|Peter Beeney||30/08/2012 11:57:20|
|1587 forum posts|
Just to slightly emphasise the point, I’ve never had an issue at all with bolting two BEC’s together, as I’ve said before, we are the founder members of ORWISC, - Our Red Wire Is Still Connected.
Briefly, what I think happens when they are in multi-connection configuration, the output voltage of one is always higher than the others, but only ever by a few millivolts. However, that’s enough to close all the others down, no output from these, so in very light load conditions one regulator will be supplying all the current that’s required by the radio and the other ESCs. The ESC load is probably only a few steady milliamps. As soon as the load increases, more current flows, the wire from the ESC has some resistance, not a lot, but perhaps just enough to drop the voltage at the receiver a few millivolts so that now the next highest output reg can join in. But of course this will soon start to drop some of it’s millivolts across it’s wire connection, but at some point they will reach a nett balance and share the load, at least to a large extent. And so on if there are more than two.
With regard to the multi-motor failure with the red wires disconnected, all the 5 volt supplies on the ESCs would be different, but only by a tiny amount; but when they’re connected, all the voltages are the same, and if they vary slightly they all vary the same. This might be the answer, however, it’s a bit of a guess, I’ve no way of knowing. But I’ve have thought the slow speed running would be down to a discrepancy within the ESC maybe. So maybe as far as gliggsy’s OP is concerned, maybe it might be worth re-connecting the disconnected red and giving it a try. Although I can’t really see it making that much difference. But from experience it does sometimes appear as though odd things seem to occur occasionally on ESC’s and their associated wiring.
I don’t think I’d particularly bother with any diodes in the leads, I’d consider that this negates a possible advantage, and if the output of one regulator went short circuit it would plonk the full battery voltage straight onto the receiver anyway. Which will quickly render the whole thing a bit hors de combat, the servos give in very easily. I’ve seen it happen, when a friend got it a bit wrong on his large powered glider. The radio got the full 24 volts head on. It came back from repair with the PCM receiver remarkably unharmed, but all 6 servos were burnt to a crisp! I’m not really into this stuff very much, but I’d have thought that a ESC with a BEC capable of delivering 5A from 50 volts might be a bit special, read expensive. Aren’t they usually opto-coupled devices? The added receiver battery definitely works: as I’ve said before, a friend used one as a ‘standby’ supply. What he didn’t realise was that the pack was doing all the work, at least until it’s voltage started to drop. Also what he didn’t know that the ESC was seriously overheating, to the point where the BEC was shutting down even without a load, but of course the battery kept it’s 5 volt supply going. That shows just how tenacious these these things can be even under these adverse conditions. On the day that he didn’t bother to switch the battery on because he’d not had time to charge it, and the model was subsequently totalled half way into it’s first flight. Because the BEC shut down, trying to cool off. With hindsight, it was pretty obvious, really.
A couple of ESCs that I tried with a ‘back’ voltage for a prolonged time, 8 volts for 24 hours, did not complain about this, and I’d say the output voltage from a switched regulator would be the same as the linear, coupled together, the highest voltage will just shut the other one down. Although I’ve not tried this particular combo, so that’s not as verified.
|Peter Beeney||30/08/2012 11:57:47|
|1587 forum posts|
This is not an ideal setup, I’m sure regulators are not designed to be paralleled together; but I reckon it’s the best of a bad job; but trying to extract an unregulated supply, the radio and servos, from a regulated source, the BEC, can only really be construed as a bit Fred Karno. I’m not convinced any one would plan this, it just happened gradually, I believe it started when some of the electric car aficionados realised there was a 5 volt supply on the controller, and they could lose the rx battery; and it went on from there. Certainly it works within reason, as long as you keep within some operating parameters; but just as an observer, I’d say that BEC woes have surfaced on a regular basis. One of the biggest drawbacks is the fact that as the main battery voltage becomes higher, thus obviously the voltage differential becomes greater, and so the applied load can only become less. - The Law of Diminishing Returns.
I’ve always considered that a battery is probably the most secure type of power supply; proved it is given the requisite amount of care and attention, that is. Which doesn’t amount to much, keeping it well charged and a capacity check from time to time. And it goes without saying that wires and connections all need to be sound and and secure. At the end of the day, anything can fail, but this would be the last to give in, batteries are the end of the line as a power supply in any emergency system; and if you think about, even using a BEC you are still using a battery after all, the flight power pack. From personal experience, these can fail, but I have to admit, I was looking for maximum performance, rather than considering the batteries. But, in the event, the rx pack did not let me down.
So, as usual, it seems we’ve still not discovered a practical reason why we should not parallel 2 BEC’s, but fortunately in general terms it’s never going to make any difference if you do or if you don’t.
|Chris Bott - Moderator||30/08/2012 13:17:10|
6724 forum posts
I must become a fully fledged member of ORWISC very soon.
I put my diodes in because the BEC's are switched mode and I just don't know enough about them to know it would be OK without. Oh, and I'm on a 4S motor battery too, so the volts dropped is not horrendous.
|Mark Powell 2||30/08/2012 13:22:27|
|430 forum posts|
You both make very good points. I have had Four ESCs fail in ten years. None were overloaded, neither in terms of motor or radio supply. Two Jeti, one of nominally Axi make, and one Chinese. I have never had a battery fail, in all my equipment, model related or not.
As I have said, Kontronik say the internal BEC, where fitted, should be thought of only as a battery backup. Yet they make much of their 'advanced' BECs and also much of how many of their higher voltage ESCs (but not the highest voltage ones) have integral BECS where others dont. Odd.
High voltage? I have ESCs up to 50 volts, and one, not so far in use (Schubeler integrated ducted fan, motor, and ESC) that can be 60 volts . None of these have BECs. The weight saving in the planes that they are typically used in would be negligible.
Battery supplies have a 'handling safety' advantage. One can test all sorts of other things while leaving the power battery disconnected. I still have a small scar inflicted by my little Comet! The plane was OK.
I use mainly Multiplex receivers, many of which have a dual battery facility as standard. But I never bother. Such 'integrated' things also introduce complication which may reduce the reliability. Do it yourself, as Chris does, you have control over what's going on.. No battery failures in ten years and one has to draw a line somewhere.
PS: You want a high power, High voltage ESC?I thouroughly recommend the new OS ones. A 100 amp/12 lipo one is only about £130 and so far is reliable. And they have a good name to take care of.
Edited By Mark Powell 2 on 30/08/2012 13:26:29
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