What happens when you connect cable to + and - of same batttery?

Spyderweb

So here’s what happened.  Son-in-law had boat winterized and stored at our marina ( my former Bayliner 285).  Boat has 2 batteries, a starter and deep cycle house.  Gets on boat 1st time this spring and blower won’t work on mbss #2.  Switches to #1 and all works.  He thinks marina screwed something up in hooking up batteries.  I’m up a week later to my boat and I check on his.  Place switch to #2 and blower and several other 12 volt things work (horn etc.) Seems ok.  He uses boat and all is normal up till this past week.

Mbss is on #2 and blower won’t turn on again.  Charger has been on all week at dock.  Switches to #1 and it works.  Comes out and ties up with me on the water.  We check voltage of deep cycle and it is 4.0.  Battery is in 5th year, is only size 24, and they routinely use fridge out on water.  Conclusion is time for a new battery.

Next morning he buys a new deep cycle at marina.  Before removing cables from old, he zip ties everything on the + and then - together so there’s no chance of screwing things up.  Goes and attaches the + and all is good.  Goes to attach the - and a heavy duty yellow cable produces a huge spark.  Cables are hidden behind battery box so it’s not clear where they are coming from.  He pulls cables up so he can see their origin, and the other end of this heavy yellow cable is attached to the + post of the same new battery.  The marina had this cable attached to the + and - of the same battery!  At this point I show up.  We both know that’s not right.  

I have him remove the yellow cable completely and line it up with the 2 batteries.  The natural bends in the heavy cable have it line up perfectly with the 2 - posts of both batteries.  Essentially it connects the 2 grounds as you would expect.

So now we wonder:

1.  How could a mechanic do this?  Wouldn’t he get a big spark too as a warning something is wrong?
2.  How did this work for so long?  He generally keeps mbss on #2, even for starting.  Just extra insurance #1 is fresh.  It’s something I used to do.  Is it possible that something shorted and that power has been coming from #1 all along despite switch being on 2.  However, when he didn’t get blower to work on #2 this last time, switching to #1 gave him power.

Liberty44140

I can’t believe it worked that long and didn’t throw switches. I made this mistake once and like you said it tossed a huge spark and blew both of the fuses in my galley fridge. 

Cableguy Greg

Not sure how a mechanic wouldn’t notice that. A mechanic should know that yellow is the new color for negative on a boat. I wonder if a green mechanic did the work. I know that I would have a serious conversation with the shop. Fortunately this ended well. It could have been much worse. 

212rowboat

Curiosity would encourage a continuity check on that cable that connected + to - to see if it is intact and not broken internally.  That's the only way this makes sense.  

Unless the battery between the connects was dead as dead could be, that cable would have become a glowing wire for a brilliant few seconds- and that's the good way it ends. The bad way is the battery decides to be the glowing part.  ...... ive not tried or heard of this, but its conceivable a wire of heavy enough gauge and a battery producing a small amount of voltage could remain connected- the wire/cable large enough to dissipate the heat by product of "leveling" charge.  

****, guys, I've seen guys weld on the trail using batteries, battery jump cables, and a welding rod... those were with healthy batteries, though.  That "natural bend" is likely the welding of wire strands before the battery was spent.  

Spyderweb

Our thought was perhaps, being a 4 year old battery this spring, and sitting all winter, when the mechanic put the cable on the wrong post, the spark was small enough that he didn't think anything of it.  

PickleRick

I've seen more than a few old batteries  with reverse polarity due to being hooked up to a charger improperly when they had very little voltage.  So low that their charger  didn't set off a reverse polarity light/beep or obvious arc/melt down of wiring.  



 

Spyderweb

The rest of the story.  In talking more to my son-in-law, he recalls after bundling the wires before removing them from the old battery, and then removing them, he remembers there was a yellow cable not bundled to anything near the (+) post so he added it to (+) bundle (not thinking of the significance of the color yellow).  He then installs the new battery, hooks up (+) 1st, (including that yellow cable) and then when he attaches the  (-) cables (including that same yellow one), WHAM!

Sooo, marina did not hook a yellow cable to both posts on the same battery.  BUT, they did hook one end of the yellow cable to the house (-), and never connected the other end to anything.  Though not the best way to do it, the original selling dealer had that yellow cable attached to the (-) post of both batteries, tying them both to the same ground.  

So this also might explain why some 12 volt things weren't always working with MBSS set on #2.