John McNamara wrote:Hi Guys,
I have just come accross this Thread. The thing most people miss is that by using two Batteries we are trying to add redundancy, and that is there to compensate for the possibility of some failure occuring. Measuring the conditions when no failure is present is futile. Yes the cross drain on batteries in normal conditions is neglible. However, the diodes are NOT there for normal conditions, but for a failure condition. There are circumstances where one battery SYSTEM can fail that will apply a dead short to a fully charged battery with devastating results.
My friend took this bad advice about not needing diodes with his first jet, because this information was written down in a magazine he preferred to believe a guy in a magazine column rather than me. I did my education in the field of electronic, wheereas the magazine correspondent admited that electricity was a mystery to him (why he was going into print on a subject that he knew nothing of, baffles me).
My friends model burst into flames. This was because there was a dead short in one switch, the switch of the other battery. This meant that a fully charged 5 cell nicad wad discharged more or less instantaneously, through the receiver and the associated wiring loom. The tracks on the receiver vapourised, and the insulation on the wires melted. Had it not been a jet, we probably would not have had a fire extinguisher on hand....
For the sake aof a couple of quid and a little time soldering this risk can be eradicated.
John
John
I realise it is a few months since your post which is largely a reply to mine from last year.
I don't agree with your last line. Yes, you may "eradicate" (well, lessen anyway) one risk but you do introduce another. Your friend's scenario was a full short in a switch - a fairly rare occurrence. OK, in that situation the diode would prevent the second battery from being discharged rapidly but it wouldn't prevent the first battery heating up the faulty switch and wiring and setting fire to the model. Yes, until the fire damages some other critical component you would likely retain control.
However, with the diode setup you are relying on the junction of a component costing a few pence to pass all of the current drawn by the model on every flight. With a stalled servo that can be considerable. In the event of a momentary short circuit the diode will fail like a fuse well before any wiring will burn out. You will have no indication that this has happened unless you have onboard voltage monitor and turn each switch off in turn.
So, as I said in my original post, you swap one risk for another. Which is more likely? A short circuit switch or burnt out diodes? I wasn't advocating one approach, just pointing out both sides of the story.
Surely a far better solution would be to fit two receivers, each with their own batteries, monitors and switches but no diodes. With each receiver controlling half of the servos you would at least have some control in the unlikely event of a whole number of failures.
"A little time soldering" you say. Fine if you can do it properly (as I realise you can!) but I have seen some real horrors with terrible joints and wire totally unsupported so that the vibration of the engine is encouraging the wire to break next to the joint. In fact it was S M Services who said to me that he fitted the little screw terminal blocks to most of his units because he never ceases to be amazed just how badly many modellers soldered!