LIPO voltage difference between two batteries

[jkflow]

Because of the resistance of the wiring between the packs. I have personally noticed this and I have also seen others mention it as well.

Sorry, I don't get that at all. I never noticed this. I think I would have by now. I am anal about the lipos.
A series circuit is a series circuit. NO matter which pack is at Neg or Pos. side.
It has to do more with the individual cells Internal resisitance if anything. And no two cells are exactly alike.
How I see it.

Technically, it makes no sense that the 'closest to the positive' would discharge faster. They are in series and the current is equal in all places of this chain. The weakest of the 2 i.e. the one with the highest IR would discharge faster (due to less capacity). This is a 50/50 chance thing, but I'm not surprised that the internet is skewing those odds.

In a series circuit, the current is determined by the total resistance. A resistor in the middle (bad connection) will limit the current equally.

For the technical inclined, this resistor loss would manifest itself as a negative voltage, i.e. the voltage 'sag' can be observed at the ESC terminals. Yes, that is real and if it's bad enough your connector will melt due to the power loss. It translates 100% into heat. Not the first time I've melted my solder, and that is at ~ 350F or so.

 

[slick2500]

Technically, it makes no sense that the 'closest to the positive' would discharge faster. They are in series and the current is equal in all places of this chain. The weakest of the 2 i.e. the one with the highest IR would discharge faster (due to less capacity). This is a 50/50 chance thing, but I'm not surprised that the internet is skewing those odds.

In a series circuit, the current is determined by the total resistance. A resistor in the middle (bad connection) will limit the current equally.

For the technical inclined, this resistor loss would manifest itself as a negative voltage, i.e. the voltage 'sag' can be observed at the ESC terminals. Yes, that is real and if it's bad enough your connector will melt due to the power loss. It translates 100% into heat. Not the first time I've melted my solder, and that is at ~ 350F or so.

Idk every vehicle I have or ever had that ran 2 batteries the positive side would always be about 0.01v per cell lower than the one on the negative side
As I said before I have seen others mention this a few times over the years.

 

[Hector_Fisher]

Technically, it makes no sense that the 'closest to the positive' would discharge faster. They are in series and the current is equal in all places of this chain. The weakest of the 2 i.e. the one with the highest IR would discharge faster (due to less capacity). This is a 50/50 chance thing, but I'm not surprised that the internet is skewing those odds.

In a series circuit, the current is determined by the total resistance. A resistor in the middle (bad connection) will limit the current equally.

For the technical inclined, this resistor loss would manifest itself as a negative voltage, i.e. the voltage 'sag' can be observed at the ESC terminals. Yes, that is real and if it's bad enough your connector will melt due to the power loss. It translates 100% into heat. Not the first time I've melted my solder, and that is at ~ 350F or so.

I think in a way, you mean to say: the amount of mah's drawn from (or put into) each "cell" is determined by the differential voltage (such as voltage drop) across that cell during a discharge (or charge) sequence. Voltage differential is proportional to the cell's resistance as measured across the positive and negative tabs of that cell. Each cell will have slightly different internal resistance due to minute changes, thereby the mah's drawn from (or put into) each cell at the final resting voltage will be slightly different.

Idk every vehicle I have or ever had that ran 2 batteries the positive side would always be about 0.01v per cell lower than the one on the negative side
As I said before I have seen others mention this a few times over the years.

In a simple circuit of two resistors and a power source, that would effectively look like the lead resistors seeing more current than the second, which would defy Ohm's law.

Not to say that the observation you or others have seen is incorrect, but I would rather chalk that up to coincidence of slightly varying IR between the cells always being the same, rather than a principle.

 

[slick2500]

I think in a way, you mean to say: the amount of mah's drawn from (or put into) each "cell" is determined by the differential voltage (such as voltage drop) across that cell during a discharge (or charge) sequence. Voltage differential is proportional to the cell's resistance as measured across the positive and negative tabs of that cell. Each cell will have slightly different internal resistance due to minute changes, thereby the mah's drawn from (or put into) each cell at the final resting voltage will be slightly different.

In a simple circuit of two resistors and a power source, that would effectively look like the lead resistors seeing more current than the second, which would defy Ohm's law.

Not to say that the observation you or others have seen is incorrect, but I would rather chalk that up to coincidence of slightly varying IR between the cells always being the same, rather than a principle.

Maybe I have a crappy cell tester, out in the field I just use one of those cheapo buzzer alarms . I'll bring along my Hobbywing LCD program card next time.

 

[jkflow]

Maybe I have a crappy cell tester, out in the field I just use one of those cheapo buzzer alarms . I'll bring along my Hobbywing LCD program card next tim

Maybe there is some RC magic that I'm missing, but I highly doubt it.
Mark your Lipos and swap them around, just for giggles.

By all means, a 0.01V error could also be just a measurement error where the instrument is rounding up/down and the difference is somewhere in the 3rd digit i.e. 3.556V vs 3.554V, you will get 3.56 or 3.55 on your display.

 

[slick2500]

Maybe there is some RC magic that I'm missing, but I highly doubt it.
Mark your Lipos and swap them around, just for giggles.

By all means, a 0.01V error could also be just a measurement error where the instrument is rounding up/down and the difference is somewhere in the 3rd digit i.e. 3.556V vs 3.554V, you will get 3.56 or 3.55 on your display.

This is probably the most likely culprit.

 

[slick2500]

I knew I wasn't crazy https://www.smc-racing.net/index.php?route=product/category&path=166

"The reason why this setup isn’t very good is because the packs will discharge down to different levels. This is due to the flow of current, the current from one pack must flow ‘through’ the second pack meaning that the current sees virtually double the resistance of the first pack. "

 

[Hector_Fisher]

I knew I wasn't crazy https://www.smc-racing.net/index.php?route=product/category&path=166

"The reason why this setup isn’t very good is because the packs will discharge down to different levels. This is due to the flow of current, the current from one pack must flow ‘through’ the second pack meaning that the current sees virtually double the resistance of the first pack. "

Idk man. I'm not going to claim to be an expert, but some of their article just doesn't make sense to me. Now if you told me this was all using the EVX-2 from the Summit, i'd agree, since those batteries are literally connected separately into the ESC (4 battery cables into the ESC) and one side gets the draw from the BEC while the other side doesn't. But when connected directly in series, if both packs were identical in IR and specs... That's where the disagreement seems to be.

This is the statement of theirs that i either don't understand or is incorrect:

They way they wrote this suggests that one pack would be draining twice as much as the other? But i've never seen anything close to that happening unless the IR between the packs is widely different. Usually they are relatively close to each other based on the slightely different IR between the packs. Some of my sets of packs are almost identical in IR, so at the end of the run in the kraton, they have the same final voltage, within 0.01 or 0.02V per cell of each other.

I'll always agree about swapping batteries around and rotating to keep things fresher. But I'd definitely like to see some sort of test to show how their claims are happening in reality. Perhaps I can set something up in series on the discharger and measure voltage drop across both batteries using a cell checker on the balance leads while discharging. If what they say holds true, that one battery should have more voltage drop than the other, then that should be easily observable.

 

[jkflow]

Current in a series circuit is the same everywhere. Yes, the pack with higher IR will always lose and have the lower voltage. The IR of that cell doesn't change with respect to where it is in the circuit.
Maybe there is something to the myth, but those explanations make no sense.

 

[Cheez]

Idk man. I'm not going to claim to be an expert, but some of their article just doesn't make sense to me. Now if you told me this was all using the EVX-2 from the Summit, i'd agree, since those batteries are literally connected separately into the ESC (4 battery cables into the ESC) and one side gets the draw from the BEC while the other side doesn't. But when connected directly in series, if both packs were identical in IR and specs... That's where the disagreement seems to be.

This is the statement of theirs that i either don't understand or is incorrect:
View attachment 301550

They way they wrote this suggests that one pack would be draining twice as much as the other? But i've never seen anything close to that happening unless the IR between the packs is widely different. Usually they are relatively close to each other based on the slightely different IR between the packs. Some of my sets of packs are almost identical in IR, so at the end of the run in the kraton, they have the same final voltage, within 0.01 or 0.02V per cell of each other.

I'll always agree about swapping batteries around and rotating to keep things fresher. But I'd definitely like to see some sort of test to show how their claims are happening in reality. Perhaps I can set something up in series on the discharger and measure voltage drop across both batteries using a cell checker on the balance leads while discharging. If what they say holds true, that one battery should have more voltage drop than the other, then that should be easily observable.

That article is a bit confusing….I plan on doing something like this , should be okay yes or no ?

 

[slick2500]

Idk man. I'm not going to claim to be an expert, but some of their article just doesn't make sense to me. Now if you told me this was all using the EVX-2 from the Summit, i'd agree, since those batteries are literally connected separately into the ESC (4 battery cables into the ESC) and one side gets the draw from the BEC while the other side doesn't. But when connected directly in series, if both packs were identical in IR and specs... That's where the disagreement seems to be.

This is the statement of theirs that i either don't understand or is incorrect:
View attachment 301550

They way they wrote this suggests that one pack would be draining twice as much as the other? But i've never seen anything close to that happening unless the IR between the packs is widely different. Usually they are relatively close to each other based on the slightely different IR between the packs. Some of my sets of packs are almost identical in IR, so at the end of the run in the kraton, they have the same final voltage, within 0.01 or 0.02V per cell of each other.

I'll always agree about swapping batteries around and rotating to keep things fresher. But I'd definitely like to see some sort of test to show how their claims are happening in reality. Perhaps I can set something up in series on the discharger and measure voltage drop across both batteries using a cell checker on the balance leads while discharging. If what they say holds true, that one battery should have more voltage drop than the other, then that should be easily observable.

I knew had read it somewhere is all I was saying.

 

[Cheez]

I knew had read it somewhere is all I was saying.

https://www.smc-racing.net/index.php?route=product/category&path=166

 

[SrC]

Idk every vehicle I have or ever had that ran 2 batteries the positive side would always be about 0.01v per cell lower than the one on the negative side
As I said before I have seen others mention this a few times over the years.

Whether 2 packs or a single brick or even 3 packs. All the cells combined will always be considered in Series. The ESC doesn't know or care how many packs are creating the total amount of cells in all. As long as there is no overvolting going on. Running the correct volts.
How I always see it at least.

 

[Hector_Fisher]

I knew had read it somewhere is all I was saying.

All good, wasn't disagreeing with you Was just picking at what SMC said.

@jkflow, agreed not sure why SMC would think a battery can be "solicited" twice as much as another. Keeping in mind current is the same throughout the circuit, they might want to revisit the topic and clarify what in the world they were trying to say.