OK, so let's see if I can remember what I've read and heard correctly and offer an opinion on this...
Yes, I agree with you that two parallel strings of 48 volts gives you options. You can put them together for a better Peukert effect (though with Lithiums, this may not be as much of an advantage as it would be with AGM's...). You can take one string off line for maint. if necessary.
You could also use one string going out, and assume that you can always make it back on the fresh string.
The disadvantage of the two separate strings is that then you really (ideally) need to have two full strings of voltage monitors to detect a cell going bad. The only way to detect a cell going bad is to monitor the individual voltages at each point.
When cells are in parallel, you can't monitor each cell - you can only look at the voltage of the paralleled set. So, if you parallel 2 cells, and then do 16 groups of two cells, you only have 16 points to monitor. The catch is that if one cell goes bad, it will draw the other paralleled cell down with it.
If you have two parallel strings of 16 cells, you have 32 different points that you can monitor. But then you have full visibility to every cell in the entire group. The gotcha in this case is that the two strings of 16 cells probably won't be exactly matched - so one will always draw the other down a little bit. Though in the real world, that probably isn't enough to matter.
It boils down to a tradeoff of how paranoid you want to be.
I think there's at least one person on this group that set up a 48V bank with no BMS at all to speak of - he just monitors the full bank voltage, and occasionally spot checks the cells - and has no problem so far (though he is reasonably careful with the bank).
This is always a fun topic to debate. It will be interesting to see if anyone strongly agrees or disagrees (or even understands my rambling...).
John
On Sunday, March 10, 2019, 9:20:33 PM CDT, tvinypsi@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:
Hi Brian. Carl Clark at Electric Car Parts Company proposed to me recently the concept of parallel 48-volt packs. One of the things about this approach that appealed to me was the certain degree of redundancy it provides. That is, the failure of one of the cells of a pack, effectively killing the pack, would not leave you dead in the water because you could take the failed pack off line and continue (albeit with more limited capacity) with the remaining good one. You might still be able to maneuver around the docks for instance.
This seem to work economically because the lower capacity cells are less expensive, it seems, per AH.
But Carl seemed to be suggesting a two 16-cells-in-series packs put in parallel rather than doubling up individual cells and putting the doubled cells in series. If I understand it correctly, using the approach you describe, if one cell goes bad, you'd have to disassemble (or reconfigure) the entire pack before you could put it back on line. Or have I missed or misunderstood something?
[-tv]
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Posted by: oak <oak_box@yahoo.com>
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