John offered:
"Side note - if one properly installs fuses on each string that is added in parallel, hopefully that would at least help protect against the case of a single dead battery causing the parallel string to do a massive discharge."
That's a good point --- and a point well taken for folks paralleling Lithium also (as in my case). Fuse size and wire lengths become important---you could figure that fusing at just over half the maximum current draw planned. However, let's say that your buddies are not collocated but instead a distance away. Path length and cable resistance may result in less current flowing thru the distant battery unless configuring the cabling such that there's a balanced path so that neither battery gets a short path. In my case, I'm running with 7 Lithium batteries (31-48v @ 3kwh each) all connected up in parallel---ultimately I'll have 10-12 of these for a 30-36kwh pack. Yet these batteries are located in 4 locations around the boat, with about 10ft separation Fore-Aft and about 4ft Port-Starboard between the subpacks. And each of these battery modules actually has 2 sub-batteries. You can easily imagine wildly different currents being drawn from each of these modules---so what size fuses do I use if maximum draw is under 250amps total? Can they all be the same size? Is it necessary to perfectly balance the cabling so each module path has the same resistance as any other? How to avoid a "zipper" or cascade fuse blowing event?
That last question is one that's really important to consider. So you choose to make all your fuse the same size, then you're cruising along and suddenly the boat motor stops. You find that all the fuses have blown on one of your buddy pairs (or in my case, all 14-24 submodule fuses). This would happen if the current rating on the fuses isn't sufficiently higher than needed or your battery configuration isn't sufficiently balanced such that one of the buddies sees current exceeding the fuse rating while the other(s) see less. So the fuse blows---you're still motoring along and don't know anything happened, then a fraction of a second later, the buddy fuse(s) blow, "zippering" quickly out in sequence as a greater amount of current ends up flowing thru the remaining buddies' fuses. This is an important design decision and one that I'm currently involved in not just for my boat but with a friend's boat's repowering.
I'll be using 20amp fuses primarily, which for the most part is total overkill. But I'll also be carrying along some 60-80amp fuses to handle the case of for some reason having to run on a couple of batteries for a time. Initially I've used industrial tubular fuses and holders, but I'm now switching over to the 8ga "Maxi" style 60-80amp fuse holders and fuses. These are available pretty cheap these days from vendors on Ebay.
In case this helps-
-Myles Twete, Portland, Or.
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Posted by: "Myles Twete" <matwete@comcast.net>
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