Ground wire sparks when connecting to chassis

[dhally]

I am replacing the house batteries on my van, and when I make the final connection from house battery ground to the chassis, I get a medium size spark.

If I touch it again right away, there is no spark. If I wait a minute, it sparks again.

Any ideas?

[Flux]

I am thinking you should always put on the ground first, pos second.

Do you have any stereo equipment like a capicitor connected in there some place?? It sounds a lot like a capicitor discharging and then charging back up a minute later and doing it again.

But it could be something to do with the battery that I have no idea about.

[daveb]

I'm guessing all the hot side is connected and the neutral is the last connection back to normal?

[rallypanam]

That's normal..

[dhally]

Originally Posted by daveb

I'm guessing all the hot side is connected and the neutral is the last connection back to normal?

Yes. It is.

[Scalf77]

sounds normal to me, i expect that you have some devices immediately drawing power when battery connection is completed, when first connected there is a higher in rush current. When you back off and touch again, it doesn't spark because devices have not bleed off power yet. If you wait long enough, they will bleed off and you will see the original spark.

-greg

[MadScience]

That's normal, unless you really manage to disconnect everything, including the separator and charger.

Also, on vehicle batteries, where the vehicle frame/body is used for ground, generally it's positive first, negative last, as Dhally was doing. That way you're much less likely to accidentally weld a wrench into place.

[dhally]

Well I guess it wasn't a problem. Got the new batteries installed and everything works fine. I did determine that the house batteries are at 12.7 volts and the chassis battery is at 12.4 volts.

The batteries failed after we accidentally left the main 12v disconnect on for a couple weeks. The CO detector puts a constant drain on if we don't turn off the switch. The batteries are 5.5 years old, so I guess we're lucky they failed in the summer not the winter.

One thing I learned, is keep your hands away from the batteries when using a floor jack to install them. Luckily no bones were broken.

**The reason I learned for connecting the negative last, and connecting it at the frame not the battery, is to avoid ignition of hydrogen fumes near the battery. This probably isn't an issue with modern batteries.

[daveb]

Originally Posted by dhally

Yes. It is.

I do the same, negative goes on last but it still can spark like you're found. Load is load and even the differential between the starting batteries and the coach batteries can cause that if the separator is closed.

[Wowbagger]

Originally Posted by Flux

I am thinking you should always put on the ground first, pos second.

Sorry, but that is very bad advice. Consider what happens if you fumble the connection. Remember that modern vehicles are negative ground, so every piece of metal is connected to the minus side of the circuit. Now, you screw up while wrenching on the connection and your screwdriver|socket wrench comes into contact with the frame while connecting the positive lead. If you hook positive up first, there is no circuit yet (you don't have the other side of the battery connected - one point does not a circuit make). No current flow, no sparks, no problem. Now, assume you had the negative side hooked up first: now you have a full circuit, and you will be looking for a new screwdriver (and likely a clean pair of underware).

To address OP's question: Likely you have several devices on the system with capacitors across the power. After sitting for a while, those caps will be discharged, so when you complete the circuit they all charge, drawing a high current for a moment - this is called "inrush current". Once they are charged, they won't draw current, and it will take a bit for them to discharge - so the second time you connect there won't be sparks. If you are worried about letting the smoke out, you can try first connecting the ground through a 12VDC incandescent light bulb - it should brighten briefly, then go out. If it lights up and stays brightly lit, you have a problem. If it lights up dimly and stays lit, you have something turned on.