# Laptop PC powered from Car Battery

Discussion in 'General Electronics Chat' started by vonsworld, May 25, 2014.

1. ### vonsworld Thread Starter Member

Apr 27, 2011
24
0
Hello

When we go camping we use a leisure battery which is 12 volts and 100 amp/hours. This is like a car battery except it is not damaged by being significantly discharged.

I've been able to perfectly run my Dell Laptop from this battery using a laptop car adapter lead with a cigar lighter plug on the end. It draws about 3amps and the leisure battery also charges the laptops own internal battery.

Now the problem! Instead of using the leisure battery I thought about running the laptop off the battery in my car. So I created an extension lead to stretch from the car to the tent. I used 6amp 240v cable, orange cable like they have on lawn mowers, and the extension is about 10 metres long.

This extension from the car powers the lights in the tent perfectly, but will not run the laptop. It appears that not enough amps are getting through and so the laptop does not recognize it is attached to an external power source.

I need 3amps at 12 volts to pass through the cable which is 10 metres long. I've measured the resistance on the cable and it is just 0.5 ohms.

I thought about Ohms law ie. Amps = Volts/Ohms, but that would mean 24 amps could flow though the cable, which just confused me further!

Please can someone explain what type of cable I need to use or how else to fix this problem?

Thanks

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,787
771
0.5Ω X 3 Amps = 1.5V
So u loose 1.5V for cable
12V - 1.5V = 10.5V

Charger is getting 10.5V which might below it's threshold value.
If it runs perfectly when u connect it directly to battery than u need thicker cables. Get as thick as u can get. This will reduce cable loss.

Use high current cables that are used for welding purposes. A standard extension code will drop too much voltage

3. ### vonsworld Thread Starter Member

Apr 27, 2011
24
0
Thanks for your advice

Using the existing cable would it be possible to build a circuit to attach to the end of the cable that counteracts the current loss. In other words to increase the 10.5v back up to 12v?

Is a regulator chip able to accomplish this on its own, or would you need an inverter circuit followed by a transformer, rectifier, and then a regulator? It would also need to be able to handle 5amps when everything is plugged in.

Apologies for the newbie questions

4. ### paulktreg Distinguished Member

Jun 2, 2008
622
124
Have a look at boost converters something like these for one that fits the bill.

5. ### #12 Expert

Nov 30, 2010
16,671
7,314
16 gauge wire, about the skinniest extension cord you can buy, has 249 feet per ohm.
60 feet of that (round trip) should be 1/4 ohm (3/4 of a volt). Look for crusty connections. Look for the labeled size of the wire.

12 gauge at 630 feet per ohm would get you .0952 ohms and lose .2857 volts.

6. ### prashanth58 New Member

Jan 30, 2010
8
1
The suggestion by Mr Paul here appears2b the best.Should u have any issues I suppose u could also try out power adapters that power L/T,s from a 12v Batt in car.They plugin2the cigarette lighter socket.these units have a selector that enables u2 set the opt voltage all the way 4m 12-24V..But Pz be careful how u set the whole thing up...As Hi currents are involved, u better b careful.I,m Not Pessimistic but its real easy2 get a red hot connectors provoking the plastics or upholstery in the already hot confines2catch fire rather easily.The bottom line is pz do ur homework b4 trying it out or have some1keep a sharp eye......Bye

7. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,787
771
No SMS typng plz. nt here.
If you cannot post complete words please refrain from posting.

Brevor and djsfantasi like this.
8. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,787
771
A boost converter will likely try to take more power from the battery.

It will increase cable loss more and you will loose more power than you need to.

Get thicker cables

9. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,900
876
I (and others) have warned this member numerous times. I even went so far as to email him/her. This user thus has earned a place on my ignore list.

10. ### #12 Expert

Nov 30, 2010
16,671
7,314
Let me translate my post #5:
Use bigger wire and make sure the connections are clean.