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View Full Version : New to Lipo: A couple of Questions


Beelzebub
12-01-2005, 05:56 AM
Hi Everyone,

I've been lurking on this forum for a month or so, and there is some great information. I'm fairly new to RC flight, and I'm hooked. I have a couple of questions in regards to Lipo's and there discharge rates.

I've read through all of the FMA guides, battery University and a number of posts and this is what I have come up with. Please tell me if I'm wrong.

I currently have two lipo's.

#1 is an Electrifly 1250 mAhr 7.4 V with a 15C max discharge.
#2 is a EPKokam 1500 mAh 11.1 - I'm not sure about the max discharge, but I'm assuming it's 15C, but if anyone has this battery I'd like to know for sure :cool:.
I understand the charging aspect, but the discharge is a little confusing.

For #1, the max discharge rate is 15C or the equivalent to 18.7A (15 X 1.25). This would get me about 4 minutes of Flight time (really 2-3 minutes, so I don't drain it completely), and would deliver 18,750mah for 4 minutes. The confusing part is that the battery chart that came with the battery says max discharge rate is 9.6A :confused: That would give me double the flight time, but the max discharge rate would only be 7.68C. Anyone know why the battery chart is different than the battery specs on the package?

#2, would have a max discharge rate of 22.5A (15 X 1.5), this also would give me a 4 minute flight time. But once again, the battery chart says max discharge current is 12A or 8C.

Are my calcs for 1 and 2 correct, and if so does anyone know why the battery chart and battery packaging would be different? Anymore info would be appreciated :cool:

Thanks.

This site is sweet!

Geoff_Gino
12-01-2005, 06:41 AM
Hi Beezlebub

Can't exactly say why the chart differs from your battery specs. Your calcs are spot on, but bear in mind that the 15C discharge should only be held for a max of 60 seconds. The battery pack normally gives a constant discharge rate and this is the important one. Try using 10C as a guideline.

Typically you would climb out at full throttle (WOT) and once you have gained sufficient height you would throttle back. I always throttle back to about 60% when the plane is about 30 feet off the ground.

Depending on your AUW (all up weight) you should typically get 8 to 12 minutes without a problem.

Geoff

qban_flyer
12-01-2005, 06:59 AM
MAX discharge rate refers to the "Burst Output" the battery will deliver safely for a short period of time. Most manufacturers do not tell us what that "period of time" is so we have to guess most of the time. That is why your chart differs from what you estimated it to be.

According to the manufacturer of one of your packs, 9.6A is the continuous "safe" rate at which that pack can be discharged at. If you discharge it at a higher rate than that for an extended period of time, chances are that either the battery will be ruined quickly or that it may even stop working in flight.

When Li-Pos are abused, they begin to swell, "watermelon" like and remain that way for good. Once a pack does that, it no longer performs the way it was intended to.

Back to burst rate, I would "guess" the burst time period to be 20 to 30 seconds, though in the e-sailplane competition world it is determined to be 45 seconds (?). :confused: :confused: :confused:

hoppy
12-01-2005, 03:37 PM
Kokam states that burst is no more than 6 sec of every 60 sec.

Be wary of manufacturers claims of C ratings -

The Kokam 1500 is an 8C continuous duty cell (advertised) which means IMO, use it at about 6C.

Go to the battery test site and check your batteries out.
http://www.wattflyer.com/forums/showthread.php?t=1661
More available here-
http://www.rcgroups.com/forums/showthread.php?t=342568
and here:
http://www.elektromodellflug.de/datenbank.htm

Rugar
12-01-2005, 03:51 PM
When Li-Pos are abused, they begin to swell, "watermelon" like and remain that way for good.

This isn't necessarily true. In fact out of a 1/2 dozen or so swelled packs I have personally witnessed (one being my own), all of them returned to normal size after cooling down. But once they have swelled, they do need to be properly disposed of. The guys I fly with like to push the limits :o.

Matt Kirsch
12-01-2005, 04:52 PM
Beelzebub,

In a nutshell, yes, your calculations are correct, but you don't have to run the batteries at full capacity the entire time, and it's highly recommended that you don't. Your actual flying time will be determined by the average current drawn during the flight, not by the C rating of the battery pack.

Regarding the electrifly statistics, it could be that the chart refers to a previous generation of cells. I say this because I have an electrifly 1250mAh pack that's about a year and a half old, and it's only rated at 8C.

Also, don't ASSUME the C ratings on battery packs. Look them up on the manufacturer's website, or ask. Making an incorrect assumption could be a very expensive mistake...

Beelzebub
12-01-2005, 05:18 PM
That's great info:cool: I would never try and run a battery at the max discharge rate constantly, I'm just trying to get a feel for what all the numbers mean. I will definately look up what the manufacterer's say is the correct C rating on these. I also am going to get an amp/Watt meter to make sure where I'm running. I don't want to have a Lipo catastrophe, or even cut the battery life span by more than I need to.

qban_flyer
12-01-2005, 06:05 PM
The guys I fly with like to push the limits :o.

YUP!

I know the type. One character I fly with used to run a single 2S1P T/P 2100s on his Twinstar.

After three flights and getting it rather hot for my taste, the pack looked like a ripe watermelon! :eek: