the priority list' tends to get forgotten though.

 

 

[RottenRow]

45 minutes ago, FlyinFlynn said:

 

The 125V as stated in the specs is most probably there for our American cousins.

 

Indeed. The data sheet states that the ratings are to UL and CSA, which are the US and Canadian versions of British Standards if you like.

 

47 minutes ago, FlyinFlynn said:

It makes no difference to the switch contact if it is 125v or 5v, it is the current that strikes the arc.

 

It is, but it’s the voltage and the contact gap that allows that arc to be maintained once struck, for a given level of circuit inductance.

 

50 minutes ago, FlyinFlynn said:

Whilst it is true there is no, or very little, regulation on the speed of operation of a slide switch the switch does at least wipe the contact during operation

Exactly. This action is intended to keep the contact surfaces clear of contaminants, and maintain as low a contact resistance as possible. Low resistance equates to low contact volt drop and low contact heat generation. The latter isn’t significant in our low energy application, but the low volt drop certainly is.

 

54 minutes ago, FlyinFlynn said:

It would be quite easy for an instantaneous current in excess of 1 amp to flow if say, something like a retract servo was stalled due to a bent leg door fouling the wing.

 

Once the arc has been struck you can kiss goodbye to your silver coating on the contact and once that happens all bets are off.

There wouldn’t be any maintained arc, even with a (say) 1A current being broken at the 6V or so that we are using, even if you were able to open the switch contacts by the tiniest amount and hold them there. The minimum voltage required to strike an arc is around 10V, which is above our system voltage. This information can be found on the internet if a fuller explanation is required.

 

I would have no issue with using a quality slide switch such as the RS example given, or the original genuine radio manufacturers’ offerings, but wouldn’t touch the el-cheapo ones that are around nowadays.

 

Brian.

[Cuban8]

I think we've gone down a bit of a blind alley with most of these (albeit interesting) technical discussions. My original gripe was with the poor  manufacturing of so called 'compatible type' switches, which IMHO are of a doubtful quality from 'seems to work OK' to a potentially dangerous item as I found with the switch that I bought and posted the video on here. They do sell hundreds of them and I guess most people get away with it - but I suspect that many do not, and experience unexplained crashes due to the variability of the product's quality.

The genuine switches that were produced by the radio manufacturers were designed specifically for use with their modelling radio products and as with Japanese made radios back in the day were of a similar high quality - for which one did pay a premium but did get the reliabilty and manufacturing integrity.

I've just bought a genuine Futaba electronic switch (all packaged, labelled, and moulded with the Futaba logo) that wasn't expensive at around £15 and trust me when I say that the quality and mode of its action is far and away better and confidence building than any copy item I've encountered. I suspect that it's new old stock as the instructions are dated 2015 so I don't know if Futaba still make them. It's also marked made it Taiwan rather than China, to where most consumer manufacturing has migrated now.

If anyone is using one of the cheapo switches of any type I urge them to inspect the item closely, wiggle the actuator tab and check for intermittent contact - similarly, ensure that the 'on point' doesn't just occur right at the end of the switch travel and could potentially be disturbed by vibration. If you have the equipment and knowledge, check the voltage drop across the closed contacts - which should be minimal - you might be in for another surprise.

If someone finds a duff item and possibly saves a decent model or avoids something more serious then all this is worthwhile.

Really can't add any more.

Edited August 10 by Cuban8

[Simon Clark]

On 09/08/2024 at 11:26, Nigel R said:

Sure. Many assumptions; it is an abnormal scenario.

 

Nimh cell resistance is usually quoted as between 30 to 100mO - going to assume 30 for now.

 

Cell #1 is charged perfectly to peak. Voltage will be quite high. e.g. it will read (maybe) 1.8V. When it is disconnected it will almost immediately drop to around 1.5V

 

IF Cell #2 has been overcharged for whatever reason. If the peak during charge was (the same as cell #1) 1.8V and then charge continued until the cell was 1.6V.

(that would be quite the overcharge, but, chargers do go wrong) When Cell #2 is taken off charge it will drops a comparable amount to cell #1, say to 1.3V.

 

Now parallel them up.

 

Initial current, (assuming the usual simple internal model of a battery)

Vdrop = 0.2V, Rtot = 0.06Ohm ( 2 cells in series with 0.03Ohm each).

 

Gives I = 3A

 

The discharge curve at this current (1.5C ish) is sharp and of course, cell #1 terminal voltage will decay quickly. How quickly? Minutes, around 5 minutes or so.

 

Within cell #2, the terminal voltage will also fall, but slower.

 

The situation will self limit. There isn't enough energy in cell #1 to sustain the scenario.

 

Meantime, further overcharge + temp rise within cell #2.

 

Significant? Maybe. Depends.

 

 

Likely outcomes

 

1) some further damage to cell #2

2) your power source is now busy self discharging and doesn't have much spare capacity to run the RX + servos.

 

Hello Nigel, I think that we will just have to agree to disagree on this subject! The manufacturer recommended charge method is to terminate charging with a negative delta peak voltage of 5-10mV. Your scenario has a pack charged to a point where this level is 20 times the maximum recommended amount! I have no data or experience of anything even vaguely close to this and so I have no idea what would happen and certainly would not dispute that if this was done then some sort of problems would likely arise.

 

I would just summarise that there are some people who think that the simple two battery/two switch system is a bad idea and presumably do not use it for that reason and then there are others, like myself who think that it is a good idea and use it without issues. It is up to individuals to decide what they think and do.

 

I have attached to this post a publication from Panasonic their 'Ni-mh handbook' that has a quite a lot of useful information, things to avoid as well as typical charge and discharge curves for various cell types. It certainly only covers a small amount of scenarios but some might find it useful.

 

Simon

id_ni-mh_1104_e.pdf

[FlyinFlynn]

22 hours ago, RottenRow said:

The minimum voltage required to strike an arc is around 10V, which is above our system voltage

Whilst that may well be true we are not just dealing with the 'system voltage'. There is the inductive  element to consider when dealing with electric motors, take a look at a motor commutator running at 5v.....plenty sparky. A motor stalled mechanically is still subject to inductive influences when a circuit is both made and broken.

 

Anyhow.... I think everyone is in agreement - don't use the cheap Chinese switches

[Cuban8]

22 hours ago, FlyinFlynn said:

Whilst that may well be true we are not just dealing with the 'system voltage'. There is the inductive  element to consider when dealing with electric motors, take a look at a motor commutator running at 5v.....plenty sparky. A motor stalled mechanically is still subject to inductive influences when a circuit is both made and broken.

 

Anyhow.... I think everyone is in agreement - don't use the cheap Chinese switches

Yes, the trouble is getting that message out into the wider model flying community. 

I mentioned my findings to a clubmate who I noticed was using one of the horrible switches that I had trouble with and his reply was "never had any bother with it, but I'll see how it goes".

Fair enough.

[Nigel R]

On 09/08/2024 at 12:48, Martin Harris - Moderator said:

This is where telemetry is so useful.  One cell shorted in a 4S 2P arrangement should allow control but report a low voltage with a sensibly set alarm threshold - worst case scenario is a 2S 2P LiFe but you’re starting from a higher voltage so still within limits for most receivers/servos. I have had this happen on a single 4S 700mAh nicd pack so I speak from actual experience. 
 

If you’re talking about a wiring short after the pack, it’s (a) unlikely and (b) probably going to melt any standard receiver pack wire very quickly, acting as a fuse!  If it happens on a single pack set-up, it’s disastrous anyway!

 

Cells can fail shorted. Yes, wiring can also short. Failover device like the 'ideal diode' linked by Phil provides protection against both. Even if a first failover devices fails with a dead short, the second failover will prevent the first problem from affecting the second pack. If a first failover devices goes open circuit, the second is there for backup.

 

On 11/08/2024 at 08:35, Simon Clark said:

The manufacturer recommended charge method is to terminate charging with a negative delta peak voltage of 5-10mV. Your scenario has a pack charged to a point where this level is 20 times the maximum recommended amount! 

 

"chargers can fail" I think I said, or words to effect.

 

Your models, your choice, etc.