During our last flying session, my brother asked me to rush to him while he was flying a 60 size nitro airplane (Calmato 60) and was on his 3rd sortie of the day.  He uses a Futaba 9C, and uses a TX lipo to power it.  He started the day off with 11.6v on the lipo, and usually it would last at least 10 more flights before it needed to be recharged (starting at 11.6v).  But this time, his TX had started beeping, and he wanted me to check if it was any timer that was enabled, or something else.  To our surprise, I saw the TX voltage at 8.1V and by the time he did an emergency landing, it was reading 7.9V !

The TX lipo he was using is a purpose specific one from DualSky (model TXB25003-FU), with a built-in-balancer. So there is no balance connector for it, just a regular servo type connector, with only two wires. 

Now the real issue was that once I took it home and checked the voltage, it was back to reading over 10volts ! 

So my question to the electrical masters here is, how come there is such a wide disparity ?  We all know about no-load voltage variance etc, but this seems out of place.  If something like this possible all the time, that would explain some of the unexplained crashes, especially the ones on 2.4GHz. Us measuring the voltage after the crash would be a mute point.

i have seen this regular on my nicds and nimhs even on 2s lipo but on the lipo it took almost 3hrs to get back 1 volt at no load

What make this more unusual is that this is a TX, not an RX.  So the usual RX issues like servo loading are not coming into play. 

I am looking for a response from people who do electronics as their day job, as they may have come across something like this, or may have a ready explanation for this type of behavior.

Not sure if this is just the sign of a weakened battery ? Dropping quickly is a sign of a weak battery, but showing +2volts later on is the question.

yes temp variations can show positive or negative voltages later . but you have to load the battery pack again and then measure the voltage.

but in this case its a bad battery pack where some cells are unable to give sustained voltages( unbalanced pack)

next remote unlikely option is corrupt software on the TX not reading the voltage correctly.

solution - better to be safe than be sorry theory holds good ...  change the pack

It is in the same TX that it showed the higher voltage, so the load is similar.

Of course we will not fly with the same pack, unless it has been put through more tests.

It was reading 10V for a long time ? Or does it drop soon when you leave the tx on ?

Dropped 0.2v in about 5 minutes. I then moved on to try recharging it.

That's odd.

Do you pull out the battery everytime to charge it ? The connector may be worn.

A worn out connector will usually show intermittent contact. The voltage would fluctuate between max. and 0V.

Unless the connector is oxidised, resulting in poor contact.

Both seem unlilkely causes from the description. Seem a battery problem, for sure.

Yep, bad idea to charge a lipo inside the TX 

So it is odd, and the battery is deemed unsafe.

I have come across connectors giving unreliable connections, not just a make or break. Take a look at datasheets, they specify how many connects/disconnects they are safe for. All connectors are not equal in this respect, some of them are good only for a few times. I have seen enough that I avoid unplugging connectors as much as possible.

There could be a similar loose contact inside the battery, but I have never come across that case so far.

Factory made battery packs have spot welded connections. It is virtually impossible for these to be loose and/or intermittent.

I also feel that contact issues due to usage alone can be pretty much ruled out.  This lipo was used for little more than a year, and would have been charged about 10 times only so far.  If a connector cannot last 10 reconnects, that is a pity.  This is a pretty good brand, so hopefully that was not the case.

Now there is no guarantee about things like corrosion of contacts; but usage cannot be it.

Anwar, inspection of the connector will usually show up oxidation/corrosion.

I still think it is your battery. Why not charge it again and observe it's Voltage vs Time reading, with the Tx on?  A reading every 10 - 15 mins should be enough.That should pretty much result in adequate data for a diagnosis.

hello,

in these kind of situations, i follow certain guidelines, which has helped me.

1. The first step to solve a problem is to be able to create it consisitently.
2. "Loose contact" is generally a term used to explain away something marginal and not consitent.
It may be a good idea to use the same batt/ connector and the xmitter, do several insertion etc to recreate the propblem if u can. if not we should find a means to recreate the problem and lot of thinking is required to do this.  one should be extra cautious not to add any more variables like charging the battery etc.
Once u are able to create the problem, then we can find a solution. We can cross check the solution and its effectiveness by un doing the solution and creating the problem again. ( Always undo the last change and create the problem).

This method is heavily relied upon to solve marginal, hardware/ firmware bugs and is quite handy to solve even simple problems.

Idf you are not able to consistently create the problem, there is no 100% guarantee that it can be solved. Then you leave it to the guess work and chance.