Li-Poly Voltage Fall off?
 

Just got to put my new Alfa Fw 190 up and had a ball. :DI am still fairly new to all this so I would like to ask that all employ the KISS theory on responces if at all possible. I have the Kokam 3 cell 11.1 Volt / 1500 mAh packs to power my MP Jet red Outrunner. Flew two of the packs for about 20 minutes each after charging about a week prior to flying. After the flights I put a digital voltmeter to them and they still showed 11 plus volts, Never really heard or felt the power "fall off" like I am used to with my Nimh packs. I just want to be certain that I am not harming these because they are rather pricey. I also tested one pack that I had not used yet and it was 12 volts. If these are 11.1 volt packs how is that possible. I have an Apache 2500 charger and charged them new at 1200 until it cut off. I want to be able to monitor these because I have read so much about how sensitive they are to overcharging and over discharging. (If that is the correct word) I just figured that after 20 minutes or so of flying they would have read around 8 or so volts instead of 11?

RE: Li-Poly Voltage Fall off?
 

Having the Apache 2500 charger is a good start! It will keep you safe from over-charging your Lithium packs and essentially warn you if you are discharging them excessively by displaying a red flashing warning if it thinks your 3-cell pack is to low. You'll then need to try an initial charge on the 2-cell jumper setting to raise the voltage a bit.

Your 3-cell Kokam 1500 pack should measure around 3x4.2v or 12.6 volts when fully charged and unloaded. The lowest voltage you should read after a flight is 9.0v. If it is lower, try to recharge the pack immediately. Typically, I see around 3.5v/cell or 10.5v after a flight on a 3-cell Kokam 1500 pack.

Good luck!

RE: Li-Poly Voltage Fall off?
 

I guess I just expected to feel or hear the battery get lower and could not believe that it still had 11 volts or better in these packs. I am used to flying the GWS setup in my Corsair and it uses the Nimh Sanyo 8.4 V/ 730 mAh. You know when they start to wind down. I flew for an easy 20 minutes with the Kokams and the Outrunner at about 1/2 to 2/3 throttle and never heard or noticed any power loss. I guess I expected less and got more. About time...

RE: Li-Poly Voltage Fall off?
 

Ahhh, yes, I see now.

Lithium cells hold there voltage fairly level until they are depleted. Once depleted, the voltage drops fast. This is far different from our NiCd/NiMH packs which are strongest in the first few minutes of flight.

In this manner, the Lithium cell effect more closely mimics a glow engine power level.

RE: Li-Poly Voltage Fall off?
 

I'll second that. My 2s 1320mah TP li-po runs strong and then nearly goes flat for about the 20 seconds of flight. I fact by the time I notice the pack is dying there is usually not enough charge left to make a power on landing. At best I can get a little bit of juice at touchdown for a flair.

RE: Li-Poly Voltage Fall off?
 

Thanks for all the good info. Both of you. I will have to look out for the dreaded voltage drop you guys were talking about. That is when you land quick. To be quite honest about it I was about worn out when I sit my bird down after the first two flights, probably 20-25min. each in duration. Like I said it was a maiden flight for the "Red 13" Fw 190. Sure is sweet flying though, everything I hoped it would be. I am ecstatic about the performance of the Kokam 3s and Outrunner motor. Now I need the P51 or the P47 with the same setup.

Thanks again,
Rex:D

RE: Li-Poly Voltage Fall off?
 

I am new into electric. If the power drops so rapidly at the end of the discharge cycle, would it be wise to skip the BEC and take the penalty of a separete receiver/servos-battery (small lipo) to safeguard the power to the controls once the "main" power battery is depleted?

Joje

RE: Li-Poly Voltage Fall off?
 

It may depend on the type of plane you are flying.
If you were flying a powered glider you would not want to rely on the low voltage cutout. Carefull timing of the motor runs or a seperate battery would be required.
I am currently flying a foam 3D plane. The plane will not stay in the air very long once the motor cuts. The low voltage cuttout and the BEC works well with this plane.