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Advanced Lipo Voltage question

Advanced Lipo Voltage question

Old 04-23-2015, 04:45 PM
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Default Advanced Lipo Voltage question

In the name of due diligence, I first searched google using "site:rctech.net" to see if the question was asked and answered. Nothing jumped out as being an answer, so...


I am trying to optimize my ESC set-up to balance performance with battery longevity. I have observed minor swelling of the batteries on particular occasions, which I believe was caused by heavy load on the batteries.

So I am experimenting with settings. Last run was at 13.5v LVC (2 x 2S 7200 mAh in my truggy).

When I checked the data from my run, I noticed that the lowest voltage was 11.7 volts! (Sample rate was every 2 sec, so it might have dropped lower.)

I know lipo voltage drops under load, but I am surprised to see actual low voltage 15% lower than my LVC setting.

So the question is, should I be worried about this?

I can lower the current limit (currently at 30A), which would help control this voltage drop, but I don't know how far I have to lower it? My batteries are SMC and claim a 70C rating, which if true, would mean a safe discharge of an absolutely ridiculous 500amps! But I have seen comments made by Danny of SMC where he admits that no current hobby grade lipos (including those he sells) actually handle more than 25-30 amp discharge.


I'll keep on testing, but doing actual tests and collecting real data is such a slow and tedious process. So if you've already done some of the work, I'd like to know what you've found on this issue!
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Old 04-23-2015, 04:54 PM
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can't you LVC for each cell ? it's better to cut when one cell reach a lower peak.

Correct me if I 'm wrong, but I think the first cell of the pack has more work. I noticed of some dead lipo's that the IR of the first cell is often higher. Coincidence ?

And regarding LVC, I noticed too that at 3.4 LVC cut ( yeah I'm a soft guy with lipos ), the cell is actually stabilizing at 3.7 V approx. But If I'm doing spped runs and really sucking out the power of the lipo, it might go to 3.5 V approx.
this on my cheap hobbyking ESC with nanotech lipo 4S 5000 95C
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Old 04-23-2015, 05:11 PM
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Are using a Tekin esc with data logging, is that where you are getting your readings?

What are the pack voltages reading immediately after running?

I'm running mine at 14.4 lvc.
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Old 04-23-2015, 05:49 PM
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I know back when I had my RX8 I was somewhere like 70% current limiter to save my packs.
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Old 04-23-2015, 06:04 PM
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Originally Posted by jjahn
Are using a Tekin esc with data logging, is that where you are getting your readings?

What are the pack voltages reading immediately after running?

I'm running mine at 14.4 lvc.

yeah, it's a tekin rx8

My voltages after running to LVC just now is 7.45 and 7.33v. (The one at 7.33v is dying, I'm pretty sure. It gets warmer and will end at a lower voltage, part of the reason I'm paying more attention to this stuff.) But to answer the question, the LVC is shutting the car down when the resting voltage is at whatever I set it at.


I tried something like 14.4v LVC, but I could only run for 2-3 minutes before the ESC would start cutting in. It was very obvious, like it would cut back to 1/2 power. If I completely let off the throttle for maybe 1 sec, and got back on it again, I got the full power again. It ran like that for several minutes. This was on a fresh, balanced set of batteries.
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Old 04-23-2015, 06:12 PM
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Originally Posted by werner sline
can't you LVC for each cell ? it's better to cut when one cell reach a lower peak.

Correct me if I 'm wrong, but I think the first cell of the pack has more work. I noticed of some dead lipo's that the IR of the first cell is often higher. Coincidence ?

And regarding LVC, I noticed too that at 3.4 LVC cut ( yeah I'm a soft guy with lipos ), the cell is actually stabilizing at 3.7 V approx. But If I'm doing spped runs and really sucking out the power of the lipo, it might go to 3.5 V approx.
this on my cheap hobbyking ESC with nanotech lipo 4S 5000 95C
unless you have an ESC that allows you to connect your batteries balance cables to it, it is impossible for the ESC to know the voltage of individual battery cells. The ESC can't tell the difference between two cells at 3.5 volts, or one at 3v and the other at 4v--both read as 7 volts. That's why it's a good idea to use a higher LVC.

The order the cells are connected shouldn't make any difference.
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Old 04-23-2015, 06:48 PM
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update: I did one more run before dark. Sampling rate was every .2 second. Current limit set to 13 amps.

This time the lowest voltage under load reached down to 11.23v, but just once early in the run. After that, the voltage didn't drop below 12 volts.


I knew voltage dropped under load, but I'm a bit surprised that it is dropping this much. And the LVC didn't seem to affect this voltage drop under load.


My other observation is that the current limit does not do what I always thought it did or was supposed to do. If the current limit limited the amps the motor could draw, then I would see less of a voltage fluctuation the lower it is set. But there was no difference in voltage fluctuation between a 30 and 13 setting, which means that the motor was still drawing the same amount of current.

Only observed difference was that at the 13 setting, it took more throttle to get the truck to move, like there was a crazy amount of expo on my radio.
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Old 04-23-2015, 06:51 PM
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Originally Posted by fredygump
yeah, it's a tekin rx8

My voltages after running to LVC just now is 7.45 and 7.33v. (The one at 7.33v is dying, I'm pretty sure. It gets warmer and will end at a lower voltage, part of the reason I'm paying more attention to this stuff.) But to answer the question, the LVC is shutting the car down when the resting voltage is at whatever I set it at.


I tried something like 14.4v LVC, but I could only run for 2-3 minutes before the ESC would start cutting in. It was very obvious, like it would cut back to 1/2 power. If I completely let off the throttle for maybe 1 sec, and got back on it again, I got the full power again. It ran like that for several minutes. This was on a fresh, balanced set of batteries.
How long after your run did you wait before checking the voltage, immediately? Those don't seem like bad voltages 14.78 is higher than your cutoff, that's why I wondered if it was immediate. Also, do you know the individual cell voltages? That could explain the lower pack voltage, a weak cell.
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Old 04-23-2015, 07:31 PM
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Originally Posted by fredygump
unless you have an ESC that allows you to connect your batteries balance cables to it, it is impossible for the ESC to know the voltage of individual battery cells. The ESC can't tell the difference between two cells at 3.5 volts, or one at 3v and the other at 4v--both read as 7 volts. That's why it's a good idea to use a higher LVC.

The order the cells are connected shouldn't make any difference.
dammit you're right ! I guess the only purpose to show an individual cell LVC is that the ESC recalculate the total LVC whenever you connect a 3s or 4s or 6S lipo ( like I do some times )
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Old 04-24-2015, 06:00 AM
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Your slight swelling is related to the batteries you are using. 7200 in a 2s pack is really pushing the maximum size you can physically fit inside of a case that can have a maximum height of 25.1mm. No matter how good you treat those cells in that size, you are probably going to see some swelling in the case.
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Old 04-24-2015, 06:21 AM
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Default Questions...

Originally Posted by fredygump
In the name of due diligence, I first searched google using "site:rctech.net" to see if the question was asked and answered. Nothing jumped out as being an answer, so...


I am trying to optimize my ESC set-up to balance performance with battery longevity. I have observed minor swelling of the batteries on particular occasions, which I believe was caused by heavy load on the batteries.

So I am experimenting with settings. Last run was at 13.5v LVC (2 x 2S 7200 mAh in my truggy).

When I checked the data from my run, I noticed that the lowest voltage was 11.7 volts! (Sample rate was every 2 sec, so it might have dropped lower.)

I know lipo voltage drops under load, but I am surprised to see actual low voltage 15% lower than my LVC setting.

So the question is, should I be worried about this?

I can lower the current limit (currently at 30A), which would help control this voltage drop, but I don't know how far I have to lower it? My batteries are SMC and claim a 70C rating, which if true, would mean a safe discharge of an absolutely ridiculous 500amps! But I have seen comments made by Danny of SMC where he admits that no current hobby grade lipos (including those he sells) actually handle more than 25-30 amp discharge.


I'll keep on testing, but doing actual tests and collecting real data is such a slow and tedious process. So if you've already done some of the work, I'd like to know what you've found on this issue!
I know you are looking to optimize your battery and run time but I would have to ask what you are running for motor and gear and what vehicle is it in. It sounds like you have something else a little off if you are hitting LVC of 14.4 within just few minutes. You may be chasing a problem that is not battery and LVC related... That may just be the results of too much motor or too much gear or something else.
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Old 04-24-2015, 07:09 AM
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Originally Posted by jmackani
Your slight swelling is related to the batteries you are using. 7200 in a 2s pack is really pushing the maximum size you can physically fit inside of a case that can have a maximum height of 25.1mm. No matter how good you treat those cells in that size, you are probably going to see some swelling in the case.



I've read what SMC says about swelling, so I agree that some swelling will occur. But what bothers me is when I can see a significant increase in swelling after 1 run.
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Old 04-24-2015, 07:13 AM
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14.4 is actually pretty high voltage cutoff for 4s packs. We are normally running ours at 12.8 or 3.2 per cell.
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Old 04-24-2015, 07:18 AM
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Originally Posted by fredygump
I've read what SMC says about swelling, so I agree that some swelling will occur. But what bothers me is when I can see a significant increase in swelling after 1 run.
I run those packs in 4wd sct and I also do the battery inspections for ROAR. There is no room for any swelling in those packs and running them in a truggy is only going to amplify it. Settting that current limiter too low is also creating issues in a truggy. It will struggle to move those large tires running it that low. Once the packs swell, they will never go back to the original size. The current limiter numbers also are not direct amperage numbers.
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Old 04-24-2015, 07:52 AM
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Originally Posted by JFuel11
I know you are looking to optimize your battery and run time but I would have to ask what you are running for motor and gear and what vehicle is it in. It sounds like you have something else a little off if you are hitting LVC of 14.4 within just few minutes. You may be chasing a problem that is not battery and LVC related... That may just be the results of too much motor or too much gear or something else.

You might be right...it might be too much motor for the batteries. But that just opens up a whole can of worms, doesn't it?


This is my dex408t with tekin rx8 and t8 2250 kv.

I have been trying different gears. Last year I used a 17t pinion. Motor temps were like 140 or 150 most of the time. The local track is a modest 1/10 dirt track, so quite small for truggy--it's constantly twisting and turning, so no opportunity to get any speed.

Last year I managed to strip out the fancy spiral cut diff gear (rear), so I decided maybe I should increase the gearing a little to reduce the strain. I've run 18,19, and 20 this year. Temps are pretty low, but weather is still pretty cool around here..


The early limiting with the 14.4 LVC was with the 18t gear. It was at home, so on a combination of gravel, short (i.e. still dead) grass, and pavement. I'm using aka i-beams, so not a huge amount of traction on grass, not like if I ran badlands.

The data logging I mentioned, showing down to 11.2 volts under load, occurred with the 18t and 20t pinion. No difference, really. Bigger gear was faster, but I think the peak current draw is based on the motor size--the duration of peak current draw is determined by gearing.


The only factor I cannot control is the motor's current draw. The T8 gen2 is a beast of a motor, and it is probably sucking more power than the batteries can supply.

A smaller motor would fix the current draw problem. But how to match a motor to a set of batteries when nobody (except Danny at SMC) is honest about battery discharge rates? And motors mfgs, like Tekin, sort of don't tell you any specs at all.



I'm investigating the possibility of adding a current limiting circuit between my batteries and the ESC. My brother is an electrical engineer, so hopefully he can help. If I can limit discharge at the battery, then the motor draw is irrelevant. The motor can try to draw 50 amps, but if the limiter won't let more than 25A through, 25A is all the motor will get. And according Danny at SMC, 25A is about all a good lipo can actually deliver.


Or is there something that I'm missing that would dial down the torque/current draw? I'm really a finesse type of guy, and I almost never use full throttle...and limiting the throttle affects top speed, but not current draw under load...
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