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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 03-25-2012, 10:54 PM   #1
dumo01
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Default New Lipo propblem to me, looking for safe terminal discharge

I have a Flight max 3S 1300 maH that had a little abuse last summer, had to open it up and resolder one of the tabs between the cells. It worked fine the rest of the summer and worked well for about 6 flights this spring until last week. Charged normally and was showing about 4.18 per cell before I hooked it up. Took off and flew about 2 minutes or so when I lost all control and nosed in. Was not too far up so no damage other than broken prop, but had nothing on the receiver or UBEC when I got to the plane. Checked the cells when I got to my field box, two cells were around 3.9 and one cell was around 3, so I figured I had one cell going bad, but I was a little surprised that there was not enough power to keep the UBEC powered up and give power to the flight controls. When I got home I hooked up to a different battery. The Receiver came on, UBEC came on, ESC armed; everything looked OK.
Replaced the prop today and thought I would check out the suspect battery for grins. The cells were still pretty much where they were before 4,4 and 3 for a total of 11 v or so. I left the voltage checker hooked up to the balance tap and plugged the battery into the ESC. Nothing out of the plane electronics and the display on the battery checker went out. Hooked up a different battery and ran up the motor with the new prop. Battery voltages were stable and all looked good.
For the cost of a new battery, I decided the suspect battery did not owe me anything so I got my discharge circuit ( 2 wires and a 12 v light bulb) and hooked it up to kill the battery before disposing of it. The bulb did not light up as I have been used to seeing in the past. Thought this over for a few seconds, well less than a minute and disconnected. Checked the cell volts and had one cell showing 5.2, both others were around 3.6 or so.
My best guess is something happened so that two of the cells are discharging onto the 3rd when the main power leads are hooked up. The battery is not puffed and did not get warm for the time it was hooked up.

I am now looking for ideas on how to discharge the battery other than hooking up a load and letting it die.
Maybe the old salt water bucket is the best idea for this. It is currently sitting outside where it should be limited in what happens if it does something funky.
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Old 03-26-2012, 12:28 AM   #2
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Did you open it up again to inspect the solder joints or test the voltage at the power leads instead of the charge taps?

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Old 03-26-2012, 04:30 AM   #3
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I have seen similar things happen with the grounding circuits in a car. The voltage will read fine, but once it has a load it will drop power. This is usually and intermitent or bad connection. Where two cells read good, the ground must be bad or loose on the third. Maybe anyways.
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Old 03-26-2012, 05:25 AM   #4
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There is a bad connection, or something that is causing a high internal resistance.

Modern digital meters, (I am making a guess you have that type) do not put any significant load on a circuit. That means the battery may be so bad it can not deliver 1/10 of an amp. Yet, at no current load at all, it reads full voltage.

Because of this, checking a circuit by voltage alone, may not find the problem.

If you can carefully check the circuit while the load ( light bulb, motor, etc.) is hooked up, then you will find the problem.

In ANY good electrical circuit, ALL connections must be clean and solid, to work properly.

I have seen a wire clamped under a screw, yet there was no electrical connection, because the screw was corroded, removed wire, scraped everything clean, re-tightened, then everything worked.

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Old 03-26-2012, 12:31 PM   #5
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The usual culprit for this sort of fault is the aluminium tabs that lead into the LiPo cell.
They are very thin and vibration can cause them to crack if there is even the slightest relative movement between the cell and the solder joint on the 'connection' board.
It can still make a touching contact to show a voltage but is quite unable to carry any significant current.

If this is the problem it cannot be repaired.
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Old 03-26-2012, 12:43 PM   #6
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Haven't open it up yet but will do that when I have some time. Might be a few days until I can hook up with one of the guys I fly with that has done more battery repairs than I have to see if he sees something I don't.

I agree with WF about a cell going bad not being able to sustain a voltage under load. That is what I expected to find. What did surprise me was finding what I thought was the bad cell sitting up above 5 v after the load was attatched for a short period of time. If there was a short in the internal wiring from the two good cells back into the bad cell, I would have expected that to be a consistent flaw, not associated with putting the load on the power wires.
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