|Robert Cracknell||23/09/2017 02:03:51|
|61 forum posts|
As someone returning to the hobby after a few years layoff (want it qualified? DEACS - remember them...??)
I am now puzzled as to how to power my Rx's which require 4.8v from Li-Po's which are, I believe, 3.7v. I am probably missing the obvious but can someone enlighten me...
Any and all advice greatly appreciated.
|Tom Thomas||23/09/2017 03:47:12|
314 forum posts
Lipo's vary greatly in voltage and size Robert.
There are a great many experienced forumites here that will help you.
Can you not just purchase a ni-mh pack for your receivers? Or do you need to power an electric setup with speed control/ motor?
Or do you need something to reduce the voltage of a lipo?
I myself am not experienced in these equations although I have bought some ARTF's which run on lipo' but of course the mating of the electronics is already done.
Hang in there and you will get all he advice you need
|Piers Bowlan||23/09/2017 06:08:27|
1239 forum posts
If you are flying a glider or IC model then, as Tom suggests, one of these 4 cell NiMh (Nickel metal hydride) batteries for a 4.8v supply are commonly used. Also, some people use LiFePO4 batteries which have a 3.2v nominal voltage so a 2 cell pack will have a voltage of 6.4v here and can be used with most receivers. These might be used in larger models where more power hungry servos are required. Spectrum receivers have a voltage range of 3.5v to 9.6v for example, (I wouldn't try running them below 4.8v as the voltage can drop under load). Check the spec for your RC equipment to be sure.
More relevant than the receiver voltage perhaps is what voltage your servos can handle? Again look at the spec. for the servos in question. Some will only tolerate 4.8V others up to 6v while there are high voltage servos -15v? LiPo batteries have a nominal cell voltage of 3.7v so a two cell pack is 7.4v but fully charged will read 8.4v. You will usually need a small voltage regulator or UBEC to step the voltage down to 4.8/6v or at least something more useable if using a lithium battery.
If you are flying a small electric model then the Electronic Speed Controller (ESC) which controls the electric motor will usually incorporate a Battery Eliminator Circuit (BEC), a voltage regulator, to reduce the voltage the receiver/servos from the motor power battery. It will also incorporate a Power Cut Off circuit (PCO) to stop the motor before there is insufficient power available for the receiver.
For larger electric models a stand alone UBEC or separate receiver battery will be required.
I hope this makes sense, no doubt someone else will be along soon to correct me!
Edited By Piers Bowlan on 23/09/2017 06:46:10
|Steve Hargreaves - Moderator||23/09/2017 14:42:40|
6592 forum posts
A 4 AA cell NiMH pack still has a lot to commend it for simplicity Robert. You can buy LSD (Low Self Discharge) NiMH packs these days which hold their charge very well (claimed 70% charge after 12 months). Search for Eneloops as they are widely available....
Lithium chemistry is frustrating in that a single cell is too low a voltage (nominal 3.7V but up to 4.2V fully charged) whilst 2 cells are too high (7.4V or 8.4V fully charged). Plug a 2 cell Lipo into a typical Rx/servo set up & you'll most likely fry the servos. As the other chaps have mentioned the way forward here is to use a small regulator that will feed a steady 5V or so to your radio when powered by a 2 cell Lipo.
There is an alternative type of battery available though...the LiFE battery....similar advantages to LiPos (light weight...high energy density etc) but the nominal voltage is 3.3V (3.6V fully charged) so 2 of these cells gives you 6.6-7V. Most "common or garden" servos are rated at 4.8 to 6.0V so a 2 cell LiFE battery is slightly over this figure but many people (myself included) use these batteries very successfully & have done for many years. Do be aware that LiPo & LiFE batteries need different chargers...you CANNOT charge a LiFE battery using a LiPo program.
An alternative method is to use so called HV or High Voltage servos. These are designed to run at 7.4 to 8.4V so are ideal for use with LiPos....you will need to check that the Rx can take the higher voltage too. Most modern Rxs can.
Of course all this presupposes you are wanting to use a separate battery pack.....If you are thinking to fly an electric powered model these often use an electronic circuit on the ESC (Electronic Speed Controller) to divert a small amount of power (at around 5V or so) from the big battery powering the flight motor to feed the radio.
Hope that helps...
|Frank Skilbeck||23/09/2017 17:00:51|
3991 forum posts
For small planes you can buy a small inline device that will boost a single cell lipo voltage to 5v.
|Robert Cracknell||24/09/2017 01:37:05|
|61 forum posts|
A BIG collective thank you to all who took the time to post. I now have a number of avenues to explore, all of which are perfectly understandable to a relic like me....
Once again - thank you.
|1637 forum posts|
I have a Graupner 3 Ch 35 Mh set Tx & Rx NIB and claims on the box to incorporate a BEC. would that be in the Rx circuitry somehow .To date I have not opened the box so do not know if there is an Esc or something else in there..Can it be done through the Rx alone ? John O/T
|Steve Hargreaves - Moderator||24/09/2017 21:50:08|
6592 forum posts
BEC stands for Battery Elimination Circuit & comes to us from the dim mists of time when electric flight was in it's infancy & the ability to tap a small amount of power from the main flight battery to run the radio thus "Eliminating" the need to carry a separate battery to do the job offered a very useful weight saving. Modern electric powertrains have made this unnecessary but still the term endures.
Rather than a BEC we should really call it a "regulator" as it regulates the voltage supplied to the Rx & servos. Such a circuit could easily be incorporated in an Rx....in use you would be able to plug a higher voltage battery into the Rx & let the electronics sort out the voltage. Not a feature I've ever come across before but absolutely no reason why it couldn't be done...
I might be tempted to ask why you would want to though....
|Allan Bennett||25/09/2017 08:57:47|
|1307 forum posts|
I agree with the others that a modern 4-cell NiMh battery is probably still the simplest option.
I personally wouldn't go for a LiPo, even though you can use a BEC to regulate its voltage to suit the receiver, because it can be permanently damaged if left fully charged for an extended period (so you have to charge it immediately before you go fly) and if you let it run down below about 3v per cell (such as if you forget to switch off your receiver), and because it needs a charger designed specifically for LiPos.
|1946 forum posts|
|Personally, I'd go for a five cell nimh pack rather than four, as a single weak cell in a X4 pack will almost certainly lose you a model after a while.|
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