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Transmitter LiPo Conversion

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Hi there everyone, I've decided to post a bit of a walk-through of my transmitter conversion to LiPo batteries. It's ultimately as simple as plug and play but I learned a few things along the way.

NB: The "David said" bits are an artefact of me using the 'quote' tags. I couldn't find a better way to format the text into blocks.

Is It Necessary? David[/b]']

Not really. In fact if you're not 100% comfortable with LiPo's and don't already have a LiPo charger then there's no point. This mod requires more attention and maintenance than other batteries in absolute terms. The balance changes somewhat if you already have LiPo equipment and LiPo battery management is part of your maintenance cycle. High capacity LiPo's will last ages, have an almost endless shelf life and are significantly lighter than the alternatives. Nevertheless it's still subjective and I'm not actually recommending a switch to LiPo's.

If you're looking for a hassle free alternative to standard rechargeables it seems these Eneloop's are actually the best bet (based on public opinion, not personal experience).

The Battery Saga. David[/b]']

As it turns out this was the biggest challenge. I needed an 11v flat pack for my Tx and I found a few but they were all around €50 which was a bit rich for what is really an experiment. I saw LRP, Venom, Reedy and others, all 3000mAh or more and nothing less than €65 delivered.

Then I stumbled upon a Turnigy 1450mAh at a local online store (this one) for €13 and thought that might better suit my endeavour. Two days later I was back on the hunt because it was out of stock (despite the web store saying otherwise). In the end this worked out well because 1450mAh, upon reflection would probably not be satisfactory in terms of run times.

I was back looking at €50 batteries which didn't appeal and I stumbled upon this LRP (which is common for around €70 shipped) for €30 on special at a store in Poland! Yay... except they never replied to my emails. :(

Then I found this interesting Photoshop:

TxLiPoLRP.jpgTxLiPoNosram.jpg

Hmm... look similar? (Check out the web sites here and here). :D

So I ordered the Nosram version for €19.90 from here rather than the €59.90 LRP version! Even cheaper than the Polish special I found for the same thing! :D:ph34r:

If you're thinking of doing this and need similar batteries then get these as you won't find a better deal as of the time of this post, I promise.

The Parts. David[/b]']

Strictly speaking you only need a compatible LiPo battery but for extra protection you can, and really should, add a low voltage alarm (LVA). This will help prevent killing your battery or worse, burning your house down. With a LVA even if you leave the Tx turned on you're sure to be alerted to bad situation, the alarms are as loud as a smoke detector!

If you get an alarm it needs to be compatible with your battery (2S or 3S - 3S in my case) and you'll probably also need a 100mm to 150mm balance cable to match.

I got these parts for less than €3:

LVA%2B3SExt%20%28800%29.JPG

You can pick them up from many places such as here.

General tools and perhaps some tape and adhesive is all you'll need to complete the task.

The Transmitter. David[/b]']

I have an old but tried and trusted Sanwa (AirTronics) MX-3FG 2.4GHz transmitter. I happen to think this is a great example of not having to spend enormous amounts for quality products. I've had this radio for about 6 years now and it has never missed a beat. It wasn't the most expensive one available (mid-range) but had every feature and function I have ever needed.

MX3FG01%20%28800%29.JPG

Disassembly is easy, 9 Philips screws (including the back trigger adjuster screw).

MX3FGOpen01%20%28800%29.JPG

Careful observers will notice in the above pic that the mould, even though this is a 2.4GHz system, includes space for an FM aerial. This is because the chassis is identical to the standard MX-3 and as you'll see later this proved beneficial.

The build quality of the remote is actually very good, careful attention to detail here:

MX3FGTidy01%20%28800%29.JPG

MX3FGTidy02%20%28800%29.JPG

MX3FGTidy03%20%28800%29.JPG

I'm quite impressed actually.

The transmitter comes with a charge port that I will need to disable because it will provide the wrong type of charge for a LiPo battery. The intention was to 'clip' the necessary wires but that proved impossible because the charge port uses the same tracks as the main power supply and I didn't want to attempt cutting or etching the PCB.

So I used the simple solution:

MX3FGChargePort04%20%28800%29.JPG

And used a cut-off from a parts sprue to blank out the charge port, simply glued on well enough to take the bumps but removable should the need arise.

MX3FGChargePort05%20%28800%29.JPG

MX3FGChargePort07%20%28800%29.JPG

Happy with that... :)

Next, onto the installation and the various options I need to consider...

Installation. David[/b]']

After considering the options, for example it would be possible to squeeze the buzzer into the battery bay, I decided to install it as follows, there's ample room in the recess opposite the wheel.

MX3FGSpace02%20%28800%29.JPG

And I can run it down the FM aerial channel I mentioned before...

Layout01%20%28800%29.JPG

So I added some high density foam to attenuate the sound (it's really loud!) and used servo tape to stick it to the Tx chassis.

Buzzer01%20%28800%29.JPG

Added a slight channel (to match the factory one on the other side) to increase the space available for the 4 wires.

Channel02%20%28800%29.JPG

Which resulted in quite the tidy outcome I think...

Bay01%20%28800%29.JPG

And I can still use the stock battery tray if required.

Bay02%20%28800%29.JPG

In the end it went it quite simply. It was a little bit of a challenge to line the wires up in the grip to seal the case properly but a long thin shaft (e.g. a hex driver), a torch and a bit of patience allowed me to line it all up and mate the sides.

So the battery arrived today:

MyTxLiPo%20%28800%29.JPG

I'm quite impressed with it. It fits the battery compartment perfectly, there's room for some movement (which makes tucking the wires away easy) but once the cover is on the battery doesn't move at all. The battery is very similar in dimensions to the standard AA tray.

LipoInstalled%20%28800%29.JPG

Unfortunately the battery comes with a JST-EH balance tap and all of my equipment, including the extension cable I installed, uses JST-XH connectors. I didn't know this before ordering (and couldn't have done anything about it anyway). On the other hand I did know that I'd need to change over the power connector to the "blue type" from the "black type" (what are they called?).

Converting the power was easy, the pin sockets are the same for both types so I just needed to slip them out (one-by-one so you don't short circuit!) and push them into a spare blue connector I took off a redundant receiver switch.

The balance connector on the other hand is a different story. The tabs are incompatible and simply can't be interchanged. I'd have to buy the parts and create a new connector (soldered not crimped) for the battery to make it XH... :( I did learn a lot about balance plugs in my research though! B)

So in the end I ordered male EH to female XH 100mm converter cable and will need to find a way to also fit that into the battery bay. It shouldn't be a problem but that's a lot of cable (4x100mm) plus two extra connectors and further €8 (shipped from Germany) which I hadn't anticipated. :(

Either way I can power up and use it now. I just can't balance charge it (which means I won't be charging it) and my low voltage alarm can't be tested yet. I do have a voltage readout on the LCD (currently at 11.6v) and a "super low" alarm at 9v which is built into the radio (for NiMH) that will tell me when I've just about killed the battery... :D I'm expecting the converter in a few days and I have plenty of charge for that amount of time!

Results. David[/b]']

Updated 18 March Below

The first result I was interested in before testing anything was the weight and I can report on that now. To be honest I expected quite the weight reduction from standard (non-rechargeable) AA's based on all the web commentary but as it turns out is wasn't as much as I expected:

WeighAA%20%28800%29.JPG

Verses:

WeighLiPo%20%28800%29.JPG

And when you add the alarm, the extension cable and the (yet to be installed) balance converter cable the net result will only be about 15 grams... nothing worth mentioning. Given that it's a 3600mAh battery, which is really quite large for this application, it's understandable in retrospect.

In the end this is a surprisingly pleasing result since the batteries provide real balance to a pistol radio, too little weight in the bottom would be uncomfortable and quite unstable when standing up.

Update 18 March

Okay so the adapter cable arrived and the entire system is now functioning as I need it to. The good news is that the adapter cable fits in the battery bay no problem and as soon as I plugged it in the LiPo alarm beeped to let me know I've wired it up okay and it's functioning. The volume is perfect, not too loud but clearly audible, even from a nearby room. The insulation created by the case and the extra foam I added works well.

I'm very happy with this setup now and will record my summary thoughts in the second post below.

More to come... (field testing) :ph34r:

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Summary

I'll update this section now that I've finished once I've done some field trials this week.

Was it worth it?

Yes, if you can get a 3600mAh LiPo battery for €20 as I did otherwise I'd say 'no'. In total this cost me €35 which I could have reduced to €29 had I not needed to purchase a LiPo balance cable adaptor. I do not already have NiCad batteries or a charger so there was no throwaway costs for me. The retail price for a 3600mAh 3S LiPo is still around US$50 and at that price I'm not sure this mod would be economical.

Is it safe?

Yes, totally. I'm familiar with LiPo batteries, have the proper equipment and know how to care for them. Adding a low volume alarm as I did is an essential addition for the €5 or so it costs to have it delivered.

What about the alarm?

Well it slipped right in and works as expected. I added some foam to the front of the horns to attenuate the noise and the result is ideal. It's not piercing but it's still loud. Quieter than a smoke alarm but louder than a bedside alarm I would say. The one I purchased came with LED's but these aren't really necessary except for verifying the initial hookup.

And the weight?

The 3600 battery is actually quite hefty and with the added cables the net difference is only around 15 grams which isn't even noticeable. In retrospect had it been much lighter it might have been a concern for the stability and centre of balance of the radio. I'm very satisfied with the weight.

Does the battery fit okay?

In this case yes but this may be an exception, I'm not sure. The battery is Nosram which is a relabeled LRP and LRP have a relationship with Sanwa who made my radio. The battery is specifically marketed for the Sanwa/Futaba/AirTronics range of radios so may not fit in others. But in my radio it fits perfectly.

Some tips.

- Make sure get get the correct battery, voltage, number of cells and size are all important.

- Check that the balance and power taps are compatible with your ones or be prepared to do some soldering or swapping of plugs.

- Do utilise a low voltage alarm.

More to come.

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Have you found that the Lipo alarm will slowly drain the pack when not in use? I have one that continuously displays the cell and pack voltage on a 3 digit LCD, I assume that after (quite a long time, admittedly) it will flatten the pack to where the alarm sounds, thus flattening it further. If I happen to be out of the house at work or on holiday, this could end up with a completely flat pack!

I use my alarm to monitor a cordless drill pack I converted from 18V Nicad to 5S Lipo. I ended up fitting the alarm on the outside of the battery pack and I just plug it in when I use the battery to avoid the constant drain.

That concern is what led me to use LiFE in my Tx conversion, I used a 3S LiFE to replace 8 AA cells, I understand that LiFE needs no cutoff (or, more accurately, the cut off voltage is so low that the Tx will stop working effectively before that point is reached). I haven't found anything concrete on this matter yet though!

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Have you found that the Lipo alarm will slowly drain the pack when not in use? I have one that continuously displays the cell and pack voltage on a 3 digit LCD, I assume that after (quite a long time, admittedly) it will flatten the pack to where the alarm sounds, thus flattening it further. If I happen to be out of the house at work or on holiday, this could end up with a completely flat pack!

I use my alarm to monitor a cordless drill pack I converted from 18V Nicad to 5S Lipo. I ended up fitting the alarm on the outside of the battery pack and I just plug it in when I use the battery to avoid the constant drain.

That concern is what led me to use LiFE in my Tx conversion, I used a 3S LiFE to replace 8 AA cells, I understand that LiFE needs no cutoff (or, more accurately, the cut off voltage is so low that the Tx will stop working effectively before that point is reached). I haven't found anything concrete on this matter yet though!

Thanks for the reminder, I needed to come back and update this post. I'll add some comments into the main post above shortly but in the meantime:

Yes, I have found this even though mine doesn't even have an LCD, it does have LED's but they're only activate at start-up and alarm conditions (I can't actually see them). Worse, my alarm causes discharge in an unbalanced fashion so it's far from ideal (it was only US$1.89 after all!).

I've quickly reached the conclusion that I need to remove the battery from the radio for any length of storage. I think it will drain in less than a month from a storage voltage to a low voltage with just the alarm connected. And as you say, if you're out when it goes off it's not much use.

And that's with a 3600mAh battery which is a far higher capacity than many Tx batteries! In fact, if I didn't get such a big battery for such a good price (€20) I'd be a little disappointed with this change. However the 3600 battery makes all the difference - I can't drain it now when I'm out, however much I try. I was going to get a €10 1450mAh originally, glad I didn't!

When I was researching this mod I looked into LiFe batteries and despite some being available for Tx's, most of what I read recommended against it unless you set your alarm at quite a high voltage. Apparently you need a significant buffer (>0.3v/cell) when using in a Tx because the discharge curve is so flat that, as you say your Tx will die before the alarm goes off. You do not want this because you lose control of the vehicle! :)

Whether LiPo or LiFe you also need to remember that each cell needs to be checked for voltage, not overall battery voltage so your alarm needs to be able to read the info coming off each cell (via the balance tap).

e.g. in a 3S LiPo if you only check total voltage the following would not set off the alarm whereas you would want it to for sure:

2.9v + 3.2v + 3.6v (= 9.7v = no alarm)

Which is also a good reason to balance your batteries... I had one of my cells drop to 2.6v on the alarm and considering the terminal voltage for a LiPo is 2.5v (safe "low" voltage is 3.0, preferred "low" voltage is 3.2) I was very, very lucky. It took me a total of nearly 30 hours on the charger to re-balance the pack! :(

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...Apparently you need a significant buffer (>0.3v/cell) when using in a Tx because the discharge curve is so flat that, as you say your Tx will die before the alarm goes off. You do not want this because you lose control of the vehicle!....

Hmmm....Isn't that exactly how a transmitter works when using alkaline cells? Most transmitters already have some sort of low-voltage warning built into them to keep you from loosing control of your model.

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Isn't that exactly how a radio works with alkaline cells? Most transmitters already have some sort of low-voltage warning built into them to keep you from loosing control of your model.

Yep, if it has no alarm - which is why an alarm is good. The question is does one want to avoid this and the answer should be "yes" hence setting the alarm higher that the Tx fade level of the batteries. Heaps of excuses not to (e.g. my ESC has a Tx failsafe and centre's the steering and brakes to a halt when it loses the signal) but it's still a situation you want to try and avoid.

Most transmitter alarms also only work on total voltage so if your batteries aren't balanced (as they discharge) then that alarm may not do what you need it to... bad puppy! :angry:

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Yes, I have found this even though mine doesn't even have an LCD, it does have LED's but they're only activate at start-up and alarm conditions (I can't actually see them). Worse, my alarm causes discharge in an unbalanced fashion so it's far from ideal (it was only US$1.89 after all!).

Which is also a good reason to balance your batteries... I had one of my cells drop to 2.6v on the alarm and considering the terminal voltage for a LiPo is 2.5v (safe "low" voltage is 3.0, preferred "low" voltage is 3.2) I was very, very lucky. It took me a total of nearly 30 hours on the charger to re-balance the pack! :(

Ooops, I meant LED not LCD, fat fingers/slow brain error!

Also, I can't see why anyone wouldn't balance charge Lipo/LiFE packs every time to be honest, it's not like it takes a huge amount of extra time and if it improves the longevity of your batteries, why not?

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Yep, if it has no alarm - which is why an alarm is good. The question is does one want to avoid this and the answer should be "yes" hence setting the alarm higher that the Tx fade level of the batteries. Heaps of excuses not to (e.g. my ESC has a Tx failsafe and centre's the steering and brakes to a halt when it loses the signal) but it's still a situation you want to try and avoid.

Most transmitter alarms also only work on total voltage so if your batteries aren't balanced (as they discharge) then that alarm may not do what you need it to... bad puppy! :angry:

Mountains from mole-hills. LiFe can be run flat. No worries about balance etc. Transmitter has low-voltage warning to protect your model. No worries.

If you're using lipo, you must use a LV-Alarm.

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LiFe can be run flat. No worries about balance etc.

That's simply not true. They can be both run flat and unbalanced. I don't know where you got that information from.

Transmitter has low-voltage warning to protect your model. No worries.

My point is that many (most?) don't have a suitable alarm (adjustable and per cell). You've predicated your statement on the incorrect assumptions above. Since LiFe batteries can become unbalanced and since some Tx alarms are unsuitable then then a proper alarm is recommended in these circumstances.

It's risk mitigation and I'm not making it a mountain at all, I'm just not calling it a molehill when it's not that either.

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Okay I see. I always thought LiFe could be run flat from the use of A123 cells in power tools. So a little research shows a 2.5v bare minimum per cell. My Futaba transmitters have an alarm or indicator at 8.5v (with multiple indicators along the way) and stops working beyond that. A 3S LiFe pack has a minimuim voltage of 7.5v If I believe the pack to be balanced.

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Yes, that's the principle although common practice is to set a LiFe low warning at 3.2v and those "pushing the envelope" would set it at 3.0v so you'd commonly set the alarm at 9.6v on 3S. 9.6v leaves a margin for unbalanced cells, 9.0v is okay if you know your cells are balanced.

An important characteristic of LiFePO4/LFE/A123 batteries that needs to be factored in is that they dump voltage rapidly from 3.2v, far faster than a LiPo. If the alarm goes off at 8.5v (2.8v / cell on a balanced 3S pack) then you might not finish the lap before they hit 2.5v. Some of these batteries also have a minimum voltage of 2.8v not 2.5v so you need to be extra mindful of that.

Most battery power tools have smart batteries in them with low voltage cutoffs, hence your prior experience. RC uses "dumb" batteries.

I learnt all this researching this Tx LiPo install actually... :) Here's a a representative thread I saved - clicky - and yes I understand planes face different risks from cars when a battery dies... :D

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