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Old 05-16-2019, 07:45 AM   #1
Rivet Master
Al and Missy's Avatar

2002 30' Classic S/O
Melbourne Beach , Florida
Join Date: Nov 2013
Posts: 3,831
Connecting/Disconnecting Batteries

This morning I had one of those blinding flashes of the obvious. It is often stated, and I have done so myself, that the safe way to disconnect/connect a battery is to disconnect the negative terminal first and connect it last. The rationale for this is that without a ground reference the positive terminal will not arc through a wrench to ground if the wrench accidentally comes in contact with the vehicle sheet metal or frame.

This philosophy has its source in automotive mechanics where, if the engine is not running, the battery is the only potential source of current. This is not true in our trailers with multiple batteries, converters, and solar chargers which is referenced to ground at the distribution panel. The positive terminal of the battery with its negative terminal disconnected is connected to the trailer’s 12 volt distribution system which is still being fed by any other batteries, the converter if shore power is connected, and a solar system, if fitted, unless it’s disconnect switch is activated. While those sources are typically protected by fuses or circuit breakers, a short burst of high current is still possible.

I apologize for repeating this inappropriate safety guideline.



"You cannot reason someone out of a position they have not been reasoned into"

Al, K5TAN and Missy, N4RGO
2002 Classic 30 Slideout
S/OS #004
2013 Dodge 2500 Laramie 4x4 Megacab Cummins
2001 Safari 25 RB Twin (Gone, but not forgotten)
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Old 05-16-2019, 08:32 AM   #2
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2012 27' Flying Cloud
W , New England
Join Date: Oct 2012
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A good reminder. When I disconnect the batteries (for example, at the end of the camping season to store them on a charger in my garage), I am not connected to shore or solar power and I remove the negative cables from both batteries first (end of camping season is a NEGATIVE thing so that’s how I remember it). Reverse that process to start the season (a POSITIVE thing so connect the positive cables first) - again, with no other connections to shore or solar power.

This has worked for me for the past 5 years. The only exception being when I had them on the charger at home - I also left the connector cables on so I wouldn’t lose them - and I bumped one of those cables across pos/neg and man that sparked and ate a little chunk of lead out of the post it hit.....scary for sure.

Are you saying the inappropriate safety guideline is failing to include the caveat that you must not be connected to any other source of power when disconnecting the batteries? If so, I don’t know that you need to apologize for that. It’s a very good reminder nonetheless!

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Old 05-16-2019, 09:35 AM   #3
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1988 25' Excella
1987 32' Excella
Knoxville , Tennessee
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Not wrong advise. Just incomplete. I turn off converter and disconnect both ground cables and then the positives. Connect positives first then grounds. If I had solar I would cut it off also.
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Old 05-16-2019, 10:24 AM   #4
Rivet Master

2007 22' International CCD
Corona , California
Join Date: Jul 2013
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Connecting/Disconnecting Batteries

Adds to the established need for a good manual disconnect switch at the positive connection of the batteries. Cuts off all the possible sources of power from the trailer side of the wiring.

There has been much discussion about putting the manual switch on the negative side of the batteries, but considering the other power sources like the converter and solar panels (like my rig has), I think from an engineering point of view the main disconnect switch really needs to be positioned in the positive lead from the batteries.

Then, once that switch is set to ‘off’ if you are working on the batteries themselves the best practice is still to pull negative cable first, then positive.

Admittedly, I have an all plastic battery box, so it’s very hard to hit stray metal around it with a metal tool, but negative post disconnect first is a long-standing safety habit with me. I also position all the removed wires so they can’t touch, and put electrical tape around the positive lug for safety.

There are a few stainless steel fasteners in the bottom of the box that are grounded to the Airstream frame. My luck would have a hot wire land right on one. Murphy’s Law still lurks!!
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.
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