Spektrum DX3r Battery Mod

[BrianG]

There is a thread over at nitrokillers where someone (Jason IIRC) used lipos instead. The problem is that the DX3r wants 6v, and lipos are too low in 1s (3v min and 4.2v max) and too high at anything above LVC in 2s (6v min and 8.4v max) to get the right voltage. Normal radios that take 8 X AAs function just fine on 3s since the voltage of 3s is ~9v min and 12.6v max.

He used a BEC and a low-voltage cutoff/alarm, but that's getting a bit pricey just for a radio battery solution IMO. Not to mention what would happen if the BEC went bad and outputted the full 2s (or higher) battery voltage. Not sure how sensitive these radios are to overvoltage, but 8.4v (2s @ 4.2v/cell) is quite a bit higher than 6v...40% higher to be exact.

[t-maxxracer32]

oh your right.

AAs are not a big deal i was just wondering if it would be a simple plugin.

guess not!

thanks

[MetalMan]

Originally Posted by BrianG

He used a BEC and a low-voltage cutoff/alarm, but that's getting a bit pricey just for a radio battery solution IMO. Not to mention what would happen if the BEC went bad and outputted the full 2s (or higher) battery voltage. Not sure how sensitive these radios are to overvoltage, but 8.4v (2s @ 4.2v/cell) is quite a bit higher than 6v...40% higher to be exact.

Here we go again . The cost isn't that much - $10 cheap switching BEC, maybe $5 for the components to make your own dead-bug style audible low voltage alarm (schematics are at RC Groups).
The chance of the BEC failing in the TX is MUCH less than if used to power a RX at a higher input voltage. So, you are much more likely to have a fried RX than TX due to switching BEC failure.

[t-maxxracer32]

well is it really worth it to buy the materials when you have 5 rechargeable AA's sitting around the house?

i think that is what Brian meant.

[BrianG]

Originally Posted by t-maxxracer32

well is it really worth it to buy the materials when you have 5 rechargeable AA's sitting around the house?

i think that is what Brian meant.

Yeah. I already had the cells from my DX3.0, so it was essentially a "free" mod in my case.

And, I don't think anyone knows for sure exactly what causes the BEC to go bad. We are assuming overcurrent can do it (which is probably true), but there could be other issues as well like a weak/faulty component, heat (it is enclosed inside a radio box after all), voltage spike, or any number of circumstances. Personally, it's not worth risking a $300 radio just to be able to say "I have lipos in my radio". Is it cool? Sure. Is it practical? hmmm.

[MetalMan]

I guess there are those who like a challenge, and those who prefer against it.



JK

[BrianG]

lol, hardly a challenge. I've never been a betting man, and it's worked well for me thus far.

[Mike_D]

Mod fight! You boys need to get along.

[BrianG]

Who's not getting along? I stated my opinion, he stated his (even though it was wrong), my opinion means more to me. End of story.

J/K. I'm sure using the lipo is his is cool and all, I just don't think it has the "bang for the buck" factor.

[MetalMan]

Well, different circumstances, different results. ALL of the Lipo-related stuff in my TX was stuff I had laying around. The switching BEC was one from an old project (caused glitching in my FM equipment), the LV alarm components I had left over from a prior order, and the 2s 2100mah Lipo was salvaged from a 4s2p Maxamps pack that had a couple puffed cells.

Brian mate, EXCELLENT mod! Nice and quick/easy to do, but you weren't kidding about those short cables lol.

I've been having serious issues with my DX3R's range with my HPI Baja and a modded Pico fail safe with a whole bunch of different receivers so I'm hoping this will fix the issues :D

Again, thanks a lot mate, much appreciated!

[Kenneth Nygaard]

Hello.
Is making my Dx3R with LiPo.
Anybody know how many volts it can handle the highest!
Can you possibly put 2s LiPo in Without the bec.
Have a bec that can provide either 5 or 6v.
It might just be better if I could escape bec while using spectrum voltage alert!

[BrianG]

Well, I've run mine off 5 freshly peaked NiMH cells, which is 6.5v and it works fine. However 2s lipo is 7.4v nominal and 8.4v "peaked", which I wouldn't try. I think it would be best to use a BEC, use 5 NiMHs, or 4 Alkalines.

[BrianG]

I was contemplating switching over to 2s lipo, but was concerned that the nominal and peak voltage of 2s would be too high. So, I contacted Spektrum and listed my mod and asked about using 2s lipo.

Originally Posted by My Email to Spektrum

I have your DX3r radio system. I know it works fine on 4 AA
batteries, but I like to use rechargeables. But, 4 NiMH cells is only
4.8v, not the 6v that 4 alkalines provide. So, I modified the battery
tray (removed the spacer) to use 5 NiMH cells. This has been working
great for the last several months but was wondering what the max voltage
the radio can take? 5 NiMH cells is exactly 6v nominal, but can get as
high as 7v freshly "peaked". I usually wait a little while to let the
battery voltage drop back down a little to be safe.

Ideally, I'd like to use 2s lipo, but 8.4v peak voltage is quite a bit
higher than the rated 6v of 4 alkaline AA's.

And yes, I know I voided the warranty in performing the modification.

Thank you!

And their response:

Originally Posted by Jeff at Horizon Hobby

Hello Brian,

Thank you for contacting Horizon Hobby.

You would be completely safe using just 4 rechargeable as the system is
designed to operate that way, modifying it to use 5 cells is not going
to electrically damage it as it will also work safely up to 6 volts. DO
NOT use a 2 cell LiPO as that will defiantly damage the transmitter.

Again, thank you for contacting Horizon Hobby, if you have any further
question, please feel free to contact us again.

Jeff
Product Support Team Member
Horizon Hobby
4105 Fieldstone Rd.
Champaign, IL 61822
877-504-0233
217-403-3582 fax
productsupport@horizonhobby.com

So, it seems that 2s is a no-go, unless regulated. But if regulated, the voltage meter won't show the true battery voltage which risks discharging them too low. Regardless, I've found that the Spektrum voltmeter is not accurate anyway, it seems to be always ~0.4v lower than what the batteries really are. But, it looks like 5 NiMH cells is fine.

Despite the system being designed for 4 rechargeable cells, the cutoff voltage of ~4v (~1v/cell) is a bit low and range has to suffer. So, I think I'll stick to using 5 NiMH cells...

[BrianG]

To bring up an old thread once again, I thought I would mention an alternative to lipo for the DX3r. Really, the only reasons for going lipo in a transmitter is flatter discharge curve and lower self-discharge. Well, how about using some LSD (low self-discharge) NiMHs? Doesn't really solve the flat discharge curve, but the high LSD of regular NiMHs can be a PITA if your radio is sitting on a shelf for a couple/few weeks. So, I've been doing a little reading and found this: http://www.stefanv.com/electronics/l...discharge.html. The cells tested here dropped only 0.04-0.07v after 7 weeks sitting on a shelf. That's a heck of a lot better than the Duracell NiMHs I'm using now. I'd probably opt for the cells that dropped the least, which is around 0.04v after 7 weeks, but even the worst ones would be better than nothing. Sanyo would most likely easiest to find, and I think I remember my LHS saying something about some LSD batteries they had. I might have to check them out.