LIpo's for tx

[Tim Mackey]

I usually advise recharging Tx Lipos at around 11.5V or at the very lowest....11V.

Remember at 11V you have LESS THAN 10% capacity remaining - pretty close to danger for a radio system. These low drain situations are not like we have on our current hungry flight motors, and the drop from 11V down to a critically low point occurs pretty quickly. I have done tests after fitting my LV alarm system to mine, and found that it fell from 11V down to 10V in about 15 minutes -  only about one more short flight! I certainly believe that if using a Lipo one should make "other arrangements" for a LV warning at the appropriate level - most TX alarms are set to sound off at around 9v or maybe even lower, and this is a disater waiting to happen. The hidden engineer menu settings allowed me some alteration of the LV point in the DX7 ... but even that was still too low.

[James Hindle]

Danny,

 I recharged my lipos from 11.5, i had used the lipo in my TX for 9 months or so for probably 2-3 hours per week (not flying mainly building with the weather !!!)

[John Cole]

Would it be better (for 8-cell Txs) to consider A123 LiFePo4 cells, with nominal voltage 3.3V and working range 2.0 to 3.5V?  A three-cell pack would give a voltage range of 6.0 to 10.5V so the max voltage would be within the NiMH max (I see 11.3V after a full charge) and the safe minimum well below the Tx cut-off (for me 8.4V)?  A123 ANR26650M1 cells offer 2300mAH.  See:

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6444 and

http://www.a123systems.com/#/products/p1

[Myron Beaumont]

John

What a revelation having followed all the links i could find .OK they might be heavy BUT !

Couldnt find their sizes though - .Important for our applications eg   three 18650 cells for a Tx flat pack (Futaba 2.4 battery compartment )  maybe there is a market for an extention type box to enable these batteries to be  used if they wont fit in ? I can always make up one for myself of course - its not rocket science after all. Comments please gentlemen .I get the feeling that lipos will go the same way as Nicads before too long . However I'm very happy with my new lipo Rx battery & voltage reducer  /

  ( just another "must have" on the lekktrikery seen maybe )

[John Cole]

18650 are half the capacity of 26650, and the latter weigh 70 gm per cell (210 gm total).

The bigger cells are 26 mm dia and 65 mm long.  18650s are 18 mm dia and 65 mm long (the original Li-ion packaging size).  You could of course use 6 of these smaller ones to get 2000 mAH capacity: 2P 3S.

Not a good shape-match to AA.  Would I think need to be external.

[Myron Beaumont]

John

Yep ! the battery compartment is 11.5 cm by 5.5 cm by about 14mm depth .& AA's are a perfect fit . The depth problem (4 mm  too much ) can be overcome But I can't find a way of fitting in 6 small batteries having done a "scale drawing" They are you say 65 mm long & can't find any scenario where I could fit them in .Not even at an angle ! In fact according to my calculations I could only get four of the smaller ones in  .Plan B is already in operation ie three sets of nicads and my meter in the flight box !

[John Cole]

Don't overlook the new-generation NiMH cells I referred to earlier: 2000mAH capacity and good charge-retention, and inexpensively available in an 8-cell AA flatpack (with Futaba plug).

 The LiFePo4 cells don't seem yet to be available in AA format (you could then use 6 in a 2P 3S config and remain within the Tx pack-space); ordinary Li Ion cells are, but not this new chemistry.

These new cells seem to be aimed at power tools where their ability to accept charging- and discharging-abuse will be every useful, and the AA size may be too low-capacity for that application.

[Myron Beaumont]

John

I've been trying to obtain a flat pack NimH for a while now for the Tx  but cant resource a single one in this  country . By the way -What do you mean by " New Generation" ? It seems from what I've read on the forum that high mAh's does not necessarily mean high amperage available.Of course this is not relevant to Tx installations ie 200 mA  consumption for example.

Thanks for info etc  Myron

[Tim Mackey]

He means Vapex and so on. ( google it )

Indeed....many NimHs are poor at current delivery

[260 Flyer]

Myron

Vapex Instant Tx pack.

Rob

[Myron Beaumont]

Thanks Rob

Very good price & spec Eh !    Just wondering which of my many chargers I could utilise . I have that many I think they're breeding under my bench  

[John Cole]

Note: the GREEN ones (lower capacity - 2100 mAH) are the ones with improved charge retention.

[James Hindle]

Just noticed the article in the December issue, well done David for pointing out the implications for using lipos in main brand TX's, it was a very honourable move bearing in mind the main brand Lipo advertiser who I have had some rather disappointing arguments with. 

[260 Flyer]

My Vapex Instant NiMH pack charges up to 11.8V which isn't far off LiPo territory. I wonder if these invalidate the warranty too?

[Tim Mackey]

Now you see this is where the argument lays... any half decent nickel based cell can show up to 1.6V fresh off charge, so 8 x 1.6 = 12.8V. Even at the more noirmal 1.5V its still 12V.

Fresh 3 s Lipo = 12.6V.... I rest my case .

From someone who has been using Lipo packs in his DX7 for almost 3 years

Caveat. As mentioned in the article I wrote on here ages ago about doing this mod..... I feel one of the main keys to success is to change the cheap and inefficient linear regulator.

However, I also accept that with the advent of low self discharge NiMh eneloops and others, this is probably a better choice. I use these in my DX6i and am very pleased indeed.

[David Ashby - Moderator]

James - I just kept it factual in wanting to reflect the UK distributor's views on the subject really.  The BMFA have promised me a statement on the subject detailing their perspective, stay tuned.....