Paralleling Battery Banks [RE-wrenches]

[Kurt Albershardt]

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--On Monday, January 01, 2007 12:16 PM -0700 Ray Walters <walters at taosnet.com> wrote:
>
> I'm working on an electric vehicle with 4 parallel strings of batteries. I'm adding all copper interconnect resistances up to balance the bank (I had to add 5 ft of #4 to one string for instance) The problem is that I don't know where to put the fuses. A fuse at each battery positive would protect the sometimes long runs under the vehicle, but the other 3 strings could back feed a potentially dangerous short without blowing their fuses. So, do I put fuses on both ends of each cable, or just 
live with the problem, as we do with most solar systems, where the main battery cables are unprotected all the way into the DC load center.

I'm not completely clear on the "other 3 strings could back feed a potentially dangerous short without blowing their fuses" scenario.  If each string has a fuse sized to protect the cable it protects, where is the problem?



> Personally I would like to see batteries with integral fuses to protect any short, even the classic "dropping a wrench across the terminals" move.

This is a great idea, but I think it would be hard to implement in a cost effective manner.  You could make a low cost fuse integral to the battery, but it would not likely be replaceable.  If you make the element replaceable, it would increase cost and size of the battery.  Then there's the whole question of fuse rating and type, given the different applications and load/charge profiles.


> My kids' Barbie Jeep had sealed batteries with an ATC fuse that plugged right into the top of battery.

We usually put a PTC or two inside our small SLA setups, with an ATC30 on the higher current ones.


I like the Bussman Telpower fuses, which are designed for DC and are about half the length of an equivalently rated Class J or L fuse.  (TPL is 2.66" long up to 250A and only 3.5" long for 300-800A sizes.)

On medium-sized battery banks we bolt the fuse directly to the battery terminal with the cable lug bolted to the other end of the fuse.  On larger banks we usually install a TPHCS pull out fuse holder.

Another possibility would be to use a cable limiter (typically used when paralleling large cables) <http://www.bussman.com/library/bifs/1042.pdf>




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