Rerouter
Member
- Joined
- Jan 1, 2017
- Messages
- 71
for any size bank of cells, (even XS1P), if you match the capacities closely, they will remain closely balanced at top and bottom, if the ESR is different you will see a drift occur if charged and discharged at widely different rates, e.g. 0.1C charge via solar, then a 0.4C discharge to run the aircon, as that higher ESR means it will appear to take more mAh to charge that it delivers when discharged. and this effect scales up with the rate of charge,
Most of your set ups appear to be testing capacity by discharging at 0.5A and are matching off of that, to this extent, if your charge and discharge currents are fairly similar you will likely never notice an issue, as the capacity has appeared smaller when you tested, and you have already accounted for the loss,
As far as the current sharing debate goes, yes V = I*R is ohms law, this doesn't really play into unequal cell current, however it more plays into C rating per cell, so lets say you have a 1000mAh cell in parrellel with a 3600mAh cell with the same esr, you add up the capacity and charge him at 2.3A (0.5C), now in this case, the current will be fairly evenly balanced, as most of the current will be shunted to the 3600mAh cell to increase his voltage at the same rate as the 1000 cell, (lithium cells do have a voltage / current ramp, and old nicd and nimh rules dont quite apply).
If you wanted to play safer, you can lower your C rating to be more in line with your lowest cells. and apart from the initial current spike, you cannot shove more current into a cell than the voltage/current curve allows. Still i'll give an example of where your concerns can lie.
we will say we have a pack at 3.5V, with a mix of ESR's between 65 milliohm, and 200 milliohm, (lets say 10 cells, with an average esr of 100) we hook our charger that will dump 10A into this pack, now when we hook him up, the average ESR will lift the voltage up to 3.6V and the cells will start taking in that charge current,
The 65 milliohm cell will see a difference of 0.1V across 65 milliohm, and pull 1.5A, while the 200 milliohm cell would pull 0.5A, this is the initial conditions,
shortly after that low esr cell will reduce its current as it will have charged up to the float voltage and be mainly stuck waiting for the rest of the cells to gain in voltage, as each cell continues charging, they will begin to balance out,
The next interesting point is when they reach 4.1V, now most chargers will begin to taper off the current, to keep the float voltage in check, the low esr cells will see little discharge blips on each step down in current as they try and bring the higher esr cells up, and they will be the last to charge, but again, the charger should taper off at 4.1V, and the esr of the cell limits how much current it can take from surrounding cells
So from my perspective, it is only when you first begin charging that you can risk dramatic current sharing differences, once the lower esr cells charge up to the float voltage things should balance out more or less evenly, and when charging nears its end, the current will taper off meaning the high esr cells are charging at a slower rate anyway.
Most of your set ups appear to be testing capacity by discharging at 0.5A and are matching off of that, to this extent, if your charge and discharge currents are fairly similar you will likely never notice an issue, as the capacity has appeared smaller when you tested, and you have already accounted for the loss,
As far as the current sharing debate goes, yes V = I*R is ohms law, this doesn't really play into unequal cell current, however it more plays into C rating per cell, so lets say you have a 1000mAh cell in parrellel with a 3600mAh cell with the same esr, you add up the capacity and charge him at 2.3A (0.5C), now in this case, the current will be fairly evenly balanced, as most of the current will be shunted to the 3600mAh cell to increase his voltage at the same rate as the 1000 cell, (lithium cells do have a voltage / current ramp, and old nicd and nimh rules dont quite apply).
If you wanted to play safer, you can lower your C rating to be more in line with your lowest cells. and apart from the initial current spike, you cannot shove more current into a cell than the voltage/current curve allows. Still i'll give an example of where your concerns can lie.
we will say we have a pack at 3.5V, with a mix of ESR's between 65 milliohm, and 200 milliohm, (lets say 10 cells, with an average esr of 100) we hook our charger that will dump 10A into this pack, now when we hook him up, the average ESR will lift the voltage up to 3.6V and the cells will start taking in that charge current,
The 65 milliohm cell will see a difference of 0.1V across 65 milliohm, and pull 1.5A, while the 200 milliohm cell would pull 0.5A, this is the initial conditions,
shortly after that low esr cell will reduce its current as it will have charged up to the float voltage and be mainly stuck waiting for the rest of the cells to gain in voltage, as each cell continues charging, they will begin to balance out,
The next interesting point is when they reach 4.1V, now most chargers will begin to taper off the current, to keep the float voltage in check, the low esr cells will see little discharge blips on each step down in current as they try and bring the higher esr cells up, and they will be the last to charge, but again, the charger should taper off at 4.1V, and the esr of the cell limits how much current it can take from surrounding cells
So from my perspective, it is only when you first begin charging that you can risk dramatic current sharing differences, once the lower esr cells charge up to the float voltage things should balance out more or less evenly, and when charging nears its end, the current will taper off meaning the high esr cells are charging at a slower rate anyway.
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