[Madidus_Scientia (OP)]

One of my friends at work has a decent audio system in his car and it would be good if we could drive it around to near where we're working to listen to music, but this will result in the battery going flat quite fast. Is there an easy way to rig it up so it can run off mains power? Would simply connecting a car battery charger to the battery while its on work?

[Geezer]

I think your best option is the battery charger. Alternatively, he could rig up a transfer switch to power it from an external 12 volt power supply, but the external power supply might be quite expensive, and the wiring is a bit of a pain.

However, before spending money on a battery charger, borrow one and do a test. It is possible (but unlikely) that the charger will create a lot of 100Hz hum on the audio.

Unless you require a rock concert to work, you could also get a big boombox and run it off the mains. But then you'll have to secure it after work.

 

[RD]

There are mains transformers available for running in-car CB radio from the mains, (13.5Volt 5Amp )...

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As they are designed for use with audio equipment, unlike a battery charger, they may be a better bet.

13.5 volt @ 5 amp is 65 Watts, (what is the power consumption of the in-car stereo ?)

[Geezer]

Er, well, if you don't mind me saying so RD, bollocks!

A car battery is a very precise voltage regulator. Why would you waste money by buying another one?

[RD]

A car battery is a very precise voltage regulator. Why would you waste money by buying another one?

Because you don't want a container filled with sulphuric acid, which smells of rotten eggs, in your living room.

The used CB transformer above is on sale for six quid,
 a lot cheaper than a stinking car battery which could (chemically) burn a hole in your carpet.

Edit:
I've missread the original question  [:I], I though the car-stereo was going to be used indoors on mains power. 

[Madidus_Scientia (OP)]

Thanks for the answers, i'll try a battery charger. Why do you think it might cause a 100hz hum?

[Don_1]

If you connect a charger to a car battery, you must open the cells. This can result in some spitting of the acid, that's why it is always advised that a battery be removed from the car while being charged.

[neilep]

Surely the easiest and most practical way is to drive your car through the wall into the lounge and then ewe won't need to go to so much trouble of yanking the car stereo from the car which might cause some damage !

[Madidus_Scientia (OP)]

No we don't want to take it out, we want to leave it in there and drive it around the back where we work to listen to it

[graham.d]

A word of warning. If you power your sound system off a battery and have a battery charger connected, make sure that you connect the sound system to the battery terminals and the charger, independently, to the battery terminals. If you connect the charger to the sound system and then connect this to the battery, there is a danger the battery would become disconnected leaving the sound system connected to the charger only. Depending on the charger design, this could put a higher voltage than is desirable on the sound system and may damage it.

[neilep]

It must be some serious bit of car audio kit if it's worth listening to from outside. Would it not be prudent to just bring a ghetto blaster into work ?

[Geezer]

If you connect a charger to a car battery, you must open the cells. This can result in some spitting of the acid, that's why it is always advised that a battery be removed from the car while being charged.

What! Do you always disconnect you battery when you are driving? The alternator is just a charger!

I think you are correct if you are using a really fast charger that pumps out a lot of amps, but a little 5A charger can be left on without any problem. When the battery is fully charged, the current drops off to next to nothing.

[Geezer]

A word of warning. If you power your sound system off a battery and have a battery charger connected, make sure that you connect the sound system to the battery terminals and the charger, independently, to the battery terminals. If you connect the charger to the sound system and then connect this to the battery, there is a danger the battery would become disconnected leaving the sound system connected to the charger only. Depending on the charger design, this could put a higher voltage than is desirable on the sound system and may damage it.

I think the sound system is already "plumbed into" the car, so they should be in good shape. You're right though. There better be a battery there or the charger might damage the sound system.

[Geezer]

Thanks for the answers, i'll try a battery charger. Why do you think it might cause a 100hz hum?

The current from the charger may have quite a lot of ripple. Assuming it uses a full wave rectifier, the frequency of the ripple current will be twice the mains frequency. The battery will work as a pretty good voltage regulator, but there might still be a small amount of ripple voltage across it. However, if the audio system is a good one, it will reject any ripple.

You should not need a very big charger. Unless he has really enormous bass speakers, I'm guessing about 5A should be plenty.

BTW - let us know how it goes. We love doing experiments using other people's money []

[Don_1]

If you connect a charger to a car battery, you must open the cells. This can result in some spitting of the acid, that's why it is always advised that a battery be removed from the car while being charged.

What! Do you always disconnect you battery when you are driving? The alternator is just a charger!

I think you are correct if you are using a really fast charger that pumps out a lot of amps, but a little 5A charger can be left on without any problem. When the battery is fully charged, the current drops off to next to nothing.

Yes, sorry I should have qualified that. I was referring to a standard 10/20amp battery charger.

[Madidus_Scientia (OP)]

It must be some serious bit of car audio kit if it's worth listening to from outside. Would it not be prudent to just bring a ghetto blaster into work ?

We'd obviously open the doors, but regardless of the practicality I was curious of the possibility

[SeanB]

The charger ripple will not be audible at all, the electrical system in a car is very noisy, with a ripple from the alternator ( or generator on older vehicles) that varies in amplitude and frequency with engine speed, voltage pulses caused by the coil/s in the ignition system, noise from motors like windows, fan and such. A battery in good condition filters out most of this, and all in car audio is designed to survive this noise. A 5A charger will probably not be able to keep the battery going flat if you turn up the volume on a biggish audio system, a 10 to 20A one will be a better bet.

A better idea is to use a second battery to power the audio system only, and have a split charger connected to the alternator to charge both systems when the engine runs. This enables you to still start the car if the battery for the audio is flat. The split charg conversion is often done for the owners of caravans and for off roaders with big power winches. You can get it ( in the UK at least) from Maplin, and supply your own second 12V car battery of appropriate capacity.

[Geezer]

He might need a bigger engine too!  []

BTW, how many amps does this audio system pull from the battery? 10 amps seems a bit extreme. Are they using it as the PA for a rock concert?

[Madidus_Scientia (OP)]

A better idea is to use a second battery to power the audio system only, and have a split charger connected to the alternator to charge both systems when the engine runs. This enables you to still start the car if the battery for the audio is flat. The split charg conversion is often done for the owners of caravans and for off roaders with big power winches. You can get it ( in the UK at least) from Maplin, and supply your own second 12V car battery of appropriate capacity.

One battery, two batteries, they'll both go flat on a 10 hour shift.

I'm not sure how many amps the system pulls, it doesn't approach anything like what some crazy people have, but it is above standard.

[Geezer]

Assuming the battery has a capacity 50 amp hours and that it's fully charged at the begining of the shift, that would imply the audio system is pulling an average of at least 5 amps.

If the battery is only partially charged to start with, it could be a lot less than 5 amps.