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Question for car and electronics guys

Discussion in 'Off Topic [BG]' started by Shroom, Apr 7, 2009.

  1. Shroom


    Dec 12, 2006
    Hey TB. I've been wondering, as I'm sure many others have, how possible it would be to build a portable power supply in which to plug your bass amp when you're out and about and away from any power outlets; preferably something contained in your car.

    I don't know too much about either topic I mentioned in the thread title, so please excuse any stupidity I might unknowingly be spouting, but it seems like it might only be a matter of a couple extra car batteries, maybe a big-ass capacitor or a few, and the right type of unit to convert the power from DC to 120 V AC with and outlet to plug the amp into.

    What say you, TB? I've always wanted to play my bass at the beach, and I'm sure you guys have some outdoor fantasies of your own.
  2. Hi.

    There's been quite a few threads about the subject in the amps section. While it can be done and has been done numerous times, a clean, efficient system gets really expensive, really fast. Not to mention heavy as hell.

    If I wanted to do something like that, I'd use one of my portable generators to supply the juice, even though I could build a portable system easily with parts and equipment lying around.

    The "briefcase" type generators are relatively silent, cheap, light and fuel efficient.

  3. If you know the power requirements of your bass, you could use a simple inverter to turn the 12vdc from your car's battery into 120vac to power the amp. I've seen as big as 4000 watts, which would power a whole band, albeit not too long on a normal car's battery. Using Optima Red Tops could net you some extra time, due to their extra storage capacity.

    If you have any questions, PM me.

  4. Shroom


    Dec 12, 2006
    I was worried about that :/

    I considered just using generators, but gas is expensive, and more importantly, I want to hear the bass, not engine noise.

    Well I was considering having a band play guerrilla shows this way, but with each band member running an individual system rather than all running out of one. I suggested an extra car battery(ies) too, to avoid the drain on the one being used by your car (considering it's a little more important). Also, what are Optima Red Tops?
  5. Optima Red Tops are batteries that use gel cells vs. electrolytic water/lead plates to store electricity. The battery can thus hold more amperage (power) and it is a more stable state with no acid. They're more expensive than regular lead-acid batteries, but good luck finding a car with a high-end audio system in it that isn't running an Optima battery.

    As far as the band goes, one vehicle could power an entire guerilla show. Add up all the amplifiers power, and then double the number to be safe to find how much inversion capacity you will need. IE:

    Singer : 100watt output amp
    Lead Guitarist: 150watt output
    rhythm guitar: 150watt output
    bass: 100watt output
    drummer: 300watt output
    total: 800watts output x2 = 1600 watt power inverter.

    These are just theoretical numbers right now, but that would let you run your show at the max and you wouldn't have to worry about burning up your inverter, since you won't be running it close to the max. But you can apply that simple formula to get you where you need to be.

    As before, don't be afraid to PM me.

  6. Shroom


    Dec 12, 2006
    You make it sounds so easy, Greg. You just wire up your battery to an inverter and that's it? Also, if you don't mind, I'm gonna keep posting here so anyone else that might be interested in this can benefit from this thread.

    I guess my main question now is, without being recharged by an alternator in your car like your car battery would be, how long could you realistically expect this type of system to last before having to pay $150 for a new battery? More importantly, what if you DID want to hook it up to your car's alternator or whatever so that it DOES get recharged when you drive?
  7. hartke20g


    Apr 12, 2006
    miami, FL
    why not just leave the car running while you play? it'd be the same as having the car running normally (albeit with more of a load). just make sure to use a car that's not noisy (exhaust, valves, belts, etc). car batteries are too expensive to use when they're not being charged simultaneously IMO.
  8. Shroom


    Dec 12, 2006
    Because that's basically like using a generator, making noise, and consuming gas.
  9. It's really not too hard, just make sure you have done your homework so you don't burn something out.

    As far as expectations on battery life, it depends on your battery's capacity first and then state of charge. Realistically, you could expect at least 90 minutes out of your battery. I haven't exactly tried what you are doing, but it's not too far removed from running a sound system with the engine off. Most high-powered systems can run for about 2 hours before the battery won't start the vehicle. That is different from my chevy truck with a fairly stock sound system that will last between 6 and 8 hours. So if you are looking to play for more than 90 minutes, ensure you have a set of jumper cables handy.

    Another option, depending on vehicle, is running dual batteries in parallel. That would probably require an alternator upgrade to keep both charged. I know lots of off-roaders like that option and it could be feasible for you, depending on what kind of vehicle you have.

  10. Shroom


    Dec 12, 2006
    So I've read what you said, Greg, and done some homework, and from what I gather, amp hours are what I want to be looking for in a battery. For now I'm not gonna worry about alternators and all that. Anyway, from what I've read, you can calculate amps from watts/volts, and amp hours are just amps*hours. I also found out the amp hours for the Optima Redtop was something like 75.

    So hypothetically, if I were running, say, a 300 watt bass amp and two 100 watt guitar amps, that would be (500 watts)/(120 volts)=4.2ish amps, which if it were running out of an Optima Redtop would theoretically last me about 17 hours ( :eek: ) ? That doesn't sound right to me, but what do you think?

    Also, for these calculations, I should be looking up the power consumption of the amps, and not the output power, right?

    Oh yeah, and would I be better off with a deep cycle battery?

    And before I forget, thanks for all the help man. :hyper:
  11. Hi.

    Well... An another way to calculate it ;).

    For simplicitys sake, let's assume first an 100W amp, running directly from the battery.

    100W output = 200W draw for class A/B amp (50% efficiency).
    Since we're talking about MI instrument signal without compressors, and not not a synth bass, let's assume 35% average draw = 70W. Higher for lower frequencies reproduction, less for high.

    70W with the engine running (14.2V reg) = 4.9A
    70W without the engine running (~12V) = 5.8A

    As the voltage drops, the current goes up until a mechanical or chemical limit is reached.

    Since the inverter has about 80-90% efficiency if You buy quality and 60-80% if You take the budget approach when powering an 100W A/B amp (35% average draw) with an inverter:

    Quality inverter draws ~5.7 / 6.8A respectively.
    Budget inverter draws ~7 / 8.3A respectively.

    Almost all the battery manufacturers recommend that 10% draw of the total capacity should not be exeeded in order to avoid deforming and chemical deterioriation. That means that one battery roughly equals 100W A/B amplifier to operate about 5 or so hours.

    So for Your example of 300W+100W+100W.

    500W = 1K draw.

    1K with 35% average = 350W

    14.2V = 24.6A
    12V = 29 A

    Quality inverter = 29 / 34 A
    Budget inverter = 35 / 41 A

    70Ah/10% = 4-6 batteries for ~5 hours of service with a 1K-1.5K inverter.

    This all is assuming that the whole system runs efficiently and the MAX battery life is one of the goals. If You double the draw to 20% the battery life is reduced and the time drops more than 50%.

    I'd go with the deep cycle(s) (yellow top) as the voltage/power curves are more suited for continuous draw.

    Since the charging of the conventional batteries is quite inefficient and slow (max current 10% of the capacity), I'd still go with the generator route. It's cheaper and IMHO more environmental friendly.

    If the budget is the same for both cases: batteries+inverter and the generator, that buys a diesel generator that can run on biodiesel or heated straight used cooking oil. On some parts of the world fuel ethanol and methanol is available, so even cheaper gasoline engines can be run environmental friendly.

  12. Found your math error. You calculated your draw from the inverter and yes that equals 4.2 amps, but you forgot to calculate the draw from the inverter to the battery, which is 12vdc instead of 120vac. That brings you up to 42amps draw, which based on the 55AmpHour measurement straight from optima's website gives you about an hour of gig time. Bring your jumper cables.

    And yes, you are looking for a deep cycle application for a battery that will last you for more than 5 shows. Don't forget a decent trickle-charger for when it's parked at the house pre- and post-show. I know from personal experience that a weak battery can truly make your life miserable until you get a good long road trip in.

    Sounds like you are all set. I suggest trying it in your front yard for the neighbors before you take it out and need a ride back from the missus, but would love to hear about the results.

  13. IanStephenson

    IanStephenson UnRegistered User

    Apr 8, 2006
    Converting 12V DC to mains AC level, then feeding it into your amp would be a pretty brute force approach. What's the first thing that your amp does with that power? It converts it from 110/240V AC to a lower DC voltage!!!!

    Much better to find out what the output of the mains tranformer in your amp is, and have the amp mod'ed so you can feed 48V DC (or whatever is requried) straight to the amp circuit boards, bypassing the transformer. Then you could just take (say) 4x12V batteries and feed them straight in with no extra components. You could even strip the transformer out of the amp to reduce weight. It's also safer, as you're not messing round with mains levels, and might even sound better.

    Of course permanently moding your amp my not be what you want - depends on the amp, and how often you need to do this. You could pick up a cheap/small amp for the purpose and have it specifally converted to avoid messing with your main rig - probably still cheaper than the AC route.
  14. Ian,

    That would be a great idea except for the fact that he trying to power the entire band, not just one individual amp. That could possibly mean 4 different input voltages.

    Also, I'm an electronics technician and I don't like to go screwing with something that deals in high amperage DC unless it's really broke and it has to be fixed RIGHT NOW. It signifigantly raises the stakes of frying either the repaired/modded component or something along the circuit due to too much draw.

    Another thing, inverters are designed to provide 120vac current to any of a number of consumer electronics and most are at least class 2 devices providing reliable power for anything requiring household AC input voltage. As long as he isn't consistently pushing that inverter to it's threshold, it should provide him many years of usage in multiple applications, including powering the laptop for those hours-long road trips to gigs so the guys who aren't driving can enjoy some movies or something, I don't know.

    But that's a pretty wikked idea that would be great for a permanent setup. Do it right and it would get noticed for the originality of it, as well as not lighting the vehicle on fire.

  15. Ripper


    Aug 16, 2005
    from experience: a deep cycle gel cell (yellow top) hooked to an inverter will power a 100 watt bass amp at 1/2-full volume for at least 5-10 hours at a time. I use this setup for marching band. we have a full cart with a battery and an old fender 100 watt combo that i plug into. i dont remember off the top of my head how big the inverter is, but its one of the larger ones, and the battery will power the amp and wireless for several hours. It had a normal car battery before, and that was getting worn so it wouldn't quite last an hour.

    we dont have any generator on the cart itself. It usually gets put on a 2a charge a day before we need it and it lasts quite well.

    i wish i had the specs on the inverter for you, but its spring break and i wont be back there until the 20th

    hope that helps
  16. Shroom


    Dec 12, 2006
    Wow! Thanks for all the input guys!

    After reading a couple of your replies, I'd like to clear some things up: First, I currently don't plan to have the system hooked up to my car in any way. I'm talking a completely separate power system with its own battery, so that if it does get drained, I can still drive home. Second, while it would be nice to power an entire band out of one system, it's not necessary. I initially only planned to use it for my bass amp. Right now though, I'm considering maybe at some point making two of these systems in order to power a band, rather than one. This of course, would have to be funded by the band :D .

    With that said, I like Ian's idea, but it's too complicated for me right now, and I don't want to permanently dedicate either my practice or gigging amp to traveling.

    So it still sounds like one or two batteries into a high-power inverter, plus the right cables/apparatus to recharge the battery at home between trips. I don't need 5 hours showtime right now. The only matter that leaves is cost, which for one Optima Yellowtop and one of these (which looks decent to me for my purposes, but you let me know), comes out to be about $330-400. I found yellowtops on amazon for as low as $130, so that's where the number comes from.

    Incidentally, I also found this, which looks like everything I really need, but it takes the fun out of making it myself, and also looks a little underpowered for powering a band (if that's what I chose to do). Anyway, it looks like one of these can be found for as low as $230, which is obviously a definite plus. What are your opinions?
  17. for what you want to do with the band i think a portable generator might suit better..a pignose 30 for your solo stuff

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