I pretty much agree with Hyena, except that you really do need to do the simple mod to at least one of the supplies to keep the current down to its rated output of around 13-14A. I have the same S-350-24 supplies and it took me all of ten minutes to open up one unit and tack on the extra resistor required to lower the current. You only need to do the mod on one the units, if they are used in series, but if you don't, you will fry one, and it won't take very long. These things are designed to be used in things like audio applications with average power needs below the rated output, but they set the current limit at 135% of this number so that it can handle current surges. When used as a charger, the fixed current limit needs to be set lower, so that it can run continuously at that level.
While I agree that you don't necessarily need to balance the cells with every charge, or even every 5-10 charges, there are a few cautions. I've found that as long as you don't discharge the cells down close to the end of capacity, the cells will stay pretty well balanced. They will eventually start to drift apart, over time, and if you do run the pack all the way down, the lowest cells will become farther apart. Without cell level low voltage protection, this drifting apart will quickly turn into dead cells, but even it the cells will get farther and farther apart, without balancing the cells, at least once in awhile.
Another thing I've noticed is that the higher the charge current, the higher the voltage will go for the cells that get full first. I have two 12s3p packs I've been using to test the new BMS stuff Richard and I are working on, and when I tried charging with two unmodified MW S-350-24s in series, set to 49.8V, or 4.15V per cell. The cells were fairly close in balance but there was enough difference that at least one or two cells in each pack hit the point that their voltages started to rise before the rest, which caused them to go way too high, too early. The voltages for most of the cells were down around 3.98-4.00V but the high cells got up close to 4.40V. That is extremely dangerous to let any cell go that high, even with healthy ones. Charging at 10A, I never saw one go over about 4.30V. The point is that if you are going to try and charge at high rates, without any sort of "help", you should lower the max voltage, to like 4.10V per cell, or lower. It may seem overly cautious but even this isn't going to be enough for cells that are weaker. In that case, the weak cells will have even less ability to accept high currents, as they get full, and their voltage will rise to the point they do go into thermal runaway, and the fireworks start. I have tried to put out fires started from exploding cells and it is just about impossible until all the cells have completely burned. The fire is white hot, enough that the Pyrex dish the packs were in exploded, and then the concrete underneath started popping off.
There are three functions a BMS will provide, at a minimum, cell level low voltage protection, cell level overcharge protection and cell balancing. If you want to keep your packs healthy for a long time, you really need to do all three, but without cell-level LVC protection, it is only a matter of time before you start killing cells. Even if you don't ever plan on running a pack down below 20%, if you have one cell, in one of the paralleled packs that is on its way out, it will pull down the others that are paralleled with it. If it is allowed to "fall off the cliff", and go down to zero, it will kill the other cells paralleled with it as well. With cell level LVC protection, the bad cell can't pull the good ones below a safe point,
What cell level overcharge protection does is simply not let any cell (or block of paralleled cells...) go over a set voltage point, during charging. In our design, what we do is generate a opto-coupled signal whenever this happens. This signal is used to "throttle" back the charge current, using the same sort of PWM circuit used in the power supply to limit the voltage (CV mode...), so that it keeps the voltage for the cell, or cells, that tripped this HVC signal, right at the set point. It is really like having individual cell CV modes. What happens is that this high cell, is going to control the current available for all the cells in the pack. Like any Lithium-based cell, what happens is that as a cell gets fuller, it can't accept current at the same level, so the voltage rises, and does so at a fairly steady rate. At a point where the cell is about 75-80% full, the voltage starts to rise at a much faster rate. For LiPos, this happens at around 4/15-4.20V, and for LiFePO4 cells, this point is at around 3.65-3.70V. What most chargers try and do is hold the voltage at this point (i.e. CV mode...), which then causes cell to start lowering the amount of current it lets in. When it gets down under about 100mA, the cell is about as full as it is going to get.
The problem is that although the first cell is full at this point, that is not the case for the rest of the cells. Once the full cell doesn't let any more current in, it is also blocking any current going into the rest of the cells. Lead acid cells are different, because they have a unique characteristic that will allow them to "absorb" a bit more current than the cell can use, so this allows the rest of the cells to have at least that much so that they can catch up, and eventually become full as well. Lithium-based cells can't do this, and the current will drop all the way to zero. This is where the balancing part of the BMS comes in. What we do is have a shunt circuit in each channel that comes on at the CV crossover set point, which is 4.15V for this LiPo version. This shunt will allow a set amount of current to be bypassed around the full cell, so that the rest of the cells will have this "extra" current in order to finish charging. With most of the Chinese BMS designs, this shunt current is typically only 100-200mA, which means for a 20Ah pack, it can take half-a-day, or longer, for the cells to all reach the same point. Our latest version will support up to about 2A shunt currents, but for 1A and above, it will need active cooling to get the heat out. We have a circuit that will control multiple small "muffin" fans, that is included.
What our BMS control board now does is monitor the charge current, until it detects that the current is down to the same level as the shunt current. That says that at least one of the cells is pretty close to full. If the cells are fairly close in balance, they will all be almost full. We now have a balance switch that if set to off, the charge current will be cutoff completely. If set in the balance position a timer started. This timer can be set with a jumper block to timeout after 15 minutes, 1 hour, 2 hours or 4 hours. Once a timeout occurs, the charge current is cutoff. This way you can let the pack balance and if you forget, it will shutoff after it times out. We also have one more saftey feature that will cut the charger off if the current ever dips below about half the shunt current setting, which would only happen if there was a malfunction, of some sort.
Anyway, this is way too long, but I hope useful in some fashion. The S-350 mod is in this thread somewhere, but you might want to PM mkeefer, and ask him. He's got lots of these 350W supplies, and might be able to sell you one that has already been modified.