LiPo voltage drop

I got 2 1,800-mAh 3-cell LiPos the other day, one with a Parkzone foamie I bought to replace one I totaled and the other off the shelf of the LHS. I charged both then realized the new plane was missing parts so I decided to discharge both to 11 volts because they'd probably be unused for a few weeks (I'm not looking to start the "how to store LiPos" discussion again -- last time was pretty thorough). Anyway, I discharge by attaching a 12-watt 13.8 volt automotive lamp with a voltmeter in parallel and watching while compensating for the drop from the lamp. One battery discharged normally. The other dropped a bit, then went immediately to about 6.5 volts. Couldn't miss it -- the bulb went quite dim. I tried the questionable battery again, and it did the same, dropping suddenly to about 7 volts. My question, obviously, is Do I have a bad battery? Seems to me the sudden drop is one cell going virtually dead. But I'm new to all this and pretty ignorant. Any suggestions? Thanks much rr

Sounds like a bad cell to me.

Dan

It's 1 for charge, 15 for discharge. Instructions on battery say to charge at no more than 1.75 amps, so I charge at 1.5. It's a balancing charger that comes with the plane and battery, supplied by Parkzone. Batteries are identical.

Sounds like it. If you're careful, you can measure each cell with a voltmeter. Access it at the balance socket. I had a cell go bad on a 3 cell lipo. It was an end cell. Gave it to my mate, who chopped off the dud, and is still using the 2-cell battery..

Trefor