Weird Lipo problem

[cameron]

Hi guys

So i was driving my truggy on Saturday and it randomly cut out. I thought that i had burnt out the speed controller or something along these lines as there was no way the batteries were below the ESC safe voltage as if they were, then the speed controller reduces the power, allowing you to (i assume) bring the car back . Well i connected a pair of batteries that were flat to test it and it did exactly as expected, it had a flashing light and only allowed for a low amount of power at the wheels. So i assumed there must have been an issue with the batteries (the car runs off two 3s lipo's in series)

Well i remembered this issue just earlier and decided to have a look at the batteries. I tested the voltage of one of the batteries and it was around 12.2v showing it hadn't really been driven much, but the other one was 0v. I figured there may have been a broken connection in the cables so i tried moving them while testing and nothing. I then connected an alarm to the balance lead and it shows there is still plenty of charge left.

So i decided to cut the blue cover off and get to the circuit board to look for a break there. I tested across the positive and negative there and there was still 0v. I also tested the cables to see if they were broken but my multimeter gave a buzzing showing that there is no break in either the positive or negative cables. My question is what is going on? How can i have 0v at the +/- leads but proper voltage at the balance leads? Surely, if one of the connection between the batteries or circuit board were gone, then the alarm would go off when connected to the balance lead? In case it helps, the battery is a turnigy 5000mah 3s 25-35c non nano-tech.

Any help will be appreciated

Thanks

Edit: Also forgot to mention that there has not been any physical damage done to the lipo when it failed, nor does it show any signs of puffing at all. And i wasn't going fast when it broke, i was turning around, i went to accelerate full power and me and my mate heard an odd noise come from the car, followed by it cutting out

 

[Sixtysixdeuce]

Pretty obvious that there's an open circuit, probably within one of the cells if you can't see it with the wrap removed.

Surely, if one of the connection between the batteries or circuit board were gone, then the alarm would go off when connected to the balance lead?

The balance leads connect to each individual cell, are independent of the power leads.

 

[cameron]

Thanks sixtysixdeuce, I had a feeling that is was a broken connection but I didn't realise the balance leads are independent of the power leads, I thought they connected to the same ± and - connectors on the board as the proper leads. I shall have a look at the battery later.

 

[Sixtysixdeuce]

I thought they connected to the same ± and - connectors on the board as the proper leads.

On a 3S, the balance leads will be common with one ground and one power, but will have two additional leads that are independent of the primary power wires in order to monitor individual cell voltage.

 

[cameron]

Ok, i think i may have found the problem, it seems like a solder join has broken. But it's odd as there is no damage to anything that was directly in contact with it. Over the circuit board, there was a really thin transparent yellow tape but that was completely fine along with the foam sort of tape over that. I would have expected this to have melted. What would cause this to break ,could this break suggest there is a problem with the battery, maybe it isn't powerful enough for the power the car is pulling from it. The ESC is a 150A controller and as said before the battery is a 5000mah 3s 25-35c battery.
The picture shows the top of the battery and the black line shows the circuit along with the black circle that is the point where i think the break is. I suppose it's a case of just rejoining it and that will be that? Each cell is showing 3.81v individually.

 

[Sixtysixdeuce]

It probably just broke from flexing; if it had been a draw issue, you'd see heat carnage.

I suppose it's a case of just rejoining it and that will be that?

Should be, although I'd test the battery after the repair by connecting it to a spare brushed motor before I plugged it into an expensive ESC.