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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2006Moving battery: cable sizing, grounding ?'s
I have a typical nose heavy compact wagon and want to move the battery from the engine compartment, to the rear of the vehicle. I have two tech details to work out. 1. What size of batt cable to use. I will be using tinned marine grade wire, from Ancor Marine. The run will be no more than 15' long. Engine size is 2.5 litre, alternator is 60 amp. 2. Whether to run the ground from the battery post all the way back to the front of the car, or just ground to the body somewhere near the battery, or ground to body near battery and also run a smaller gage ground all the way forward. In case anyone is concerned about fumes from the battery, I will be using a battery that has provisions for adding a vent hose. Jim >I have a typical nose heavy compact wagon and want to move the battery >from the engine compartment, to the rear of the vehicle. I have two [quoted text clipped - 13 lines] > >Jim Just for convenience, I would use welding cable. The kind used for Arc Welding. As for grounding, it depends on how your vehicle chassis is assembled. If you can get wires long enough to reach from the engine to the trunk area, read the resistance of those leads. Subtract that test wire reading in ohms from what you get when you measure from the engine block to a non-painted area in the trunk compartment. If you have less than 1 ohm, after subtracting the DVM test leads resistance, you're good to go with grounding to the chassis in the trunk area. On an unpainted area. IOW, scrape/file down to bare metal, tap, put lugs on the B minus, and bolt on the cable. STILL! ( this is important ) run the welding cable from the battery B minus up to the ENGINE BLOCK and bolt it on there, where it is probably already bolted on. You want two grounds. One back to the engine block One to the chassis of the vehicle, if you can get under 1 ohm chassis resistance from the front of the vehicle to the back. This is possible if you have a Unibody welded chassis like I do. On an older vehicle, you can put ground straps ( braided cable shielding ) between the front end and the passenger compartment, and another strap or straps between the passenger compartment and the rear of the vehicle. My ideas. Lg ... >As for grounding, it depends on how your vehicle chassis is assembled. >If you can get wires long enough to reach from the engine to the trunk >area, read the resistance of those leads. Subtract that test wire >reading in ohms from what you get when you measure from the engine >block to a non-painted area in the trunk compartment. If you have >less than 1 ohm, after subtracting the DVM test leads resistance, Unless you use a two dollar meter, the leads' resistance is going to be less than a half ohm. Most 3 1/2 digit DVMs only offer a 200.0 ohm range as the lowest, giving 0.1 ohm resolution. Unless you have a three hundred dollar meter with a 4 wire probe (no current through the sense wires), you're going to have a tough time measuring the resistance of a battery cable. Most of the resistance is going to be in corrosion at the connectors. IIRC, a battery cable is typically 4 or 6 gauge wire. 6 gauge wire is .47ohms/1000', 4 gauge is 0.24ohms/1000'. The existing + cable to starter is probably #6 and is 2' long. Since my new cable will be 7.5 times longer, to keep the voltage drop the same I will need to use cable that that 7.5 times less resistance per 1000 feet. That would be either 2/0 or 3/0 cable - massive stuff, heavy. So, I think I have to be prepared to accept some voltage drop, the only question is how much is acceptable. If I was to not run a full sized ground from the battery forward, I would still run a bigger wire from the body over to the engine block. Jim >... > [quoted text clipped - 17 lines] >gauge wire. 6 gauge wire is .47ohms/1000', 4 gauge is 0.24ohms/1000'. > > Unless you have a three hundred dollar meter with a 4 wire probe (no current > through the sense wires), you're going to have a tough time measuring the > resistance of a battery cable. Most of the resistance is going to be in > corrosion at the connectors. IIRC, a battery cable is typically 4 or 6 > gauge wire. 6 gauge wire is .47ohms/1000', 4 gauge is 0.24ohms/1000'. You could always cheat and pass a known current through the conductor, then use Ohm's Law to calculate resistance... but as long as your voltage drop wasn't too horrible it wouldn't matter what the resistance is, now would it?..<grin> Jim >> Unless you have a three hundred dollar meter with a 4 wire probe (no current >> through the sense wires), you're going to have a tough time measuring the >> resistance of a battery cable. Most of the resistance is going to be in >> corrosion at the connectors. IIRC, a battery cable is typically 4 or 6 >> gauge wire. 6 gauge wire is .47ohms/1000', 4 gauge is 0.24ohms/1000'. > You could always cheat and pass a known current through the conductor, >then use Ohm's Law to calculate resistance... but as long as your >voltage drop wasn't too horrible it wouldn't matter what the resistance >is, now would it?..<grin> If you're measuring voltage with a high impedance meter (even cheap meters are typically 10M), the voltage drop will be negligible. Get a 0.01 ohm shunt, and wire up the battery cable to a car battery and a nice load like a headlight. Measure the voltage drop at the 0.01 ohm shunt and you'll know the current through the entire circuit. Measure the voltage drop across the battery cable and you'll be able to easily calculate it's resistance. >... >>As for grounding, it depends on how your vehicle chassis is assembled. [quoted text clipped - 13 lines] >corrosion at the connectors. IIRC, a battery cable is typically 4 or 6 >gauge wire. 6 gauge wire is .47ohms/1000', 4 gauge is 0.24ohms/1000'. It depends on the vehicle. Is it bolted together or just spot welded like my Unibody. An older vehicle with a FRAME is likely to have more continuity problems than a Unibody like mine where everything is joined together as one piece. Still, think of I=E/R, or better yet, P=(E)(I) I move 260 AMPERES of current through my vehicle when I hit the starter motor. And use 66 amperes Plus, when I'm winter driving with the lights and heaters on. You don't need a lot of resistance to see that some places on the chassis can become *hot spots* in the literal sense if they produce resistance with that kind of current flowing through them. That is why I recommended welding cable for the main battery terminal connections. I wouldn't try this *experiment* with anything less. But it ain't my car, so... Lg >I have a typical nose heavy compact wagon and want to move the battery >from the engine compartment, to the rear of the vehicle. I have two >tech details to work out. >1. What size of batt cable to use. I will be using tinned marine grade >wire, from Ancor Marine. The run will be no more than 15' long. Engine >size is 2.5 litre, alternator is 60 amp. does the car going to have to start in cold weather? The starter is the #1 user of current from the battery and may draw a few hundred amps. Do a google search for "AWG resistance" to get an idea of what the resistance per foot is for various wire thicknesses (gauge). Find out the resistance of the current wiring; then calculate the wire gauge needed to get the same resistance for a longer run. Dunno about your battery, but my new auto battery weighs virtually nothing. And moving a battery to the back is percentage wise not much of a change in weight distribution. Or ... seems like a lot of trouble for nothing. Does remind me though, that my old VW bug had the battery under the rear seat ... but that was for practical reasons ... close to the alternator (in the rear) but not enough room in the engine compartment. > Dunno about your battery, but my new auto battery weighs virtually > nothing. And moving a battery to the back is percentage wise not much [quoted text clipped - 3 lines] > the alternator (in the rear) but not enough room in the engine > compartment. It's not nothing. 20 pounds from the front to the back is a 40 pound "net" weight change. On a 3000 pound car, that's 1% of the weight. Although, on a FWD wagon I don't see the point - for drag racing a FWD car, you WANT weight in the front. And, unless it's a Subaru wagon it's probably not being autoxed. But it's the OP's car, he can do what he want with it. He can mount the battery on the roof if he wants. :) Well, if I really really needed to remove ... and that's remove, not relocate ... 20 lbs from the front of my jeep, there are smarter "weighs" do it than relocate the frikken battery. Jim is answering some of the questions posted: I want to do it to shorten the braking distance. It is a brand new compact wagon of Japanese unibody design and US manufacture. The battery will be inside the rim of the spare tire, under the rear floor. Everything, including the rear lights, is grounded thru the body - no separate ground wires except for a heavy one from the engine block over to the battery minus terminal. The proposed positive cable will be under the car, with heavy duty PVC insulation and covered by the black nylon sheathing typically used for car wiring. Small sections subject to movement or slight abrasion will be inside lengths of rubber hose. I have done this before, and have one installation that is now 20 years old with no abrasion problems. However, that old vehicle does not have a computer so probably is not sensitive to the voltage spikes that occur when you shut off the starter motor. Thus, I am interested in dnoyeB's suggestion to run separate hot wires to the alternator, computer, and starter. Any idea what gage each should be? (Would like to keep the weight down.) Some posters have identified other cars that have the battery in the rear. Does anyone know if they run separate wires for alt, comp, starter, or take other precautions? Jim >Well, if I really really needed to remove ... and that's remove, not >relocate ... 20 lbs from the front of my jeep, there are smarter >"weighs" do it than relocate the frikken battery. > > > Jim is answering some of the questions posted: > [quoted text clipped - 24 lines] > > Jim Moving the battery isn't going to decrease braking distance. It may increase it though. Consider that 80% of the braking force of a vehicle in on the front end of the vehicle. Also putting the battery in the spare is a REALLY bad idea. If it leaks you can plan on the spare becoming useless. You also will be voiding the factory warrantee by moving the battery from the factory location. > Jim is answering some of the questions posted: > [quoted text clipped - 24 lines] > > Jim If you really want to shorten the braking distance then there are a couple of simple tricks that might provide some improvement. Moving a 20 pound battery from the front to the rear will have NO positive impact on braking distance, nor will it have any measurable (positive or negative) effect on handling. The tricks for shortening braking distance are as follows: 1. Make sure your braking system is in top condition, properly bled, using manuracturer pads and ABS if installed is fully functional. 2. Make sure you have the proper size tires on all 4 wheels, that they have good tread left and are inflated to the proper pressure. 3. Make sure your suspension system is in top condition as well with shocks and/or struts operating properly. Moving a battery from front to rear is a complex task that could be expensive and might result in an inoperable car and possibly damaged electronic components if you screw up the ground. Before starting this project I think you need to list both the real measurable benefits and compare them to the extensive list of costs and risks. > Jim is answering some of the questions posted: > > I want to do it to shorten the braking distance. Then this is a bad idea. Wiring has weight. Especially the gauge of wiring that will be required here. > It is a brand new compact wagon of Japanese unibody design and US > manufacture. The battery will be inside the rim of the spare tire, [quoted text clipped - 13 lines] > hot wires to the alternator, computer, and starter. Any idea what gage > each should be? (Would like to keep the weight down.) You can tell the gauge by looking at the gauge there now. That will tell you how to fuse it as well. I wouldnt worry about the engine controller. I suggested alternator and starter because these are heavy gauge wires and could reduce the size of your main Jumbo wire. You can probably get an electrical manual for yoru vehicle from HELM. and redesign the power distribution system to be rear battery. Overall this will increase your vehicle weight though. > Some posters have identified other cars that have the battery in the > rear. Does anyone know if they run separate wires for alt, comp, > starter, or take other precautions? > > Jim Alternator has to be seperate because its use is special. I doubt engine controller is seperate. Wouldnt make sense. Starter is always dedicated because its so big. Rear batter vehicles, and I believe Cadillac has one, will have rear Power Distribution centers so they can fuse the wires right away. (You can drop the gauge of a wire after you fuse it, reducing weight) Note that the cost of wiring tends to be associated with the operations performed on it and not the size of it. smaller wire primarily saves weight, and secondarily cost. However, rear battery vehicles will also bring the wiring into the passenger compartment right away. They wont go under body to the front and distribute from there. Typically you want your PDC right next to the battery. Some Toyotas even put it right onto the battery. >> Well, if I really really needed to remove ... and that's remove, not >> relocate ... 20 lbs from the front of my jeep, there are smarter >> "weighs" do it than relocate the frikken battery. >> >>  SignatureThank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 > I want to do it to shorten the braking distance. Moving the battery to the rear will not affect your braking distance in any measurable way. Want to shorten the braking distance? Find and install upgraded brake rotors and pads, and go to ultra-high-temperature, non-silicone-based DOT 5.1 brake fluid. > The proposed positive cable will be under the car, with heavy duty PVC > insulation and covered by the black nylon sheathing typically used for [quoted text clipped - 6 lines] > wires to the alternator, computer, and starter. Any idea what gage each > should be? (Would like to keep the weight down.) New battery bracket Battery ventillation provisions Bunch of extra heavy-gauge wire Insulation for bunch of extra heavy-gauge wire Weight savings? Negative. You've latched onto what I'm afraid is a pointless idea based on a misunderstanding of how things work. >> I want to do it to shorten the braking distance. > > Moving the battery to the rear will not affect your braking distance in > any measurable way. Want to shorten the braking distance? Find and > install upgraded brake rotors and pads, and go to > ultra-high-temperature, non-silicone-based DOT 5.1 brake fluid. and tires. Gotta get some sticky rubber on there. 6 piston calipers and 14" rotors won't make a lick of difference with $29.99 walmart allseason rubber on there. In fact, I'd start with rubber before spending two grand on upgraded brakes or moving around the battery. Unless of course the OP is the kind of guy who considers a 4" exhaust tip as a valid performance upgrade. Ray >> I want to do it to shorten the braking distance. > [quoted text clipped - 24 lines] > You've latched onto what I'm afraid is a pointless idea based on a > misunderstanding of how things work. Well, there *are* perfectly valid reasons for wanting to locate the battery in the trunk... off the top of my head, better F/R weight distribution and/or the desire to reduce polar moment of inertia, and/or desire to locate as much mass as possible over the rear axle. None of which would really be perceptible in a "compact Japanese station wagon" without having the automotive sensitivity of a F1 driver, and each desire would likely dictate a different eventual location, anyhow. nate Signaturereplace "fly" with "com" to reply. http://home.comcast.net/~njnagel I appreciate all the suggestions, and the criticisms too, although still have no concrete suggestions as to cable sizes. So I will look at sizes used on rear battery cars. I will be doing test of high speed braking distance before and after conversion. The following four tests are planned, in the sequence noted: 1. No modifications. 2. Battery in rear. 3. Battery back in front, cables to rear still present. 4. Battery in rear. All testing will use a driver who will not know the position of the battery. By the way, I wasn't gonna mention it, but since someone brought up changes in polar moment, the battery relocation to rear will actually increase the polar moment, a change that is desirable unless you have the fast reactions of an F1 driver. Jim >>> I want to do it to shorten the braking distance. >> [quoted text clipped - 35 lines] > > nate > I appreciate all the suggestions, and the criticisms too, although still > have no concrete suggestions as to cable sizes. So I will look at sizes > used on rear battery cars. Maintain current cable sizes. The only size you really need to change is the starter and perhaps alternator. You probably cant use 2 wires for the starter so 1 jumbo is your only real choice. If you tell gauge and length of current wire I can give new gauge if you also give new length. If you enjoy this type of thing, best solution is as I suggested. Move not just the battery but the power distribution center as well. Thats the thing that holds several maxi-fuses and relays. Its a total redesign of power distribution. Its a really big job. Im with the guys who suggest getting wider tires. If this is front wheel drive I have to wonder about removing weigt from over the driving tires. > I will be doing test of high speed braking distance before and after > conversion. The following four tests are planned, in the sequence noted: [quoted text clipped - 12 lines] > > Jim SignatureThank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 > I appreciate all the suggestions, and the criticisms too, although still > have no concrete suggestions as to cable sizes. So I will look at sizes [quoted text clipped - 17 lines] > > Jim You must be my brother. Surely there can't be TWO of you. Common sense says that any difference will be negligible and not likely to be measureable without some very expensive precision test equipment, and that further, any difference (if you *could* measure it) would be offset by the additional weight of the extra wiring. If you were able to accurately measure any changes, considering the weights involved you would also have to adjust your testing to account for whether or not the driver has taken a crap before the tests. If you really want to waste that much time and money you can pay me to let you come over and cut my grass and rake my yard. At least then you'll get some results you can actually see. > I appreciate all the suggestions, and the criticisms too, although still > have no concrete suggestions as to cable sizes. So I will look at sizes [quoted text clipped - 15 lines] > increase the polar moment, a change that is desirable unless you have > the fast reactions of an F1 driver. Have you considered what this will do to the warranty on your car? > my old VW bug had the battery under the rear seat ... but that was for > practical reasons Yeah, it was because those pathetic excuses for cars had no heater to speak of, so if it was really cold, you just got someone heavy to sit in the back seat, pushing the seat springs into contact with the battery terminals. Presto: Instant heat, lots of it, and regardless of engine temperature. > I have a typical nose heavy compact wagon and want to move the battery > from the engine compartment, to the rear of the vehicle. I have two > tech details to work out. I have used www.custombatterycables.com Vince has a pretty nice site, and you may find a lot of information there. By the way, his cables were spectacular, and have the price of OEM: > I have used > www.custombatterycables.com > > Vince has a pretty nice site, and you may find a lot of information there. > > By the way, his cables were spectacular, and have the price of OEM: Merde! I hate these autocorrecting routines. They were HALF the price of an OEM. >I have a typical nose heavy compact wagon and want to move the battery >from the engine compartment, to the rear of the vehicle. I have two [quoted text clipped - 13 lines] > > Jim CAUTION. It will work with big cables BUT be very careful of the installation/ INSULATION of the hot wire. It will be big and run a long way. If it ever wears thru the insulation the car will be gone before you smell the smoke. Because the starter draws large currents there is no practical fuse and a short of the hot wire to chassis will be deadly. When the battery is near the starter the unfused wire is very short and mostly in the air between the battery and the starter solenoid. SignatureJohn G Wot's Your Real Problem? > I have a typical nose heavy compact wagon and want to move the battery > from the engine compartment, to the rear of the vehicle. I have two [quoted text clipped - 3 lines] > wire, from Ancor Marine. The run will be no more than 15' long. Engine > size is 2.5 litre, alternator is 60 amp. Your best bet will be running multiple wires. 1 to the starter, 1 to the fuse panel under the hood. depends on the year of your vehicle. Also you will need a good strong wire to the alternator since that is very very important for charging your battery without loosing volts and efficiency. Of course if you ave room, run the 1-2 jumbo wires back to the former battery location, thats simplest. The best is to move the whole fuse panel when you move the battery, but I don't know the car to say how well/easy this will be. Without fusing you increase the fire hazard of your vehicle. Will you run these wires under the carpet in the car? I don't think there is any inobtrusive place to run these wires. > 2. Whether to run the ground from the battery post all the way back to > the front of the car, or just ground to the body somewhere near the > battery, or ground to body near battery and also run a smaller gage > ground all the way forward. Didnt tell us the car type or if its truck, unibody, etc. I would say look at the rear lights and see if they are grounded to frame or to wire runnign back to front of car. I really need to know the grounding strategy of the car to give a solid easy answer. without that i have to recommend treating the ground just like the B+ sans the fuses. Do not ground the battery at the rear. It could work or you could end up with ground loop issues in different systems. Safest way is to stick to existing architecture and simply compensate for voltage drop by using multiple wires. > In case anyone is concerned about fumes from the battery, I will be > using a battery that has provisions for adding a vent hose. Battery should not be in passenger compartment. Maybe you want to give more reasons why your doing this and we can give more options. > Jim SignatureThank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 > I have a typical nose heavy compact wagon and want to move the battery > from the engine compartment, to the rear of the vehicle. I have two [quoted text clipped - 11 lines] > In case anyone is concerned about fumes from the battery, I will be > using a battery that has provisions for adding a vent hose. In addition to the details of involving venting of hydrogen gas to the outside have you provided for some sort of drain? > > I have a typical nose heavy compact wagon and want to move the battery > > from the engine compartment, to the rear of the vehicle. I have two [quoted text clipped - 14 lines] > In addition to the details of involving venting of hydrogen gas to the > outside have you provided for some sort of drain? Most battery boxes designed for trunk mounting provide a vent hose intended to be routed through the trunk floor. There is no drain as the idea is that the plastic or stainless box will contain any inadvertant acid spills. nate > > > I have a typical nose heavy compact wagon and want to move the battery > > > from the engine compartment, to the rear of the vehicle. I have two [quoted text clipped - 21 lines] > > nate Good point. Which makes me think of another point. He is putting this in the rear of a wagon. I wonder where he is putting it.....hopefully not on the storgage compartment floor. I'm not sure I understand what the problem is that will be solved by this exercise. |
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