Battery Tender to fuse box positive - sparking and lights flash

[ljrullman]

Sold my '67 and bought a '15 Ecoboost Premium. I have a 12 amp / 1.5 v Battery Tender that has served me well and will continue to use. The '15 is not my daily so I'd like to put it on the Tender between drives, but because the battery is under a cover I hoped to install a ring terminal harness with quick disconnect so I'm not having to remove the battery cover each time. I've seen examples of others installing the harness to the positive terminal under the fuse box lid (see photo) but when I try to connect, it sparks and the headlights flash. I have not installed it because I'm afraid I might damage something. Am I doing something wrong? Should I just install it to the battery post itself? Thanks in advance.

Sponsored

 

[ORRadtech]

You can connect directly to the battery but it will negate/confuse the Battery Monitoring System (BMS).
I don't understand why you would get a spark though. Unless you are directly connected to your tender. Do you have a quick connect in the tender to battery line? It should not be connected to the tender while you are wiring it in. Also, you should connect any positive lead first before connecting the ground side.
You should only see a spark if you are completing a circuit. And if you're doing this right there won't be a completed circuit until you plug in the tender quick connect.

 

[Snakebyte]

I'm not sure about your Battery Tender, but mine has LED's that tell me if the polarity is wrong. (I tailor made a Weatherpack firewall-to-battery cable and got the polarity wrong, and the battery tender notified me immediately of the problem. It sounds like you are probably using off-the-shelf components, so I'm guessing that is not your issue. Do you have another vehicle you can confirm with that the Battery Tender is not defective.

 

[Postal Bob]

Connect the positive lead to your battery, and the negative lead to a ground on the chassis. This will allow the BMS to still function.

 

[ljrullman]

Thanks for the replies so far. The quick connect harness was *not* connected to the Tender and I was careful to make sure that the negative loop/harness was not touching anything (e.g. not grounding). It was sparking and causing the headlights to flash on just by touching the quick connect positive end to the post (screw) under the fuse cover. On my old '67 I just attached the alligator clips to the battery positive and negative posts so I thought I could do that here too if needed (and not negate the BMS) - at least that's the way the advertise them.

 

[ORRadtech]

If you connect both leads to the battery it will maintain the battery. But that bypasses the BMS sensor in the neg battery lead.
My only WAG for the sparking is vehicles wires on the post are loose and sparking when they are inadvertently bumped.
Avoid the issue by disconnecting the negative battery lead from the battery then wire in the pigtail. Technically, the battery should be disconnected anytime wiring is being worked on anyway.

 

[jloshotz]

Check for continuity leg to leg on your little whip. Sounds like it could be internally shorted. Only thing that even remotely makes sense to me

 

[ljrullman]

Thanks everyone. Maybe I'll try 1) disconnecting the battery, 2) wire on the positive quick disconnect ring harness to the positive screw under the fuse box, then 3) reconnect the battery. Think that will work? First time poster in this forum - I really appreciate the advice!

 

[Mach VII]

Thanks everyone. Maybe I'll try 1) disconnecting the battery, 2) wire on the positive quick disconnect ring harness to the positive screw under the fuse box, then 3) reconnect the battery. Think that will work? First time poster in this forum - I really appreciate the advice!

I would recommend what @jloshotz suggested and test the quick disconnect with an ohmmeter to see if it presents as a short, if you proceed as you are suggesting you might not be able to disconnect circuit quick enough to prevent damage to something else.

 

[Skye]

https://www.mustang6g.com/forums/threads/batter-tender-battery-connection.172738/page-2#post-3580978

^ This is a thread I commented in prior, with photos and references.

Regarding my comments in the thread on resetting BMS, if you have an older battery, I recommend NOT doing that. The car knows the existing unit, and that's cool. I reset mine at the time as the car was only a few weeks old. BMS is reset during the installation of a new battery. Otherwise, it's not needed. Some will reset it as part of their t-shooting process, which I don't think is needed here.

Yes, when doing anything electrical with the car, disconnect the negative (-) post (the source of electrons) from the battery, first. Both (-) and (+) if you like. When reconnecting, put them back in reverse order. For your safety and the car.

You didn't mention what you're connecting, the tender. Might be a good time to consider a upgrade or replacement. Make sure it's working well, compatible with the car and the type of battery. Most are. Some are not.

You didn't mention where you're connecting. The thread above can help with that.

Edit,

When re-connecting the car's battery, things will be silent for close to 30 secs. Afterwards, you'll hear multiple sounds of things (sensors, relays, pumps and modules) charging back up. That'll stop after a few moments.

 

[ljrullman]

Thanks, What @Skye posted and showed via his pictures was exactly my plan. Even so, I'll try the Ohm meter first and circle back with what I learn. Thanks.

 

[ljrullman]

Final report (I think). Checked voltage on (new) battery - just over 12v. Checked voltage of battery with Tender attached - up a little (still under 13v). Removed tender and unplugged it. Then, checked ohms - no resistance. Feeling assured, I removed negative cable from battery post, attached positive ring harness to positive screw in fuse box, then negative ring harness to ground bolt on strut tower, then attached the quick connect cables, followed by plugging in the charger. Red light came on (charging), no sparks, no headlight flashes. I think my issue was as some of you suggested - loose screw / just touching it then pulling away; battery cable had to be removed before installing harness; etc.). Anyway, I just came back from a 60 mile drive with the top down on a beautiful sunny day feeling grateful for all your advice.

 

[Garfy]

Where you connected your leads is the same place I connected mine and I did NOT get any sparks, etc. during the hookup. BTW, I think you meant 12V 1.5 amp, not the other way around.

 

[ironsurvivor]

Final report (I think). Checked voltage on (new) battery - just over 12v. Checked voltage of battery with Tender attached - up a little (still under 13v). Removed tender and unplugged it. Then, checked ohms - no resistance. Feeling assured, I removed negative cable from battery post, attached positive ring harness to positive screw in fuse box, then negative ring harness to ground bolt on strut tower, then attached the quick connect cables, followed by plugging in the charger. Red light came on (charging), no sparks, no headlight flashes. I think my issue was as some of you suggested - loose screw / just touching it then pulling away; battery cable had to be removed before installing harness; etc.). Anyway, I just came back from a 60 mile drive with the top down on a beautiful sunny day feeling grateful for all your advice.

This is exactly how my battery tender is connected and I have not had any issues.

 

[Qcman17]

When I did mine years ago I didn’t disconnect the negative battery cable just connected my pigtails to the fuse positive and strut negative. My lights flashed a few times when reconnecting the nut on the fuse box but it’s been fine for almost 6 years now.
But yeah if I was doing it again I’d disconnect the battery negative and then connect to the fuse box. That’s definitely the smartest way

Sponsored