My 76 F-150 long-wheelbase truck has a regular cab. All the wiring has been removed in anticipation of a full replacement and some new components. This post is about one aspect of that project - relocating the battery.

There are a number of good reasons to relocate the battery away from its stock location. The primary motivation here is to make more space in the engine bay and improve the organization of wiring, with resulting improvement in appearance and serviceability down the road.

I have been scouring the internet for text and video to guide me. For example, this article from Speedway Motors entitled “Battery Relocation to Trunk or Other Area of Your Vehicle” is very detailed and seems to reflect today’s best practice.



So here is what I did. Installed a stainless steel drop-down battery box and wired it accordingly. Here it is in the raised position and …



… here it is in the lowered position.



Bulkhead connectors bring the wiring inside the frame, where they will be safer. Circuit breakers protect the new wires against overcurrent and electrical shorts.

That’s the conclusion. The rest of this post explains and illustrates chronologically how and why I did this.



There are still many questions that I need to address, but this post is my starting point. No doubt I will adjust course along the way. First off, I should report that I lucked out and got a good deal on this fancy stainless steel drop-down remote battery box some time ago. I have it mocked up attached to the frame using an existing hole and a C-clamp on the passenger side underneath the bed. This seems to be a popular spot for relocating the battery on trucks. It has two positions, down as shown here and …



… raised up, which would be the operating position. Lowering it enables servicing and easy replacement so the bed is not an obstacle. This part of the frame is flat on the top and pretty close to level. Not a bad location, it seems. A spring-loaded hook on each side of the box secures both the raised and lowered position. That will require placing the AWS #1 battery cables in such a way that it can be lowered but not create a problem when raised to the default operating position.



Fortunately, the bed is off, so I have good access to the flat area of the frame behind the passenger side of the cab and in front of the rear wheels, where this battery box can be placed. These two chalk marks define the best options for locating the drop-down battery box. The inside width of the box is only 10.5”, so that will make finding a properly sized battery be one of my first decisions.



Working with the mounting bracket separately, this position aligns with the bottom of the frame rail and is the lowest possible. I plan to bring this up closer to the top of the yellow tape to keep it all on a flat surface. The six mounting holes will be 7/16” or slightly less and will accommodate 3/8” button-head stainless steel bolts, washers, and nuts. This takes care of the vertical.



Looking at my other dent-side truck, an F-100, I can see how things look when the bed is still attached. I think the halfway point between the bed rails and between those two large holes on the frame will be a good location for the battery box.



Those same large holes are found on this part of the F-150 frame. I moved the mounting plate up 1/4” from the mocked-up position, centered it, and then used it as a template. A sharp scribe was used to mark the correct position of the 7/16” mounting plate holes for the 3/8” stainless steel button-head bolts. This takes care of the horizontal.



Here is the mounting plate all buttoned up.



The complete battery box in the down position and …



… here in the up position where it will spend most of its lifetime. The difference between positions is 4-1/2”.



This is an old battery I had hanging around. I had to trim the ledges on the sides to get it to fit. The inside dimensions of this battery box are 10.5” wide by 7” deep. A Group 24 battery such as the one that was originally in the engine bay will not fit. Thus it will be important to make sure that no part of the battery that I buy for this location exceeds those dimensions.



Lowered with the mock up battery in place we see the AWG 1 cables that come with the Painless battery relocation kit purchased a long time ago. Clearly this is going to be a bit of a challenge with these cables having to adjust to both up and down positions. Perhaps I should be looking at batteries with side terminals.



I managed to find a pair of terminals that will convert a top post battery to a side post model. The Painless kit included two lead posts that I mocked up here. It seems rather clunky and redundant to me. There has to be a better way.



I may not use this end of the Painless wires.



I created these two 15/16” holes to bring power inside the frame rails where they will be safer en route to the starter and other electrical components in the engine bay.



These two bulkhead fittings came with the Painless kit. Note that I’ve mocked up four 1 X 3/8 wire lugs. Two on this bulkhead and two on the side mount terminals on the battery. That will enable a neater-looking install.



Here is what that looks like on the inside of the frame. It does seem unnecessary to insulate the wire from the negative terminal, which will be grounded to the frame and engine block as well as other places. In all of my reading about this, grounding is emphasized as crucially important. Contact with a substantial area of bare metal is key.



Positive and negative battery cables were assembled with a hydraulic crimper and sealed with heat shrink. This is a bit cleaner than the double-lead connectors shown previously. The AWG 1 wires supplied by Painless are a bit stiff, whereas the same gauge welding cable is more flexible. Thus, I may modify this arrangement somewhat in the near future.



I finally got a new GSM battery that fits nicely. The flat stainless steel battery hold-down strap had to be modified slightly in order to work. Note that it is now bent upward at the far end so that it grips the back of the battery box.



Unhappy with the stiffness of the AWG 1 wire supplied by Painless, I purchased 5’ each of black and red AWG 1 battery cable. It is much more flexible and will work better for me in going from the battery to the frame bulkhead. The stiffer AWG 1 wire will be fine inside the frame rails for long straight runs from the battery bulkhead to the engine bay power distribution blocks.



I was also dissatisfied with the flimsy #10-24 retaining bolts so upped those specs with 1/4-20 X 3/4” cap screws and stainless lock nuts. This does raise the risk of a short due to something bridging the nut with the main stud but this may be minimized with the application of ATV silicone sealer and silicone boots.



The first leg of the ground circuit is done. I used an existing hole further back on the frame to establish the first ground connection From here it will go straight to the engine bay.



This is the first ground which is not far away from where the bulkhead connection enters this side of the frame. I will spray a little satin black on a bolt and washer covering it to create …



… a neat but minimal area for this connection.



A little dielectric grease along with this silicone boot will forestall corrosion.



The AWG 1 wires from battery to bulkhead have been redone with the new, more flexible welding wire. This is the raised position where the battery will spend 99% of its life.



There is no contact between the battery cables and the battery box at this time.



Silicone Stud Terminal Cover Protector Boots complete the outside the frame wiring. Next up is figuring out how to complete the wiring on the other side of the frame in ways that don’t promote fire and electrical dysfunction. Grounding first and then the hard part – the hot wires.



The battery positive will be split into two wires using this distribution block, which is fed by the bulkhead connection shown. One AWG 1 wire will be dedicated to the starter and alternator circuits. The other AWG 4 wire will handle everything else. Note that the stainless steel locknuts have been replaced with 1/4-20 nylon acorn nuts, which eliminate the possibility of the short circuit envisioned earlier.



Heat shrink and a nylon boot insulate this connection. Each of the positive wires exiting this distribution block going toward the engine bay will have a resettable circuit breaker instead of a fuse. In addition to being resettable they can also be used to disconnect the battery.



Unfortunately, I painted myself into a corner with the battery box being flush mounted to the outside of the frame right where those circuit breakers need to be. They need to be as close to the battery as practicable.



The solution that I came up with was to use the six bolts holding the battery box to take on the additional task of supporting a mounting plate for the circuit breakers. Here I am mocking up the solution with longer 3/8” bolts.



This creates amounting surface that provides enough space for bolts to go through to lock nuts on the other side.



I rounded the corners and applied primer to the surface.



I will only use two of these circuit breakers; probably 200 on the AWG 1 wire and 100 on the AWG 4 wire. This is close to the ampacity of those two wires, but I will be able to move to higher amperage if need be.

NOTES: Ampacity (pere capacity) is the maximum electrical current, in amperes, a conductor (like a wire) can carry continuously without overheating or damaging its insulation, determined by factors like wire material, size, insulation type, and installation conditions, and is crucial for electrical safety, preventing fires and system failures.



Voilà! They mount nicely without interfering with the battery box.



The circuit breakers do not use lug-type connectors, so ferrules need to be used instead, and that requires a different kind of crimper as shown.



This completes this part of the battery relocation. Of course, there will be more to do in the engine compartment, but there are a number of other things that will have to be done before that. Thus, the last step will be to prettify the work before moving on.



There are about 15 holes in the frame that are not needed, so I get the opportunity to use a copper backing plate to weld up the holes with a MIG welder. This is a first for me.



These copper plates have little magnets in them to remain in place. The weld will not stick to copper.



The top and bottom holes were easy.



Side holes were a little harder due to gravity working against me. Several short passes are better than trying to fill a hole in one pass, as can be done on the top and bottom.



An angle grinder and a surface conditioner were used to bring the metal to this stage. It’s ready for a thin coat of body filler, sanding, primer, and paint.



Cheese grating.



Dry sanding (80 grit)



Primer, wet sanding (240 grit)



Satin black, two coats



Reassembly. First, the battery box mount with all new stainless hardware. The bolts are 3/8 X1.25 inches (replacing the 3/4” bolts used initially) with flat washers on the battery side and both flat and lock washers on the inside of the frame rail, followed by a 3/8” nut. The height of that nut and those washers raise the circuit breaker panel enough to accommodate the hardware that secures the circuit breakers.



Painted a granite color, the raised up circuit breaker panel gets an additional set of washers and nuts to secure itself. The bulkhead connectors and positive side distribution block are now in place.



The ground cables connected to the bulkhead go in two directions. One goes rearward to be connected to bare metal on the frame. The other goes forward toward the engine bay. The two positive cables also go forward toward the engine bay. I will stop at this point to work on other aspects of this project; things that will make completing this wiring easier and more effective.



The rear frame ground. Stainless bolt and washer on bare frame steel. Dielectric grease is supposed to ward off rust and corrosion.



On the outside of the frame, all of the wires and the battery have been put back in place. This is the lowered position.



This is the raised position. Mission accomplished.

 

Excellent job. Write up too. If I ever build a street pickup, that'll be where I install the battery.

 

Nice work.
Have you started/driven it? No sure where you are in your build.
Long crank will pop a 200 A breaker.
Had a 250 A on mine. Ended up moving battery back under hood.
Fords with battery in trunk are not fused to starter.

 

@smoky_diesel Given a host of changes initiated by or thrust upon me, the truck is quite some distance from running. I did spend a lot of time researching both circuit breakers and fuses. What I learned is that, depending upon the design of the device, it will suffer an overvoltage condition for more or less time than other brands in its class. For example, a Megafuse is labeled "slow burn" to indicate that it is more persistent than most fuses in its amp range. Ditto for circuit breakers.

I went with circuit breakers because they can be reset instead of replacing a blown fuse. Lazy. Note that I have two circuits, one for the starter and another for everything else. Thus, I could trip the starter circuit breaker, and the truck would continue to run if it was already running and if the battery was reasonably healthy.

You are correct, of course, since a high-torque starter can draw 250-300 amps. Grinding away on the starter will overcome any fuse or circuit breaker eventually. The blown fuse or tripped circuit breaker is telling us something that we really need to pay attention to. They only protect the wire.

There is another aspect of this that I didn't get into called TVS for Transient Voltage Spike. The most troublesome source of TVS on sensitive electronics (ECU, digital instruments, digital audio, etc.) occurs when the battery is disconnected while the engine is running. The alternator, which is still spinning, produces voltage that has nowhere to go. Trying to find an alternate route to earth can endanger some sensitive electronics. As well, battery interruption will do a frontal lobotomy on anything that is remembering data, like modern fuel injection systems. Note that this is not the same worry that racers fret about and resolve with a proper battery cut-off switch at the rear in case of a crash and burn. The recommended 4-pole shutoff for that problem cuts the alternator off as well as the battery.

 

Looks amazing, clean and professional.

 

Awesome write up, and great detail.

Im running a diode kit that was from Painless, but wasn’t able to find the part just now. The description of the product indicated it’s supposed to act as overvoltage protection from the alternator to the rest of the system. It’s connected across the battery posts

This obviously wouldn’t protect from a complete battery shutoff of both breakers, but would help if the starter breaker trips while engine is running and the starter was drawing power.

I wonder if one solution would be to run a relay that interlocks with your ignition circuit to shut the truck off if both breakers let go? I can tell you from experience that running an interlock that kills power for “safety” will find a way to trigger at the worst possible time (like pulling out into traffic), so beware.

I’ll pop the hood today and see if I can find better info.
I want to say it’s just a Zener diode probably designed to allow voltage to pass to ground at 16 volts(?), but my low volt controls part of the brain is not fully awake yet.

Maybe wrap some 3/4” hose around the battery cables for rub protection off the battery tray? Or tie wrap moment?

 

It’s got 5KP15A and 1631 written on it.

 

@motorsickle1130 Yes, diodes are the usual remedy. I'm not so sure about the efficacy of the Painless solution. I will need to take another look at it. Your understanding of diode function is correct. They act like a one-way valve in hydraulics. A good example is the starter solenoid that most of our trucks have mounted on the inner fender. It's an electromagnet that connects a circuit and then disconnects under spring tension. The collapse of that magnetic field causes a TVS that is easily contained with a small, standard diode going from the terminal that gets hot on start and then to ground. The folks who build aircraft for a hobby are pretty keen on this. The alternator is a bit of a different story. Many diodes are not fast-acting enough. We're talking nanoseconds here. Balmar has a product that claims to be fast enough to deal with a runaway alternator. Zenner diodes are a special case: whereas standard diodes are one-way, Zenner diodes have a directional bias. Finally, it should be noted that some devices claim to have internal circuitry that protects against TVS. MSD, for example, makes this claim, but you'll find folks with experiences that contradict that claim. Of course, we're splitting hairs here since involuntary battery disconnection while the engine is running is a rather rare event. BTW, in my correspondence with Painless, I found them to be rather myopically focused on deliberate battery disconnection under racing conditions. I was not sure they understood and cared about other scenarios. They wanted to sell me this. I deferred for the time being. Is this what you sourced?

 

Searching the painless site for these two numbers turned up nothing but I did find Alternator Backfeed Diode Kit Part No 30720 which is for GM multi-wire alternators. It addresses a GM problem where the engine continues to run after switching ignition key off. I have a one wire alternator so access to pin #1 of a voltage regulator is not possible for me.

 

That’s not what I installed; I’m running a big alternator and starter from Powermaster, with a master fuse between the alternator and original style ford starter relay (that’s now just another control relay for the fuel pump and ignition). The powermaster starter has its own built in relay and requires constant power to retract the gear (don’t love that). Installing a 5 second delay relay is on the to do list.

That relay looks like a decent idea, though pretty spendy, but also a vampire load and dead battery waiting to happen. I suspect there’s a simple solution, might just take a while to come up with a design that’ll work with the materials available for a reasonable cost.


I also was only able to locate the GM 6amp rated solution and not what I have installed.

I had bought my wiring and associated several years ago, and wasn’t entirely sold on the diode I installed, but it seemed like cheap insurance. Also if the diode shorts, and turns into an arc welder, the leads on each end are quite small and would become fusible links in short order.

 

Nothing wrong with breakers including auto or manual reset type. I use them over fuses for large currents (>30A).

One downside with a fuse or breaker on a heavy load and a long wire is voltage drop. The motor get less voltage, and uses more than rated current because the rotor is not moving as fast as it should. Also starts from locked rotor current vs running.

TVS are usually only effective right at the point something needs to be protected. So they are everywhere in automotive electronics, at the connectors. A vehicle has hundreds them. How do you plan to add one? The amount of energy in a alternator disconnect or starter load dump is very large. 100x a leaded TVS.

I don't see a diode across the battery doing anything either. The impedance of the battery is far below the diode even in forward conduction. To be effective the diodes have to go to everything else, same as TVS.

Hope this helps.

 

If you want to protect the alternator disconnect scenario, you can get a shut off switch with a 2nd pole, and connect in series with the alternator field wire. The 2nd pole should open first and begin to unload the alternator before the battery impedance suddenly goes away and voltage sipkes.

Or move battery underhood. That is what I did specifically for NHRA rules. Smaller, lighter battery, short cable, no fuse, no switch contact. Cranks faster.