[Gary B]

The point of this thread is to show the sort of currents that can be drawn by powerful servos for very short periods so that if you are using 4 cell batteries that will suffer voltage drop such as high capacity AA nimhs you can't be complacent because your ammeter showed an average 0.1 amps for example over 1 second.

Sorry, been away from this topic for a while but I feel that I must reply to someone who has such an eloquent tone of phrase when replying to what was meant to be a slightly tongue in cheek reply in the first instance. 

I agree you can't be complacent with high instantaneous currents but I doubt anyone would use a 4 cell AA NiMH with high power servos so surely what we are really talking about is other system components to handle high currents, i.e. receivers, connectors, regulators and cables etc. both from a current handling perspective and voltage drop (which although related are seperate issues).

What would you consider a valid time frame for this type of measurement?

[Harry formerly mpx]

I doubt anyone would use a 4 cell AA NiMH with high power servos

I'm not so optimistic!  There are plenty of people on this forum who have had to be told about it.  The basic rule used to be that to handle higher currents you used batteries of higher capacity, but that was from a time before manufacturers started squeezing large capacities previously associated with sub-C and even D cells, from AA cells, many people still apply the rule or fit large capacity because it seems a good idea, without knowing that the rules have changed and that high capacities can actually work against them. Even the top world aerobatic pilot Quiqui Somenzini lost a big model to a Spektrum brown-out, so the voltage drop to dangerous levels due to inadequate power supply can happen to even very experienced people.

What would you consider a valid time frame for this type of measurement?

I have no idea, I was hoping someone knowledgable would jump in with info on how quickly a battery can drop its power, such as there must be a delay or it is simultaneous with the current.  I also hoped someone might know if rx electronics, without a capacitor on the power input, will continue to work for some amount of time without switching off if the voltage drops below the threshold, e.g will it cope with 1/1000 second failure but not 1/100 second power failure?

[dickw]

.......I have no idea, I was hoping someone knowledgable would jump in with info on how quickly a battery can drop its power, such as there must be a delay or it is simultaneous with the current. .....

It's not so much the battery dropping it's power, more a case of how quickly it can increase its output to match the new current requirement.

Think of it as if the battery is producing a fixed level of power, and then the current increases. At a fixed level of power the voltage must fall as the current increases - the battery has to quickly ramp up it's output to match the new power requirement. It normally does this so fast you do not notice it, but I have no idea what the actual time scale is.

Put simply - the voltage drops simultaneously with the current increase, and will settle at a voltage proportional to the current and the internal resistance of the cells.

The internal resistance of the high capacity AA cells is usually higher than the sub C cells hence lower volts at high currents for the AAs.

Dick

[Cornish Pixie]

So if I am reading this correctly the best fix of all is to feed the RX and servos from separate power sources, so that the receiver always sees a good voltage .

Simon

[Pup Cam]

So if I am reading this correctly the best fix of all is to feed the RX and servos from separate power sources, so that the receiver always sees a good voltage .

Pretty much the conclusion I came to in my great hunt the glitch (Post 35) exercise on my DVII earlier in the year.    

Having separate supplies for the voltage sensitive low current devices i.e. Receivers and the high current, much more voltage tolerant high current devices i.e. the servos makes a good deal of sense.    

Alan

[Harry formerly mpx]

So if I am reading this correctly the best fix of all is to feed the RX and servos from separate power sources, so that the receiver always sees a good voltage .

Simon

yes, that's a very good way but you must be careful not to double your probability of loss of control due to battery failure!  With 1 battery you have X probability of it failing causing loss of control, with the system relying on a battery for each of Rx and servos you have in effect 2X probability of a battery failing and either will cause a crash.  To get back to the same level of probability you started with you need to carry 4 batteries, 2 for Rx and 2 for servos.

Another way is to carry just 2 batteries but of sufficient capacity and pass each through a diode, they have to be 5 cell batteries to compensate for the diode's small voltage loss, this way you get two battery redundancy, probability of a diode failure incredibly low, and the greater starting voltage means it is very unlikely that a battery will be dragged down to the rx switch off voltage.

[Cornish Pixie]

Another way is to carry just 2 batteries but of sufficient capacity and pass each through a diode, they have to be 5 cell batteries to compensate for the diode's small voltage loss, this way you get two battery redundancy, probability of a diode failure incredibly low, and the greater starting voltage means it is very unlikely that a battery will be dragged down to the rx switch off voltage.

This is the system I run in my petrol powered models at the moment.

Simon

[tsr]

Just as well you mentioned the diode otherwise you double the risk.

[tomkfly]

Is a diode needed? Have a look at this thread and follow the link, posted in reply No1,  to a battery FQA website.





   Tom

[Harry formerly mpx]

Tom, decent quality nicads or nimh batteries actually fail incredibly rarely during use.  Problems in mechanical items such as loose wires in switches are at least as likely, maybe more likely, (I once had a  switch from a well known brand that seemed a bit intermittent, I opened it up and found the wire had not actually been soldered onto the tag) and if these float about and cause a short then you do need the diode to protect the other battery.
So the website is correct in that a diode is not needed to protect against battery failure, but it neglects the other components in the system.

[tomkfly]

Tom, decent quality nicads or nimh batteries actually fail incredibly rarely during use.  Problems in mechanical items such as loose wires in switches are at least as likely, maybe more likely, (I once had a  switch from a well known brand that seemed a bit intermittent, I opened it up and found the wire had not actually been soldered onto the tag) and if these float about and cause a short then you do need the diode to protect the other battery.
So the website is correct in that a diode is not needed to protect against battery failure, but it neglects the other components in the system.

I agree the switches and harness components are the most likely parts to fail.
Therefore, I personally tend, in my larger models, to use either  a single sub C or A123 cell pack, with deans connectors and fed through two switches. Although it is probably not necessary, I also use a diode to drop the voltage with the A123 packs.




  Tom

[tsr]

If battery one fails beause it develops an internal short circuit and there is no diode then we get heat and more heat and potentially fire if it is in a parallel circuit it makes it worse. If it just fails though high resistance or by becoming an open cell then the parallel battery will just keep on working and the Amp hour rating that it can supply the voltage for will be shorter.

I have had the internal short circuit thing happen it is very exciting and it gets very hot and takes a lot of current and isn't fussy where it gets it. Fit a diode if you are going to do this my models are cheap ones yours may not be, and having 5 seconds of your model trailing smoke as you try and get it down may look spectacular but trust me when its yours you keep waiting to see that orange flower of flame that says your model isn't going to make it. I made it by the way but the cell was bloomin hot and the foam had started to melt around it and was fuming as was the battery casing.

[Cornish Pixie]

I like the Diode fitted. For my mind it dose two things. First it keeps both systems separate from each other so whatever happens to one will not effect the other. Secondly it just drops the voltage a bit and it seams to make a difference to some of my servos. I am using 5 cells buy the way.

Simon

[Cornish Pixie]

Ok  so what about this scenario.
Two RX's are fitted to the model.
Two battery packs are fitted with there own switches.
Battery pack one has two outlets fitted after the switch with a doide in each outlet. One outlet plugs into one RX and the other outlet into the second RX.
Now both RXs are fed from battery one , but if there is an electrical short on one RX it will not affect the second ?
Now do the same with battery 2 . Will this then give twin battery redundency to both RX's without any chance of one RX failure afecting the other power wise.


Simon

[Harry formerly mpx]

I think that's ok Simon.  The only thing I would watch out for is that using 2 rx allows twice as many servos to be connected direct to the rx so if you are using a large number then the bateries and wiring and switches need to be capable of handling it all.  If one switch for example failed, then the remaining battery and single wire to the single switch needs to be capable of handling the max power consumption without fading.  this is true of tw battery 1 rx as well, I mention it simply because 2 rx makes it easy to have an awful lot of servos!

[Cornish Pixie]

The second receiver is for a bit of redundancy .
RX 1 has one elevator , one aileron and throttle
RX 2 has one elevator, one aileron ,rudder  and choke (second means of stopping engine)
So no more servos fitted.

After not finding out what happened in my crash I am using this on my 1/3rd scale models
Its just deciding the best thing to do with the battery's and switches.

Simon

[Harry formerly mpx]

Simon with that sort of size of model I would be looking at a power management system such as by Emcotec or Powerbox, both make a redundant receiver system, a device which you plug two rx into and which provides stabilised power etc.

[Cornish Pixie]

Simon with that sort of size of model I would be looking at a power management system such as by Emcotec or Powerbox, both make a redundant receiver system, a device which you plug two rx into and which provides stabilised power etc.

These are Slow flying scale models.
All have been flying for many years on basic set ups with no problems at all.
I think we get a bit carried away with some of the gear we fit.
Take my 1/3 rd cub. Flew perfectly well for 2 seasons on a set of JR 591 servos in it and a pair of 2000ma Sub C cells through a S&M battery backer.
Zenoah 62 powered.
Only changed the servos as they were starting to get some play in the gear train.

My 1/3rd pup is very similar , fitted with a set of HS645mg servos and a pair of 2000ma battery's. No problems at all.

My only worry now is a reviver failure , hence the twin set up.



Simon

[Harry formerly mpx]

I think we get a bit carried away with some of the gear we fit.

Quite agree.  A couple of years ago I was wondering about ensuring the power system and radio redundancy on my jets was sufficient, was chatting to a top international jet flier who said he preferred to keep it simple - one rx, two batteries plugged striaght into rx no backer, and never had an issue with it in many many years.

If you carry two rx then the redundant system from emcotec or powerbox might give an advantage.  With your system if one rx fails or goes into failsafe then you have control but it's less than perfect, via the other rx, with the RRS you have full control.  Just something for you to consider.