Battery/PSU questions!

[cheesehead]

My friend is building a carputer, but has run into a couple of snags.
He intends to use a setup that requires about 120 watts. By using a sealed lead-acid battery, he intends to keep it in standby mode while the car is off so it does'nt have to restart every time you turn back on the car.
Anyway, I'm looking for a few things:
1. A good 12 volt DC power supply for a computer.
2. A good way to keep the computer battery charged while plugged into the car, and a way to seamlessly transfer power draw over to the computer's battery when the car is turned off.
Thoughts?

 

[DrPizza]

Don't they make an cigarette lighter/accessory adapter for such purposes? In my grand caravan, we have several sockets. Some are on only when the car is on. Others are on all the time. (and unfortunately, my spotlight was plugged into an "all the time" outlet, resulting in a huge burnhole in the back floor)

 

[AnthraX101]

Originally posted by: Cheesehead
My friend is building a carputer, but has run into a couple of snags.
He intends to use a setup that requires about 120 watts. By using a sealed lead-acid battery, he intends to keep it in standby mode while the car is off so it does'nt have to restart every time you turn back on the car.
Anyway, I'm looking for a few things:
1. A good 12 volt DC power supply for a computer.
2. A good way to keep the computer battery charged while plugged into the car, and a way to seamlessly transfer power draw over to the computer's battery when the car is turned off.
Thoughts?

The first way you can do a power supply is get a 12v DC -> 120v AC converter. This just plugs into your existing car power and converts it to 120v AC that any PS can handle. The main advantage of this is it's simple and cheap. The disadvantage is that you're going to be wasting a lot of power doing the conversion and it's going to be hard on a standard PS to deal with such dirty power.

The second way is a standard 12v DC power supply. The standard in carputers is the 12v DC-DC Opus power supply. This is going to be a bit more expensive, but will be easier on your hardware and more configurable without custom boards.

Your best bet with keeping the secondary battery charged is to simply wire it into the car as a normal second battery in parallel with the main one. Then use a battery isolator to keep your main battery charged when the power starts to drop too low. You're probably going to need to design some voltage sensing equiptment that will shutdown the computer when the battery starts to run dry, otherwise you run the risk of killing your battery and shortening its lifetime.

One option other then this rather complicated setup is to simply put the computer in "hibernate" mode when not in use. A computer in hibernate mode can take between 10-30 seconds to boot back up, this might be an acceptable delay.

AnthraX101

 

[Cashmoney995]

If the cig lighter switches off when the car is off then your most obvious, and easiest option is to wire an inverter to the cig lighter, plug a UPS into that inverter and then have the computer running off the UPS.

 

[cheesehead]

To continue:
-How can one regulate the power going into the battery? The battery that I intend to use is only rated for about a 2A charge rate; the car can put out up to 10.
-A 12 volt power supply will be used, primarily due to the inefficiency of using a power converter (often 60% or less efficiency) with a power supply (as low as 30% efficiency.) 12 volt power supplies can have up to 70% efficiency, it appears.

 

[smack Down]

-How can one regulate the power going into the battery? The battery that I intend to use is only rated for about a 2A charge rate; the car can put out up to 10.

just use a resistor to limit the current. To be safe asssume the battery is at zero volts and the car is at 12 volts with no internal resistance. V=IR where V is 12 volts and I is 2A gives 12V/2A = 6 ohms. Most likely the battery has 6 ohms of internal resistance so you might be able to do with out it just get a multimeter to check.

 

[sao123]

Power any electrical device from your automobiles vehicle

 

[CTho9305]

Have you measured the power consumption of the computer when it's in standby? You might drain the battery anyway.

 

[AnthraX101]

Originally posted by: Cheesehead
To continue:
-How can one regulate the power going into the battery? The battery that I intend to use is only rated for about a 2A charge rate; the car can put out up to 10.
-A 12 volt power supply will be used, primarily due to the inefficiency of using a power converter (often 60% or less efficiency) with a power supply (as low as 30% efficiency.) 12 volt power supplies can have up to 70% efficiency, it appears.

Er, are you sure your car is only going to give you 10A? Any decent car alternator will be in the 40-60A range, freqently as high as 100A. Just use a standard 12v car battery as your source, you will get a heck of a lot more juce out of it.

I take it from your 2A limit you're using a lithium-ion battery? Don't mess around making a charger for one of those, they can catch fire and explode.

AnthraX101

 

[smack Down]

Originally posted by: AnthraX101

Originally posted by: Cheesehead
To continue:
-How can one regulate the power going into the battery? The battery that I intend to use is only rated for about a 2A charge rate; the car can put out up to 10.
-A 12 volt power supply will be used, primarily due to the inefficiency of using a power converter (often 60% or less efficiency) with a power supply (as low as 30% efficiency.) 12 volt power supplies can have up to 70% efficiency, it appears.

Er, are you sure your car is only going to give you 10A? Any decent car alternator will be in the 40-60A range, freqently as high as 100A. Just use a standard 12v car battery as your source, you will get a heck of a lot more juce out of it.

I take it from your 2A limit you're using a lithium-ion battery? Don't mess around making a charger for one of those, they can catch fire and explode.

AnthraX101

I'm guessing from your post you didn't read the part about a sealed lead battery