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Powering mains appliances with a 12V battery and inverter

Hi. I have the following setup at home:

[UK mains power]
|
|
|------->{guitar amp, 10 Watts RMS}
|
|
|------->(adaptor giving 9V, 200mA)------->{three 9V pedals, total 150mA}

I'd like to find a way to power this from a battery. I know I will need an inverter, and should expect maybe 90% efficiency from that. Basically I think what I want to know is how many amps my equipment will need, so that I can see what amp-hour rating I need a battery to be, right?

And apparently I shouldn't discharge the battery below 50% or so, in which case we can double everything at the end.

I hope this makes sense. So for example if it turned out that the gear needed 5 amps in total, then a 50 amp/hour battery would do for at least 3-5 hours, right?

Thanks for any guidance here!

Reply 1

Stonebridge

Your main amp is 10W
The 3 pedals are 0.15A at 9V which is 0.15 X 9 = 1.35W
Total power = 11.35W

On a 12V battery that would be 12 x current = 11.35W
Current = 11.35/12 = 0.95A

Reply 2

cttp_ngaf

Original post by Stonebridge

Your main amp is 10W
The 3 pedals are 0.15A at 9V which is 0.15 X 9 = 1.35W
Total power = 11.35W

On a 12V battery that would be 12 x current = 11.35W
Current = 11.35/12 = 0.95A

Cool, thanks. I could never get my head around this sort of thing but I'll take yourne word for it.

I guess I could use any old **** like this, then?

http://www.maplin.co.uk/Module.aspx?ModuleNo=49038

Reply 3

cttp_ngaf

hmm, small solar panels could provide that kind of power, it seems.

Reply 4

cttp_ngaf

annddddd could I just string a bunch of AA batteries together if their sum voltage and capacity is sufficient? That could be a cheaper option and easier to charge them (a 12V battery will need some kind of special charger, it seems)

Reply 5

mfaxford

I wouldn't use AA batteries. They're likely to be around 1ah at best and so you might get around 1 hours use out of them but they're unlikely to like that rate of discharge for a sustained period.

In terms of the car starter that would probably work but it will take a long time to recharge (I think the chargers they come with suggest 36 hours)

Solar panels could work but you'll need a fair amount. The quoted power is generally in bright direct sunlight. So in normal use you'd probably need 3-4 times the number you'de think for the required power.

Sealed Lead Acid batteries can be charged in around 4 hours but you need something suitable to charge them with.

Although the question really is why you'd want to run off a battery ? There might be a better solution!

Reply 6

cttp_ngaf

Original post by mfaxford

Although the question really is why you'd want to run off a battery ? There might be a better solution!

It's going to be for busking. Any other suggestions would be welcome.

Currently I'm thinking of something like this:

Sealed Lead Acid Battery 12v 7Ah 151x65x101mm

Nice and small - much better than a "car battery" which I was originally thinking of. I will build this, plus an inverter (borrowing one first of all, then they're about £12) into a waterproof box with sockets on the side to plug my gear into.

Reply 7

Stonebridge

I've been thinking about this and took a look at my own 10W guitar "practice" amp.
The plate on the back says its power consumption is 75W. Clearly not all this gets converted to sound.
So regarding my initial calculation, it needs to be severely modified.
75 watts at 12V will require just over 6 amps. (=75/12)
Check the power rating on the back plate of your amp. It may be a little less or a little more.

Reply 8

cttp_ngaf

Original post by Stonebridge

Ah, yes, I was about to post this - I had heard that the output wattage can be quite different to the actual power consumption. My amp says:

Output: 10 Watts RMS 8 Ohm
Input: 30 VA

From what I've just read, in an inductive load, which I assume a loudspeaker is, VA do not correspond directly with Watts, is that right? BUT, the additional factor will only decrease the resulting answer in Watts, it seems.

So if I budget for 30W from the amp, and say 32W overall

32W/12V = 2.7A

Reply 9

mfaxford

In terms of the other bits of kit if they're on a DC 9v supply you'd be better off finding a DC dropper for 12v to 9v rather than going through an inverter and ac/dc adapter. (You might be able to do that with a few resistors but it's not going to be that efficient.)

Personally I'd also look to see if either the amp can be run off a DC supply or look at getting one that can run from DC. It would be more efficient to do that and safer (no having to worry about 240V AC)

Reply 10

Stonebridge

I agree with mfaxford.
And surely it would be easier to simply run the pedals with their own 9V batteries rather than using a 12V battery and then converting this to 240V AC, then using a power supply to reduce this back to 9V DC. It seems a bit pointless.

Reply 11

cttp_ngaf

Original post by mfaxford

Original post by Stonebridge

Well, I would have no idea how to modify my amp to run straight off batteries, so I'd have to buy a new battery-powered amp. That would cost me about the same as the full setup of a 12V battery inverter, so we can disregard that cost factor. But, I like my current amp and know it well, so I'd rather keep it in use if possible.

To power my pedals with batteries I'd need 2 x PP3 and 6 x AA batteries, which would need to all be charged between uses along with the amp's battery.

Is there a way to get a 9V supply from a 12V battery? I don't know if my pedals have any inherent ability to regulate the power put into them so it would need to be safe and not damage them or anything.

So basically, if I'm using an external battery to power the amp then I might as well use it to power the pedals as well, it's a lot less fuss, inelegant as it may be. And then since to do that I'd need an inverter anyway, and since I already own a mains--->9V adaptor I might as well just use that.

Unless there is some easy way to convert my amp into running straight from a battery, but it seems risky.