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Power from Car Battery

Discussion in 'Automotive Electronics' started by grrr_arrghh, Jun 1, 2004.

  1. grrr_arrghh

    grrr_arrghh New Member

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    Hi All.

    I am thinking of building a battery operated hi-fi. My intention is to get a reasonable quality car stereo system (CD, amp, couple of speakers, prob a sub woofer), and power it from a car battery.

    Firstly, how much power can you get from a 12v car battery? How long would this last? (obviously this depends on the power consumption of the stereo, but any rough estimates would be useful)

    Secondly, my intention would be to recharge the battery from a car/mini-bus, so it would be easiest if this could be done via the cigarette lighter socket. Also, the battery would probably have to be left on charge, so I would need a circuit that would stop the charging process after the correct amount of time (I have seen circuits like this on the net, but never for a 12v supply). How would I go about doing this?

    Lastly, has anyone built something like this before? Any tips to consider? (it will be used outdoors, when camping)

    Thanks a lot

    Tim
     
  2. ante

    ante New Member

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    Tim,

    A car battery can be anything from 50 – 90 Ah. However if you drain it time after time you will kill it, if using only about 30% of the capacity and then charge it you can get away with it. Temporarily it can supply up to 300A and continuously 40 – 60 A until the voltage starts to drop. You can charge directly from the lighter socket when the engine is running but only when it’s running.

    Ante :roll:
     
  3. grrr_arrghh

    grrr_arrghh New Member

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    You mean literally just connect the socket to the battery? Which connection is which?

    hmmm, not news I wanted. 30% of the capacity will probably not last a decent length of time. Is there a better battery than a car battery? (the only reason I wanted to use car batteries was that it is what a car stereo is designed to run off, and i thought they had a higher Ah and current supply than most batteries)

    I know for a fact that people have run these type of things from car batteries before (out scout unit used to have one), but I don't know how they did it, or how long it lasted.

    Thanks for the info

    Tim
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You can buy special camping type batteries, which are designed to accept more abuse than a standard car battery - a car battery should only rarely be run until it's flat.
     
  6. Oznog

    Oznog Active Member

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    There are "deep cycle" batteries designed to sustain deep cycling with less damage. Wal-Mart has some of around 100 amp hrs for $60.

    However, these batteries generate hydrogen during charging, will leak acid if tipped over, and grow nasty chemical residue on its terminals. So they're not supposed to be used inside a sealed compartment, inside a car's cabin, or inside a house. You haven't said exactly what you want to do.

    There are sealed lead acid batteries, these are inherently better at sustaining deep discharge, can't leak, and don't generate hydrogen unless something's gone very wrong. There are large ones but they're expensive, at least twice as much. They generally don't do as well for extremely high current discharge, and are always more limited in how fast they can be charged.
     
  7. grrr_arrghh

    grrr_arrghh New Member

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    That sounds like what I am looking for - shame they'll probably be more expensive than a simple car battery.

    I want to make something that my scout unit can take on camps with them - so the battery would be in the open air, but preferably it would be in a box with the rest of the stereo. Does that help at all?

    I still would like some opinions on the whole charging thing. Can I just connect the battery direct to the lighter socket? How would I know when it had finished charging? Is there a circuit that will stop it charging when it is full again?

    thanks very much

    Tim
     
  8. pkkp

    pkkp New Member

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    I just bought a new "deep cycle" battery for my car. It's made by Optima and the model is Yellow Top (as the name says, the top is yellow). The battery is designed so that it can be used for cars with many electronics. You can get more information from their web site.
     
  9. Klaus

    Klaus New Member

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    {quote}
    I still would like some opinions on the whole charging thing. Can I just connect the battery direct to the lighter socket? How would I know when it had finished charging? Is there a circuit that will stop it charging when it is full again?


    You can connect any car battery to the cig lighter socket (check for correct polarity!!) as long as you like while the car is running. Think about it, the car's own battery is continually connected to the alternator's output while the engine is running.
    It will NOT overcharge as the alternator has an inbuilt regulator preventing this. You should disconnect the extra battery if the lighter socket does not turn off when the ignition key is out (most do).

    Charging a deep cycle battery (or car battery) from the lighter socket could be a problem though, if the battery was discharged very deep . It would want to grab as many amps from the alternator as it could get during re- charging but most lighter sockets have a 15 Amp fuse. So, you will keep blowing the fuse if you plug in a low battery for charging.

    Its actually a much better idea to rig a secondary battery management system as they use in campervans. They have the aux. battery connected, via a battery isolator, straight to the main battery (with thick cables). The isolator prevents discharging the main battery and then not being able to start the car :oops:

    The cautions given above, about not discharging lead acid batteries (any type) too deep is very true. A cheap car battery might last only a few times if you keep running it flat. An expensive deep cycle battery can do a few more cycles of that treatment but for it to last a long time one should NEVER discharge it below 50% capacity, preferably recharge it when its down to 75%. That way it'll last for hundreds of cycles. And, you should re- charge it as soon as possible, letting a lead acid battery sitting around in a discharged state is a sure way to shorten its life. If its not used for some time it still should be re-charged at least once a month.

    Your Amplifier's power consumption will depend very much on the volume of the music and the time its on for. Its virtually impossible to give you any figures, you'll have to trial it to find out.

    So, you see, for a long lasting set up you'll need a quite big battery if your singalongs go on for hours :D.

    Happy days,

    Klaus
     
  10. grrr_arrghh

    grrr_arrghh New Member

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    hmm, this whole not discharging a battery too much could be a bit of a problem. Oh well, i'll just have to try it and see how long it will last until it is 50% discharged. I might decide to drop the whole idea, if it doesn't last long enough.

    lol, i don't think you could quite describe them as singalongs, scouting has changed quite alot recently (its not so much camp fires and campfire songs, as fosters and heavy metal...)


    Thanks for all the info, I'll just have to try it and see. Maybe i'll have to resort to a generator instead.

    Thanks again

    Tim
     
  11. weegee

    weegee New Member

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    I would have to agree with pkkp - the optima yellowtop/redtop batteries are the ones for you - they use spiral cell technology and can be Deeply discharged time and time again - and the WILL NOT LEAK - or your money back.

    I used to use one of these in my tent - now it powers all the ICE in the car - and charges via a relay when the engine is running.
     
  12. grrr_arrghh

    grrr_arrghh New Member

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    although I like the look of the 'optima' batteries, I can't fail to be put off by the price (around $150). Thats more than I inteneded to spend on the stereo...

    Tim
     
  13. weegee

    weegee New Member

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    want to know whats worse - there £140 here aswell
     
  14. Dean Huster

    Dean Huster Well-Known Member

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    I wouldn't get too worked up over buying the premium battery. It's not likely that you're going to drive your battery into the ground anyway unless you intend on running this system 24/7. I'd stick with the standard deep-discharge RV battery from Wal-Mart. They're also used for trolling motors on boats. I've had the same battery in my travel trailer now for about 10 years and it's still going strong. I'll probably go ahead and replace it this year just because I don't want to be surprised when it does fail.

    If you use a regular lead-acid (e.g., the deep-cycle RV battery) in your system, just be sure that you have plenty (as in LOTS) of ventilation for the battery. It would be very dangerous to coop it up where the hydrogen couldn't safely vent.

    Another alternative would be a pair of 6v golf cart batteries in series. You might be able to find those in used but good condition at a place that repairs the carts. Golf cart batteries (as a pair) usually have nearly double the available power of a standard RV battery. A lot of RV owners are converting their batteries over to golf cart packs.

    Dean
     
  15. magickaldan

    magickaldan New Member

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    If I was doing this I would build a small gas powered generator from an alternator, and a quiet engine.

    My 2cents. Or just build one to charge it when not being used or when the battery gets low.

    One of those super quiet engines you can't really hear. That would be cool.
     
  16. Oznog

    Oznog Active Member

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    Honda's inverter generators- like the EU1000i- are the super quiet ones, but they're pricey. A big deep cycle battery is something really heavy to be camping with.

    The thing we're overlooking here is, do you really need so much power? I'm not sure exactly what job you're trying to accomplish. A megaphone makes a crapload of sound for a long time on a single 9V battery. A boom box can make a lot of quality sound off of few "C" cells. Going for something a bit more hi-fi power, there are Class D amplifiers which run off of 12V and are efficient enough to have quite reasonable quiescent and full power current draw.

    Driving deep bass to huge subwoofers takes an inordinate amount of power nonetheless. Do you need this? Are you thinking of driving monitor speakers? In this case, the size of the battery's not bad compared to the size of the speakers?

    What area are you trying to project sound over? What quality of sound? How many hours of continous use?
     
  17. grrr_arrghh

    grrr_arrghh New Member

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    hmm, we use a boom box at the moment (we have tried several different makes of 'boom box'). It takes 8 C-cells, and is only just loud enough if everyone is sitting around it quietly. The batteries don't last much more than maybe 6-10 hours.

    I'm not bothered about the weight, its not going to have to be carried in a rucksack. The hydrogen does worry me slightly. Especially around fires, gas stoves etc (isn't hydrogen highly explosive...?)

    the only reason that I was looking at a car battery was that my scout unit used to have one the things I want to build, and that was powered from a car battery, so it seemed like the obvious choice (also, I am trying to power car cd players, speakers etc, so i would need 12v)

    I suppose I could not bother with the sub, and just have some normal speakers, if it will increase the power consumption a lot.
    Not monitor speakers, just standard car speakers (well, a bit more powerful than standard car speakers, but nothing out of this world)

    err, well quite loud, as the sound is easily lost in the open air, and loud enough so that other people on the campsite can hear us (we have a reputation to keep up).

    err, just reasonably high quality, so that when we turn up the volume it won't distort... and preferably i would like it to last at least for 10 hours without needing recharging, but it would probably only be on for around 3 hours at a time (although I could recharge it after each 3 hours, trekking it the car park with a great big battery in your hand is really inconvenient)

    Thanks for all the idea everyone - I really appreciate the input and feedback

    Tim
     
  18. weegee

    weegee New Member

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    The optima batteries are expensive but they are the BEST for this type of application - they weigh the same as any other car battery and WILL NOT give off dangerous fumes or leak acid.
     
  19. k7elp60

    k7elp60 Active Member

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    There are several problems with charging a 12V lead acid battery from a cigarette lighter socket. First wether the lead acid battery is sealed or not the charging voltage is still the same. Most manufacturers recommend the charging voltage for standby use to be 13.5 to 13.8 volts. For cyclic up to 14.7 volts. In the cyclic mode they recommend removing the charger or reducing the voltage back to the standby voltage when the battery is fully charged.
    I have found thru many years of experience that setting the charge voltage to the standby voltage you can leave the charger connected on a continous basis.
    The major problem with charging a 12 volt battery from the cigarette lighter socket is that if it is left connected and one tries to start the car, the internal battery my supply excess current and burn the wires between the cigarette lighter socket and the automobile wireing.
    I have overcome both problems, that of the auxiallary battery trying to start the automobile and the over charging, but it depends upon the vehicle's charging system.
    Some vehicle's supply about 14.7 charging volts, some of the newer ones actually reduce the charging voltage after the vehicle battery is charged. These will not work with my circuit.
    What I did was match 3A schottky rectifiers(1N5820). I did this by passing
    2.5 amps of current thru each diode and monitoring the forward voltage drop. When I had 5 that had very close forward voltage drops I put these in parallel. In series with the diodes I put a 15A resetable circuit breaker such as the Potter and Brumfield W58 series. This assembly was put in series between the + cigarette lighter plug and the auxillary battery.
    Two things happenend, 1 the battery never got overcharged, and 2 the vehicle was protected from the auxillary battery trying to supply current to the vehicle's electrical system. I went one step further, I installed a small DVM in the box so I could monitor the auxillary battery. This system worked in three different GM vechicles.
    The only drawback is if the auxillary battery charge is low it may discharge the vehicle battery if the engine is not running. So I had a little reminder on my key ring to unplug the cigarette lighter plug when I turned of the engine.
     
  20. 2camjohn

    2camjohn New Member

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    How about using a normal car battery and one of those solar charging cells.

    That would charge the battery during the day and stop it going into deep charge after several days use.
     
  21. grrr_arrghh

    grrr_arrghh New Member

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    hmm, a solar cell is quite a good idea (it would be in the great outdoors most of the day), and it would help lessen the ammount of petrol I would use recharging it. Shame they're so expensive (deep cycle battery = £140, solar cell = £70 or £80, grand total 210-£220!!)

    I think alot of people in the forum can agree with that. Nice.
     

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