switch from 12V battery to power supply

[credfern8807]

I'm wanting to run a car stereo from a computer power supply but also a battery when an outlet is not available and be able to select which one I want to use. I have a couple of ideas so far... 1) simply use a spdt switch rated above 10A (fuse on stereo) to switch between batt/psu/off. 2) use a spst switch for a simply on/off and a spdt relay to have the power supply be the normal power for the stereo and the battery power the system when the psu is off. Also, I am wanting to run a fan in the box to cool the stereo and was wondering if I could somehow wire this to the same switch / switch-relay system or if it would be better to use a separate switch / switch-relay for the fan control. Which of these configurations is best and/or are there any better ideas for this system?

 

[Reloadron]

I'm wanting to run a car stereo from a computer power supply but also a battery when an outlet is not available and be able to select which one I want to use. I have a couple of ideas so far... 1) simply use a spdt switch rated above 10A (fuse on stereo) to switch between batt/psu/off. 2) use a spst switch for a simply on/off and a spdt relay to have the power supply be the normal power for the stereo and the battery power the system when the psu is off. Also, I am wanting to run a fan in the box to cool the stereo and was wondering if I could somehow wire this to the same switch / switch-relay system or if it would be better to use a separate switch / switch-relay for the fan control. Which of these configurations is best and/or are there any better ideas for this system?

This is one of those ideas that looks good on paper but can have a few drawbacks. The radio is fused for 10 amps but in reality on average how much current does it draw? It will depend on the volume you play it at but maybe on average 3 to 5 amps? If your battery is a SLA (Sealed Lead Acid) type how much current can it supply and for how long? The typical SLA batteries, 12 volts, as used in UPS (Uninteruptable Power Supplies) units are 7.5 amp hour batteries. In reality, depending on the radio and a few variables that battery fully charged would likely not even give you an hour of play time. OK, add another battery in parallel. Now you have a 15 amp hour battery still 12 volts but you just doubled the weight. A typical 12 volt 7.5 AH SLA battery weighs in at about 5.8000 lbs and using two gets you above 15 pounds in batteries alone.

That aside, yes, it can be done. Remember when choosing a switch (or relay) to make sure the switch or relay contacts are rated for your DC current.

You may want to consider the use of an automotive type relay. They are inexpensive and designed for DC loads.

Ron