Using Li-Pos to power your R/C gear

Using Li-Pos to power your R/C gear

An Orca 3s 11.1v 2600mAh Transmitter Li-Po battery

Using Li-Po's to power your receiver and transmitter? Why would you want to do this? Ok, let me start by looking at some advantages –

 

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Li-Po’s are always ready for use, they have hardly any self-discharge, unlike NiMH cells.

They’re lighter than equivalent capacity NiMH cells (usually an advantage in aeromodelling).

They wont drop voltage to the level that the airborne radio may stop working.( see below )

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Larger capacity means longer flying sessions.

Li-Po batteries are getting cheap – cheaper than NiMH batteries in some cases.

and the disadvantages?

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Li-Pos need more care needed when handling – especially charging/discharging.

Other components are generally required to adapt to purpose.

A Li-Po compatible charger/balancer is required.

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Before we get stuck in, lets do a brief, and hopefully simple to understand overview of our needs, and how the normal set-up operates. For simplicity I’ll make some generalisations here…..

The average club-size model has around 4 or 5 standard servos, powered, along with the radio gear, by a 4-cell ( 4.8V ) nickel battery pack, of maybe 1000 – 2000 mAh capacity. Some set-ups use a 5-cell battery (6V) and it is generally the servos that must be checked for compatibility with this higher voltage. As almost all current radio gear is designed to be powered directly by this normal 4-cell battery, no other components (apart from of course a switch harness) are required. Charge the pack up, (some can be fast charged as an alternative to the normal overnight slow charge) and go fly ! Simple.

Apart from routine checking of all wiring, switches, and especially in the case of the NiCd the occasional discharge/charge cycle that’s pretty much it.

Now, with the Li-Po route, it is a little more complicated – but I for one feel the advantages mentioned above far outweigh the disadvantages. Without getting into too much about Li-Pos generally (that’s another pretty well documented topic elsewhere) a few things need to be considered.

A single Li-Po cell has a nominal voltage of 3.7V, this means the average middle road voltage of the cell, fully charged it is 4.2V and when empty, approx 3V. Nickel is 1.2V nominal, 1.4V full, approx 1.0V empty.

It can be seen therefore that a single Li-Po cell is too low a voltage to power our radio. Two cells together in series, is a bit too high (8.4v when fresh off the charger) so we need to adjust things a bit. The normal way to achieve this is with a device commonly known as a B.E.C. (Battery Eliminator Circuit) This component is basically an electronic regulator which can receive a varying level of ‘Input’ voltage, but always supplies a steady (usually fixed) ‘Output’ voltage.

BEC FOR ME?

There are things to watch for when choosing such a device – one obviously being that it supplies the correct output voltage required. Many have a little switch which can be set to give either 5V or 6V but we will aim for 5V for now, as this is approx the voltage from a loaded 4 cell nickel based battery after charging.

Another factor is the current capacity of these devices. Many cheaper and smaller ones can only handle 2-3 amps, and although small lightweight models using only a couple of regular or small servos may be fine, a larger model – especially if using digital servos, may well consume more than this. I would say that for most everyday types, a 5A capable device should be fine.

So, having got our BEC regulator, we hook the input to our 2s (8.4V when full) Li-Po pack, and the unit has a fly-lead with a plug on the other end which simply plugs into our receiver where the battery normally connects. Voila !

Having previously suggested that you should use a 5A capable BEC, I have actually just installed 4A versions in my last two conversions, but as they are not 3D extreme machines, and do not have digital servos I am happy that this current handling limit is fine. These particular devices also have a great built in extra. A loud, low-voltage warning alarm. More on this later……

Now, I should return to one of the claimed advantages listed earlier : “They wont drop voltage to the level that the airborne radio may stop working”

Now this is potentially true because in the case of the flight pack system, the BEC device I use has a low voltage alarm, that sounds when the pack drops to 6V. This is really good news for 2 reasons… 1) 6V is the absolute lowest level at which our 2s flight pack should be re-charged, Li-Pos should NOT be allowed to drop below approx 3 volts per cell as this can do irreversible harm. 2) Even at this 6V level, the BEC device will still supply 5V to your receiver and servos for a short period of time

So effectively you never have a flat battery onboard.

Just another note of caution, please remember to disconnect the onboard pack at the end of your flying session, to avoid the possibility of a very small trickle discharging of the pack which could allow the battery to fall below this critical 6V level. This is just good practice really, as in all electric flight procedures. Also you must use a good Lithium capable charger for any Li-Pos. A balancer with the correct multiplug fitted for your chosen pack is another worthwhile investment. Remember also that at 7.5V your 2 cell LiPo pack has less than 20% capacity, and will soon get down to 6V !

This BEC unit is being used in an electric flight application

THE TRANSMITTER

Now for even better news about this converting lark – I’ve also done my Transmitters! BRC Hobbies (where incidentally I got my BEC/Alarm devices too) are now supplying 2500mAh 3s Li-Po packs which are a drop-in fit to the transmitter, and they are actually CHEAPER than your average NiMH pack !!

I elected to cut the wires from the existing pack, then I added 2 x gold 2mm connectors (my standard electric flight connectors of choice) to the pack, and a matching pair to the short tails remaining in the Tx case. This enables me to easily and quickly remove the pack from the case for charging when required – which is not very often!

Freshly charged, the pack reads a voltage of 12.6V – absolutely the highest it is likely to reach, and in fact after a few minutes resting, this drops to around 12.5V.

Now I know this is above the usual voltage of an 8-cell nickel pack but bearing in mind that even this could easily show 12V when freshly charged, I am not worried about the extra .5V and in practice, it has proved to be fine. The joy of being able to fly for hours at a time, and still see the Tx meter showing over 11V at the end of the day is great, and matched only by the satisfaction of knowing that I can charge the pack up and leave it to stand for months on end, knowing that it will be ready for instant use whenever required. Having read of a few instances where the main power regulator has failed when used with this higher voltage battery, I also elected to change this component in my DX7 set for a better switching, high power unit. I strongly suggest you do also if you are contemplating the modification.

Of course, doing this mod will undoubtedly void your warranty if you still have one. There should be no real danger of flying past the safe operating minimum voltage of the Tx, as again, the Li-Po pack should be charged when it reaches its safe low level, which in this case (3s pack) is around 11 V or slightly higher – the ususal 9V “empty” figure is only applicable in high discharge useage, and for safety’s sake you should NOT get anywhere near this before re-charging. Keep an eye on your Tx meter as normal practice, most have an alarm to warn you of impending low level, often just around the 9V threshold, but as mentioned above do NOT wait for the pack to get below around 11V before you recharge.

SQUEEZING ‘EM IN

OK back to our aeroplanes! In order to obtain the capacity I require (2000mAh)and for it to fit into a particular space within a certain fuselage, I had to adapt 2 separate 2s packs, and I attach a picture showing how I did this.

Basically, the 2 packs are joined in parallel EG: negative to negative and positive to positive, this doubles the capacity of each individual battery pack, but the overall voltage remains the same.

The balancer plug was removed from one pack, and extension tails fitted to basically parallel them up. I doubt that I shall bother to balance very often, as they are only being discharged at a comparatively low rate, and also are unlikely to get too close to the safe minimum level either, but without the balance plug, it would be impossible to do at all.

A word of warning now. Firstly, you must ensure that both individual packs are each charged and balanced to the same levels before you attempt to join them in parallel. Secondly, cut and join each wire as required individually, covering the joint with heat-shrink insulation immediately after joining – this avoids any risk of the exposed ends touching other wires and shorting out another cell, which is BAAAAD!

NOT FORGETTING

Of course, it may well be that in your circumstances a single battery pack of suitable capacity (I suggest a minimum of around 2000 mAh) will fit just fine, so this pack modification will be unnecessary. The only thing to remember is that you want a 2s pack, not a 3s. Some regulators (BECs) are fine with a higher input voltage, indeed the version I use in my Spitfire can have an input of anything from 6V up to 30V, yet I still use only a 2 cell pack. A couple more things to bear in mind now…..

A higher input voltage has to be reduced even more to bring it down to the 5V output we want, and this generally produces wasted energy and heat.

The BEC/alarm device we are discussing here relies on using a 2-cell pack in order for the alarm function to work correctly, so frankly, it would be daft not to do so!

So, really that about covers it. Obviously there are several aspects that need considering before you decide to go the Li-Po route, and rather than elaborate too much, I will summarise some of these factors.

Good soldering skills are a must when fitting connectors and adapting packs etc

The balance point of your aircraft should be considered and checked after fitting a Li-Po pack, remember it may well be lighter than the old pack.

Be especially vigilant with your charging procedures and always remove packs from the airframe and Tx case before charging if at all possible. If this is not possible, then at least be present during the whole charging procedure to monitor things safely.

There is no proof that using Li-Pos will eliminate the chance of the dreaded ‘black wire disease’ so again be vigilant and regularly check all wiring, switches, and connectors etc for problems. One online UK R/C hobby supplier has now started stocking a 3-wire cable which has the negative wire sheathed in clear plastic, which can help to show the effects of this corrosion before it is too late.

I’m very pleased with the results of my conversions, and heartily “commend the idea to the house!“ If you'd like to leave a thought or have a chat then why not join me in the forum.

Tim Mackey

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