Fuse question

[madlabs]

Hi all,

There have been a few threads about fusing, and my battery bank only has a fuse where the cable connects to the bank on the + side. I know I should have a fuse on every battery. I was wondering that since I have 10X6v batteries, in series/parallel configuration for a 12v, 1200Ah bank, can I put a fuse between the series connections, in the cable that connects the + of one batt to the - of the next? Lame ascii art to explain below.

+++++++++++ to bus
6v        6v
-           -
|           |
fuse     fuse
|           |
+          +
6v        6v
----------------------to bus

The reason I ask is that mechanically it would be much easier for me to do it this way.

Thanks!

Jonathan

[ghurd]

I do not know what everyone else will say...
but I have done it that way.

For what I usually do, the charging amps and load amps are pretty small compared to the battery capacity.
I used in-line MAXI fuse holders.  Seems like they come with #8 wire?
(need to be sure they are long enough, because some are pretty darn short)
I soldered a terminal on both ends of the fuse holder, heat shrink over the end of the terminals where the wire is, and used it as the connecting wires.
2 birds with 1 stone.
G-

[rossw]

I do not know what everyone else will say...

I for one say that (individually) they're simply series circuits, and the current in a series circuit is the same everywhere, and therefore the fuses in the middle of the bank as drawn - while unconventional - is perfectly servicable.

There are a few reasons why its often not done like that, one is that (some) people thoroughly insulate the battern interconnects and terminals. Fuse blocks you can open provides a convenient place to measure volts for one string. Some people (I won't call their intellect into question) seem unable to comprehend anything thats not straight out of the rulebook, so if you ever had an electrical inspector go over the work, they may knock the non-standard fusing arrangements.

[madlabs]

Thanks for the reassurance. With any luck an inspector will never get a look anyway.

I am only concerned that it is electrically valid and am glad to hear that it's ok. As I mentioned it will be mechanically easier for me to attach there.

Thanks!

Jonathan

[SparWeb]

Not lame; excellent ASCII art.  It's perfectly clear.
Fusing the batteries the way you show it, you can use a much smaller rating of fuse for the draw on the system.
200Amps / 5 = 40 Amps.  Cheaper fuse than the 200A fuse.  But of course you have to get more of them so it will cost more. 

The problem is that faults usually occur when the system is loaded, heavily, so if something is wrong with one string causing that fuse to pop, the resulting load re-distributed to the rest of the strings could cause them to pop, in a cascade.  It's not the end of the world, but dealing with 5 blown fuses is more annoying than just one.  You won't know which string of batteries were the original culprit.  I wonder if an inverter would respond badly to having its DC supply interrupted in that way.

Offsetting that is the (small) possibility that a plate shorted inside one 6v battery would not kill all the rest in the system by draining them all at high-current until they are flat and cooked.

[ghurd]

I wonder if an inverter would respond badly to having its DC supply interrupted in that way.

Should not have any effect on any inverter I ever worked on.

Most inverters are fused to interupt the DC supply.
Many use parallel fuses.

The type of inverters with the fuses inside, or inside and soldered into the board, I try to use a lower amp fuse in the supply line where it is easier to change.  Idea being the external fuse blows before the internal fuse.
G-

[Madscientist267]

This is actually a common method in such implements as UPS units where two 12V batteries are connected in series. The fuse acts like the jumper that connects the two batteries together, and guarantees that just about no matter what circumstances are encountered (except a puncture of the cover over the fuse), the circuit will open and protect the wiring.

Only thing I would add to this is having a master link in addition to your individual fuses, at just a slightly lower rating than the collective of the individuals. For example:

+++++++++++ ~ +++ to bus
6v        6v         Master
-           -
|           |
fuse     fuse
|           |
+          +
6v        6v
---------------------------- to bus

If you have 6 sets, each fused at 20A (for example), use a 100A master link.

This prevents an expensive and irritating blow-fest (as sparweb pointed out) amongst your individuals should something crazy happen downstream of the master link. One fuse (particularly as currents go up) is cheaper than 6, and certainly simpler to replace!  

Steve
 

[madlabs]

Well, I do have a 250 amp DC breaker just in front of the inverter, and all other loads from the DC buss have a fuse on the hot side. With the fuses inbetween the batteries I'm thinking I should be well protected.

Thanks for the input folks!

Jonathan

BTW, new blades and hub almost done. The new blades are snexy!

[Madscientist267]

Just remember, the 250A isn't protecting the wiring leading up to itself, so a short before the master fuse would be a PITA.

One way or another, you're protected, but it's worth mentioning since it would bring up that whole nuisance multi-popping thing.

Steve