Cleaned up the design, other design had some sus connections. I also wired up the BMS wrong
Replaced with a common-port BMS
The cell block and interconnect arrangment is much better the new way; you can get lower interconnection resistance like this.
BTW, you can buy rolls or lengths of "plate' interconnect in various widths, includiing ones wiht "fused" cell connects if you want those (they also make them without that--just flat sheets, some with indents to reach down below cell holder lips, etc). Probably some nickel-plated copper ones out there too if you have a welder that can do those and need the current capability.
Batteryhookup carries some; they're avaialble lots of places you just have to make sure you're getting the real metal you're after (rather than nickel plated steel or whatever, like you find in the really cheap stuff). These greatly simplify the pack welding / construction.
Each foot equals 15 cell connections long by 2, 3, 4, 5, or 6 wide. For example 2p wide would be 30 cell connections per foot (2 wide x 15 long) and 6p would be 90 cell connections per foot (6 wide x 15 long) Introducing a custom cell level fused nickel design that can be spot welded with ease...
batteryhookup.com
Are there really no drawbacks connecting 2 separate batteries (with their own BMS systems) with a fused Y-cable?
There's drawbacks to pretty much any way of making and using batteries.
There's more resistance in the connections, there's more connections to fail or be made wrong, etc. But there's also advantages. (see below)
FWIW, for any +/- comparison for any project, you can make a list of all the properties of each method, then for each one that's different between the methods, check which one is "better" for your specific application, and by how much (rated say, 1-10). Whichever method has more +'s with better numbers is probably the best way to do it for that application.
Can that work just as well as one big battery?
Yes. It is actually much more like one big battery than trying to use one of those battery-paralleling-devices, because there is nothing to interfere with current flow.
My biggest concern would be voltage sag, but I am not aware of how much sag is affected by having one big battery vs connecting 2 in parallel.
There will be more sag with two in parallel that are half of a bigger single pack, because there are more wires and more interconnects.
But: If you are using two BMS, one in each pack, that are identical to what you would have used for the single big pack, then there's *less* resistance to current flow in the BMS FETs themselves, as you now have twice as many in parallel, and so half the resistance.
One advantage to two separate packs is that *as long as each can fully support the system load, by itself*, you now have redundancy should a BMS fail, or a cell in a parallel group fail in a way that drains the other cells in that group.
If each pack can't support the full system load by itself, then that's not really an advantage, as it will be more likely to shutdown under load, or be damaged (or aged faster), if you have to use it by itself.
View attachment 364990
Would something like this work? (I would add fuses to all connector ends if I do go this route)
Yes.
I would simply add a bolt-on fuse in either the + or - wire on each branch of the Y. Bolt on fuses are more secure, less holder resistance (even if you use a holder vs bolting directly to the wires), if a bit less easy to change roadside than other fuse types.
Like this type; just make sure it is rated for *at least* the full charge voltage of your system (if it's not, it could continue arcing across after blowing, and cause a fire at the fuse itself from the plasma arc, rather than preventing one....).
I use this type of fuse bolted to the actual terminal of my batteries for the SB Cruiser trike, with a ring terminal crimped to the wires that go to the Anderson SB-50 connector that plugs into the trike itself. Mine are only rated for 32v, like the ones in the images above, but the datasheet linked is for a 70v version. Mine might not break the circuit (14s pack), but the 70v version would.
(I use the same batteries, two in series, for my formerly-ac-powered lawnmower, but the fuses probably wouldn't work as designed in this case; too high a voltage for them).
Whatever fuse you get, make sure it is rated per it's manufacturer spec sheet (like the one linked above) to blow in the amount of time you need it to for the current you need it to blow for, but *won't* blow for the current you need it to keep operating for.
There is a chart in each spec sheet (page 3? of the one linked, for instance) to let you do this.
If you get fuses with no spec sheet, you can't really know when or even if they will blow (or not blow) for a given current, or even what voltage they're good for. (regardless of the markings on the actual fuse). So stick with brand names with documentation.
