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Using Li-Po batteries to power your R/C gear

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Danny Fenton13/08/2007 08:17:00
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Hi Tim,
I also have gone down the route of Lipo for my Rc gear. I do not use it as an airbourne pack, yet But I think I will when I source one. I have used BECs in my electric models as mose ESC do not support more than three servos, ie insufficient current capacity. I wrote to Macreggor and asked there oppinion before modifying my PCM 9XII Tx and I am sure Tony Heaton will not mind me quoting his reply:

"It is a very good question to which unfortunately I do not have a stock answer. It is not something that we have tried here but... I would imagine that if the max voltage is going to be 12.6 V this is only marginal over what 8 dry cells would produce.

The main regulator on the 9XII is capable (according to the technical specs from the manufacturers) of taking a max input voltage of 35V at 500mW dissipation. The smoothing capacitor is rated at 20V.

So it would look like the main 5V supply for the processor etc is going to be safe.

The RF modules use similar regulators within to supply stable voltages to the oscillator/logic section, but the raw voltage is used by the RF Power Amp. The specs on these show a max volatge of 45 V so again all seems to be well.


I think that after looking at the specs of the components in use, it seems to be fine to use a LiPo pack."


I have been using the Tx for a few weeks now and the ability to just be able to turn the Tx on and now it is charged is wonderful
Danny
Tim Mackey13/08/2007 10:47:00
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Hi Danny. I have had several dealings with Tony over at macgregor, and a fine chap he is too! He rightly could not "approve" the use of a 3s LipO In the JR gear, but they say a nod's as good as a wink :)

One or two minor errors crept in during RCME's editing of my article, and I should straighten them out here.
1) BRC did NOT supply the items mentioned, however I understand that he does stock similar, although I am not sure about the particular BEC device with built in alarm.
2) I referred to using 2 x packs in parallel, and there were several pictures in the original article - these pictures were apparantley of unsuitable format or whatever for the website to use, so this and others, got replaced with RCME's versions! I will try and post the picture concerned here now.
http://img530.imageshack.us/img530/2279/2s2pht4.jpg


Robert Bouw13/08/2007 10:50:00
13 forum posts
Hi Tim,

A very clear and well written article, thanks very much. One question that came to mind when reading through the advantages was: Would it be possible to achieve the same results using Li-Ion cells? Li-Ion generally gives 3.3V per cell, thus a 2S Li-Ion would be 6.6V. Would the 6.6V eliminate the need for a BEC?

Also, would it be "safer" charging Li-ion vs Li-Po?

A concern that has been mentioned to me in the past regarding Li-Ion vs NiCad was the discharge "curve" of a NiCad being much more progressive vs a sudden drop in the Li-Ion case.

Many thanks again.

Kind Regards
Robert
Tim Mackey13/08/2007 11:14:00
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Hi Robert, and thanks. Having never actually used Lithium Ion stuff, I am afraid I am not really in a position to answer your question with certainty.I converted all my electric stuff to Lithium Polymer years ago, and never really bothered with the Li-Ion stuff when it first appeared, as it was (IS) a good bit heavier than the Polymer versions, and I am very happy with my LiPO stuff, never having suffered any sort of "danger" or otherwise with them - ( clutching melamine covered woodchip desk fiercely )
However, 2s Li-Ion @ 6.6V is still JUST over the rating for many servos which are usually rated at 4.8V nominal operating voltage. It is usually the servos that are of concern regarding high voltage, and many receivers are quite happy on the higher volts...indeed my Spekky receivers are happy up to 9V.
Charging them is reputed to be "safer", but then as I say.... being a fairly regimented sort of chap, I never have any worries about charging my LiPos.
Yes the discharge curve is different, and nickel stuff does tail off a bit more gradually, but with the margins involved on both the flight pack and tr packs, again it is something I dont worry about. Incidentally I have just returned from an early morning ( 7AM )session up the GtOrme sloping, and after a 1.5 hr session, the tr still reads 11.2V. I last charged it around 3 weeks ago, have flown approximately 5 hours over the 3 weeks, and been using it to set up some new gear in one of my planes...including accidentally leaving it on for around 3 hours yesterday, and it is still around what I normally got from a full nickel pack !
I might give it a .25C charge later today....
Danny Fenton13/08/2007 14:03:00
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Hi Timbo, I think I can see where your cells came from LOL, same place as mine. Maybe thats why they didn't show the pics.......
Tony did say a bit more than I reproduced, but without his authorisation I was not going to repeat it. Lets just say I am glad I chose JR and if Tony is anything to go by at Macregor I will continue to use JR. Cracking service.
I use Lithium nano phosphates as power packs (A123 to most) as Rx packs even they would be a little high at 3.6 (max) per cell. They are also a bit heavy when used in packs less than 3 cells. You must also be careful to keep your model out of the sun if you are using LiPos they do not like getting warm! Do not leave a model in the conservatory after a session, it was cold and dark when it went in, but not so the next day. :(
There are also some new NiMh on the market that don't discharge on the shelf (well very slight discharge) A friend has had some of these on the shelf for three weeks now and still read fully charged
some good stuff coming along in the electric scene :)
Bladerunner13/08/2007 17:53:00
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129 forum posts
If the Tx being used supports 8-cell series NiMh/NiCd battery packs, then it will work with Li-Ion/Lipo.
8-Cell series NiMh/NiCd termination voltages can be as high as 1.5V - 1.8V per cell for a total pack voltage of up to 14V, when using charge currents between 1C-3C. This then quickly settles to between 1.4V - 1.5V per cell. Tx's are designed to cope with these peak cell voltages(although many of us still use the old trickle chargers which don't get to such a high peak).
Within the engineering fraternity, 8-Cell NiMh/NiCd is often regarded as the general equivilent of 3-Cell Li-Ion/Lipo, because the min and max levels equate so closely.
The only additional advice I can add is that ideally an on-board battery protection circuit should be installed in the battery pack. These often have min/max cut-off's set at 2.6V and 4.3V respectively which yields a pack range of 7.8V to 12.9V.
Tim Mackey13/08/2007 18:47:00
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I think the other post was referring to RX packs - there is, as stated no need to add any other components to a pack for the tx. With reference to "approval" from the manufacturer...then it is perhaps understandable that they are a little nervous at "approving" the use of LiPos in their transmitters just at the moment. I am sure it will not be too long before we see mainstream suppliers issuing LiPo packs for their units, and maybe even actually supplied with them fitted as standard.
Could you elaborate on the onboard protection circuits bladerunner?
Minimum cut off at 2.6V ( presumably you mean per cell ) seems very low for a LiPo - or are you meaning LiIon ?? Also, how does a max cut off work then ? I always use the intelligence of a dedicated Lithium charger to determine when the pack is correctly "filled".
The tr pack I use actually claims to have a PCM circuit built in...but I can find out no detail of this anywhere.
Danny Fenton13/08/2007 18:57:00
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I noticed the mention of a PCM circuit on my 2500 Hextronic pack but apart from dismantling it cannot see any evidence of it. That is my biggest fear.....leaving the Tx on
Tim Mackey13/08/2007 19:08:00
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DITTO
Danny Fenton13/08/2007 19:22:00
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what is it? great minds think alike, or is it fools never differ LOL
winchweight13/08/2007 21:27:00
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A good article thanks Tim. I have recently gone over to a Lipo Tx pack in my FF9 and it's brilliant. I haven't charged it in 3 weeks and it's still showing 11.1v!
Tim Mackey13/08/2007 23:24:00
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Good in' it !
Danny Fenton13/08/2007 23:30:00
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This should have been in print in the magazine Tim, useful relevant information, the sort of articles we used to get many years ago well done
Toby Cross14/08/2007 07:11:00
3 forum posts
Gentlemen,

I have searched for weeks on an article on how to power my aircraft on LIPOs and what I needed to reduce the voltage. Finally Timbo has written an article which I understand!! So a big thank you for that.

BEC's would appear to similar to voltage regulators. Als hobbies sell two of them made by dual sky for 9.99 and 19.99. The interesting point is the amps they provide. The 9.99 is only rated to 3 amps. My current project a Cap 232 80" ir running 2 digi servos, 3 high torques and 2 standard futaba servos. It's a 3d ship and I reckon 3 amps might be exceeded.

Cheers again for the article,

Toby
Tim Mackey14/08/2007 08:43:00
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Good morning, and thanks for the comments :-)
Toby, I would say that 3 Amp is definately too low for what you have planned. I have a rather favourite IC Spitfire which also uses high torque and digital servos, and I use a somewhat different system on this. It is called a powerbox digiswitch. Although basically still "just" a regulator in principle.....it does feature a clever integral digital switch ( which cannot be accidentally operated through vibration etc ) and an externaly mounted heatsink which allows a greater current flow, easily up to 5A from memory. Plus it has an inbuilt voltage monitor which indicates the level of your 2s LiPo pack, and will prevent switch on at all if the pack is considered too low. Although more expensive than a simple standalone BEC, I feel the extra features make it ideal for more "treasured or demanding" models. It is also very well built using excellent quality materials. Here is a link to the manufacturer.
http://www.modellbau-deutsch.com/e/powerbox_systeme/digi_switch/start.php

Other more elaborate versions are also available with even greater current capability. I hooked a DVM inline with my supplying pack, and physically moved the control surfaces ( several at a time ) to induce force feedback into the servos, and also moved the sticks to full deflection with hand pressure applied to the surfaces to check the typical current drawn in the worst case scenarios. I only ever recorded a peak of around 4A, but it might be advisable to do something similar. I have heard tales of extrememly high currents being drawn through some models systems....over 10A in one case!
PS AL's hobbies are also distributors for these items, so if you dont fancy buying direct like I did, then give Al a try.
Bladerunner14/08/2007 10:26:00
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Hi folks.

I really enjoyed reading the article, and pleased that it was so well spelled out. Well done Timbo. It won't be long before I totally convert to Lipo on all my gear - only my Tx remains NiCd.

W.R.T the protection circuits(aka PCT), I was refering to standard Li-Ion(3V - 4.2V per cell) technology, as you would find in your laptop or pda. These cells characteristics are very much the same as our typical Lipo cells, except they cannot sustain the high discharge currents safely. The battery packs have a mandatory built-in PCT as a minimim. Many also have an electronic fuel guage that measures capacity amongst other things. The two primary functions of a PCT are:
(1.) Safety of the user.
(2.) Protection of the battery pack.

The PCT prevents over-charge(voltage too high), over-discharge(voltage too low), short-circuit's and excessive temperature from damaging the battery pack. It includes a switch mechanism that can either disable charge, discharge or both. The circuit is self-resetting, so as soon as the fault condition is removed, the switch is re-activated. They are available for various cutoff configurations, depending on the application. Voltage settings range from 2.2V per cell to 2.7V per cell minimum, and 4.2V per cell to 4.35V per cell maximum. Products are designed to work within these ranges.

One major exception to the above is the new A123 Lithium Ion cells, which have a much lower operating voltage range.

Trust this helps.

Toby Cross14/08/2007 11:09:00
3 forum posts
Thanks again Timbo, I'm getting my wallet out as we speak!
Tim Mackey14/08/2007 16:07:00
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ooops sorry Toby :-)- you will be pleased with this item !

Hi Bladerunner, thanks for the feedback, and also that great explanation of the PCT. As mentioned by myself and Danny, the packs I have for my transmitters states in the website description ( would not really like to state the site I purchased them from, but it has a "U" and a "H" in it
and suggests that we should be united in our hobby - LOL ) that they have inbuilt PCT. I have tried in vain to elicit more info from them...but nothing has materialised. Considering the pack was the equivalent of just over 13 ( 3s 2500 m/a ) I felt it was excellent value even without any fancy PCT. If indeed it has such a device then it is absolutely superb !! Should there be some visible sign of such a device being present within the pack...as there does not appear to be anything "out of the ordinary".
if anyone would like me to give them the link to the product /site please PM me via my profile. TY
Danny Fenton14/08/2007 16:22:00
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Look again LOL it has an "H" and a "C" now.......
Tim Mackey14/08/2007 16:50:00
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Sorry Danny boy....you've lost me ??

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