alright guys, when i build my carputer i would like it to run on a separate sealed lead acid battery. But the best solutions i can think of are 12v Hobby Batterys. See the link below for a good look at the sepecifacations.

I would like it to charge when the car is on, maybe through the altenator but it is a 7Ah battery. Will this be enough for what i need todo. The most time i'll be leaving the carputer on is maybe an hour or two.

One more question, would this battery be elligable to be charged by the altenator on a switched link, so when the car is off the relay clicks and the connection to the rest of the car is disengaged?

http://www.hobbico.com/fieldequip/hcap0800.html




Thanks,

Shane

 

7ah means it will deliver 7 amps for 20 hours at 80 degrees while providing at least 10.5 volts. At least thats what it means if hobby batterys are rated the same as automotive batterys.

You could use the alternater to charge the battery as long as it was hooked up in parralel to the car battery. The alternator may be to powerfull for the small battery thou, which would cause battery longevity problems. To isolate it from the rest of the car you could use a relay that has closed contacts only when the ignition is turned on.

What kind of current draw are you looking at from the carputer? You could probably get away with using your regular car batterys as long as the carputer has a sleep function. New cars have over 100 cpu's and they still draw less than .050 amps with the key off after the cpu's go to sleep.

 

Check out www.mp3car.com, LOTS of information there about this stuff. Using just a battery to power the carputer isnt going to work, look for a DC->DC power supply that turns your 12V DC power from your car battery to the power connectors needed to run your motherboard (ATX) and other periphials (molex connectors).

 

It shouldn't hurt to have a backup battery for the carputer, however. You could have a separate charging line to that battery and let it float otherwise, keeping it independent of the main battery.

That would also serve, IIRC, to keep its voltage supply more constant...

 

Just to clarify : 7ah means that it can deliver 7/20 = 0.35A for 20 hours. If you take 7A out of the battery then it will actually last only about 35minutes. The faster you take power out of a battery the less total power it gives you.

Si.

 

I'm going to try the same thing. i'm using a lawn tractor battery. they're about 10 amp hours and they're $20

 

are they sealed though, that hobby battery is completely sealed. I dont want to be instlaling a second battery in the back fo the car so it can lead acid all over the rear, not a good idea.

 

no, it's not sealed, but it's real easy to attach a vent tube to the one I have and route it outside. I've abused the crap out of one of these batteries and it still works great.

 

Why not beef up your charging system and run an ac>dc converter? Then you can use the standard power supply for the computer

 

thats a thougth but..... the dc<ac converters supply themselves with enough charge for saya big screen TV, but my computer only uses maybe half that,. i'm wasting half the energy. Its a complete battery waste.

 

Basically the point Im trying to make is that in order to have the corrrect voltage and amperage you are going to need to use the PS from the computer anyway. Why not just do it the easy way?? I'm guessing you are going to use the entire case and all? How are you planning on situating the machine in the car??

 

An interesting topic, however it is not Thirdgen related but I will allow it to remain open for discussion.

 

You can get a sealed acid gel battery like an Optima to keep the computer powered and still run it off of the alternator. Then set up a power converter like ljnowell suggested and plug in the computer's power supply. The second battery isn't really necessary for this... it could all be set up with the main battery if it's healthy.

 

To tell you the truth, as long as you had a decent battery (not an everstart or duralast) and a decent alternator (100 amps) you wouldnt have any problems at all. The computer doesnt use very much energy. The two biggest problems you will face with a project like this are heat and vibration. If you dont use the PC's case, and try to integrate the boards, you will have heat issues. And I dont even want to think about the hard drives heads smashing off the platters when you hit a bump. ouch.

 

one advantage of using a second battery is it isolates the computer power supply from the low voltage condition when the starter is engaged

 

Depends on how he wires the second battery. If he uses a distribution block to isolate the second battery it would, but if he just hooks it up in parallel it wont. I wouldnt think he would be running the computer while starting the car anyway though.

 

many set their car computers to boot as soon as the ignition is turned on, the reason is to get it booted asap. Yes, it would be pointless without an isolator.

 

I don't know if this helps any, but 2 years ago in my old mini-van I had an amp and sub in the back that had a bad buzz from the charging system. So I tried filters etc. and ended up putting a new car battery in the back in a plastic marine battery box, and wiring a relay that would switch on when I powered up the amp. This relay would cut the battery in back from the van's charging system and apply power to the sub.

I hardly ever had the radio off while driving the van, and the battery never went dead. So the battery had to be charging off the van's system while the van was shut off.

The wire I used to connect the rear battery to the charging system was 14 Ga stranded and I put a 10A fuse on it up at the front battery.

This setup took up only the little extra space that the extra car battery required and worked very well.

I know you already know this but I am going to state it for safety's sake, make sure the second battery is well secured if located inside the car with you and your passengers.

 

Maybe they rate batteries differently than when I graduated Electronics School.

7AH "should" mean the battery can SUPPLY 7 amps for ONE HOUR.

At that time the individual cells would be depleted to a voltage of approximately 1.1 volts.

The math is pretty linear. It will supply 3.5 amps for TWO HOURS.
500ma for 14 hours.

I think the ambient temperature the calculations are valid for are around 70F.
Not absolutely sure on the temps. though.

Colder the battery capacity would be less.... hotter more.

Hope this helps.

 

I wish that the maths was linear, but it ain't. 7aH should indeed mean that it can supply 7 amps for 1 hour but it doesn't work out that way.

From a 7aH lead acid battery you will only be able to draw 7amps for about 35minutes at most. If you don't beleive me then either try it or ask one of the battery manufacturers.

Because most applications for batteries use the power over a long-ish period they have specified that the aH rating is determined by a 20hour discharge and using that current value to calculate the theoretical aH rating.

It is one of many areas of lead acid batteries that isn't great. Another that is occasionally forgotten is that they are only something like 65% efficient when charging. ie. only 65% of the power you use to charge one actually gets stored.

If you want a brief tutorial on the subject then this link is fairly good :

http://www.vonwentzel.net/Battery/00.Glossary/

Si. :lala: