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

 

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:

 

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

 

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.

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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.

 

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

 

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.

 

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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 ops:

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 .

Happy days,

Klaus

 

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

 

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.

 

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

 

want to know whats worse - there £140 here aswell

 

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

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

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.

 

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?