Help calculating remaining flight time with LiPo's
 

Hi,

I've just bought 3 cell 2100 hi-discharge Kokam LiPo's. By searching the web, I found that fully charged they are 3.7 v per cell and are considered discharged at 3.0 v per cell.

I can find the total flight time from Motocalc, but I don't know what the discharge curve is for LiPo's. Does anyone know if the discharge rate is linear? or is constant until discharged?

Example, if I check the voltage after each flight, and I find the voltage is 3.35 per cell, does this mean that there is still 50% capacity left in the batteries?

Any help would be appreciated.

Thanks for your help.

Best regards,

Wayne Miller

RE: Help calculating remaining flight time with LiPo's
 

the voltage you listed is nominal per cell. fully charged, they'll go to about 4.2v/cell. flight time would have to be calculated based on average current draw: if you fly wide open the whole time, then motocalc gave you that number. if you're flying @ about 5.25A, you'll get ~15 min. there's no way to calculate exact flight time. throttle variations, props, wind, you name it will effect how long you can fly. i can say that the discharge curve is relatively linear. i get good power throughout my flights, until LVC kicks in. when i go to charge the packs i find i'm putting nearly capacity back into them.

RE: Help calculating remaining flight time with LiPo's
 

Hi ReallyUglyStik,

Thanks for the information.

From your comments, I understand that LiPo's should be considered fully charged at 4.2v and should never go under 3.0v when in use and they discharge fairly linear.

Again, thanks.

Wayne Miller

RE: Help calculating remaining flight time with LiPo's
 

whatever charger you are using should stop charging @ 4.2v, that's correct. as for the drop dead voltage, that seems to be a hot topic. i've gone with 3v, which seems to have the majority's vote. i think i've also seen posts stating that mfg's recommend no less than 3v, also.

p.s. that's 3v under load. when you take it off the plane and check the voltage, it's gonna be higher.

RE: Help calculating remaining flight time with LiPo's
 

Hi ReallyUglyStik,

Thanks for getting back to me. I have a couple of more questions.

Do you allow the ESC and BEC to control the cutoff voltage, or do you time the flight? and have you ever used the LED voltage indicators?

Thanks,

Wayne Miller

RE: Help calculating remaining flight time with LiPo's
 

i use the LVC. sometimes i quit before it kicks in, but i've never actually timed my flights. i've had good performance and no problems flying until the lvc stops me.

RE: Help calculating remaining flight time with LiPo's
 

Hi ReallyUglyStik,

I have two controllers, one has the LVC for LiPo's the other doesn't, I have a timer on my TX with audible beeps, so once I know my flying time, I can keep track of with it.

By the way, I have an MPJET 25/35-20 rated only in NiMh, it is rated at 6-9 cells and 7-15 amps for max efficiency and 25 amps shory time current. With a 9X6 prop it draws 14.1 amps WOT.

I'm running it on 3 LiPo's and notice it gets very hot, but no burning smell. It is not so hot that I can't hold it, but pretty darn close. Would you know if this is normal?

If you don't know, I may have to open a thread to see if anyone else can help.

Thanks,

Wayne

RE: Help calculating remaining flight time with LiPo's
 

you're asking if it's normal for the lipos to get hot? well, in this case i don't have much experience. my packs only get warm. i've heard (but don't remember where) that they shouldn't get hotter than 100F...or C...i can't even remember that, now. You might try popping over to rcgroups (just do a search on google for it). the electric forums are very active there, and you'll most definetly find an answer to this and any other question you may have regarding the care and feeding of your leccy flyers...

jimmy

RE: Help calculating remaining flight time with LiPo's
 

Hi ReallyUglyStik,

Actually I was wondering if the motor should get hot, and if so, how hot is safe?

Thanks,

Wayne