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Discussion Starter · #1 ·
I've done a lot of reading on these forums over the last few weeks and I know this has been asked before, but everyone has their own opinions and I can't seem to find a definitive answer. I know there's not one right answer so just looking for some people to chime in with their thoughts on this setup.

I have a USB power socket mounted to my handlebars but haven't connected it yet. I'm interested in using it to charge my phone / camera while riding. It doesn't need to work while the bike is off. There are a few ways I could do this:

A) Direct to the battery - a few friends have suggested this and say I am overthinking the whole thing. I connected a multimeter in series with the USB power supply to detect the current draw when nothing is plugged in and came up with 16 ma. My battery is rated at 14 Ah (Shorai lithium) so this gives me roughly 30 days of juice. That seems plenty but what if I go out of town for a while and leave the bike? For peace of mind, I'm thinking I should connect it to a powered source.

B) Splice a live wire (I see it's common to use the brown one) using bullet connectors or otherwise, so that it turns on with the key. This seems like the most obvious choice, but I see a lot of discussions about relays and am trying to understand what the benefit is.

C) Splice a live wire but also connect it through a relay to the battery - I found an excellent write-up here by a forum member (jb882) on how to do this, however I don't really understand what is the purpose of a relay in a situation like this. It seems a relay is more intended if you have a switch / button to toggle the accessory, so that the button circuit is a lower voltage than the power circuit. But if I'm not using a switch, and just want it to turn on and off with the key, is there any reason to bother with the relay?

I'm sure I got some things wrong here as I'm very new to electrical wiring, but I am interested in learning, so appreciate any advice you guys can give me. What option would you go with for a simple USB socket intended to charge your phone on rides?

Thanks
 

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While 16ma is a very low amount of current, it is the worst kind of drain for a battery. If you kill a battery by a fast drain, it will recover with a charge (perferably at a rate of no more than 10% of the rated AmpH capacity) with less damage than one that dies a slow death. Not to say a slow drained battery will not recover, but that it is worse on the battery. My understanding is a very slow drain is even worse on Lith-ion than lead acid.

If you hook directly to the battery, you definitely want to put an inline fuse in place. Also, I would advise putting an inline toggle switch in place as well to either shut the charger down or just unplug the charger if it is the type with the SAE plug. After all, why for any amount of time, have any kind of drain on the battery if not necessary?

As far as a relay is concerned, it is no more than a switch operated magnetically with low current (small control wires) to close the high current switch to allow a higher current draw directly from the battery to the appliance it feeds. While you could use a relay to operate your USB charger, I would have to agree with your friends that it is just over kill for such a small amount of current draw, even when your camera or phone is charging.

Marty
 

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Relay does seem overkill for such a low current draw.
I simply added a handlebar mounted rocker switch between battery and USB port.
Thus I can use USB to charge phone or camera with bike turned off if I so desire.
I wired it in using thin 18 gauge wire to act as a poor man's 2.5 amp fusable link which should melt into an open circuit should there ever be a short circuit.
 

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Discussion Starter · #4 ·
Thanks for your responses.

While 16ma is a very low amount of current, it is the worst kind of drain for a battery. If you kill a battery by a fast drain, it will recover with a charge (perferably at a rate of no more than 10% of the rated AmpH capacity) with less damage than one that dies a slow death. Not to say a slow drained battery will not recover, but that it is worse on the battery. My understanding is a very slow drain is even worse on Lith-ion than lead acid.

If you hook directly to the battery, you definitely want to put an inline fuse in place. Also, I would advise putting an inline toggle switch in place as well to either shut the charger down or just unplug the charger if it is the type with the SAE plug. After all, why for any amount of time, have any kind of drain on the battery if not necessary?

As far as a relay is concerned, it is no more than a switch operated magnetically with low current (small control wires) to close the high current switch to allow a higher current draw directly from the battery to the appliance it feeds. While you could use a relay to operate your USB charger, I would have to agree with your friends that it is just over kill for such a small amount of current draw, even when your camera or phone is charging.

Marty
My "balanced" idea (#2) was to connect it to a wire that's powered only when the bike is on, such as the headlights or tail lights, which would act as a switch. If I understand correctly, by splicing into the headlights without using a Relay, the risk I run is harming the circuit of the headlights if something goes wrong. This seems like a fairly common and low-risk approach however. Would I need a fuse in this situation or the fuse on that circuit is sufficient? (I assume I don't).

Relay does seem overkill for such a low current draw.
I simply added a handlebar mounted rocker switch between battery and USB port.
Thus I can use USB to charge phone or camera with bike turned off if I so desire.
I wired it in using thin 18 gauge wire to act as a poor man's 2.5 amp fusable link which should melt into an open circuit should there ever be a short circuit.
I considered adding a physical switch but would rather keep space clear on my handlebars, and I know I'd forget to turn it off anyway, so that was my idea to connect it to a powered wire such as the headlights.
 

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I'm not sure how much ampacity is left from the stock fuse after everything else is operating on the bike in stock form.

highbeam = 60 watts
front running lights = 8 watts each so 16 total
blinkers = 21 watts each so 42 total
tail light 8 watts
stop light 21 watts
Indicator bulbs 3 watts each I think so 9 total

Total 156 watts divided by 12 volts = 13 amps at any possible time if stopping with blinkers on.
This being strictly lightbulbs, and I can't say as I don't know how many additional watts the bike uses for ignition.

If this is going to cause you any heartburn, and not knowing if you are running a dual port charger, go the relay route you intended and just let the headlight circuit operate the control side of the relay but I would suggest fusing the high current side at the battery. You may want to add additional switched items in the future and you will already have your relay sitting ready.

Marty
 

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I simplified mine. I don't use any power from the motorcycle. I have one of those power banks to run my stuff. I run my tomtom and cell phone off of it. One charge last me a day. This is only good if you remember to charge the power bank. I carry 2 smaller ones but have never had to use them. This way I leave the very reliable tw wiring alone.
 

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I simplified mine. I don't use any power from the motorcycle. I have one of those power banks to run my stuff. I run my tomtom and cell phone off of it. One charge last me a day. This is only good if you remember to charge the power bank. I carry 2 smaller ones but have never had to use them. This way I leave the very reliable tw wiring alone.
I used one of those while I was in the mountains a few weeks ago. I could run my cell on map program till at about 50% then plug into the powerpack and keep the map program going and it would top it back off before end of ride usually. Belonged to the friend I was riding with since he had hooked up a charger on his before we headed to TN whereas I failed to do so.

I wonder if our paths crossed any while we were up there Apolloha!

Marty
 

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I used one of those while I was in the mountains a few weeks ago. I could run my cell on map program till at about 50% then plug into the powerpack and keep the map program going and it would top it back off before end of ride usually. Belonged to the friend I was riding with since he had hooked up a charger on his before we headed to TN whereas I failed to do so.

I wonder if our paths crossed any while we were up there Apolloha!

Marty
Idk. I've never seen another tw around there. I ride around the colletsville, linville, blowing rock, and grandfather mountain areas. Almost totally dirt roads. Fun during the day. Scary at night.
 

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Idk. I've never seen another tw around there. I ride around the colletsville, linville, blowing rock, and grandfather mountain areas. Almost totally dirt roads. Fun during the day. Scary at night.
No we didn't make it that far east. I was thinking for some reason you and your wife took a long weekend and had rode over to the more western NC while we were up there. My bad!

Marty
 

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Test that plug with something plugged in. Mine will draw almost 2amps with my phone charging. Testing it empty doesn’t tell the while picture.


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