LIPO voltage difference between two batteries

[AndiW]

Hi, after driving with the Outcast 8S EXB the two 4s packs always have about 0.1V difference per cell. One battery 3.5, the other 3.6 for example.
The batteries are brand new 4s 6700mah, that i've just used maybe 5 times yet.
Is this normal or is something wrong with the lipos?

thanks in advance

 

[Johnny b]

Normal. Just be sure to always balance charge and store.

 

[AndiW]

Normal. Just be sure to always balance charge and store.

Ok thanks

 

[Hector_Fisher]

Short answer: yes it's fine.

Long answer: Most batteries have a "band" where most of the battery's mahs are. Outside of that band, the voltage changes much more for every mah lost. This is even more pronounced at the end of the charge. The below is the pretty common discharge curve (or at least what they mostly look like). You'll notice voltage is much more volalite at the end.

 

[AndiW]

Short answer: yes it's fine.

Long answer: Most batteries have a "band" where most of the battery's mahs are. Outside of that band, the voltage changes much more for every mah lost. This is even more pronounced at the end of the charge. The below is the pretty common discharge curve (or at least what they mostly look like). You'll notice voltage is much more volalite at the end.
View attachment 291952

Yes I know this stuff. It's a lot more noticable with fpv drones since they don't have lvc and you can see the voltages.
I've just only used single 6s or 3s packs in the past and not two packs in series and didn't know how much difference is normal.

 

[razorrc]

Normal. Just be sure to always balance charge and store.

Storage charge, not balance charge. You want them at about 7.6V/cell when you put them away.

 

[Radiocontrolsoul]

Storage charge, not balance charge. You want them at about 7.6V/cell when you put them away.

7.6?

 

[AndiW]

Storage charge, not balance charge. You want them at about 7.6V/cell when you put them away.

Ok next time i storage charge i'll set it to 7.6V. I'll have a fire extinguisher ready too (;

 

[Johnny b]

Storage charge, not balance charge. You want them at about 7.6V/cell when you put them away.

I always plug in the balance lead when both charging and putting in storage. I balance charge each time. Is this not correct? I'm still new to lipos and electrics.

 

[AndiW]

I always plug in the balance lead when both charging and putting in storage. I balance charge each time. Is this not correct? I'm still new to lipos and electrics.

You are correct, you always balance charge lipos with the balance lead pluged in if you don't want to damage the battery or burn your house down. The save voltage to store where the cells don't get damaged is between around 3.6-3.9V per cell. 7.6 was just a typo I assume. I usually charge to 3.8V so with 4s that would be 15.2V total.

 

[Radiocontrolsoul]

I always plug in the balance lead when both charging and putting in storage. I balance charge each time. Is this not correct? I'm still new to lipos and electrics.

You will always use the balance plug, but when storing them set the charger to storage not balance charge

 

[slick2500]

Another thing to note the battery that is connected to the positive side of the esc will always drain slightly more than the one connected to the negative side.

 

[Johnny b]

You will always use the balance plug, but when storing them set the charger to storage not balance charge

Understood...that's what I been doing. I use 3s lipos so I store them at 3.82 per cell.

 

[razorrc]

7.6?

D'oh, sorry. 3.7-3.8V/cell.

I always plug in the balance lead when both charging and putting in storage. I balance charge each time. Is this not correct? I'm still new to lipos and electrics.

Balance charge means CHARGE them fully, with all cells balanced to 4.2v each. Storage charge means bring them to 3.7/cell.

 

[chilly81]

Another thing to note the battery that is connected to the positive side of the esc will always drain slightly more than the one connected to the negative side.

That's interesting, I wonder why that would happen. I've only recently started running dual packs in series so I'll have to keep an eye out for that. Do you have any reference for that or is that just your experience?

 

[slick2500]

That's interesting, I wonder why that would happen. I've only recently started running dual packs in series so I'll have to keep an eye out for that. Do you have any reference for that or is that just your experience?

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.

 

[Johnny b]

You will always use the balance plug, but when storing them set the charger to storage not balance charge

Understood...that's what I been doing. I use 3s lipos so I store them at 3.82 per cell

Another thing to note the battery that is connected to the positive side of the esc will always drain slightly more than the one connected to the negative side.

I tried really hard to understand this then finally became aware that it applied to people that run more than one battery at a time! Sometimes I surprise myself!

 

[RedOctobyr]

Another thing to note the battery that is connected to the positive side of the esc will always drain slightly more than the one connected to the negative side.

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.

I only run single packs, not pairs in series. So I haven't gotten to observe this myself, and to see it stay with the positive-ESC-side pack, even if I was to swap the "locations" of the 2 packs.

But I confess this is not intuitive to me, even with the clarification. The packs are in series. By definition, the same amount of current flows through each pack, and each cell within each pack. Nothing comes to mind that would make the packs behave differently.

What am I missing?

 

[chilly81]

I only run single packs, not pairs in series. So I haven't gotten to observe this myself, and to see it stay with the positive-ESC-side pack, even if I was to swap the "locations" of the 2 packs.

But I confess this is not intuitive to me, even with the clarification. The packs are in series. By definition, the same amount of current flows through each pack, and each cell within each pack. Nothing comes to mind that would make the packs behave differently.

What am I missing?

I don’t see how it would affect it either. two theoretically, identical batteries, in series with a resistance. Shouldn’t matter what that resistance is… Both batteries are seeing the same current so should behave the same.

I have a pretty good understanding of the electrical basics up until you start getting into capacitance, then you’re generally lose me. So I wouldn’t say it’s impossible, but I don’t see any reason it would occur, and tend to doubt it has to do with the resistance of the connecting wire if it is actually happening.

 

[SrC]

Another thing to note the battery that is connected to the positive side of the esc will always drain slightly more than the one connected to the negative side.

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.