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Dec 07, 2019, 11:23 AM
32.4316 N, 80.6698 W
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Lipo Voltage Drop


I installed a small voltage sensor a few days back and looking at the logs I see a significant voltage drop during climb outs. Prior to the sensor install I normally swapped out batteries when the resting voltage was around 3.7 volts per cell measured by my meter. That was easily 3 full climbouts with generous reserves.

Upside to the telemetry sensor is I dont have to drag out the voltmeter. Downside is I get to see the voltage sag. Below you see the voltage drop during a climb to 150 meters. GensAce 1300 3S driving a Hacker A20 4.4:1 at 35 amps. Graph on left is 1st climb with a fresh lipo, second on the right is the 3rd climb with same lipo. Do you think this type of voltage drop is normal?
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Dec 07, 2019, 11:56 AM
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jtlsf5's Avatar
Could be due to a lowish C rating battery. Lower the rating, the harder the battery works to deliver the amps needed, resulting in voltage sag. As long as the ESC isn't set with too high a cutoff voltage you'd never know. Might also be a factor of not letting the motor unload before starting the hard part of your climb. Try letting the plane run horizontal for a few seconds allowing the motor to get to speed before putting more load on it via climb. Then compare to these traces.

JT
Dec 07, 2019, 12:29 PM
solastagia
kcaldwel's Avatar
Quote:
Originally Posted by mark426
I installed a small voltage sensor a few days back and looking at the logs I see a significant voltage drop during climb outs.
What is the time scale on the voltage log graphs? Are we looking at the full motor run (30 seconds?), or a clip at the very start when the motor is winding up?

What is the effective kV of the motor/gearbox, and what pitch and diameter is the prop?

Kevin
Dec 07, 2019, 12:52 PM
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Aaro's Avatar
Hi
What comes to typical F5J setup (35..45A) the drop is what I consider normal. Because of this, we usually set ESC's possible cut off -voltage very low, eg. to 2.5 V / cell. If you run a 400...600W setup full power for 30 seconds the cell voltage drops easily below 2.8 V/ cell (under load) with 650 mAh...850 mAh batteries we use.
Aaro

Quote:
Originally Posted by mark426
I Do you think this type of voltage drop is normal?
Dec 07, 2019, 05:03 PM
32.4316 N, 80.6698 W
Thread OP
Quote:
Originally Posted by kcaldwel
What is the time scale on the voltage log graphs? Are we looking at the full motor run (30 seconds?), or a clip at the very start when the motor is winding up?

What is the effective kV of the motor/gearbox, and what pitch and diameter is the prop?

Kevin
Lowest voltage (10.1v) is 3 seconds after start. Maintains 10.2 volts for remaining 16-17 seconds of climb. Recovers to 12.2 volts withing a second or two.
Hacker A20-6XL 4.4:1 2500 kv GM16-8 Last check 35-36 amps at full power. Claimed too be 45C by GensAce/Tattu.

Second and third climbs are nearly the same but the min voltage drops slightly.
Dec 07, 2019, 05:29 PM
If it flies, I can crash it.
rocketsled666's Avatar
IMO either you don't have adequate wiring/connectors between your pack and ESC, or the battery you're using is underrated (C rate) or "tired". A 45C 1300 should not show that much drop at 35A load. Even if GensAce/Tattu is under-rating their packs, you should still have some decent margin, unless they're underrating by ~50%.
Dec 07, 2019, 05:33 PM
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Leadchucker's Avatar
Looked tat some of my Castle Link data for my sailplanes and I'm right in the same ballpark under load with TP 1300 mah 3S 70 C lipos.
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Dec 07, 2019, 05:41 PM
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scirocco's Avatar
I think you can do better.
I don't have plots handy but logging with a Hyperion E-meter I found a 1400mAh 3S Turnigy held 11.1V to 37.6A for the first 15 second climb, and was still at 10.5V at 33.4A at the end of the 5th climb. In a 3.6m Pulsar 9.5m/s climb.
This was a 114g 40C Nanotech car battery. The basically identical Multistar 40-80C perform the same.

But there could be another factor - if your battery is cold it's performance will be heavily degraded
Dec 07, 2019, 05:48 PM
solastagia
kcaldwel's Avatar
Yep, a 45C battery is probably more like 22C at best when new. 1.3Ah x 22C = 29A. Anything more will see a big voltage drop when new, and it only gets worse. The voltage drop is because the battery can't supply that much current very well. Some good 70C packs will actually deliver 35C when new, some won't.

You could try going down a size or two on props, and actually pick up some performance and save your batteries. A 14x9 might work well.

Everyone seems to be happy operating with big voltage drops at the very limit of the real battery pack performance, but I find the pack lifetimes very short when treated like that. Cost of doing business with light F5J gliders I guess.

Kevin
Dec 08, 2019, 04:34 AM
Registered User
Wondering why all people still use this fake marketing C-factor, that is vaporware, say nothing, can't be re(a)liable measured, while nobody rely on the real thing called IR, that say everything about a battery capability.
And also too little talk about temperature, that climb exponentially with temperatures under +5C. There could be even SIX times ! increase between 25C and -15C... this is 6x times voltage sag between summer and winter.
So these are the details to start any talk wondering about "normal" drop...

And a solution if flying a lot in cold season gliders, i.e. short power sessions followed with long motor stop, long enough to battery become cold: consider kapton foil warmers installed in the battery neighbourhood, to keep them permanently in the 25C ballpark. Something like this:
https://www.ebay.com/itm/2pcs-HS-Pol...-/123728504074
The rectangular L80W20 model should suit almost all batteries used in F5J, imo.

This solution is not necessary for platforms where motor is permanently running, like acro/3D/multi, because battery produce internally enough heat, too much in the summer...
Dec 11, 2019, 11:39 AM
a.k.a VintageFan
I’ve done static tests with various motor prop combos and a drop to around 10V-10.5V at 35A is pretty typical for 3S. As others have pointed out the motor/prop/battery/ESC combination will have an effect. Also as renatoa points out temperature can have a very big effect.
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Dec 11, 2019, 12:11 PM
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Mr. Wiz's Avatar
What you are experiencing is exactly why I try to prop my planes with 4S battery. It’s just easier on all the gear to make your power with a higher voltage.
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Dec 11, 2019, 09:04 PM
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scirocco's Avatar
I wouldn't be too disappointed by 10-10.5V if pulling 40C (ie 34A out of 850mAh), but the OP setup was only 35A from a 1300mAh battery, ie under 30C and that is definitely not state of the art, even for cheap packs.

If you can't get over 11v under load at under 30C, it's time for a battery change.

Unfortunately discontinued https://hobbyking.com/en_us/multista...___store=en_us , easily holds over 11.1V at 30C, even at over a year old. Hopefully the replacement will achieve the same https://hobbyking.com/en_us/turnigy-...ck-w-xt60.html. At $15 surely worth a try.

4S, although indeed lower current for the same power is not a free lunch solution, as to maintain the same battery weight you need to reduce capacity and thus end up pulling the same relative load (C). Ie 3S 1300 @ 30C is 39A (say 429W @ 11V) . 4S 1000 will weigh the same, and 429W @ 14.67V requires 29A , ie still about 30C. Where increasing voltage can help significantly is ability to use smaller lighter ESCs, and perhaps in packaging for very small spaces, eg 2 x 2S in series.

Choice of 3S vs 4S - at constant component weight - is more about matching prop and Kv than efficiency or load on components, other than ESC size as stated and that in itself may be a sufficient reason to change
Dec 12, 2019, 03:43 AM
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Tuomo's Avatar
Quote:
Originally Posted by scirocco
I wouldn't be too disappointed by 10-10.5V if pulling 40C (ie 34A out of 850mAh), but the OP setup was only 35A from a 1300mAh battery, ie under 30C and that is definitely not state of the art, even for cheap packs.

If you can't get over 11v under load at under 30C, it's time for a battery change.

Unfortunately discontinued https://hobbyking.com/en_us/multista...___store=en_us , easily holds over 11.1V at 30C, even at over a year old. Hopefully the replacement will achieve the same https://hobbyking.com/en_us/turnigy-...ck-w-xt60.html. At $15 surely worth a try.

4S, although indeed lower current for the same power is not a free lunch solution, as to maintain the same battery weight you need to reduce capacity and thus end up pulling the same relative load (C). Ie 3S 1300 @ 30C is 39A (say 429W @ 11V) . 4S 1000 will weigh the same, and 429W @ 14.67V requires 29A , ie still about 30C. Where increasing voltage can help significantly is ability to use smaller lighter ESCs, and perhaps in packaging for very small spaces, eg 2 x 2S in series.

Choice of 3S vs 4S - at constant component weight - is more about matching prop and Kv than efficiency or load on components, other than ESC size as stated and that in itself may be a sufficient reason to change
I just posted about this to the other discussion.

You make me think about how much chemistry I can carry in a plane. The logical outcome of this to go to 2s (or even 1s!) setup to make it possible to power rx and servos directly from drive battery.

I practice, due to many reasons, higher voltage seems to be an advantage in F5J. Many consider 3s as std F5J battery but actually it is actually the lowest practical limit. Some have tested 2s setups but those has generally failed. No wonder there is plenty of testing going on with 4-6s F5J setups.
Dec 12, 2019, 09:32 AM
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Mr. Wiz's Avatar
Why on a large F5J plane is anyone overly concerned about the weight of one additional cell? If it’s a fit situation, I understand but otherwise, I wouldn’t sweat it. Beside, a smaller prop weighs less so there is a little offset.
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