So my intuition tells me that method B is better/safer, can someone with an electrical engineering degree or something confirm? :)
A would need to have each 3s group balance charged individually, disconnected form the pack, while B can charge all the batteries at the same time with just 2 balance wires.
Theoretically both configurations have the exact same output current and voltage.
Thanks!
Parallel in series is better.
This prevents cells differing in voltage.
ty
And you need only 1 balance charger not 3
I believe B is drawn incorrectly. If you want paralleled cells first and then connect those parallel cell groups in series, there should only be one red series wire between parallel sets of cells.
That said I believe you can run into as many potential problems with either connection pattern. In a parallel pack one cell can go bad and not be detected until each cell is disconnected and measured individually.
Is this a theoretical exercise or is there a practical reason you need a best case answer?
They are wires…all they need to do is connect one set of parallel batteries (the three in the blue circle) to the next parallel set of batteries…
The shape of the wire does not matter, it just conducts electricity.
If you look at the wires on the left or right ends of each setup, both A and B do exactly the same, connect 3 terminals together.
I also endorse the second approach, easier to maintain/keep balanced. What is more, previous to the pack assembly, we could make a sort of simple capacity binning test by charging the cells up to a given voltage/full charge, and then discharging them with the same load for a given time, for example. This way we could measure each cell's end of test voltage (mV accuracy I'd suggest), and elaborate a quick cell ranking. After that, we would arrange the cells in xS balanced groups of nP cells, having thus pre-balanced the pack. I mean, if done correctly, the capacity/power delivery of each parallel group would be balanced, I believe.
The consistency of cells from some manufacturers rewrappers may not be 1st class so, this approach would certainly help. Also, if some “black sheep”/dud is found, we'll now before assembling the whole shebang.
My ¢2. :-)
Cheers ^:)
My 18V Makita packs are 5 parallel pairs in series. Each pair gets monitored.
In the left side, if one cell cannot provide much current without dropping voltage, then that whole row can't provide much current. The other two in the row will need to stay at relatively higher voltage to make up for it.
If the lower rows are at 2.5V per cell, the upper row will have the bad cell at maybe 2.0 V and the two good ones at 2.75V each, not fully utilized.
In the right side, though this still happens but the effect is spread over all rows. This means the two cells parallel to the bad cell may also end up undervoltaged, but not by as much. And the other 6 cells all end up above cuttoff, but not as much as in the left option.
The practical difference here other than easy of re-balancing, is if you a) stress the bad cell really hard (left option) or if you b) stress two good cells with it. Which is better? Stressing a bad cell hard is probably more dangerous. But stressing the two good cells may mean you ruin your good cells too.
In the long run if you're balance charging, the right setup probably wins or at least is easier. If you're pack charging without balancing, good luck. But if you're individually charging anyway, I'm not as sure. It will be easier to figure out which cell is bad and maybe replace it if you have the left setup. Then I'd think it's best to replace a whole row (not column) if you really want to do things right. And for the right setup, if one cell damages the other three in the column, if you replace the column as you might be tempted to do, you're going to have balance problems again as that column out-performs the others. Having a row out-perform is ok.
Of course big factor in this is simply not running series packs down to the edge. Any imbalance is going to mean some cells are over the edge.
Thanks for the info everyone.
Any batteries used will be identical and tested, and they all have almost the exact same internal resistance.
Balance charging for lipos is definitely a must