Question on adding mah capacity by adding batteries

[BHChieftain]

I thought this was a very illuminating thread on the matter. Esp. see the section that has the experiment with water containers connected in // towards the bottom (no matter the size of the container, they will all fill at the same level, or if water is drained from the system no container will drain before the others).

http://www.rcgroups.com/forums/showthread.php?t=932319

Chief

[T.S.Davis]

That's a good example. That's a static display though. An actual flowing condition would produce different friction losses for each inlet style. We are talking about a flowing condition. It's during that flowing condition that the voltage available is different on a 4s5000 and a 4s2700.

Picture one just like that but with only two identical openings at the top. In our case the inlets are each pressurized. The pressure is different for each inlet.

In the OP's first post he's suggesting a combo that under load would make the two inlets each have a different pressure. Different voltage in this case. The higher pressure inlet will push water up into the lower pressure inlet. Even if you poke an opening at the bottom to make the flowing condition. The higher pressure will push water up the low pressure tube and out the new opening at the same time. But the low pressure cells can't fight the higher pressure cells. The juice is flowing back into that low pressure tube until the high pressure cells get low enough to allow flow from the lower pressure cells.

Is this making any sense or am I losing it? .....my head hurts a little now. I'm no expert here.

......and all this pushing pulling balancing act is happening in the aorta supply line to the most fragile part of our setups. That being our precious ESC's of course. Frashizzle.

[T.S.Davis]

Holy smokes. 77 pages of how to charge in parallel.

Filling the containers is a touch different than sucking them dry. I think.

[BHChieftain]

That's a good example. That's a static display though. An actual flowing condition would produce different friction losses for each inlet style. We are talking about a flowing condition. It's during that flowing condition that the voltage available is different on a 4s5000 and a 4s2700.

Picture one just like that but with only two identical openings at the top. In our case the inlets are each pressurized. The pressure is different for each inlet.

In the OP's first post he's suggesting a combo that under load would make the two inlets each have a different pressure. Different voltage in this case. The higher pressure inlet will push water up into the lower pressure inlet. Even if you poke an opening at the bottom to make the flowing condition. The higher pressure will push water up the low pressure tube and out the new opening at the same time. But the low pressure cells can't fight the higher pressure cells. The juice is flowing back into that low pressure tube until the high pressure cells get low enough to allow flow from the lower pressure cells.

Is this making any sense or am I losing it? .....my head hurts a little now. I'm no expert here.

......and all this pushing pulling balancing act is happening in the aorta supply line to the most fragile part of our setups. That being our precious ESC's of course. Frashizzle.

Yes-- we also need to keep in mind that while water in pipes makes a good analogy it is just an analogy and there are also other variables in play. I think there is commentary on that thread that talks about the batteries practically should be in similar charge states (which would also keep the voltage differences at a min) and in similar condition, and to your other point, discharging them under load causes more wacky voltages differences than charging at 1C.


Chief

[BHChieftain]

Found this video experiment-- too bad he did not run it until he hit LVC, that would have been interesting to see.

http://youtu.be/vHj3B5qqjtY

But note, I need to retract these statements on using mixed capacities in //-- from what I've seen the data supports // CHARGING with different mah capacities (and I personally do this between 5000 and 5500 packs, same brand, same C rating, same condition, 1C charging rates), but as T.S. Davis points out the battery behavior is different under load (voltage changes as packs discharge). I would need to see or run some experiments for me to be confident...
Chief

[T.S.Davis]

Interesting. The voltage fluctuated withing the packs too.

[78MaicoRider]

I believe the internal impedance of the batteries would have the largest negative effect of packs in parallel, regardless of capacity.