DX7S NIMH battery melt down

[Monkeyboy]

Went to turn on my DX7S today to upload some settings and it yelled at me saying the battery was low. I plugged it in to the wall to get it charging. and still yelled at me.

I pulled the battery cover off and found one of the cells in the NIMH pack had melted.
What would have caused this?

The transmitter has been sitting for a few days.
I'm going to get another pack for it, and hope the radio isn't toast

[JetPlaneFlyer]

Cells sometimes go bad, this isn't the only report I've seen about the DX7/DX8 battery going down.

If I were you I'd replace it with a LiPo battery. Hobbyking have them: http://www.hobbyking.com/hobbyking/s...dProduct=19481

Note that the DX7S and DX8 use the same battery.

[kyleservicetech]

Originally Posted by Monkeyboy

Went to turn on my DX7S today to upload some settings and it yelled at me saying the battery was low. I plugged it in to the wall to get it charging. and still yelled at me.

I pulled the battery cover off and found one of the cells in the NIMH pack had melted.
What would have caused this?

The transmitter has been sitting for a few days.
I'm going to get another pack for it, and hope the radio isn't toast

That is strange. Don't thing it would have been the transmitter, since I believe the transmitter power input is fused inside the transmitter.

Did you recharge it with the DX7s wall charger? Or a separate charger? If a separate charger was used, FYI the Spektrum charger input has reverse polarity, the pin is negative, the outside is positive.

[Beemerider]

Originally Posted by JetPlaneFlyer

Cells sometimes go bad, this isn't the only report I've seen about the DX7/DX8 battery going down.

If I were you I'd replace it with a LiPo battery. Hobbyking have them: http://www.hobbyking.com/hobbyking/s...dProduct=19481

Note that the DX7S and DX8 use the same battery.

+1 on that. I replaced the battery in my DX7s with a lipo. Fly for weeks now rather than a few days on a charge.

[flydiver]

Any consideration of lipo in a TX should be weighed against how likely you are to forget to turn it off. A set of Eneloops will work as well, maintain their charge, can be charged with the stock charger (wall wart), and be recovered if you run them dead.

The lipo.....it'll likely stay dead. Happens all the time. It's one of their major drawbacks in a TX.

[kyleservicetech]

Originally Posted by flydiver

Any consideration of lipo in a TX should be weighed against how likely you are to forget to turn it off. A set of Eneloops will work as well, maintain their charge, can be charged with the stock charger (wall wart), and be recovered if you run them dead.

The lipo.....it'll likely stay dead. Happens all the time. It's one of their major drawbacks in a TX.

Agreed. My new DX8 has a Nih battery in it. Simple to charge, no worries about balancing. The DX8 also has an alarm if it is left on for more than something like 5 minutes or so.

I put together a 15 hour timer for my wall charger so it won't be forgotten and left on for a week or so.

[Beemerider]

The 7s has an alarm that goes off after a few minutes of inactivity. The 7s also has internal charging circuitry to handle both nimh and lipo cells. The lipo was the best choice for me.

[JetPlaneFlyer]

Originally Posted by Beemerider

The 7s has an alarm that goes off after a few minutes of inactivity. The 7s also has internal charging circuitry to handle both nimh and lipo cells. The lipo was the best choice for me.

Also there is a low voltage cut-off* set to 6.4v by dafault, so it's impossible for the battery to go flat.

*Actually the manual says it's a cut-off but the screen calls it an alarm so I'm not 100% sure what happens if you let the battery go down to 6.4v. Maybe it gives an alarm then cut-off if you don't do anything? I've never let the battery go that low to find out but either way you would have to be exceptionally dumb and/or stone deaf to get a flat Lipo.

I prefer the LiPo for two reasons:
1 - it lasts much MUCH longer than any NiXX battery
2 - it has virtually zero self discharge rate so you never get caught out with a low battery when you want to fly. I appreciate that Enerloops have a low self discharge rate too but the standard Spektrum battery isn't an Enerloop and the cost of an Enerloop (if you can get one) would be far higher than the Hobbyking LiPo, but without the advantage of increased run time.

having said that....The stock NiMh is fine but you just need to remember to charge it regularly.

Steve

PS.. if the consequence of forgetting to turn off/running the battery down too low is your primary driver for battery selection then you obviously wont be using LiPo's to power your model either

[Monkeyboy]

Im back up and running with a new NIMH battery for now.

the same day my battery went bad I got an email letting me know the lipo battery was back in stock at hobby king.

Ordered the battery along with all kinds of other fun stuff just to make the shipping worth it.

[FlyWheel]

Originally Posted by JetPlaneFlyer

Also there is a low voltage cut-off* set to 6.4v by dafault, so it's impossible for the battery to go flat.

*Actually the manual says it's a cut-off but the screen calls it an alarm so I'm not 100% sure what happens if you let the battery go down to 6.4v. Maybe it gives an alarm then cut-off if you don't do anything? I've never let the battery go that low to find out but either way you would have to be exceptionally dumb and/or stone deaf to get a flat Lipo.

I would say if there is a shut off it better be some time after the alarm sounds; cause if it does shut down and your pride and joy is 1'000 or so feet up you may be well beneath Bandini Mountain!

Posted via Mobile Device

[A.T.]

Originally Posted by FlyWheel

I would say if there is a shut off it better be some time after the alarm sounds; cause if it does shut down and your pride and joy is 1'000 or so feet up you may be well beneath Bandini Mountain!
Posted via Mobile Device

Always start out with a fully charged battery and keep an eye on your Tx battery indicator.
Once any transmitter batter falls so low the the LBW sounds, then the battery starts to rapidly loose power which reduces range in a corresponding way.
Start landing immediately LBW is heard as in majority of cases, especially with NiCD/NiMH batteries user has only 1 max 2 minutes.
. 101 LiPo vs. NIMH discharge curves
Much more information available below
"Radio Systems, Accessories, Alterations and FAQ" at
Alan's Hobby, Model & RC FAQ Web Links

Alan T.

[kyleservicetech]

Originally Posted by A.T.

Always start out with a fully charged battery and keep an eye on your Tx battery indicator.
Once any transmitter batter falls so low the the LBW sounds, then the battery starts to rapidly loose power which reduces range in a corresponding way.
Start landing immediately LBW is heard as in majority of cases, especially with NiCD/NiMH batteries user has only 1 max 2 minutes.
. 101 LiPo vs. NIMH discharge curves
Much more information available below
"Radio Systems, Accessories, Alterations and FAQ" at
Alan's Hobby, Model & RC FAQ Web Links

Alan T.

These tests were conducted at 9C which is about 36 Amps or so. Transmitters run at perhaps 0.1C or o.2C depending on mfg.

As such, you will have much more warning than a minute or two when a Nih battery starts to quit. It is reasonable to run a discharge test one the order of 9c or much higher on a LiPo designed for electric motor use. IMHO, it is not reasonable to run a Nih battery used for a transmitter at the same discharge rate.

Again, if you are using Nih cells for your receiver and servos, you've got to be careful if your model has a big handful of high current high torque servos. I'd never use those "AA" cell receiver batteries, even a 5 cell pack on any model over about 1000 watts or so. I"ve measured peak currents of 14 AMPERES on my giant scale Big Stick with its seven Hitec 645MG servos. That peak current was measured with my $$$$ Fluke 87V digital multimeter with its peak reading function.

Those uBEC switching power supply type Castle Creations 10 Amp uBEC or similar is the way to go on the larger models. In fact, it's a good idea to use the CC uBEC along with a 2 cell LiFe or A123 battery as a backup on those giant scale models. (I've got a thread in www.wattflyer.com on how to do this.)