Author Topic: 12V Electrical Question  (Read 818 times)

Shinysideup

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12V Electrical Question
« on: February 18, 2016, 09:40:56 PM »
OK, all you EE types out there...

Since I commute in freeway traffic every day, splitting lanes much of the time, I really need a horn that's louder than stock. Something to make it through 70 mph wind noise, tempered glass, and over a Lexus's sound system with enough volume to wrench the car driver's attention away from her text messaging.

I've always used the Stebel Nautiuls Compact air horn. This unit draws a maximum current of 18 amps. I replaced the horn fuse on the OEM 12V fuse block with a 15 amp fuse and the horn works, but it cuts off the bike. Doesn't blow the fuse. Obviously the bike's protective circuitry is doing its job, but having my bike quit on the freeway is no bueno. :(

So I need to know if the following idea would work:

I buy the smallest lithium motorcycle battery (small and light) and wire the horn to the battery using a relay, so the current doesn't go through the handlebar switch. Once I've done that, I have two choices:

a) I can simply hook up the battery to a tender each night when I plug in my Empulse for its daily charge.

or

b) I somehow hook the battery up to the Empulse's 12V supply, so the bike can continuously charge the battery while the bike is running.

The huge gap in my knowledge lies with option b): How can I do this without damaging either the battery or the Empulse's electronics? Would I need to sense the battery voltage and put some kind of gizmo in the circuit that would charge only when the voltage drops below a certain point and stop charging when it reaches a preset maximum? Would I need a diode to keep the battery from feeding current into the 12V Empulse electronics? Will the battery catch fire and burn up my Empulse?

Am I totally crazy? (Don't answer that: my wife already has long ago taken that option!)

vtbrammorider

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Re: 12V Electrical Question
« Reply #1 on: February 18, 2016, 10:14:35 PM »
OK, all you EE types out there...

Since I commute in freeway traffic every day, splitting lanes much of the time, I really need a horn that's louder than stock. Something to make it through 70 mph wind noise, tempered glass, and over a Lexus's sound system with enough volume to wrench the car driver's attention away from her text messaging.

I've always used the Stebel Nautiuls Compact air horn. This unit draws a maximum current of 18 amps. I replaced the horn fuse on the OEM 12V fuse block with a 15 amp fuse and the horn works, but it cuts off the bike. Doesn't blow the fuse. Obviously the bike's protective circuitry is doing its job, but having my bike quit on the freeway is no bueno. :(

So I need to know if the following idea would work:

I buy the smallest lithium motorcycle battery (small and light) and wire the horn to the battery using a relay, so the current doesn't go through the handlebar switch. Once I've done that, I have two choices:

a) I can simply hook up the battery to a tender each night when I plug in my Empulse for its daily charge.

or

b) I somehow hook the battery up to the Empulse's 12V supply, so the bike can continuously charge the battery while the bike is running.

The huge gap in my knowledge lies with option b): How can I do this without damaging either the battery or the Empulse's electronics? Would I need to sense the battery voltage and put some kind of gizmo in the circuit that would charge only when the voltage drops below a certain point and stop charging when it reaches a preset maximum? Would I need a diode to keep the battery from feeding current into the 12V Empulse electronics? Will the battery catch fire and burn up my Empulse?

Am I totally crazy? (Don't answer that: my wife already has long ago taken that option!)
I'll have to check but I think your 12v is always on at the DC DC side.  Getting a bit more complex you could probably tie the battery into your old horn circuit and use a switch to control charging your battery or not.  But then you have an extra switch.....

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Chocula

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Re: 12V Electrical Question
« Reply #2 on: February 19, 2016, 12:32:35 AM »
Disclaimer, my degree is in Mathematics, not EE.

Before using a battery, you would want to make sure the voltage from the Empulses DC power supply is stable and does not exceed the voltage for your battery to keep it from overcharging.  You would also need to make sure that there was no parasitic drain when the systems is off to avoid overdischarging your battery.

Since you only need a very short duration burst of power, I would think a largish capacitor would work better than a battery.  You may need some additional circuitry to prevent the capacitor from drawing too much power when it is initially charging.  A capacitor would be much more tolerant of voltage fluctuations, getting completely discharged, etc..  Adding a resistor could make sure the entire horn + capacitor could not draw more power than the bike can provide, at the expense of the horn not working when the capacitor is not charged.  As long as your capacitor is large enough, you should be fine.

Chocula


Shinysideup

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Re: 12V Electrical Question
« Reply #3 on: February 19, 2016, 02:33:27 AM »
I like the idea of a capacitor. With my rudimentary knowledge (from building a crystal radio as a kid) I even had the thought that I was using the battery as a capacitor in this application.

Unfortunately, I have NO idea of what spec I capacitor I need. Or how to design the "additional circuitry" or how to specify the resistor needed.

Maybe I'll just have to stick the battery on a suitable tender each night... not elegant, but doable.
« Last Edit: February 19, 2016, 02:37:12 AM by Shinysideup »

kingcharles

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12V Electrical Question
« Reply #4 on: February 19, 2016, 05:33:43 PM »
A flux capacitor?
Once you go EV, gas is history!

Chocula

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Re: 12V Electrical Question
« Reply #5 on: February 19, 2016, 08:33:49 PM »
1 Farad is 1 Ampere second per Volt. 1 F = 1 As/V

Reference https://www.physicsforums.com/threads/amp-hours-to-farads-conversion.256751/


You also need to consider the voltage drop as the capacitor discharges.

Something like http://www.amazon.com/Lanzar-LQ19CAP-Contaq-Farad-Capacitor/dp/B001D21DNM/ref=sr_1_9?s=electronics&ie=UTF8&qid=1455931856&sr=1-9&keywords=capacitor+1+farad might be a good starting point.

Shinysideup

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Re: 12V Electrical Question
« Reply #6 on: February 21, 2016, 10:12:59 PM »
Thanks, Chocula. It's STARTING to make some sense for me!

But my math is troubling:

If 1 farad equals 1 amp flowing for 1 second and if the horn pulls 18 amps, and if the cap you linked to is 1.9 farads, then wouldn't I need 10 such capacitors to meet the 18 amp draw by the horn? At $45.49 per capacitor, this would total $455 which is bit much, even for me the farkle queen, for a horn! :o

I'm searching for better solutions. Seems like a lot of the 12V capacitors on the market for car audio (to add bass oomph) are listed at 10 farads and 20 farads are frauds, according to many comments on Amazon by folks who bothered to actually measure their capacity. In many cases, they were but fractions of a farad.
Still looking...

Shinysideup

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Re: 12V Electrical Question
« Reply #7 on: February 22, 2016, 01:51:41 AM »

Chocula

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Re: 12V Electrical Question
« Reply #8 on: February 22, 2016, 12:37:36 PM »
Probably but that thing is huge!  Your capacity would depend on how long you need to continuously blast the horn and should still be able to draw about half of the power from the bike.  If it toggles on and off quickly, so that it is only on 50% of the time, a much smaller cap should work and might get you a more noticeable sound.

A simpler solution might be to just strap one of these to your handlebars and call it a day.   :)  Replace the air can as needed.
http://www.amazon.com/Falcon-Safety-Super-Sound-Horn/dp/B015YVLY2W/ref=pd_sim_200_5?ie=UTF8&dpID=413rjJR%2B4ZL&dpSrc=sims&preST=_AC_UL160_SR101%2C160_&refRID=1WK2B0RR109C291H024K

Shinysideup

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Re: 12V Electrical Question
« Reply #9 on: February 22, 2016, 01:48:38 PM »
Great idea! I was so focused on getting the electrics to work that I forgot about these. For years, I rode a bicycle in San Francisco traffic with an air horn (refillable bottle) and they work really well. For the very few times I really need a loud horn on the freeway, the cannister would last be a very long time. So I'll say goodbye to ultracapacitors, but it's been fun learning about them.