Wiring relay question

[stangn99]

This is probably a stupid question but I'd like to do this job just once.

I recently purchased a USB charger to use on the bike, along with a relay so it "activates" when I turn the key. The relay has 4 pins, and I understand one of the wires needs to go to ground.

My question is, should I be using the negative terminal on the battery for ground, or somewhere on the bike chassis, and why?

I plan to do this:

PIN 86: License plate light
PIN 85: Ground
PIN 87: USB charging device
PIN 30: To positive terminal on battery

The relay I purchased has a built-in fuse for reference:

 

[Jampy00]

Typically you should have a "ground" location.

 

[bitzz]

Where's 85?
That will work if 85 goes to ground and 86 goes to light for a trigger
You can use the frame as a ground... if you minimize the resistance, going directly to the battery is usually better

 

[stangn99]

Where's 85?
That will work if 85 goes to ground and 86 goes to light for a trigger
You can use the frame as a ground... if you minimize the resistance, going directly to the battery is usually better

My bad, I made a type. 85 PIN to ground.

I'll try to locate the ground. I thought maybe going to the battery would be fine because it's a low draw device (2.1a usb charger).

Thanks for the feedback

 

[Brian P]

Chassis ground, or engine-block ground, or battery-terminal ground, are electrically connected and function identically. Do whatever is easiest.

Bear in mind that a battery-terminal ground is the most secure, because there is as little as possible in terms of connection-points that could corrode and work loose and give trouble, and as little as possible that could lead to resistance or electrical interference. An engine-block ground will be next-most secure, because there will be a large cable (the ground strap) connecting the battery terminal to someplace on the engine block.

 

[RC-31]

The ground for the relay really isn't that big of a deal. It's just activating a tiny electromagnetic coil to pull the contacts. Put it some place convenient.

Putting too many connectors at your battery makes servicing the battery needlessly difficult. It can also interfere with the battery cover and may just look messy.

What is more important is the ground that goes on the other side of the load; i.e. the ground for the USB charger itself. In this case it's also not such a big deal because it's not going to be a high draw device.

If you were hooking up auxiliary lights or an air compressor for a horn, then the device ground is very important.

In your case, I wouldn't even bother with a relay at all. I run heated grips without a relay and have for decades and on many bikes. Never ever had a problem. Just pick the existing fuse wisely.

 

[Brian P]

Appears that the original poster's main reason for using a relay is to provide switching with the ignition. It's not a bad idea. The coil of a relay, with insulated wiring to and from it, is a reasonably fault-resistant device. Heated grip elements, not so much. Connecting through a relay removes some of the potential side effects of an abnormal current draw if they fault out in some way. A blown ignition or lighting fuse, because you fed heated grips through a shared circuit without using a relay to isolate the fault to its own circuit, would suck.

 

[RC-31]

I didn't say to run it without a fuse. And it depends on your particular bike but an easy example is to hook up (since we're talking about these), heated grips to something like the gauge illumination circuit. If there is a fault with the heaters then that fuse will pop and you will only lose the back lighting on your gauges. You won't lose anything super critical and you'll be aware of a fault.

I agree that you wouldn't want them connected with the general bike illumination circuit (marker lights,) or the gauge power circuit (which would most likely take out the OIL warning light,) or the horn circuit.

A defective relay coil can also be a fault and blow the fuse to which it is connected.

Cheap relays can also stick so that the device continues to draw current even with the key off.

The OP could also install a new module that adds a couple new circuits and have that auxiliary fuse box feeding several new devices. Easternbeaver.com makes such modules. I don't know how much space the OP has to place these extra components but in the case of having limited space on a classic bike and adding something small like a GPS power feed or a USB phone charger, adding a 30a fused relay is overkill.

There is also the matter of properly running a new hot lead directly from the +12V battery post. The fuse needs to be as close to the battery as possible so there is no unfused portion of that wire that could short to ground anywhere.

 

[Bobo]

Appears that the original poster's main reason for using a relay is to provide switching with the ignition. It's not a bad idea. The coil of a relay, with insulated wiring to and from it, is a reasonably fault-resistant device. Heated grip elements, not so much. Connecting through a relay removes some of the potential side effects of an abnormal current draw if they fault out in some way. A blown ignition or lighting fuse, because you fed heated grips through a shared circuit without using a relay to isolate the fault to its own circuit, would suck.

I would skip the relay and wire your charger straight to the tail light wire with an inline fuse fuse holder and a 1 or 2 amp fuse. The charger draws barely anything. You’re making it more complicated than it needs to be

 

[stangn99]

I wanted to thank you all for the feedback and suggestions. I managed to wire the relay nicely using the negative terminal on the battery as @Brian P mentioned (it was close and easy to access).

I usually plug my bike into a battery tender after every ride (overkill, but my last battery lasted 8 years doing this). I do this mostly because there are times when I don't get a chance to ride for weeks. I read USB chargers draw current even when no device is connected, so I wanted to make sure if I forgot to plug the tender, I wouldn't end up with a flat battery after weeks of not riding.

As well, I thought if at some point I wanted to add heated grips I would simply repurpose the existing leads (ditch the USB charger) and wire in the grips instead.

Anyway, it's all working as intended now, and my charger only turns on when the key is turn to "ON"

Thanks y'all

 

[RC-31]

Side note: is your battery tender lead fused?

 

[stangn99]

Side note: is your battery tender lead fused?

Yes, it's got a 7.5a fuse. I have the SAE cable sticking out from under the seat, so it's easy to plug into the tender anytime I'm not riding.

I also bought of those SAE to USB things for emergency charging, so I just plug it into the battery tender port that sticks out from under the seat.