Each battery is actually made up of four 3.65V cells, wired in series, to give you one big 14.6V battery. 3.65V x 4 = 14.6V.
The charger will typically apply something like 14.6V to the battery to charge it. As the charger charges the battery, each cell individually charges. However, they might not have all started at the same voltage, or due to individual resistances they charge at slightly different rates. The result will be that they each finish charging at a different time. Here's the problem though, the BMS is designed to prevent damaging the cells by overcharging them (above 3.65V per cell). On these Eco-Worthy batteries it does this by stopping all charging across all cells as soon as any single cell hits 3.65V. That's great, but what if the fastest charging cell hits 3.65V when one cell is still at 3.55V and two other cells are still at 3.40V? The cells are out of balance and because they are not fully charged you get less of the advertised capacity. That can also lead to long term degradation because some cells are getting charged more fully than others.
The good news is that the BMS is also supposed to balance out the voltage across the cells. The problem is that typically when you use a charger the cells charge too quickly for the BMS to balance them out before one of them hits 3.65V and the BMS stops all charging to prevent damage to the highest charged cell. That's the point of the charger, to get as much power into the battery as fast as it can safely do so. The charger has no way to know about balance, it just knows to provide a constant voltage. So that's where the bench power supply comes in. You hook it up, set the voltage to a lower voltage than 14.6V, like maybe 13.80V, and limit the current (amps) to maybe .80A-1.0A or so. That will slowly charge the cells and give the BMS time to use resistors to limit charging to high cells and to bring low cells up. I'm not an expert and I suspect that it may also be able to bring high cells down, at least that is what I observed a few weeks ago. You slowly up the voltage and eventually you will have a mostly balanced battery. I believe there are also active BMS systems that can shunt power from one cell to another, but that is not what eco-worthy or any cheap battery brand uses. Again, I am no expert, just a noob to batteries figuring this out for himself.
This is where the importance of having bluetooth comes in. The app will show you the voltage of each individual cell. Without it you would have to cut open the battery to find that out using a multimeter. You can also take a multimeter and if your fully charged battery is giving a DC voltage of less than 14.6V then you know that the BMS cut off charging before all the cells hit 3.65V. That doesn't tell you which cells are out of balance though.
I've heard that expensive brands like battleborn balance the cells from the factory before shipping. In my experience, eco-worthy does not balance them before shipping and their support will tell you that everything is fine.
I should say that maybe you could get it balanced using a charger if you went through a bunch of charge/discharge cycles (not 100% to zero, maybe 100% to 90% to 100% to 90% to 100% to 90%). The problem is that a charger is going to charge it from 90% to 100% in an hour or so but it would likely take several days to actually balance so you would need a lot of cycles. A bench power supply puts you in control without the BMS stopping charger which stops balancing.
Also, I assumed people know what a bench power supply is. Basically, think of a charger but that gives you total control over the voltage and current (amps). They both take AC power from the power outlet and convert to DC. The charger voltage is likely fixed at 14.6V (or maybe they vary, I don't know) for the charger, but the power supply lets you set it. Thus, you can use the bench power supply to slowly charge your battery which gives the BMS time to balance the cells.
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