TX Battery Replacement
I’ve had my Spektrum DX7 system for about 4 years now and noticed lately that the stock 8 cell NiMH battery pack was getting a bit tired. This is a 1500maH which lasts a long time, but it seemed I was charging it more often and the final voltage after charging was less than in the past. So, I started looking for a replacement.
For charging the NiMH pack, I used an old FMA Direct Einstein charger, and it took sometimes 12 hours, typically overnight because it could only charge at 0.1A. This was ok in the old days but now my expectation is different and I want to charge in an hour or less like my other packs. My DX7 transmitter manual says to not fly below 9.0v on the battery but does not list a maximum. My habit was to charge the NiMH when it got to 10v just to be safe.
How About LiPos? I have some friends using 3S LiPos for this application, but the 11.1v just seemed like too much to me and I have read online that some people have had trouble with this. I found the following link which included a comment from the Horizon Hobby tech support guy recommending against 3S LiPo as their higher voltage can burn up the RF deck. No RF (Radio Frequency) means the tx does not tx anymore. One industrious individual must have looked up the internal regulator datasheet and determined it has a max rating of 25v so it’s not the limiting factor. OK, I think I’ll look for another path.
So, 3S LiPo is too much (3*3.7=11.1v), maybe 3S A123 batts which are 3.3v/cell would be ok (3*3.3=9.9v). I pinged my buddy at www.rclipos.com and he told me everyone he knows is going with the LiFe batts these days. “LiFe” while also describing the chemicals involved from the periodic table has also been branded by Hyperion, clever move. Li stands for Lithium and Fe stands for Iron. Another point of reference is that LiFe is the same chemistry as A123, although A123 is also a brand name.
These Hyperion tx batts and the ones I bought. One source is below.
At the bottom of the page I even found an adapter for my radio, and it all fits nicely in the transmitter cavity. Woo hoo!
Hyperion LiFe Tx Pack
Hyperion Brochure and Datasheet
I’ve used A123 batts before on some planes constructing my own battery packs like my electrified Sig Fazer and Ritweing TL60, TL40); in this case I would use 5s A123 to be about equivalent to 4S LiPo. Below is a comparison pic.
4SLiPo(top) and 5SA123
The A123 chemistry shows a more flat discharge curve than LiPos (which is good) but when they start to die, they fall off the cliff very quickly (which is bad).
I use a Hyperion EOS Sentry to monitor the state of charge of my LiPos and it works well, but the flatness of the A123 discharge will likely make a123 use a bit shaky on the % capacity function, but I expect the direct voltage reading should be ok.
There is a nice paper here from Hyperion on the topic of A123 charging and FMA Direct chargers.
I’ve been using the LiFe batts in my tx for about a couple months now and am monitoring the voltage closely. I have not charged them at all, still at 10.0v on my DX7 display. I think I’ll buy a spare for my flight box since the storage discharge is very low. I don’t want to find out at the beginning of a flying session that my txhas an issue. With a spare, I can just swap them out when it’s time.
DX7 Change battery voltage alarm
There is a voltage alarm on the DX7 that is set to 9.0v by default, which is the minimum acceptable battery voltage for tx operation per the manual. I came across this video showing how to change this default setting. You have to get into some rather hidden menus.
I’m using this method to alarm at 9.5v on the tx battery as a safety mechanism when using the LiFe batts.
As always, moving away from a stock setup takes a little bit of work, but if you’re open to it, you can extract additional functionality from our rather complex equipment. Just another tool for the toolbox.