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"the maximum battery voltage of the ESC?"

yes that one

"Surely then these voltage spikes will also destroy any extra capacitors you put in circuit, which being electrolytic will have a low voltage."

its the current that causes the voltage, if you buffer the current with a capacitor you don't get the voltage, imagine filling a tiny pond with a small hose (level rising rapidly) vs filling a lake with the same hose (level rises not at all)

"Finally, the lithium battery itself acts as a capacitor surely?"

yes - in precisely the wrong place to be helpful.

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Excuse my ignorance - but if you are going to fit a safety plug in the battery lead to the ESC, shouldn't you fit it to the negative wire? I was always told (and taught) to connect the positive lead first when installing a car battery to avoid arcing and possible electrical shorts - won't the same principle (for the prevention of arcing) work for our purposes?

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Sorry but your argument makes little sense.

I'll stick with the old wives tale theory. It may have been accurate years ago, but if it is still true then why is there nothing in the instructions of any of my ESCs about the issue? Its is also interesting that all references to the problem seem to go back to the guy Geoff included the reference to 10 years ago.

Also can you explain why I've never had an ESC failure in 10 years? Most of my ESCs are Hobbywing ESCs of one sort or another and all have survived long leads over an extended period of time.

One ESC manufacturer, I seem to remember, did comment on long battery leads - Castle Creations. However if you follow some of the links from Geoff's reference you will find the following comment by them:

12" between where the wires solder on the ESC, and where they solder on the battery tabs on the pack. Anything over that and you want to add capacitors. Rule of thumb is for every ~4" over that you should add one extra cap that's the same voltage and capacitance rating that is on the controller.

I got this great and specific Info from Joe @ Castle Creations Tech Support

Thanks Joe

**LINK**

Now we have a definitive measurement, which makes a whole world of difference. Clearly you can have up to 12 inches from the battery to the ESC, not as has been commented ANY extension! (Indeed the poster in Geoff's reference gets this wrong.) It is perhaps interesting to note that there is nothing in the current Castle Creations ESC instructions about long battery leads.

 

Edited By Andy48 on 10/04/2018 16:21:32

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"Sorry but your argument makes little sense"

Whose argument?

If you don't want to fit caps to the ESC if you have long battery leads, by all means don't. The world won't stop. If the input caps on your ESC are a bit borderline in size, you may experience a shorter lifespan of ESC when using a long lead.

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Why not buy British? If you call them and have a conversation with a real person that knows their subject, you may be enlightened that this issue was engineered out years ago and its not a problem with their products.

Of course if you buy from a far eastern supplier with poor support you can take a risk and it might just die on you or last years.

If in doubt, ask an expert (and that's not me, I just made a phone call)

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Posted by Chris Walby on 10/04/2018 17:19:28:

Why not buy British? If you call them and have a conversation with a real person that knows their subject, you may be enlightened that this issue was engineered out years ago and its not a problem with their products.

Of course if you buy from a far eastern supplier with poor support you can take a risk and it might just die on you or last years.

If in doubt, ask an expert (and that's not me, I just made a phone call)

I would buy British if I could. Who is this mystery manufacturer of EScs? Are they suitable for our motors?

Dick

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Posted by Andy48 on 10/04/2018 16:18:54:

Sorry but your argument makes little sense.

....... It may have been accurate years ago, but if it is still true then why is there nothing in the instructions of any of my ESCs about the issue? Its is also interesting that all references to the problem seem to go back to the guy Geoff included the reference to 10 years ago.

Also can you explain why I've never had an ESC failure in 10 years? Most of my ESCs are Hobbywing ESCs of one sort or another and all have survived long leads over an extended period of time.

...............

Edited By Andy48 on 10/04/2018 16:21:32

Its related to how hard you push your ESCs as well as cable length so if you don't get close to max current then longer leads may not be a problem.

At least one modern ESC producer does suggest extra capacitors and even supplies them - have a scroll down this page.

Dick

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"so it's unlikely to be just a Nixx problem."

Battery type is indeed immaterial. The problem / effect is induced voltage caused by rate of change of current in the input wires. The things dictating the size of that induced voltage at the input to the ESC are the current, and wire length (and to a degree gap between pos & neg wires).

And (of course) the caps on the input of the ESC.

Don't ask me for the exact physics (this is all going back about 20 years).

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Very nice and probably adds a good measure of safety, but to my mind, not as good a solution as a removable link. The more complex the solution, the more chance there must be of failure. There's also the complacency factor of relying on a "clever" device to factor in.

The golden rules must be, first and foremost, to think carefully about what you're doing, never ever under any circumstance reach through the prop arc with a battery connected and regard any connected model in exactly the same way as you would an IC model with its engine idling. If you can't connect your batteries without putting your anatomy in peril, fit a removable link - or an engine!

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I'm with Martin Harris on this one That electronic switch is hardly any different in operation than an ESC which has been 'locked out' either by the throttle-cut switch on the trannie or by a built-in on/off switch in its receiver signal wire or something similar a la Jeti.

An arming plug is simple and, hence, to my mind, less likely to go wrong.

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Posted by Gordon Tarling on 11/04/2018 18:15:42:

BEB - I feel that I should point out that switches ARE available and have been for some years. All of my larger models where battery access is not immediate are equipped with one of these - LINKY For those that baulk at the cost, what value do you place on losing a digit or worse?

I'm guessing that that's just another MOSFET being used as an isolation switch. I think my transmitter throttle enable/disable switch coupled with a verbal warning is as effective and reliable. I think Bruce, who first raised the query about a switch, was thinking of a mechanical, hand operated device.

However, in the end, safety devices rely on their humans being sufficiently conscientious to use them. The safety plug is useless if you forget to remove it just as (say) the bow doors on the Herald of Free Enterprise depended on the man responsible for closing them not being asleep in his cabin and no system in place to check the doors were safe for sea.

On the conversions I've done where the battery access is vis a top hatch I don't bother with a plug because it's just as easy to disconnect the battery connection itself. That isn't the case on my DB Gypsy Moth, hence my initial query.

Geoff

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Posted by Geoff Sleath on 11/04/2018 22:05:59:

However, in the end, safety devices rely on their humans being sufficiently conscientious to use them. The safety plug is useless if you forget to remove it just as (say) the bow doors on the Herald of Free Enterprise depended on the man responsible for closing them not being asleep in his cabin and no system in place to check the doors were safe for sea.

I think this is a very valid point. Having safety plugs on all my models means that I now have a fixed routine so that I only insert the safety plug when actually on the flying field in a position to take off, and always remove it as soon before the plane is moved from the flying field. Thus it has become almost automatic, and the only time I really have to think about this is when I am testing a model in the workshop. Indeed the safety plug clips on to the transmitter strap hook as a double check.

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Now I remember why I didn't pursue physics at A level.

To try to summarise; there's no mechanical switch; safety plugs are the way forward; too long battery leads, say 8"+, = unwelcome inductance; battery leads should be close together; safety plugs between ESC and motor are marginally useful; er, that's all folks.

BUT I've seen various plug types. I try to stick to Deans simply for consistency although I recall seeing an ... XC 90? with spark suppressor; any clear superiority here for the safety plug? How important is it to keep battery wires similar to each other in length if interrupting one to fit a safety plug? Plug on the + or the - cable?

Finally an observation after two years in my third modelling career. Electric accidents are usually due to unrestrained models. Nobody dreams of starting any sort of i/c plane without a proper restraint. Some modellers arm electrics without restraint.

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I think we are in danger of loosing the plot a bit here TBH! OK if you want to run a 24" wire from the ESC to the battery you are likley to have problems. But if you can arrange matters so that your arming plug is sort of broadly speaking "en-route" to the ESC anyway then any increase in length will be marginal at best and I'm confident we don't get problems.

Personally I'd worry less and fly more! wink 2

BEB

Edited By Biggles' Elder Brother - Moderator on 12/04/2018 13:37:49

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Posted by Bruce Collinson on 12/04/2018 13:29:02:
 
.....\. How important is it to keep battery wires similar to each other in length if interrupting one to fit a safety plug? Plug on the + or the - cable?.................
.

Not important at all. different lengths won't cause any problems.

No problem with safety plug on either +ve or -ve, Your choice to best suit the installation.

Dick

Edited By Dickw on 12/04/2018 14:31:02

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Bruce: there's always been a variety of battery connectors. I started my electric flight attempts in the days of (mostly) 7 cell nicads (nominally 7.2v) and brushed motors. I arbitrarily chose to use 4mm bullet connectors with the male on the battery positive lead. At the time it seemed I was in the majority but some chose the other polarity.

I'd never heard of Deans connectors but they are very good! I was a beginner like you but electrically experienced as a retired electronics engineer. When the wonderful world of inexpensive brushless motors and lightweight LiPo batteries arrived I continued to use 4mm bullet connectors but I got fed up having to cut off perfectly good XT60s on every 3S 2200 pack I bought and converted all my 3S models to XT60s. I still use 4mm bullets on 4S and 6S packs. XT90s are, as the name implies good for 90 amps (presumably continuous) and that's a lot of current! I still have to change connectors sometimes because even those who use 4mm bullet types also don't agree on the polarity convention. You can't win and it's best to adopt a standard and put up with it. They're all sound and if you are able to solder there's no problem except your time.

Battery packs bigger than 4S tend to spark as they are connected and that, whilst harmless, can eventually erode the connector so there are connectors which have a small resistor which discharges the stored energy more gently so the spark doesn't happen.

Safety plugs have their place but aren't always necessary IMO. The battery lead length still seems to be a dark(ish) issue but I've never seen any esc instructions which actually set a lead length value and batteries come in non-standard lead lengths. I started this thread because I thought it might be handy to have the safety plug in the motor side so I could safely connect the battery and still be able to check the electronics. It seems that may not be a good idea.

Now I'll get on trying to make my sprung undercarriage look something like DeHaviland intended on the Gypsy Moths I'll start a thread on that in a day or two!

Geoff

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Geoff,

Really grateful. I'll try to get out more, hopefully this Sat but am minded to fit a plug to the Acro Wot which awaits my pleasure as soon as I turn sixty....something .... tomorrow morning and which is specced for electrickery courtesy of 4 Max ESC, motor and prop and the LiPos from my Wot 5 (4S 5A, biggest that will fit without major modification). A very "sweet" setup in the Wot 5, hoping for similar in the Acro Wot.

BTC

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Posted by Geoff Sleath on 12/04/2018 14:53:25:The battery lead length still seems to be a dark(ish) issue but I've never seen any esc instructions which actually set a lead length value and batteries come in non-standard lead lengths.

Copied from the Jeti Advance Pro ESC manual:

connection cables to the flight battery can be extended to a maximum length of 20 cm

More specific example from the high end Mezon manual:

 The distance between the motor and the controller should not exceed 10-15 cm . The cable lengths to the flight battery may be lengthened up to an additional 20-25 cm. You can lengthen the battery cables further, if you solder, in parallel to the cables, electrolytic capacitors (so called ESR low internal resistance capacitors with corresponding voltage values and capacities of several hundred microfarads). Add one capacitor for every 25 cm of additional battery cable length.

From the YGE 200 Hotliner FAI manual:

In the FAI types the cables to the motor are usually cut to a minimum and directly soldered. On the other side the total line length, from the controller to the battery, for all types should not exceed 20 cm. If long cables cannot be avoided,  every 20 cm a  330μF / 50V Low ESR capacitor should be soldered into the line, or add our Capacitor Module YGE Cap's Type 7

...and as I've only checked 3 non-far east manufacturers with known reputations (nothing obvious on Castle Creations) I'm sure there are other examples.

Edited By Martin Harris on 13/04/2018 00:43:43

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Obviously some high end esc manufacturers do actually comment and specify lead lengths but, as I said, I haven't seen any on the budget escs that I use successfully. I always try to over specify by around 25% on maximum current anyway. The only Jeti esc I've used were for brushed motors and the only Castle Creations esc I have is a 10 amp one which generated so much noise it made its use with a 35Mhz receiver impossible. I returned it to the manufacturers - the new one they provided was only marginally improved IIRC (it was a few years ago).

The only members of my club who use high end electric flight equipment are those flying aerobatic models (F3A?) and none of the ones I've asked even know how much current their motors draw (on 10S LiPos) so they're unlikely to be aware of potential battery wire length issues either. Considering they're flying models costing several thousand pounds that surprises me.

Geoff

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...... but the salient point is that you CAN extend battery leads up to at least 8" or 20cm with no adverse consequences, which should be enough for most of us. So often on here you are given the impression that battery leads must not be extended at all.

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Summary

The thing to worry about with long battery leads is the turn-on
transient. The turn-off transient can pit, even weld, the contacts of
the circuit breaker but will not over stress the capacitors in the
ESC. The turn-on transient can double the battery voltage at the ESC
for which the capacitors, possibly even the FETs, are probably not
rated.

Rather than use an extra capacitor when using long leads, it would be
better to protect the ESC by using a power zener diode to clamp the
excess voltage. A zener capable of dissipating a few watts (e.g. 3W)
in the steady state should easily absorb the transient power without
the need for a heat sink. Choose a zener voltage to match the S-rating
of the ESC plus 1S if you trust the supplier. Better yet, see what the
voltage ratings of the capacitors are and use a matching zener diode.

Reasons

The battery leads are a pair of parallel conductors that should be
kept close together to minimise magnetic coupling to nearby
components. They act like a transmission line and have an equivalent
inductance of about 10nH per centimetre. So, battery leads about a
metre long have an inductance of about 1 microhenry.

I guess the total capacitance in a typical ESC is about 100
microfarad. Together, the inductance and capacitance form a series
resonant circuit that resonates at about 16 kHz. The Q factor of this
resonant circuit is only about 5 if the net series resistance is about
20 milliohms (4S battery plus ESR of the capacitor(s) in the
ESC). Neglect the parallel loading of the ESC: it barely matters if
the motor is off. Arcing at the contacts would increase the series
resistance and decrease the Q factor.

When the battery is first connected, the currents in the leads
increase and the voltage on the capacitance also increases. If the
total series resistance were zero, the voltage across the capacitance
would rise and overshoot, oscillating about the battery voltage with a
peak amplitude of the battery voltage. Hence the factor of two, the
battery voltage plus the oscillation amplitude, mentioned in the
opening paragraph. With a finite and low Q factor, the oscillations
would quickly die away leaving the capacitance charged to the battery
voltage.

But surely this applies to any length of lead? Yes but the Q factor of
the connection is lower for shorter leads and this will decrease the
amount of overshoot.

The reason why the turn-off transient is less stressful is because
a) the current from the battery is usually minimal when disconnecting
b) the energy stored in the capacitance would typically be an order of
magnitude greater than that in the inductance so the ability of the
latter to cause overshoot is minimal.

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That's very interesting, Bryan. Thanks for that.

One term I don't understand is the S factor of the esc. Is that the number of cells? ie the maximum input voltage? So the zener should be chosen to be the rated maximum voltage plus one extra cell - ie (say) 4v for LiPo?

The input capacitors on the esc I have on my desk (Black Mantis 60 amp) are 2x220mfd in parallel (so 440 mfd) 35v working.

You estimate a 4S LiPo as having an ESR of less than 20 milliohms. In the measurements I've made 5 milliohms per battery is quite low and they can go up as high as 10 milliohms/cell before they're virtually useless as sourcing a decent current. So I think your estimate is perhaps a little on the low side.

Geoff

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