LiPo flight failure

[reddrock]

Had an interesting flight this weekend. Launched a 3" diameter x 3' long rocket on a J1299P motor. Rocksim estimated 48 Gs. Ebay held a G-WIZ Deluxe (rated at 50 Gs) and Adept 22 altimeter. No brand GPS in the nose cone. Motor CATOed around 900' blowing the fin can apart and separating ebay/payload section and NC from motor section. Electronics fired the main at selected altitude and the payload section landed softly, but NC separated. Retrieved the NC 300' away and found the 1000mAh 1S LiPo burned. Luckily the GPS circuit was not burned and tested fine with a new battery.

My conjecture is the battery couldn't take the G force and shorted out internally, causing the fire. But I had two 9V LiPo batteries powering the G-WIZ Deluxe that had no problem, and the LiPo powering the Adept also had no problem.
Anyone see this sort of thing before? Got any other ideas?

Just glad I didn't fly the J1799P.

 

[David Schwantz]

my guess is the screw at the bottom of the LiPo shorted out the cells. I have launched a rocket, with LiPos that Rocksim puts at 283 G's and my batteries held up.

 

[reddrock]

Maybe, but screw at bottom and top of battery has nylon spacer around it. So only the screw head could make metal contact with the battery. If you look close you can see the white nylon around the screw.

 

[Tractionengines]

The screw at the bottom is my thoughts too. If not from the metal screw itself... maybe the round screw/spacer was still enough of a stress concentration to distort the end of the battery. A "shelf" under the battery spreads out the forces instead of concentrating it at a point.

Only other idea is it looks like a tie wrap was around the middle. Maybe it was pulled too tight...

 

[reddrock]

I like the force concentration on the shielded screw. The force of both motor burn and CATO would be in the right direction. I've done shelves before, so may go back to them. Or maybe just quite flying those 54mm Warp 9 motors....

 

[Rocket86]

That force concentration is really a huge stress concentration. Calculate the force acting on the screw, the battery mass times the 48G or whatever it was, and divide by the very small line contact between the battery and screw. It's a large force divided by a very small area, resulting in a stress that's probably way more than a soft Lipo case can withstand. And if there was a flexible circuit board/ribbon at the bottom it could easily have been cut.

 

[Voyager1]

my guess is the screw at the bottom of the LiPo shorted out the cells.

The screw at the bottom is my thoughts too. If not from the metal screw itself... maybe the round screw/spacer was still enough of a stress concentration to distort the end of the battery. A "shelf" under the battery spreads out the forces instead of concentrating it at a point.

Only other idea is it looks like a tie wrap was around the middle. Maybe it was pulled too tight...

I agree, the single point of contact at the base bruised the LiPo with those Gs. A shelf is definitely better. Even if the cable tie wasn't too tight to start with, the LiPo would have swelled up when heating up. The GPS tracker wouldn't have caused the current surge that fried the LiPo, so it does point to a physical shock that did it.

Is it really necessary to for you to use a 1000 mAh LiPo for your GPS? I've never used anything larger than a 400 mAh for my GPS trackers.

 

[OverTheTop]

It looks like either the screw or the cable tie triggered the cremation. I use cable ties, but I put Velcro on the battery and sled. The Velcro takes the shear forces and the cable tie just holds it in place. That way the cable tie doesn't need much tension.

A 48G flight probably deserves more support around the battery.

 

[ghostfather]

High G flights need support around and under a lipo to keep it from tea-bagging, which will also short it internally. Looking at the photo, I would also guess that the pressure point of the screw underneath could easily cause a short

 

[Rocketjunkie]

A 48G flight probably deserves more support around the battery.

Who knows what happened during the cato. It could have been momentarily 1000gs. The damage probably occurred at the time of the cato.

 

[OzHybrid]

Designing your electronics to survive a CATO is a lot different to designing to survive a flight. But as Stewart says, velcro stuck to the battery and sled and cable ties. Get rid of the metal screws top and bottom as they concentrate the toad to a point on impact.

 

[manixFan]

For what it's worth, I now enclose my batteries in 3D printed boxes that completely protect the battery, with a cover held in place by a couple of screws. They are fairly thin walled, but plenty strong enough to keep the battery in place without putting any direct stress on the battery.


Tony

CAD example: (the small tabs act as wire guides)

 

[cerving]

Velcro won't hold the battery in place in a high-G flight, but you'd be amazed what a few wraps of masking tape can do. Glue some balsa stringers to the sled with CA to make a little box around the LiPo, hold the LiPo in place with a few wraps of masking tape, and voila! easy battery mount. If you need to replace the battery, all you need to do is to snip off the tape. I've done this with 80G flights... and nothing moves a millimeter.

 

[OzHybrid]

Velcro won't hold the battery in place in a high-G flight, but you'd be amazed what a few wraps of masking tape can do. Glue some balsa stringers to the sled with CA to make a little box around the LiPo, hold the LiPo in place with a few wraps of masking tape, and voila! easy battery mount. If you need to replace the battery, all you need to do is to snip off the tape. I've done this with 80G flights... and nothing moves a millimeter.

That's very European of you Chris. Milimetre indeed. What's that in fractions of an inch for the American audience? Just over 1/32 or just under 3/64?

 

[OverTheTop]

Velcro won't hold the battery in place in a high-G flight,

I have used the Velcro/Cable Tie thing in flights above 50G. Works. Remember the Velcro only takes the shear forces. Velcro on its own, or with cable ties of questionable lineage, would not.

 

[OzHybrid]

I have used the Velcro/Cable Tie thing in flights above 50G. Works. Remember the Velcro only takes the shear forces. Velcro on its own, or with cable ties of questionable lineage, would not.

I'm going to have to do a family tree for my cable ties now???

 

[OverTheTop]

I'm going to have to do a family tree for my cable ties now???

You can choose your friends but not your relatives.

FYI, these are the ones I use for serious situations (like holding big batteries down ). They have a stainless steel barb and I have never had one pull out. Panduit Barb Ty. I am sure there are similar offerings from other manufacturers.
https://www.panduit.com/en/products...es-mounts-straps/cable-wire-ties/bt2s-c0.html

 

[Sooner Boomer]

Here's what I did. Sled fits inside 54mm coupler. The cover for the battery section now has a lip that covers the front portion of the batteries.

 

[Handeman]

At our last launch there was a rocket that failed to deploy it's main because the apogee charge was too large and the large shock created when the cords hit the ends caused the tie-wraps to break and the battery got disconnected. I've also had tie-wraps break on me before, although the battery didn't get disconnect, fortunately.
I have since gotten away from tie-wraps for holding batteries whenever possible. I will use electrical tape before tie-wraps. If I do have to use tie-wraps I make sure they are large one rated at 50 lbs. or higher.

I use 3mm plywood for my sleds so I cut a U shaped piece of 1/2" plywood that is screwed and glued to the sled to hold the 9V battery. A second is put on the back side if redundant altimeters are used. Another piece of 1/2 ply is attached across the top with enough of a gap to allow the power wires to exit. This boxes in the 9V and an aluminum strap, held in place with a couple wood screws, holds the battery in the box. Not as pretty or neat, but much stronger than tie-wraps or plastic battery holders. YMMV.

 

[tsmith1315]

What's that in fractions of an inch for the American audience? Just over 1/32 or just under 3/64?

Even better, it’s about 1/25.4th, which is approximately half a fine flaxen hair over 5/128ths.

 

[user 30299]

At our last launch there was a rocket that failed to deploy it's main because the apogee charge was too large and the large shock created when the cords hit the ends caused the tie-wraps to break and the battery got disconnected. I've also had tie-wraps break on me before, although the battery didn't get disconnect, fortunately.
I have since gotten away from tie-wraps for holding batteries whenever possible. I will use electrical tape before tie-wraps. If I do have to use tie-wraps I make sure they are large one rated at 50 lbs. or higher.

I use 3mm plywood for my sleds so I cut a U shaped piece of 1/2" plywood that is screwed and glued to the sled to hold the 9V battery. A second is put on the back side if redundant altimeters are used. Another piece of 1/2 ply is attached across the top with enough of a gap to allow the power wires to exit. This boxes in the 9V and an aluminum strap, held in place with a couple wood screws, holds the battery in the box. Not as pretty or neat, but much stronger than tie-wraps or plastic battery holders. YMMV.
View attachment 567310

Darn it Handeman, I just got done with 9-volt batteries. Now I have to test zip ties?





I'm in the process of changing the ties over to white and green ones. A few people have noticed that the black
ties don't seem to hold up well (break) in cold weather.

I have not had any tie failures - except when I pull them too tight. As you can see I use multiple ties.
Warp motors are my favorite kind of motor . . . .

My Lipo batteries always go in a battery case. I won't zip tie around a Lipo.

 

[FredA]

Now I have to test zip ties?

I've had them break when used to hang onto 9V's.
Since then I've backed them up with a layer of duct-tape then zip-ties. This combo has been unshakeable - so far.

 

[Handeman]

Darn it Handeman, I just got done with 9-volt batteries. Now I have to test zip ties?

View attachment 567379

Sorry for that. It's just that the small, easy to use zip ties really don't have a high breaking strength. It doesn't take a lot of shock force to break them. The larger, stronger ones, are definitely harder to install and use, but they are much more durable. I also use the 9V battery clips to connect to the batteries instead of battery holders. That way, if the battery does come loose in the av-bay because of broken tie-wraps, there is some chance the clip stays connected to the battery instead of a disconnection when the battery comes out of the holder. I know of one flight where that has saved me.

 

[coachgabbi]

For what it's worth, I now enclose my batteries in 3D printed boxes that completely protect the battery, with a cover held in place by a couple of screws. They are fairly thin walled, but plenty strong enough to keep the battery in place without putting any direct stress on the battery.


Tony

CAD example: (the small tabs act as wire guides)
View attachment 567235

 

[coachgabbi]

I too have incorporated that technique. I utilize an RRC3 and the preferred battery is a 9volt quality alkaline battery. This printed box holds each very well.

 

[OzHybrid]

I too have incorporated that technique. I utilize an RRC3 and the preferred battery is a 9volt quality alkaline battery. This printed box holds each very well.

Can you please use a format we can all access. stl and a jpg photo of what it looks like would be the most common.

 

[coachgabbi]

 

[cls]

I really like the printed boxes! years ago I had tried every commercial 9v battery holder, none were suitable for rocketry. so I went to drilling holes in the ebay sled and using zip ties.

at the time, Radio Shack sold heavy duty 9v connectors with #22 wires, excellent!