Faulty esc..... Or is it?

[DogBone]

My esc seems to do the low voltage cut out at 3.8v per cell. Its brand new so I contacted the shop to ask about a replacement. They think the esc is working as it should be. They may well be right but I also don't want to be fobbed off due to my lack of knowledge on electrics and lipos. Some help would be appreciated. 

Here's a few things they said:

 

It’s not that the LVC is cutting in at 3.8v it’s that by the time you come to check the battery its cell voltages have recovered back up to 3.8v.  The LVC would have been triggered because the measured voltage at the time the LVC cut in was down to 3.4v per cell.

 

A LiPo cell operates in a safe range between 3.6v per cell and 4.2v per cell.  Beyond those ranges can cause a chemical change in the battery which will reduce its lifespan and ability to hold a charge.  These cell voltages are then multiplied up by how many cells there to give you the overall pack voltage.

 

If you have a battery at say 4.2v per cell and you put a high current demand on to it then that voltage will sag down to say 3.9v or 4v (for arguments sake).  Once you release that demand for current the voltage will bounce back up to around 4.19v or maybe even 4.2v again.    As the capacity of the pack is used up the resting voltage is lower and the sag can be exaggerated.  Again, this is also exaggerated with a cold battery as it loses its ability to pass current effectively and as such the voltage sag seen is higher.

 

In this instance, if your battery is sitting at around 3.8v and you hit the power then the voltage is likely sagging down to 3.4v which is then triggering the LVC threshold and the ESC is going into limp mode.  You then bring the car back, remove the battery and check it when you get home.  The voltage will have bounced back to 3.8v per cell.

 

If you lower the punch setting to level 2 and give it a try then there will be less of a current hit triggering a false LVC.

 

 

Does this all sound correct. I clearly have a lot to learn about lipos and esc's! 

[Yrkoon]

you have a lot of good info there but the lvc cutting in at 3.8 is way to high, not sure m8, i know i got terrible voltage sag on a few of my models during the snow and lvc's cutting in way quicker than normal but im getting normal run times now in the higher temps.

 

i was getting short run times on my kronos recently untill i changed my esc lvc to the low setting instead of mid which improved things, when my kronos ran out of juice on mid lvc it was at storage mode basically 3.60 a cell and on low it is going to 3.3 a cell and giving me a longer run time.

 

is there a setting you can adjust for lvc cut off volts?

[DogBone]

Not on the 3s esc. It's only options is lipo or NIMH mode. It was suggested in another thread putting it in NIMH mode and running a separate lvc but if it's faulty and guaranteed I'd rather just get a replacement. 

[Yrkoon]

ah k m8, i would not be happy if one of my escs starting hitting lvc at 3.8 😞

 

what esc is it?

[lorrylemming]

Lipo batteries do sag as described above. Lower spec ones sag more, for example my race packs will run right down to 3.59v before the LVC kicks in, which is set for 3.6v. But, my cheapo hobbyking lipos cut out and rise back up to around 3.8v as you've described.

Please could you link the battery you're using, should give an idea of what quality it is. As the LVC is fixed you're a bit stuck, you could use an alarm, buy higher quality batteries or replace the ESC.

[DogBone]

It's this one. I have two and they both do the same. They are cheap ones. 

[DogBone]

8 hours ago, Yrkoon said:

ah k m8, i would not be happy if one of my escs starting hitting lvc at 3.8 😞

 

what esc is it?

Its a  blx 100 which is stock 

[Yorkshireman]

Run it in lipo mode then with an alarm fitted (cheap as chips on ebay).... then check voltage as soon as it cuts.

[DogBone]

Yeah I think that's going to be the only way of knowing. 

[zlab]

Could also be due to to long/to thin wiring from the battery to ESC.

For example, by the calculator if you have 12AWG wiring in total of 30cm (each wire 15cm), the resistance is around 0,003 ohm.

Then let's say on the connectors we have 0,001 ohm in total (just an assumption)

Internal resistance of the battery 0,003 ohm for each cell,so for a 3S Lipo, this gives us 0,009ohm for the pack.

Let's add a 0,001ohm for the ESC itself (depends on where on the PCB the actual measurement is done)

 

So a total of 0,014ohm. Doesn't seem much? Now introduce a 100A current spike in there and you get a voltage drop of 1,4V in total on the ESC voltage measurment. So if we have a 3S battery, the ESC sees that as 0,47V per cell.

Of course the current draw is much less in average, BUT if the ESC is not done properly (not averaging the measurement  but instead just trips on the voltage drop) then yes, this can be a problem....

 

 

So it would be good if you could provide some more information, like:

-cross section of the wiring (in AWG or mm2)

-length of wiring

-internal resistance of the battery

-are connectors OK (how warm are they after the run?)

 

Edited April 9, 2021 by zlab

[DogBone]

😳

[DogBone]

😅 Thats way over my head but I'll see if I can find out! 

[Glenn75]

There was a faulty batch of blx100 Esc doing exactly what you have described it was a software fault, arrma are replacing them under warranty ,I have two blx100 both lvc cut in at 3.2v I also use the voltz lipo but 5000mah

[DogBone]

1 hour ago, Glenn75 said:

There was a faulty batch of blx100 Esc doing exactly what you have described it was a software fault, arrma are replacing them under warranty ,I have two blx100 both lvc cut in at 3.2v I also use the voltz lipo but 5000mah

Maybe I will just contact arrma direct and see what they say. If they say it's fine then I'll buy a separate lvc 👍