Need Help Solving Electrical Problem

Submitted: Thursday, Oct 06, 2005 at 06:49

ThreadID: 27021 Views:2893 Replies:9 FollowUps:4

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Member - TonyG (Qld)

Hi All,

I was hoping I might be able to get some advice from those that might know before I take my car to the local auto electrician.

EXISTING SETUP

I have a 3.0TD patrol running a 2nd battery

Running a Rotronics system for the 2nd battery

Have a 115 amphour 2nd battery

Have 5 off 12v sockets running through the car directly off the 2nd battery

I am running a 50l Waeco fridge, and toshiba laptop computer and 3 kids dvd players off 2nd battery sockets

PROBLEM

With car both running, and when turned off, 2nd battery supplies enough power to run fridge and kids stuff.

When I connect toshiba laptop (draws approx 6-7 amps) the power supply to laptop turns off (I think beacause it is not getting enough power)

I have tried the same and using a older laptop (which draws approx 3 amps, and all works fine)

I have successfully run laptop when no other items are plugged into car. Laptop also runs fine in my other car straight from 12 cig connection.

I concluded that fridge pulls 3-4 amps, dvd players pull 1-2 amps, and I know that the laptop draws 6-7 amps

From the above, I have concluded that I need say 15amps of power to be supplied from 2nd battery. I am assuming that I am only getting below 10amps

QUESTION

Is it possible to increase the amount of amps supplied by 2nd battery to run all these items.

Does this mean bigger fuses, or bigger alternator??

As you can tell, I am not an electrical wizard.

All advise would be greatly appreciated

Thanks

TonyG

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Reply By: Member - TonyG (Qld) - Thursday, Oct 06, 2005 at 06:52

Thursday, Oct 06, 2005 at 06:52

Hi All,

Sorry but forgot to mention that it has been installed with 6mm wiring, but I have not checked fuse size yet

AnswerID: 133226

Reply By: Mark T - Thursday, Oct 06, 2005 at 07:15

Thursday, Oct 06, 2005 at 07:15

I would suggest that the lap top is shutting down as the voltage is dropping to a level that is below the laptops minimum to work. You can't "increase the amps".

What you need to do is to is measure the voltage at the laptop 's 12 volt connection and see what it is. You could also start the engine and run at around 2000 rpm and check the voltage then as the alternator will be charging. (Ensure your fan belt is tight though as many cars run around with loose fan belts which ensures under charging!)

Depending on how old and what condition your battery is in, the state of battery charge might come ino play here. If you're drawing 15 amps for some time out of the battery, then the voltage will drop simply because of the load. It all depends as I said on the state of charge and age/condition of the battery. My assumptions are based on heavy duty wiring and you state 6 MM wire which should be fine.

Your 115 amp hour battery.. many people think that you can draw 115 amps for 1 hour. Actually there are 2 rates, the 10 hour rate or the 20 hour rate. The 10 hour rate (best) means you can draw 11.5 amps per hour for 10 hours or the 20 hour rate means that you can draw 5.75 amps per hour for 20 hours.

Anyhow after all this stuff.. get a digital multimeter and measure the voltage at the laptop's 12 v power supply and see what you get.

Hope this all helps you and doesn't confuse.

Mark Taylor

AnswerID: 133230

Reply By: Member - Geoff M (Newcastle) - Thursday, Oct 06, 2005 at 07:38

Thursday, Oct 06, 2005 at 07:38

As Mark Taylor said, the problem will be voltage drop. If the battery is in good condition then look at the following,
Measure what voltage you are getting at the laptop with everything else connected.
Is everything running off the one 6mm cable feed?
Does the negative wire of the 6mm supply go back to the battery or have you screwed it under a convenient bolt on the body? (Works best to run it back to the battery)
Are the DVD players 1-2 Amps each or that's the total, my guess 1-2 Amps each?

Therefore you are pulling worst case 4 + (3 x 2) + 7 = 17 Amps down that 6mm cable, or 17 x 12 = 204 watts. More than both of your driving lights combined!!
Spilt the load. One 6mm feed for the fridge, seperate 6mm feed for the DVD players. And a third 6mm feed for the Laptop.
All with the negative leg run back to the battery.

Let us know what you find,
Geoff.

Geoff,

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AnswerID: 133231

Reply By: Ken - Thursday, Oct 06, 2005 at 08:51

Thursday, Oct 06, 2005 at 08:51

Tony, All good advice, there is a possibly a poor connection somewhere in your wiring so that when the additional current draw of your new laptop is added to the laod there is insufficient voltage available at its powersupply. As stated measure the voltage at the laptop supply with the load connected though. If you just measure it at the socket with no load it will show 12v. The meter will not draw enought current to cause the voltage drop that is happening with the laptop plugged in.
Also just as a precaution measure the voltage at the 2nd battery [ with the motor off ] with and without all the loads connected. This will give you an indication if there is a problem with the battery either not charged or faulty. It should be a little over 12 volts if it is OK. While you are at it also check the voltage with the motor running to make sure it is being charged by the alternator and the voltage should be 13 -14 volts.
Good luck Ken

AnswerID: 133242

Follow Up By: V8troopie - Thursday, Oct 06, 2005 at 15:17

Thursday, Oct 06, 2005 at 15:17

Use a digital volt meter, set to DC volts. Battery by itself should measure 12.6V after resting a while, recharge if its anywhere near 12.0V. With alternator running there should be at least 13.6V at the battery, better is 14V. The battery won't take a decent charge if its only fed 13volts.
Tenths of volts make a BIG difference in 12V battery systems.

The other advise about running a seperate wire (+ and -) for the laptop is what I would do, protected by a 15A spade fuse in a good quality fuse holder at the battery end. Do not use a cig lighter type plug/socket, they are unreliable for laptop applications.
Klaus

FollowupID: 387496

Reply By: Peter 2 - Thursday, Oct 06, 2005 at 09:28

Thursday, Oct 06, 2005 at 09:28

As suggested run seperate cables for each socket, preferably use sockets other than fag sockets, they will not handle a continuous 6amp load. Check any connections and plugs for heat (check with hand, fingers etc), heat equals resistance which stops current flow and causes voltage drops.
Any joins should be soldered rather than connectors which corrode and loosen over time.

AnswerID: 133248

Reply By: Member - Jeffrey - Thursday, Oct 06, 2005 at 09:56

Thursday, Oct 06, 2005 at 09:56

Hi TonyG,
In my humble opion I think your flogging the horse with to much load..yes and I also hate paying for something I might be able to do myself,heavier wires? bigger battery,?maybe a dedicated battery.I think maybe you need a GOOD auto elect to sort it out properly,but then you face finding a good one,one who really knows his stuff.hope it all works out for you.
All The Best In Health And Wealth
Jeffrey (AKA JD)

AnswerID: 133257

Reply By: VK3CAT - Thursday, Oct 06, 2005 at 19:01

Thursday, Oct 06, 2005 at 19:01

Hi Tony.
Firstly you state that the cable used is 6mm. Is this the ISO standard measurement of the cross sectional area of the cable as per 240 volt cables or is it automotive cables that often refer to the overall diameter of the cable and insulation?
There is a huge difference.
Using AS3000 as a reference, the voltage drop at the end of a 6mm squared cable 6 metres long with a load of 17 amps is 0.765 volts. Thus it should be adequate for this use.
If however it is automotive cable, the actual cross sectional area may only be 1.5 to 2.5mm squared. Using the above figures, this would produce a voltage drop of 2.958 volts for a 1.5mm square cable or 1.836 volts for a 2.5mm square cable.
By your description, I think that you must be using automotive cable.

The problem may be solved by either installing a bigger cable or installing individual smaller cables to each of the appliances.

A further thought.
6-7 amps for the laptop seems high. This may be because the lap top, when plugged into its mains adaptor, runs at a voltage of 15 - 18 volts and at a lower current, thus there are inefficiencies when converting direct from the cigarette lighter. A way around this may be to use an inverter and the standard laptop 240 volt adaptor.
Cheers Tony (M)

AnswerID: 133316

Follow Up By: Member - TonyG (Qld) - Thursday, Oct 06, 2005 at 19:15

Thursday, Oct 06, 2005 at 19:15

Hi Tony,

Thanks for the feedback.

To clarify your questions

The 2nd battery, rotronics, cable and 12v outlets were installed by Opposite Lock in NSW. They told me they would use 6mm2 cable which they said was minimum for 2nd battery setups. I can only assume this was done as they said.

I know the laptop pulls 6 amps, beacause I purchased a 12volt adapter, and it says it supplies 8amps. I have to lower the processor speed on the laptop power setup to get it to work.

I will look into the inverter, thanks for the suggestion

TonyG

FollowupID: 387524

Follow Up By: Spade Newsom - Thursday, Oct 06, 2005 at 22:17

Thursday, Oct 06, 2005 at 22:17

Laptops that have DVD Roms draw much higher than lap tps without. Mine draws up to 7.1amps.

FollowupID: 387580

Reply By: GaryInOz (Vic) - Thursday, Oct 06, 2005 at 19:29

Thursday, Oct 06, 2005 at 19:29

Other thing to consider is the laptop itself. You may find that it needs at least 14.4 volts to work (or some other figure above 12-13 volts, read the manual)

DSE sell a product that effectively boosts the voltage output, specifically designed for laptops. It will use a bit more current doing it this way, but you will have more reliable operation of your laptop. From memory they are switchable from about 14-18 volts, other benefit is it is a regulated output with no alternator spikes.

As noted above the voltage loss with everything fired up is about 0.7 volt, take this away from your nominally 12.6 volts would leave you very marginal for voltage to the laptop.

AnswerID: 133319

Follow Up By: Spade Newsom - Thursday, Oct 06, 2005 at 22:28

Thursday, Oct 06, 2005 at 22:28

My lap top runs on 19 volts. The car adapter converts (Inverts??) the 12 volt to 19 volt. Your adapter should already do this. If it doesn't than maybe Gary In Oz might have nailed this one.

Our previous laptop was approx 14+ volts as described by Gary In Oz. Using a simple adapter, the car whilst running, produced close enough to the correct volts from the alternator for the computer to run. If the engine wasn't running then battery itself didn't have the volts to do the job. Look under the laptop and it should have the both the DC Volt and Amp requirements.

Hope this helps.

FollowupID: 387586

Reply By: VK3CAT - Saturday, Oct 08, 2005 at 16:57

Saturday, Oct 08, 2005 at 16:57

Tony, just another thought. Not sure on the type of battery isolator that you are using but if it of the transistorized type, then you can get 0.7 volts drop acroos each PN junctions making an additional 1.4 volts drop. Using solonoids or MOSFETS can remove / reduce this problem. I just use a 100 amp isolation switch.
Cheers again, Tony M.

AnswerID: 133649