I have a fairly old Spektrum DX7 which uses an 8 pack of NMH batteries. That should be around 9.6 volts I think. when charged the Tx shows a voltage of close to 11 volts that drops of pretty quickly to around 10.5 volts, and then slowly goes down as I use the Tx.

The charger is a very slow overnight type as supplied with it.

I was wondering if I could change this over to a 3S lipo pack at 11.1 volts. it means I could use a fast 1 hour charge (from flat), and a 2000mha lipo is a lot cheaper than having a 8 cell NMH 1700 mha pack made up by the local battery shop - last one cost me $AU70.

I’d probably have to factor in half a roll of duct tape to hold it in place.

Is this a viable project! Or am I missing something?

A 3S LiPo hot off the charger can be 12.6 volts, and stays there a while, before slowly dropping. A NiMh may read high at 10v (or a bit more) initially, but it very, very quickly drops. Nominal voltage for the TX is 9.6v. If it doesn't produce a puff of smoke, the stress of the higher voltage will greatly shorten the life of the TX.

Use a LiFe cell instead. They have a lower initial voltage and are much closer to the nominal 9.6v. Many sources for a TX replacement pack. Some may even fit! You will need a charger that has an LiFe setting.
Still the 1 hr charge time, MUCH safer from fire, less need to reduce to storage voltage when not in use (a good idea anyway, but not as detrimental if you do not).

LiFe, beyond lower voltage, also may not have as high a "C" rating - so not favored for motors. But in this instance, where high current is not needed, is a better solution.

There are LiFe TX batteries available for JR/Spektrum transmitters. Only drawback is you need to remove them to charge them. You could make an adapter to plug into the side of the TX but you still need to pull the cover to access the balance plug. Lipo may come off the charger with more voltage then the TX can handle.

Thanks both for the advice.



Given the option I would rather avoid puffs of smoke, that said it might make an opening for that coveted Spektrum NX8.



I will look into sourcing a LiFe pack. My charger will do these batteries.



Just need to source a Spektrum battery plug, or just open the unit and solder in a different socket to suit the battery. Now, where’s my duck tape?

FYI, the normal servo plug will plug into the TX power input connector. You would just need to be careful as it is not polarized and if you get it backwards the TX is toast.

So, I have had 4 different lipo packs wired for TX use, they all have come with a standard servo type lead,,, the Futaba TXs only have two pins for it battery connection, unless spectrum is somehow different, if you plug it in backwards, one pin just doesn't plug into anything,, you can't plug it in reverse polarity with out purposely modifying the plug

My DX7 has a small 2 pin connector so a servo type lead will not fit.there looks to be a compatible plug called a Micro JST that could fit. I may order in a pack from eBay and see if they work. At worst I am out of pocket a few dollars.

Unless something changed, I ran a LiFe battery in my DX7 ( 10 years ago ) that had a standard servo plug. They do look different but the pin size and spacing were the same, just two on the TX PCB as opposed to 3 on the servo connector.

Here are a couple of pix of my Spektrum DX7 showing the small 2 pin plug and socket, against a standard 3 pin servo lead.

Yep, same as mine. I clipped the two tabs off the TX connector and connected the servo connector to the two pins being careful to get the polarity correct. At the time I did not have my old battery otherwise I would have just spliced the old connector onto the new battery.

^^^^^
Aaah,, that makes more sense now,,


This is the last one I bought looks like it comes with that connector, the last product review is mine

If you cut off the tab on only the positive/right side in the Tx, then this forces the servo lead connector to be offset to the right. If it up the correct way then the Negative and Positive will be correctly aligned with the spare Signal contact sticking over the edge where the tab was cut off.
If placed upside down, then the Signal contact will be on the Negative terminal and the Positive will still be on the positive terminal, so no harm will be done by getting the connector the wrong way around, it just won't work.

So trick is to not remove the tab on the left/negative terminal.

I hope my logic is sound.

But then what if you wired the servo lead as Neg, Pos, Neg? Then it would not matter which way you plugged it in as it would work both ways (making sure that the tab on the Tx left/Negative side stays intact.