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General Discussion A place to talk about anything R/C or otherwise.

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  #1  
Old 2017-10-20
MichaelLevy MichaelLevy is offline
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Internal resistance discussion

There has been some discussion about lipos internal resistance on another thread and I felt it is important enough to give some explanations on the subject. Without going into the science which is not all that clear to me, IR measurement is an indispensable tool to determine your lipo batteries health. I bought a cheap Imax B6AC charger at Hobbyking for $21 US which measures each cell's IR. It is worth more than its weight in gold as it has saved me more than once from losing my planes. I check the battery's IR after each charging cycle.

https://hobbyking.com/en_us/imax-b6-...plug-copy.html

Measuring only the voltage can lead you to think that your battery is fine and fully charged until you take off or at some time during the flight when your plane suddenly loses power. You may think your battery is discharged but it's not, one or more cells may be dying and your voltmeter did not detect it. The IR measurement is instantaneous and real time and will let you know when a cell is starting to sag. With high C rating batts (50C to 135C), the IR should be 005 ohms or lower, lower C rating batts may have a higher IR of 010-020 ohms, but all cells should show about be the same reading. So for example, you have a 4S batt and three cells read .005 ohms and one cell at .020 ohms, that cell is going. This means that you will drop to 3 cell power at some time in the flight, possible on takeoff or at the worst possible time. I bought the copy Imax B6AC which is cheaper, you can get the original, more expensive but make sure you next charger can give you IR reading. You will never regret it.

Note: As batteries get older or mistreated the IR will go up! I never fly under 3.8 V each cell and always keep them in storage at 3.8V, charge them the day of flying or the day before. I used to puff and kill batteries, not anymore.

Last edited by MichaelLevy; 2017-10-20 at 11:26 AM.
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Old 2017-10-20
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Sempai-mj Sempai-mj is offline
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Thanks for the info, that's happened to me twice on a 3 cell battery.
Checked the voltage before takeoff 12.6
Right after take off battery dies.............
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Old 2017-10-20
MichaelLevy MichaelLevy is offline
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Quote:
Originally Posted by Sempai-mj View Post
Thanks for the info, that's happened to me twice on a 3 cell battery.
Checked the voltage before takeoff 12.6
Right after take off battery dies.............
Happened to me, right over the Forest of Doom...
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Old 2017-10-20
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Thank you Michael, extremely interesting info!
In the last few years we all had to become amateur electrical engineers!
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Old 2017-10-20
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Thank you Michael, extremely interesting info!
In the last few years we all had to become amateur electrical engineers!
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Old 2017-10-20
MichaelLevy MichaelLevy is offline
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Another reason why it's good to know the IR of you battery before flight. Suppose you are using an old 4S battery, still flyable but not too trustable with a just measured IR of .010, .012, .010 and 30 ohm respectively. You've flown it before with no problem. You take off and very soon after, the power starts to drop rapidly. In cases like this my standard protocol was to find the closest open place to land cut power to preserve whatever is left for a final flair, hoping to limit the damage. This is the wrong response when dealing with a suspected IR issue because in fact there is a lot of power left in the battery, but you are basically flying on 3 cells instead of 4 and have quite some time to fly. The best response, even counter intuitive, is to gun the throttle, using maximum available power and get into a better final approach.

I had a similar experience with a fully charged 6s batt on one of my jets. Had initially plenty of power, immediately did one touch and go after take-off and lost power on the second take-off. In this particular case, when I brought the battery home and recharged it, the weak cell had a IR of .075 ohm, up from about .028 ohm before the flight! That cell was totally finished. So yes, things can deteriorate quickly once the IR starts to climb abnormally in one cell.

Last edited by MichaelLevy; 2017-10-20 at 04:50 PM.
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