You are correct that a BMS that limits via voltages on a series pack, would always be good even as the cells age and loose capacity. Under 2..5 volts would always be bad, over 3.5 volts would always be bad. No matter the total capacity. The other James was talking about some mythical BMS that monitored via percentages, to my knowledge so such thing exists.
Not to say he does not have a point. With Lead or Lithium I only recommend building packs in series. Because things do get more interesting when you start talking parallel, strings. If you are not monitoring every cells individually, then one of the cells in your parallel string could become very out of balanced, do to a difference in internal resistance. So let's say you have four cells in a parallel string. They start out at full charge:
3.5, 3.5, 3.5, 3.5 = a voltage of 3.5 for the string, your BMS is happy. But after a few cycles at full charges because one of the cells has more internal resistance you now have:
3.6, 3.6,3.2,3.6 = a voltage of 3.5 for the string, your BMS is still happy but three of your cells are too high and one is too low. At full discharge the low high resistance cell could even reverse and start ruining other cells. This is why Tesla packs individually fuse each and every cell in there parallel strings.
If I can I personally avoid parallel strings, with Lithium or Lead. So my advise is either buy bigger cells and build a series only battery pack, or if you do build in parallel monitor every cell. This goes for lead as well if you have Lead battery's in parallel I recommend you charge each battery separately not as a whole. So if you have a 12 volt pack made of four 12 volt batteries, charge each one with a separate charger.
Obviously, you don't always have a choice, but if you do, the less cells in parallel the better.
> On Sep 17, 2018, at 12:22 PM, oak oak_box@yahoo.com [electricboats] <electricboats@yahoogroups.com> wrote:
>
> Wait...
> Shouldn't the BMS be monitoring the cell voltage all the time?
> If the BMS is watching for a low cell voltage of 2.5V on all cells, then shouldn't you be fine?
> Even with cells tied in parallel at the low level, all the voltages would be the same. If you had one really weak cell, I would expect it to either bring down the whole set, or maybe "charge" off the other cells in that group. Either way, when the voltage gets to the minimal threshold, the BMS should shut off power. Right?
>
> John
Posted by: "james@deny.org" <james@deny.org>
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