Battery Safety Question

Discussion in 'General Electronics Chat' started by demodregon, Feb 25, 2010.

  1. demodregon

    Thread Starter New Member

    Feb 25, 2010
    4
    0
    I work on military vehicles with a 24v series-parallel battery configuration. The ground is located off the starter from the negative side of the battery. We are taught to disconnect the negative terminal before working on the vehicle and leave the postive terminal hooked to the battery. I may be mistaken, but I learned way back that the flow of electricity is nothing more than an inbalance of potential energies. My question is, would it be wiser to keep the negative cable connected and remove the positive cable? As opposed to removing them both, ofcourse.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Here is the deal. When removing a negative-ground vehicle battery, I teach to remove the negative lead FIRST. This is because if you remove the Positive lead first, and your wrench accidently touches vehicle ground with the other end of the wrench on the battery positive pole, you have a short and a mess. If you put the wrench on the negative lead and the wrench accidently touches ground, nothing happens.

    If just disconnecting the battery without removing it, disconnect the negative lead based on the safety reasons above, and let it go at that. Even if you short some branch circuit to ground while working on the vehicle, with the battery negative lead disconnected, no current can flow, so nothing happens.

    Ans to address it on a purely "circuit current flow" theroretical basis, it matters not where you break the circuit: At 24V, no current can flow through an air gap!
     
  3. demodregon

    Thread Starter New Member

    Feb 25, 2010
    4
    0
    Thanks. Honestly, I have never had a problem with disconnecting the positive lead, being a conventional flow believer, but when they told me that i HAD to disconnect the negative lead on military vehicles, I figured there was some sort of reason, like battery configuration. I asked why but no one could tell me.

    by conventional flow believer, i mean that is how i read schematics, and understand signal flow.
     
    Last edited: Feb 25, 2010
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hello Demodregon,

    Tell me, are the vehicles you're working on using 6TMF batteries, or have they gone to the newer AGM batteries?

    We have a few retired military vehicles at a local Vietnam War Museum; a 5-ton, a couple 2-1/2 tons, an M113 APC, a HMMVEE with an Avenger on it, and a half-dozen M151A1/M151A2 jeeps. If we're not going to be operating them for a period of time, we'll disconnect the negative cable, for the same reason that MikeML gave.

    One of the guys disconnected just the positive cable in the HMMVEE; when someone went to tighten the terminal back down, they contacted the battery box with the wrench. It melted a spot on the battery box.

    The 5-ton uses four 6TMF batteries connected in a series-parallel arrangement. When we're charging the batteries, we'll disconnect the negative side AND one of the jumpers.

    One of the problems with using four batteries is that if one battery develops a shorted cell, it will drain the batteries on the other parallel string, causing them to fail earlier than they would otherwise. It also causes the battery that it's in series with to be overcharged when the engine is running. Thus, one bad battery will quickly kill the rest of them - and those batteries aren't cheap.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    MikeML is exactly right as to why removing the negative terminal is the preferred method. I learned it decades ago the hard way -- with sparks and a frisson (as they used to say on Laugh In, look that up in your Funk & Wagnall's :p) when the wrench contacted the auto's body.

    I wish the bureaucratic mentalities would tell people the reasons behind a rule so that we wind up with more educated people...

    Years ago I remember someone insisting that the negative lead for the jumper cables be connected to a bolt on the engine of the car with the dead battery rather than the negative battery terminal -- but he couldn't tell me why it should be done this way. I had never done that, as I always just connected to both terminals on the dead battery. Then about 25 years ago a friend told me about an experience he had where he was jumping his battery and it blew up in his face. Since he wore glasses, he was very, very lucky not to get acid in his eyes. Then I made the connection between the sparks and hydrogen...
     
  6. demodregon

    Thread Starter New Member

    Feb 25, 2010
    4
    0
    The Army is going over to the stryker batteries. They were using the ones that you had to pour acid in, then they used the ones that you pour water in. There's no telling what they are going to be using within the next ten minutes, but that's the army for you - trying every new fad that's out there. NICE PIC btw, Sgt Wookie. I still wear my Marine boots with the Eagle, Globe, and Anchor branded on the side. I catch so much crap for it. I tell them they can take them off me when I die...Semper Fi! hehehe.

    Well, thanks again for the help. This only goes to show that i need to study up on my electronics.
     
  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Just to clarify something based on the OP comment: if you disconnect the negative cable at the battery (or batteries), that's fine. But if you were to disconnect the negative from the bolt near the starter, then you would have a dangling live wire that could spark if it makes contact with any part of the chassis.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Stryker batteries? I Googled that, but didn't come up with anything conclusive.

    I know that an Army Reserve Avenger unit north of here was starting to switch over to AGM (absorbent glass mat) technology batteries about a year ago as they used up inventory of the older technology batteries. AGM batteries are much better than the plain old lead/acid; if they take a hit, they won't leak acid everywhere. You can charge them much faster, too. The maint spec (TM 9-6140-200-14) for the old 6TMF and other batteries calls for a maximum 5A charge rate to limit the internal thermal rise; with those big batteries it takes quite a while to get 'em charged back up.

    I don't see the AGM type batteries going away anytime soon - they really are much better than the older style. Just the elimination of the acid spill hazard would probably have been enough justification to switch types, but they really do perform much better, and are better able to withstand shock/vibration.

    I see that you were Radar in the Corps - were you in a fighter squadron? I was a radar/missile fire control tech on F-4J and F-4S, mostly at MCAS Beaufort a long time ago. I did stuff with the F/A-18's too - but that was after I got out.

    I don't wear my stuff very often at all - except maybe an old sateen cover when I'm out & about. I've run into a number of interesting Marine Veterans that way. One elderly gent I met in the parking lot of a grocery store was a Cpl in Infantry in the Pacific during WWII, and was an eye witness to "The Great Marianas Turkey Shoot"
    http://en.wikipedia.org/wiki/Battle_of_the_Philippine_Sea

    Do you remember the Navy NEETS courses? You probably went through those modules when you were training for radar. You can find them online, here:
    http://www.phy.davidson.edu/instrumentation/NEETS.htm

    You might appreciate them more nowadays than you did when you were first going through the training.
     
  9. demodregon

    Thread Starter New Member

    Feb 25, 2010
    4
    0
    Yeah, I had it wrong. It's Hawker not stryker. Anyway, here is the link to what we used after the acid batteries then the hawker.
    http://www.absbattery.com/images/optima/51_Right.jpg

    http://www.milbatteries.com/hawker/p/hawker.jpg

    HEY! Thanks for that NEETS stuff, I forgot what it was called. I still have all of the paperwork from the schoolhouse, and about 3 other classmates' notes. I could definately use that.

    I was stationed in Camp Pendleton, under Headquarters and Headquarters Squadron Air Station. I repaired and maintained the airfield systems along with the precision approach radar.
     
Loading...