Car Audio on a Cart - Alternatives to Battery Power?
 

Alright, let it be known that this year, 810 has joined the list of teams who put car stereos onto their carts. Before the flaming comes, I'd just like to say that Joel J. said it best in this thread when he said something to the extent of a sound system on your cart is not a bad/illegal thing until it is being blasted in the pits - something we're not going to be doing. We're building this to increase moral on the team, give the less-advanced kids something to do that they'll enjoy working on while the robot design is being finalized, and lastly, to have something to play music in our work room. With that said, on to my question.

Are there any good ways of powering a car-audio-based system (headunit + 2 20watt 5's + 2 30watt 7x9's, no amp) other than a battery? We plan on having this going during our building phase, so we don't really want to have to charge a battery 3, 4 times a day. We also don't want to dish out $50 or $60 for a car battery. Specifically, we want something that can just plug into an outlet -- like an ac power brick, but can deliver a lot more current. It's a 120watt headunit, so does that mean it needs 10amps? I've heard that ratings for headunits are only peak readings (tricks the manufacterers play to make them seem more powerful than they are), but actual rms power is much much less. Anyone know how much less, and what would be a good current value to shoot for? Finally, does anyone know where to buy an ac adapter that can supply this much current?

Or, for teams that have done this using car batteries, how often do you have to recharge your battery?

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

I believe that real car batteries will not survive long if you just take them out of the car and discharge them. They usually can only stand being empty a couple of times before they are shot.

Here's a quick list of power supplies that you could probably use, I'm sure you could find cheaper ones though:

http://www.universal-radio.com/catalog/hamps.html

I think the MFJ ones are probably the most cost-effective. Remember that you can always use a 12V power supply around the shop for other things too, so it's a good idea to have one. If you had a power supply and a battery (like a robot battery), you could rig it up so that you could just switch power sources when you were "mobile." I definitely wouldn't charge the battery with the power supply though :ahh:

Good Luck :)

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

thanks for the link, but those are kinda out of our budget. Is it possible to buy something like a 12v, 10amp power supply for around $50 at most?

What about powering the stereo through a battery while the battery is hooked up to a battery charger? My gut instinct tells me thats a really bad idea, but can anyone explain why?

Better yet, if anyone has created such a cart, how did you go about the task of powering your system?

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Last year on 212 we had a head unit simmilar to the one you described in power ratings and a 300 watt amp all running off of a lawnmower battery we bought at sears for 20 bux it lasted a whole day. The ratings on head units are peak power, i.e. when bass kicks and the volume is all the way up. We had 2 batteries and made it easy for us to change them, but the die hard battery lasted all friday at nats, so that was alot of play time.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Why not use a Robot battery?

They're only like $30.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

You could use a old computer powersupply. Just rip off all of the wires, and solder it to the PCB! I used the sam idea to make a 12v PS for some R/C pit equipment.

-Jon Pannell

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

I am assuming that your cart has a on-board 12V battery. Just buy a 10 Amp battery charger, and have the battery on-charge while you're listening. Don't use a charger as a powersupply because the quality of power is very poor (lots of noise, and no voltage regulation - could hurt the AMP). By using a battery, it acts sort of like a reservoir in a plumbing system- It takes the erratic power from the charger (pump) and uses it to "fill" the battery (reservoir), and then you can draw as much/as little electricity (water) from the "reservoir" at the same quality as pure DC power. The battery essentially acts as a crude voltage regulator/power stabilizer.

I like sound systems on carts - post some pictures when you're done :)

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Well, how much amps can a computer power supply supply? If the rating is say 250watts, does that mean 250/input voltage (250/120), or does that mean 250/output voltage (250/12)? Besides, I was under the impression that each rail in a computer power supply is designed for a different amount of current, and if I were to open one up, I'm not sure where to tap in before the power supply splits up into the individual rails. If you've done it before, I'd appreciate some explanation on what to be looking for and how to find out how much current the power supply can take.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Yes, different rails are rated at different amperages. It would appear that a decent 250W power supply is rated at 13A on the 12V rail. you'd tap into it from the normal molex connectors, I'd think. Not real reason to rip the thing apart that I can think of. 12V would be the yellow wire, BTW. If you need more amperage I suppose you could up the wattage of the PS.

Keep in mind that the computer PS is going to do a poor job of charging a battery since the charge voltage is 13.8V or so. Using a normal battery charger should be fine, since car amps are designed to run off the 13.8V an alternator puts out anyways. I'd say just go with that idea.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Well, with car audio being a specialty of mine, I think I can offer some advice right now. You're not going to be drawing a lot of power, so i'd have to say, the easiest thing to do, would be to buy a simple power supply. Since you're not running an amp, you probably won't exceed anymore then 150 or so watts, so I would use a power supply such as this one (http://www.millionbuy.com/pyrps12k.html). I know it's rated at more then 150, but this is just to be safe. Besides, at that site, the prices only range about $5 an amp above or below. (If that makes no sense, I apoligize, it is 4am here in New York :) )

And if the deck is rated at 120watts, then that means that each outlet (or speaker connection) is rated at 30watts peak. The RMS power is probably more along the lines of 15-20. The RMS is what you're really concerned about. The peak power is just when say a bass note hits, it jumps to that for a split second (u can also eliminate this if the deck has a high pass filter on it).

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Radio Shack has power supplies for CB radios that are fairly inexpensive.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

It says Output: 13.8V DC... a car electrical system is designed for 12v, no? Would the constant 1.8v extra hurt it in any way?

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

No they put off more than 12v, if you look at most 12v batteries, like the one that comes with the robot when its fully charged is about 13.5v, a good thing to know if you are in charge of making sure batteries are charged.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

How bout fuel cells?

Is internal combusion an alternative?

What about propane, like on a forklift?

Seems like a propane "people transporter" would make an excellent robot cart. They use them in airports, to transport the elderly and handicapped...

Just a thought...

Good luck this year!

-Andy

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

The type of battery used in cars has a terminal voltage of 13.8 volts when fully charged. It is generally referred to as a 12 volt system simply to delineate it from 6 volt systems used in some cars or 24 volts used in some trucks. Auto accessories are designed for operation up to 15 or 16 volts so that they will not be damaged should the voltage regulator in your car charging system fail and provide a higher voltage.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Ah, alright... many thanks for the explanation and everyone who posted suggestions on power supplies and the like.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

You do realize you have to tell us what it finally came out to be, otherwise we will all go craaaaaazy......errrrrrrrrrrrr

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Are you still lookign for a power solution for your robot cart? I designed you a VERY SIMPLE(4 compenents) circuit that wil met your needs. Its costs $28.1(plus parts shiping) and when used with a lawn tractor battery(about $20 from home depot) will give you more power than you will ever need.

Here is what you will need along with part numbers from http://allelectronics.com
TX-12625 $20 power transformer
FWB-352 $2.50 bridge rectifier
EC-648 $0.65ea( you need 2 of them, so $1.30) 680uF capacitors

LCAC-129 $3.00 7.5' power cord
ACS-3 $0.65 plug for power cord


Note: Don't use this with SLA batteries(like the robot battery). The charge curent is way too high and it will signifiganly shorten their lifespan.

Tell me if you(or anyone else) are/is interested and i will post a schematic.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Yes, I would be interested in this... maybe if you have some time, could you also post some of the basic theory behind how it works?

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Sure. But that will prolly be tomorrow, bc i'm up pretty late with homework as it is.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

I'm guessing that this design uses the transformer to step down 120 volts AC to 12 volts AC, along with a full-wave rectifier to transform 12 volts AC into 12 volts DC and a couple of filter caps to eliminate ripple. Am I right?

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

you get a cookie. Basically thats it. Its pretty simple, but thats all that is needed for car audio stuff, which generally isn't too picky.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

When using a transformer a capicitor will make it sound sooo much better.

Basics
 

Here is a basic description of the circuit. The fist part of the circuit is the transformer. One side(the primary) is hooked up to the wall plug. The transformer takes the 120V AC from the outlet and steps it down to about 12.6V. Both the input and the output from the transformer will look like the wave shown in fig1, with the difference being that the input will have a magnitude of 120 volts, while the output will have a magnitude of 12.6 volts. The next stage of the circuit is the rectifier, which internally consists of an arrangement of 4 diodes. The the output of the rectifier is the absolute value of the input, and looks like fig B. The purpose of the rectifier is to convert the AC from the transformer to DC. As you can see in fig B, the rectifier output is a series of pulses, which need to be smoothed out into a continous wave to be of use. This is the job of the capacitors. They act sort of like a tank for electricity. when the voltage is on, thye take in electricity. When the voltage drops, they provide power. Finally we end up with soemthign that looks like fig C., a steady relatively clean(ripple free) 12 volts. When plugged in, the circuit will power whatever is hooked up to is, as well as charging the battery(up to 12V). When it is unplugged, power will come from the battery. This circuit is one of the most basic types of power supplies and is a bigger version of what you would find if you cracked open a "wall wart"( the big annoying things that use more plug space than they should). This circuit has only 1 problem, an it is that it will only charge batteries up to 12V. A fully charged battery is normally about 13.8V. This van easily be fixed by replacing the 12.6 volt transformer with a 14-15volt transformer.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Thanks for the info and the explanation... I'll look into this.

One question though - how do you determine how much current this thing can handle?

Off topic, about those pesky "wall-warts," I found this product from CyberGuys to be the solution: Liberator. It's essentially a 1ft extension chord. At first I thought it was a cheap gimmick, but I bought one anyways. I then proceeded to buy about 5 more. These things do wonders for ya!

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

The transformer you buy should be rated to pass a certain amount of current. It should be safe to use that value as the theoretical maximum current rating of the circuit.

If you're feeling more adventurous, connect an ammeter in series with the positive supply lead and a variable load, such as a potentiometer. Slowly decrease the resistance of the load and record the ammeter's reading when the magic smoke leaks out of any of the components. (NOTE: This is a BAD IDEA and should not actually be implemented.)

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

The rectifier ratings are important too. They should be rated for at least the current of the transformer. Btw if you run this circuit without the battery, then it would be a good idea to up the filterign capacitors to a combined value of 5,000uF to 20,000uF depending on how much you will be stresing it. The battery is actually doing most of the smoothing in the above circuit.

O yea, this circuit it rated for about 25A constant (25X12=300watts!!). Your system will probably never ever reach these current even momenarily, and this thing is way overspecified, but you never know, you might want to put like 50 neon lights on it or something. The reason for the overspecification, is that the only place that i could fond that did not want 2 arms, your soul, and your first born son for high current transformers was All Electronics. And the transformers they carry jump from 5A to 25A.

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

James et. al.,
The circuit shown is a fine and simple power supply but...
The output is unregulated so the voltage does vary quite a bit with load changes and with line voltage. (audio produces some wild load changes.) There is no provision for limiting current when charging the battery. A dead battery will take as much current as can be pulled from the transformer. For a fixed current transformer, adding output capacitors will likely draw the output voltage down below 12 volts. There is no protection fuse shown. Be sure to protect the circuit by fusing the input to the transformer. And one thing that hasn't been said yet, a 25 amp transformer weighs a lot!

I still like one of these for your application...
http://www.radioshack.com/product.as...ct%5Fid=22-506
http://www.radioshack.com/product.as...ct%5Fid=22-504
or if you need 25 amps...
http://www.radioshack.com/product.as...ct%5Fid=22-504

Re: Car Audio on a Cart - Alternatives to Battery Power?
 

Yeah, I would go with a store-bought power supply for reliability and less headaches :)