Page 1 of 2

Automotive dual battery question

Posted: Tue Feb 24, 2004 8:30 am
I have a diesel pick-up with dual 750 CCA batteries wired in parallel. The passenger side battery has a 2/0 cable running directly to the starter. The drivers side battery is connected to the pass. battery thru a 5 foot length of 4 gauge wire. According to the shop manual, starting current can be as high as 700 amps. My question is, does the pass. side battery take more of a load than the drivers side battery because of the realitive small 4 gauge connection? How is the current draw split between the 2 batteries or is it equal? Thanks in advance for any thoughts.

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 9:06 am
The battery with the thickest cables takes the greatest load during start-up. The thinner cables resist the current flow from the passenger battery while cranking the starter.

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 9:57 am
The further the distance, the bigger the cable needed. The two most likely draw apx the same current, based upon size AND distance.

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 11:11 am
This will screw up the estimates (And probably show how little I know) According to my estimate,
with a copper resistivity of 1.6 microhms per cubic centimetre the resistance of a 4 ft long piece of 4 gauge wire is the order of one thousandth of an ohm hence, a 10 volt drop across it would represent a current of 100 amps. There

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 11:11 am
This will screw up the estimates (And probably show how little I know) According to my estimate,
with a copper resistivity of 1.6 microhms per cubic centimetre the resistance of a 4 ft long piece of 4 gauge wire is the order of one thousandth of an ohm hence, a 10 volt drop across it would represent a current of 100 amps. There

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 11:16 am
This is going to screw up the estimates (Or show how little I know) - according to me, with a copper resistivity of 1.6 microhms per cubic centimetre the resistance of a 4 ft long length of 4 gauge wire woulf be the order of 1000th part of an ohm. Hence, a 100 amp current through it would produce a volts drop of 10 volts. There is no way the differnce between two, fairly well charged 12 volt batteries, with one on load, is going to exceed 10 volts so the likely, bettery healthy, starting current in the lead will be the order of 40 - 60 amps

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 11:58 am
WOW… <p>[ February 24, 2004: Message edited by: Edd Whatley ]</p>

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 1:55 pm
Chris Smith,<p> Not sure I follow you. The 2/0 cable to the starter is approx. 4' long. Are you saying that the wire size between the 2 batteries makse no difference and they will always share "equal current",ie, 350 amps?

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 3:25 pm
Where's that guy with the strap-on ammeter?
The large, close cable (passenger side) will carry most of the current.

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 4:26 pm
You have to include the internal resistance of the battery to make a valid calculation. Since internal resistance will be higher than cable resistance the two batteries will be sharing a nearly equal load.

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 5:34 pm
The batteries are connected in parallel. The drivers side 4 ga. wire carries only the current from that battery. It connects to the passenger side battery. The 2/0 carries the current from both batteries. If both batteries are healthy, the current will be pretty close to the same from each battery.

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 5:34 pm
the smaller cable only has have the current flow in it thus smaller cable to the other battery the larger wire carries all the current for the starter 700 amps
joe

Re: Automotive dual battery question

Posted: Tue Feb 24, 2004 8:12 pm
If driver's side battery is connected to passenger's side battery, the cable from passenger's side battery to starter carries current from both batteries.<p>Passenger side battery does supply more starter current than driver side, but its not too lop-sided. And, if engine doesn't start, driver side battery will partially recharge passenger side battery between tries.<p>700 amps is only when you first turn the key. As starter motor speeds up, the current decreases. At full cranking speed probably well under 100 amps. Then driver side batt is still providing less current, but is more of an equal share.

Re: Automotive dual battery question

Posted: Wed Feb 25, 2004 3:56 am
My, my! Let's expand on the proportions. Say the passenger side battery is 2 feet away from the starter with 2/0 cable. The drivers side battery is connected by 4 ga, in say, the next county.
Same current from each battery? I don't think so. Most of the current from the drivers side is lost as IR drop in the longer, smaller cable.
Put another way- disconnect the parallel connection between the batteries. Connect just passengers side battery to starter and measure the cranking current. Then, connect only the drivers side with the smaller wire from the battery in the next county. It won't even crank!

Re: Automotive dual battery question

Posted: Wed Feb 25, 2004 6:11 am
What threw me off is that in almost all cars, the stock (OEM) battery is on the driver's side. I did not catch that the vehicle was a diesel. So, If I were to add an extra battery, I would tend to leave that one alone and connect the extra one to it. I did not catch that both batteries were stock.<p>If I were you, I would connect the batteries together with 0000 gauge wires.<p>[ February 25, 2004: Message edited by: Joseph ]</p>