Voltage drop on load - Lipo battery

[andrenoites]

Hello to all

Does the golden rule to never let lipo voltage under 3,65v apply to idle voltage or never under no circumstances (like for example load voltage, when motor is at full throttle voltage keeps under 3,65v for a few seconds).

Tks

 

[Ykick]

3.6V/cell resting is my interpretation. As far as how low can you go under load? Kinda depends how much current you're pulling but somewhere between 2.7-3.3V would be my suggestion.

3.6V is fairly conservative but one cell in a series string will always hit the "empty cliff" well before others so if you don't utilize some form of monitoring, alarms and/or LVC cell protection you might enter very dangerous and damaging voltage range before realizing it.

Another problem's wih aging weak cells. I just replaced one today resting comfortably at 3.8V but asking 6A from it sagged below 3.3V before I terminated the load. In other words crap high IR. (internal resistance). Deeper the cell is discharged the more it sags which internally heats, etc.

One useful item is CellLog 8S and logging software on Win computer. Connect it to balance leads, start logging, ride around. Download and view the individual cell voltage sag graph under usual riding load which reveals a ton of useful info about cell health.

 

[andrenoites]

Tks for the reply

I was asking because I'm configuring my low voltage cutoff in the controller:



I'm running 4x 6s5000mha c20 lipos with a 1680w cyclone motor at 48V (35A with 100A peak)

maybe 44V could be a good value for under voltage?

 

[dnmun]

where did this golden rule originate. a lipo cell is not fully discharged until the cell voltage is at 2.7V.

you can charge your cell up to 4.2V and then discharge it to 3.65V while measuring the current and Ah, and see how much charge is left in the pack.

 

[Ykick]

Just be aware the LVC in your controller will only be sensing the total pack voltage and NOT individual cell voltages. I will remind you some cells hit the "empty cliff" before others and when that happens you need to know and/or use a conservative LVC to avoid going into a critical zone.

 

[wesnewell]

where did this golden rule originate. a lipo cell is not fully discharged until the cell voltage is at 2.7V.

That's just total BS. Test I've run show they are totally drained at 3.32V. I wish you'd quit putting this garbage out there. Manufacturers suggest not to go below 3.5V for longevity. 3.5V is about a 3% SOC. You wouldn't ever want voltage to reach 2.7V even under a heavy load. Most RC people that pull max loads set their LVC to 3.0-3.1V. For a lowly ebike 3-4C load, setting LVC to 3.5V per cell will allow you to use 95-97% of the charge.

you can charge your cell up to 4.2V and then discharge it to 3.65V while measuring the current and Ah, and see how much charge is left in the pack.

No need. I can tell you how much will be left. About 7-10%.
http://endless-sphere.com/forums/viewtopic.php?f=14&t=47294

 

[fivari]

The LVC has to take into account the voltage sag under load. And then, all depends on the IR of te cells.
I have this homemade pack from recycled laptopbattery cells, that are not comparable with the Lipo-pack with 20C characteristics and higher. Even at 1C, the voltage sags down quite heavily under load. However, once the load is removed, the voltage jumps back to much higher values.
I put my LVC at 3V per cell.

 

[RedLine19K]

where did this golden rule originate. a lipo cell is not fully discharged until the cell voltage is at 2.7V.

That's just total BS. Test I've run show they are totally drained at 3.32V. I wish you'd quit putting this garbage out there.

Thank you! The energy in a LiPO below 3v is insignificant. The damage done by deep discharges is not. It is important to understand load and resting cutoff voltages to get the best life out of batteries. It is also important to recognise when people post nonsense.

Decent Lipos won't sag under 3.3v at 20C

And unless cells are monitored individually, some will most likely be overdischarged while average pack voltage is ok.

edit: the important part of the graph is missing. here's the link:
http://neptronix.org/posts/lipotutorial.html

 

[dogman dan]

Lets put it this way, in a 10 ah pack, once you pass 3.5v you have about two blocks before you are taking a very real risk of damage. 3.5, 3.3, Hell the difference is measured in yards.

2.7v is not so much the fully discharged voltage, as the voltage you don't go below to avoid creating a pack doomed to catch fire.

3.6v on the other hand, I have noticed that if I don't go all the way to 3.5v discharge, the packs tend to stay more balanced. And again, below 3.5v, you may as well just start pedaling your ass off, you didn't make it. Ideally you will stop when you are at 3.6- 3.7v on most rides, or even higher.

Figure out what your voltage drop under load is, 2v, 4v, whatever. Then set your lvc so you expect to stay above 3.3v, 3.4v, resting voltage, something like that. Then,,,,, don't even think of riding till the lvc pops. If you do, some cells will likely be still at 3.5v, and others will be 2.7v or below.

That LVC is not going to save your entire pack. It's only going to save some of the segments of it.

 

[andrenoites]

where did this golden rule originate. a lipo cell is not fully discharged until the cell voltage is at 2.7V.

you can charge your cell up to 4.2V and then discharge it to 3.65V while measuring the current and Ah, and see how much charge is left in the pack.

Sorry but I have to agree with wesnewell and redline19k, 2,7V is way to low. You will kill the battery quickly.

Just be aware the LVC in your controller will only be sensing the total pack voltage and NOT individual cell voltages. I will remind you some cells hit the "empty cliff" before others and when that happens you need to know and/or use a conservative LVC to avoid going into a critical zone.

Tks Ykick, I will follow your advice. I always balance charge the cells but still I will configure at a conservative 44V cutoff (3,66V per cell). Maybe needs to be lower because of the voltage drop on load.
I will do more testing once I get my watt meter.

Dogman your last post confirms my idea of setting the LVC at a conservative 44V cutoff. If I understood right, it is advisable to keep load voltage above 3,5V?

 

[Ykick]

Yes, power meter is imperative. If budget/time allows and you have access to Win computer CellLog 8S (logging version) is an extremely useful device. Never leave home without one!

8M version can be useful too but the 8S logging feature is so very helpful to view cell voltage sag under riding conditions.

Once you qualify your RC Lipo bricks deliver close to labeled capacity (for now) I'd suggest LVC about 42V, if I understand your total series cell (12S?) count correctly?

Good luck, go slow, small steps, patience & understanding. If something seems weird or "not right" check it out, ask questions. No room for errors...