I am attempting mount a screen (about 12v 500ma) into my dash of my car.
This will be on 24/7 so will be constantly discharging the car battery.

Looking for some advice, I assume my car battery is 40Ah so after 80 hours my battery will be dead right?
Is there much overhead in doing this with this calculation?

Will the car still start if it has 3/4 total drained (after 60 hours of discharging)?

 

You car battery is a starter type, not a deep cycle. They are not rated in AH but CCA. (Cold Cranking Amps)

They use a spongy mush of lead rather than a solid plate. This allows for very high amperage short bursts..for starting a car. You may actually find that the screen lasts longer than you anticipated. with a low draw, the spongy starter batteries have some time to self charge while the little amp draw is happening.

As for your calculations, the Ah rating is usually at 1 amp for 10 hours. but 2 amp wont be 5 hours..more like 4.5 hours. The HARDER you draw from a battery, the more heat is produced. The more heat, the more resistance.

So with your little draw, you would get more than the standard Ah rating. BUT car batteries are usually at around 13ish volts not 12. And during the whole discharge time, that voltage drops steadily.

If you have a device that REQUIRES 12v, and its a 40Ah battery, you may only have 11.8v in it after 30 hours.

Its crazy. And you never want to discharge a battery more than about half. It leads to plate sulfation and other damage.

So you typically want to double your Ah rating for your needs.

If you need 1a for 10 hours, get a 20Ah battery.

Also, for your needs, you may want to add a deep-cycle battery to car. You can connect a trickle charger to it that connects to your cig. lighter port. It will recharge the battery as you drive.

 

40 ah is on the small side for car batteries, however unless it's a deep cycle battery, discharging it more than half its capacity (even once) will be very hard on the battery.

ETA: I'm too slow on the typer.

 

Ok I think I will try it and see what happens. I never go even 3 days without driving it so it will get a refill when I take it for a spin.

Another question, on a 2010 Honda Civic I was told that if I do drain the battery too much, it will do some funny things to the ECU and the ECU will have to be manually reset.
Do you know if that's true or not?

 

I couldn't imagine that being true. You would have to do a manual reset every time you left the lights on and killed the battery, or replaced the battery, or disconnected it to install something (radio, cb, cd player, etc..)
But, who knows. I would read your owners manual regarding battery maintenance.

 

If it makes you feel more comfortable with the situation you could always install one of those cheap solar cells to counter effect the slow drain on the battery.

 

Yeah I might try that.

Also retched, what you mentioned here:

Also, for your needs, you may want to add a deep-cycle battery to car. You can connect a trickle charger to it that connects to your cig. lighter port. It will recharge the battery as you drive.

This is a good idea I didn't think of this.
What kind of battery should I be looking for do you think.
Lithium ion, nickel, etc?

 

Silly question, but why leave the screen on permanently?

I have a second, deep cycle battery installed in my 4x4 that is used to run a fridge on camping trips. It is connected through an 'intellegent' solenoid which disconnects the main battery when the engine is switched off. As soon as the engine is started again, a timer is started which engages the solenoid after 5 minutes of recharging the main battery. After the timer elapsed, the solenoid connects the two batteries in parallel and both batteries will get charged while driving. Here is a link to the manufacturer of this particular solenoid:http://www.nationalluna.co.za/intelsol.htm

 

Silly question, but why leave the screen on permanently?

It's actually a mobile phone modded to be a dedicated media player (Android G1).
The screen does timeout, but the phone itself is on and that is what is pulling a constant 1A 12v.

Actually it pulls 1A 5v but I run it through a 7805 to drop the voltage but as heat so it's still 1A 12v.

I never turn it off for simplicity, plus it syncs to my nas using rsync overnight with it's wifi when near my house, etc.

 

You have just won the nerd achievement award of the day.

But very cool. Good Idea. Your car media player syncs to your home system. You could eventually set a way for the song you were listening to when you leave the house to continue when you get in the car. Now that would be neat.

 

Does anyone still remember having to rewind a tape to play your favorite song.

Sorry, back to the topic....

 

Whats a "tape" ?

I remember moving the stylus on the record player, and even selecting tracks on my 8 track..

 

You have just won the nerd achievement award of the day

Funny!

Anyway, any deep cycle will suit your needs. They are sold at most places that carry vehicle batteries. You could even pick up a cheap one at wally-world (aka wall mart)

 

Discharging even a deep-cycle battery beyond 30% will adversely affect it's service life.

Discharging it to 50% repeatedly will cut it's service life by 2/3.

[eta]
A car we used to have developed a fault which caused a constant 80mA drain on the battery with the ignition off and key removed. It would drain a fully charged Optima battery within a week to the point where the car would not start.

 

Cool thanks we over here have Super Cheap Auto and Autobarn so I will check them out over the weekend for deep-cycle batteries.

Do you know what they are made out of? Are they li-ion for example?

 

They are lead-acid. They use a solid lead plate.
The starter batteries use a lead mush/mesh to have more surface area for the acid to react with. That is good for pumping out lots of AMPS quickly, for starting a car.

The deep cycle, are slower drain.

 

Ah ok that makes sense.
Will try it out.
Thanks