Hooking up two batteries together on my 73 c10????? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

luifer76 · 09-07-2011, 09:54 AM

Kind of a dumb question and i have an idea on how i should do it but i just want to get some feed back or ideas fom all you guys to make sure i do it right as some of you seen my 73 c10 is bagged and running into a problem with my battery going dead everytime i fill up my air tanks i know that compressors could draw a lot of current specially when you running multiple compressors .....the battery doesn't completly die but but you can tell it drains it....now i only run the compressor when the truck is running since i added a toggle switch to be able to trun them off any time i want to..... long story short i want to run two batteries with the same altenator and i need to link both batteries together any ideas or pics would be great thanks.......

juiceman650 · 09-07-2011, 10:15 AM

It should be just be a simple (+) to (+) and (-) to (-) for both the batteries. In other words, hook the (+) of one to the (+) of the other and the (-) to the (-). That's how we do it with car audio. Other input could be helpful though.
Posted via Mobile Device

luifer76 · 09-07-2011, 12:55 PM

Quote:

Originally Posted by juiceman650

It should be just be a simple (+) to (+) and (-) to (-) for both the batteries. In other words, hook the (+) of one to the (+) of the other and the (-) to the (-). That's how we do it with car audio. Other input could be helpful though.
Posted via Mobile Device

thats what i thought i just wanted some input from some of you guys thanks again....

JuicyJ · 09-07-2011, 10:22 AM

positive to positive and negative to negative is right. that's the way we do it here with the diesels and hydraulic lift school buses.

partsstoreguy · 09-07-2011, 10:32 AM

i just ground the drivers side - to the body or frame then run the + across to the passenger side + and the the passenger side ground to the engine in the factory location and the passenger side + to the starter like factory.

tucsonjwt · 09-07-2011, 12:12 PM

Either way will work. I ran both positive and negative cables through holes in the radiator core support and tie wrapped the cables to the underside of the radiator top panel, so the cables are out of sight.
You might want to check out a high output alternator, but you should also have heavier cables if you install one. Heavier cables for the battery and to the starter are also a good idea.
Two batteries will cost more than you might think: You should buy two new batteries or the old one will bring down the charge in the new one. You will need a battery tray for the second battery - the OEM ones are pricey but are way better than the aftermarket ones. You will need the extra set of cables and post adapters.
If you drive your truck a lot you will successfully keep a charge on two batteries. If you don't drive your truck much (like me) you will have difficulty keeping one battery well charged, much less two batteries. This is where the high output alternator comes in. You can search the internet for high output alternator and get advice on this, mostly from h.o. alternator vendors and car audio sites. Still, a high output alternator puts out most of its amps at higher RPMs, so city driving alone will not produce a lot of amps.
I have a high output alternator (140 amp, I think) and I find that for some reason the primary battery gets most of the charge and the secondary battery gets less of a charge - I have no isolator between them, which would be designed to charge the primary battery first, then the secondary battery. If you want info on isolators you can search that on the internet also, but I don't like isolators. I used a Ford solenoid between the two batteries instead of an isolator when my isolators kept failing. I had a toggle switch on the dash at one time to use two batteries for start up, but I don't think that is necessary.
Bottom line, two batteries ultimately won't help you much in the long run if you are providing the same amount of current as you were to one battery - you will just have two batteries with the same level of charge instead of one battery. Again, if you drive your truck a lot you will keep a good charge on both batteries, even with a stock alternator.

luifer76 · 09-07-2011, 12:53 PM

Quote:

Originally Posted by tucsonjwt

Either way will work. I ran both positive and negative cables through holes in the radiator core support and tie wrapped the cables to the underside of the radiator top panel, so the cables are out of sight.
You might want to check out a high output alternator, but you should also have heavier cables if you install one. Heavier cables for the battery and to the starter are also a good idea.
Two batteries will cost more than you might think: You should buy two new batteries or the old one will bring down the charge in the new one. You will need a battery tray for the second battery - the OEM ones are pricey but are way better than the aftermarket ones. You will need the extra set of cables and post adapters.
If you drive your truck a lot you will successfully keep a charge on two batteries. If you don't drive your truck much (like me) you will have difficulty keeping one battery well charged, much less two batteries. This is where the high output alternator comes in. You can search the internet for high output alternator and get advice on this, mostly from h.o. alternator vendors and car audio sites. Still, a high output alternator puts out most of its amps at higher RPMs, so city driving alone will not produce a lot of amps.
I have a high output alternator (140 amp, I think) and I find that for some reason the primary battery gets most of the charge and the secondary battery gets less of a charge - I have no isolator between them, which would be designed to charge the primary battery first, then the secondary battery. If you want info on isolators you can search that on the internet also, but I don't like isolators. I used a Ford solenoid between the two batteries instead of an isolator when my isolators kept failing. I had a toggle switch on the dash at one time to use two batteries for start up, but I don't think that is necessary.
Bottom line, two batteries ultimately won't help you much in the long run if you are providing the same amount of current as you were to one battery - you will just have two batteries with the same level of charge instead of one battery. Again, if you drive your truck a lot you will keep a good charge on both batteries, even with a stock alternator.

thanks for the info both of the batteries are new i got an extra battery tray allready and i have a 120 amp altenator i think i may be fine with that altenator i think the extra battery will solve my problem since im running 4 380 compresors to fill up two 7 gallon tanks and everytime i get to run the compressors to get to at least 160 psi or 200 psi the one battery starts feeling it.....

Keith Seymore · 09-07-2011, 03:13 PM

Agree -

Positive to Positive, negative to negative in theory; in practice, it ends up being negatives routed to ground (ie, frame), postive on one battery to the postive on the other, then one positive routed to the starter.

Here's what it looks like in the trunk of my Chevelle:

Negatives are to the rear, and you can see them poking through the trunk floor headin' to the frame.

The positives are the forward post, and you can see the cable (inside the convoluted conduit) running across the rear of the trunk and through the latch area.

The side post/top post batteries make it a little easier, since it provides a place for all this stuff to hook up to.

Lastly, everything goes through that Moroso master cut off switch before heading to the rest of the car.

K

lilpoindexter · 09-07-2011, 08:15 PM

Has anyone rigged up a setup where one battery works when the car is shut off...so that even if you run that one down when the engine is off, you can still start the car with the other battery?

Redcap · 09-07-2011, 09:22 PM

Quote:

Originally Posted by lilpoindexter

Has anyone rigged up a setup where one battery works when the car is shut off...so that even if you run that one down when the engine is off, you can still start the car with the other battery?

That is just a simple isolator setup.

jimbonice · 09-07-2011, 10:36 PM

Quote:

Originally Posted by lilpoindexter

Has anyone rigged up a setup where one battery works when the car is shut off...so that even if you run that one down when the engine is off, you can still start the car with the other battery?

That is typically the most common way of setting it up, either with a diode type isolator or with a relay. For example I run the stereo and trailer\camper plugs to the 2nd battery. I can run it down and still start the truck on the main battery. I personally prefer the diode type of isolator, but I'm sure there will be many opinions here as to what is best. In this situation, where you have very high current draw, it may be better for you to have your batteries hooked directly together to provide the amps to run the compressor. The only downside is that the batteries will discharge each other down when the truck is off.

*cr34t3d_by_d4rkd3n* · 09-07-2011, 11:46 PM

Quote:

Originally Posted by lilpoindexter

Has anyone rigged up a setup where one battery works when the car is shut off...so that even if you run that one down when the engine is off, you can still start the car with the other battery?

i hate this idea. whats the point of having a seperate battery? u want more consistant voltage and more amperage? run 2 batterys in parallel and call it good. if not ur just drGging abound 70lbs for what?? " if u need it??" an alternator loves 2 batterys. its not harder on it.. its eaiser. less draw=less to recharge.

rcbildr · 09-08-2011, 04:32 AM

[QUOTE=cr34t3d_by_d4rkd3n;4887806]i hate this idea. whats the point of having a seperate battery? u want more consistant voltage and more amperage? run 2 batterys in parallel and call it good. if not ur just drGging abound 70lbs for what?? " if u need it??" an alternator loves 2 batterys. its not harder on it.. its eaiser. less draw=less to recharge.[/QUOTE

It comes in handy on trucks with campers. You can run the camper off one battery and still be able to start the truck on the other one. Somebody correct me if I'm wrong, but I think usually the 2nd battery (the one that runs the camper) is a deep cycle battery and the other is just whatever you need to for the vehicle...it can be done that way because of the isolater. I have a camper on my C30 that I want to set up this way when time an money allows.

On my diesel burb I need 2 batteries to start the truck and they are wired in parrallell..although in reality I can get away with just one battery...gotta luv the optima red tops.

thrtsxchambers · 09-08-2011, 12:31 PM

With the isolator setup the main battery is not being used so if it goes dead it does not kill the main battery to still be able to start the truck. the isolator separates the batteries until the isolator has power. I have always used this setup for high power add ons to vehicles like a stereo system or airrride. both ways will work but the isolator helps keep the main battery for starting the vehicle.
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tucsonjwt · 09-08-2011, 01:46 PM

Isolators work as described but the ones I had (diode type) always failed after about one year of use. More expensive ($100+) isolators are supposed to last longer.

jimbonice · 09-08-2011, 09:23 PM

Quote:

Originally Posted by cr34t3d_by_d4rkd3n

i hate this idea. whats the point of having a seperate battery? u want more consistant voltage and more amperage? run 2 batterys in parallel and call it good. if not ur just drGging abound 70lbs for what?? " if u need it??" an alternator loves 2 batterys. its not harder on it.. its eaiser. less draw=less to recharge.

When you're parked in the bush a 100 miles away from civilization, it can come in kind of handy if you want to start your truck after a week of camping. It's a long walk home trying not to become grizzly snacks.

*cr34t3d_by_d4rkd3n* · 09-07-2011, 11:42 PM

+ to + - to -. if u went +to- ud create a 24v system. u want to create a "parallel bridge"

*cr34t3d_by_d4rkd3n* · 09-07-2011, 11:43 PM

oh. and i have 1370 cold cranking amps

BLE 'BURBAN · 09-08-2011, 02:22 AM

I've always ran 2 batteries by hooking them up + to + and - to - . Now what MOST PEOPLE do from here is simply go directly to the starter and ground off of one battery only. Which is fine if you want to double your reserve capacity.
But if you want to DOUBLE your COLD CRANKING AMPS... Hook the 2 batteries up the same way as described above BUT go to the starter (+) off of one battery and ground (-) off of the other battery.

thrtsxchambers · 09-08-2011, 03:10 AM

I had the same problem on my truck. I used the isolator hook up and a 100amp alternater and have had no problems since. I have one battery running the truck and one runs my compressor, amp and switches. I also run optima yellow top batteries that can be recharged without damage to the battery.
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*cr34t3d_by_d4rkd3n* · 09-12-2011, 04:44 AM

Quote:

Originally Posted by BLE 'BURBAN

I've always ran 2 batteries by hooking them up + to + and - to - . Now what MOST PEOPLE do from here is simply go directly to the starter and ground off of one battery only. Which is fine if you want to double your reserve capacity.
But if you want to DOUBLE your COLD CRANKING AMPS... Hook the 2 batteries up the same way as described above BUT go to the starter (+) off of one battery and ground (-) off of the other battery.

Hahah WHAT?! U need to open some books. 2 batterys parralled stays 12v and combines both batterys cca. The way u describe leaves insuffent grounds. BOTH BATTERYS NEED A CRAP LOAD OF GROUNDS AND EITHER A COMMON BUT EQUIL + JUNCTION POINT or EQUAL POSITIVE BRANCHES OFF POSTS. To say neg off one and pos to starter is a frog leap away from a in series system. Your 2000 posts are impressive, but if they contain incorrect instructons whats the point?
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BLE 'BURBAN · 09-19-2011, 02:47 AM

Quote:

Originally Posted by cr34t3d_by_d4rkd3n

Hahah WHAT?! U need to open some books. 2 batterys parralled stays 12v and combines both batterys cca.That's exactly what I said. The way u describe leaves insuffent grounds. Not at all if you will look at the attached diagram below.BOTH BATTERYS NEED A CRAP LOAD OF GROUNDS AND EITHER A COMMON BUT EQUIL + JUNCTION POINT or EQUAL POSITIVE BRANCHES OFF POSTS. To say neg off one and pos to starter is a frog leap away from a in series system. Your 2000 posts are impressive, but if they contain incorrect instructons whats the point?
Posted via Mobile Device

The diagram below is how I've always hooked up my batteries to DOUBLE MY COLD CRANKING AMPS.
I've ran like this for over 30 years and have never had a problem.

*cr34t3d_by_d4rkd3n* · 09-12-2011, 04:37 AM

Instead of running an iso and power lines and bein that far n the bush seemsnlike a risk. When wed go to the desert wed just bring a generator. And for the cost and reliability id do that every day vs isolator this and that. Just another system to fail.
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*cr34t3d_by_d4rkd3n* · 09-19-2011, 03:36 AM

Ok excuse my languare i get irritated when one csnt see my point of view. Think of it this way... If 1 battery has 1 ground. 2 batterys should have ??...2 min. Ur method would work acceptably if it was an ac set up. But sense its dc the flow of power has to constantly work. Ur method increases reserve capicity. Think of it like hydrolics. Return line is always bigger than feed line.
Posted via Mobile Device

*cr34t3d_by_d4rkd3n* · 09-19-2011, 05:53 AM

the way u show is using the series principal. (flowing thru another) ur set up will work. but used to pull a winch on say a 4000lb 45 sec pull will turn that ground wire to a welding rod. the set up u show is best for a trunk sense grounding 2 batts at the sheet metal when theyre sitting right next to eachother is dumb. the more grounds u have the better. simple rule for every hot, the ground should b equil and 50 percent "fatter" .

in an engine compartment running that long of a ground is highly dangerous. surrounding positives will wanna jump to it. its better to ground both batterys to sheet or frame and jump the positives. but remember. share the load. brancch off each batt equil or else all will try to feed off one cable.

09-07-2011, 09:54 AM	#1
luifer76 Registered User Join Date: Jul 2010 Location: Southern California CA Posts: 1,088	Hooking up two batteries together on my 73 c10????? Kind of a dumb question and i have an idea on how i should do it but i just want to get some feed back or ideas fom all you guys to make sure i do it right as some of you seen my 73 c10 is bagged and running into a problem with my battery going dead everytime i fill up my air tanks i know that compressors could draw a lot of current specially when you running multiple compressors .....the battery doesn't completly die but but you can tell it drains it....now i only run the compressor when the truck is running since i added a toggle switch to be able to trun them off any time i want to..... long story short i want to run two batteries with the same altenator and i need to link both batteries together any ideas or pics would be great thanks....... __________________ nOtHiNg LiKe ThE gOoD oLe ChEvYs....

09-07-2011, 10:15 AM	#2
juiceman650 Registered User Join Date: Jan 2011 Location: San Bruno, CA. Posts: 348	Re: Hooking up two batteries together on my 73 c10????? It should be just be a simple (+) to (+) and (-) to (-) for both the batteries. In other words, hook the (+) of one to the (+) of the other and the (-) to the (-). That's how we do it with car audio. Other input could be helpful though. Posted via Mobile Device

09-07-2011, 10:22 AM	#4
JuicyJ Registered User Join Date: Jun 2010 Location: San Antonio, Texas Posts: 123	Re: Hooking up two batteries together on my 73 c10????? positive to positive and negative to negative is right. that's the way we do it here with the diesels and hydraulic lift school buses. __________________ America is all about speed, hot nasty bad ass speed

09-07-2011, 10:32 AM	#5
partsstoreguy Registered User Join Date: Aug 2011 Location: sayre ok Posts: 24	Re: Hooking up two batteries together on my 73 c10????? i just ground the drivers side - to the body or frame then run the + across to the passenger side + and the the passenger side ground to the engine in the factory location and the passenger side + to the starter like factory.

09-07-2011, 12:12 PM	#6
tucsonjwt Registered User Join Date: Jul 2007 Location: Tucson, AZ Posts: 3,189	Re: Hooking up two batteries together on my 73 c10????? Either way will work. I ran both positive and negative cables through holes in the radiator core support and tie wrapped the cables to the underside of the radiator top panel, so the cables are out of sight. You might want to check out a high output alternator, but you should also have heavier cables if you install one. Heavier cables for the battery and to the starter are also a good idea. Two batteries will cost more than you might think: You should buy two new batteries or the old one will bring down the charge in the new one. You will need a battery tray for the second battery - the OEM ones are pricey but are way better than the aftermarket ones. You will need the extra set of cables and post adapters. If you drive your truck a lot you will successfully keep a charge on two batteries. If you don't drive your truck much (like me) you will have difficulty keeping one battery well charged, much less two batteries. This is where the high output alternator comes in. You can search the internet for high output alternator and get advice on this, mostly from h.o. alternator vendors and car audio sites. Still, a high output alternator puts out most of its amps at higher RPMs, so city driving alone will not produce a lot of amps. I have a high output alternator (140 amp, I think) and I find that for some reason the primary battery gets most of the charge and the secondary battery gets less of a charge - I have no isolator between them, which would be designed to charge the primary battery first, then the secondary battery. If you want info on isolators you can search that on the internet also, but I don't like isolators. I used a Ford solenoid between the two batteries instead of an isolator when my isolators kept failing. I had a toggle switch on the dash at one time to use two batteries for start up, but I don't think that is necessary. Bottom line, two batteries ultimately won't help you much in the long run if you are providing the same amount of current as you were to one battery - you will just have two batteries with the same level of charge instead of one battery. Again, if you drive your truck a lot you will keep a good charge on both batteries, even with a stock alternator.

09-07-2011, 03:13 PM	#8
Keith Seymore Registered User Join Date: May 2009 Location: Motor City Posts: 9,226	Re: Hooking up two batteries together on my 73 c10????? Agree - Positive to Positive, negative to negative in theory; in practice, it ends up being negatives routed to ground (ie, frame), postive on one battery to the postive on the other, then one positive routed to the starter. Here's what it looks like in the trunk of my Chevelle: Negatives are to the rear, and you can see them poking through the trunk floor headin' to the frame. The positives are the forward post, and you can see the cable (inside the convoluted conduit) running across the rear of the trunk and through the latch area. The side post/top post batteries make it a little easier, since it provides a place for all this stuff to hook up to. Lastly, everything goes through that Moroso master cut off switch before heading to the rest of the car. K __________________ Chevrolet Flint Assembly 1979-1986 GM Full Size Truck Engineering 1986 - 2019 Intro from an Old Assembly Guy: http://67-72chevytrucks.com/vboard/s...d.php?t=342926 My Pontiac story: http://forums.maxperformanceinc.com/...d.php?t=560524 Chevelle intro: http://www.superchevy.com/features/s...hevy-chevelle/

09-07-2011, 08:15 PM	#9
lilpoindexter Registered User Join Date: Sep 2010 Location: los angeles Posts: 1,134	Re: Hooking up two batteries together on my 73 c10????? Has anyone rigged up a setup where one battery works when the car is shut off...so that even if you run that one down when the engine is off, you can still start the car with the other battery?

09-08-2011, 04:32 AM	#13
rcbildr Registered User Join Date: Sep 2000 Location: Gold Bar, WA Posts: 1,216	Re: Hooking up two batteries together on my 73 c10????? [QUOTE=cr34t3d_by_d4rkd3n;4887806]i hate this idea. whats the point of having a seperate battery? u want more consistant voltage and more amperage? run 2 batterys in parallel and call it good. if not ur just drGging abound 70lbs for what?? " if u need it??" an alternator loves 2 batterys. its not harder on it.. its eaiser. less draw=less to recharge.[/QUOTE It comes in handy on trucks with campers. You can run the camper off one battery and still be able to start the truck on the other one. Somebody correct me if I'm wrong, but I think usually the 2nd battery (the one that runs the camper) is a deep cycle battery and the other is just whatever you need to for the vehicle...it can be done that way because of the isolater. I have a camper on my C30 that I want to set up this way when time an money allows. On my diesel burb I need 2 batteries to start the truck and they are wired in parrallell..although in reality I can get away with just one battery...gotta luv the optima red tops. __________________ My Projects: 1960 GMC 1/2ton with 305 V6 (daily driver) 1963 Chevy 1/2ton 2wd...converting to 4x4 1967 Suburban 1/2ton 2wd My Girlfriends Projects 1985 Chevy Suburban 3/4ton 4x4 with 6.2L diesel, 4" lift, converted to TH-400, Warn Premium manual hubs, & Wildcat 315/75/R16 tires. (daily driver) 1978 Camaro Type LT with a Marine 350 & vortec heads, Rochester Carb, & roller cam 1978 Camaro Z28

09-08-2011, 12:31 PM	#14
thrtsxchambers Registered User Join Date: Nov 2009 Location: Salisbury, NC Posts: 316	Re: Hooking up two batteries together on my 73 c10????? With the isolator setup the main battery is not being used so if it goes dead it does not kill the main battery to still be able to start the truck. the isolator separates the batteries until the isolator has power. I have always used this setup for high power add ons to vehicles like a stereo system or airrride. both ways will work but the isolator helps keep the main battery for starting the vehicle. Posted via Mobile Device

09-08-2011, 01:46 PM	#15
tucsonjwt Registered User Join Date: Jul 2007 Location: Tucson, AZ Posts: 3,189	Re: Hooking up two batteries together on my 73 c10????? Isolators work as described but the ones I had (diode type) always failed after about one year of use. More expensive ($100+) isolators are supposed to last longer. Last edited by tucsonjwt; 09-08-2011 at 02:25 PM.

09-07-2011, 11:42 PM	#17
cr34t3d_by_d4rkd3n Account Suspended Join Date: Jan 2011 Location: Allen, TX Posts: 188	Re: Hooking up two batteries together on my 73 c10????? + to + - to -. if u went +to- ud create a 24v system. u want to create a "parallel bridge" Attached Images

09-07-2011, 11:43 PM	#18
cr34t3d_by_d4rkd3n Account Suspended Join Date: Jan 2011 Location: Allen, TX Posts: 188	Re: Hooking up two batteries together on my 73 c10????? oh. and i have 1370 cold cranking amps

09-08-2011, 02:22 AM	#19
BLE 'BURBAN In the Forgotten far North. Join Date: Jul 2007 Location: Anchorage, Alaska Posts: 2,627	Re: Hooking up two batteries together on my 73 c10????? I've always ran 2 batteries by hooking them up + to + and - to - . Now what MOST PEOPLE do from here is simply go directly to the starter and ground off of one battery only. Which is fine if you want to double your reserve capacity. But if you want to DOUBLE your COLD CRANKING AMPS... Hook the 2 batteries up the same way as described above BUT go to the starter (+) off of one battery and ground (-) off of the other battery. __________________ 1987 R3500 CREW CAB DUALLY (BIG RED)Acquired 06/12/2015 1990 chevy suburban V2500 5.7L My cluster Mods-Nov 2007 overhead console Stereo install Round 2 Aug 2009 Heated/turn signal mirror upgrade

09-08-2011, 03:10 AM	#20
thrtsxchambers Registered User Join Date: Nov 2009 Location: Salisbury, NC Posts: 316	Re: Hooking up two batteries together on my 73 c10????? I had the same problem on my truck. I used the isolator hook up and a 100amp alternater and have had no problems since. I have one battery running the truck and one runs my compressor, amp and switches. I also run optima yellow top batteries that can be recharged without damage to the battery. Posted via Mobile Device

09-12-2011, 04:37 AM	#23
cr34t3d_by_d4rkd3n Account Suspended Join Date: Jan 2011 Location: Allen, TX Posts: 188	Re: Hooking up two batteries together on my 73 c10????? Instead of running an iso and power lines and bein that far n the bush seemsnlike a risk. When wed go to the desert wed just bring a generator. And for the cost and reliability id do that every day vs isolator this and that. Just another system to fail. Posted via Mobile Device



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09-19-2011, 03:36 AM	#24
cr34t3d_by_d4rkd3n Account Suspended Join Date: Jan 2011 Location: Allen, TX Posts: 188	Re: Hooking up two batteries together on my 73 c10????? Ok excuse my languare i get irritated when one csnt see my point of view. Think of it this way... If 1 battery has 1 ground. 2 batterys should have ??...2 min. Ur method would work acceptably if it was an ac set up. But sense its dc the flow of power has to constantly work. Ur method increases reserve capicity. Think of it like hydrolics. Return line is always bigger than feed line. Posted via Mobile Device

09-19-2011, 05:53 AM	#25
cr34t3d_by_d4rkd3n Account Suspended Join Date: Jan 2011 Location: Allen, TX Posts: 188	Re: Hooking up two batteries together on my 73 c10????? the way u show is using the series principal. (flowing thru another) ur set up will work. but used to pull a winch on say a 4000lb 45 sec pull will turn that ground wire to a welding rod. the set up u show is best for a trunk sense grounding 2 batts at the sheet metal when theyre sitting right next to eachother is dumb. the more grounds u have the better. simple rule for every hot, the ground should b equil and 50 percent "fatter" . in an engine compartment running that long of a ground is highly dangerous. surrounding positives will wanna jump to it. its better to ground both batterys to sheet or frame and jump the positives. but remember. share the load. brancch off each batt equil or else all will try to feed off one cable.

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