My truck currently has one battery but is set up for two (extra battery tray and cables). I am thinking of adding a second battery. I'm assuming that the benefit will be more power for cold weather starting and accessory use, and the downside will be cost and extra weight.

Anyone have any experience with a two battery setup? Is there any problem with hooking up a new battery in with an older one?

 

Hello, Yu have'nt stated what year truck you have but i thought I would reply to this anyway, I have a 1979 F-150 ranger 4x4 with a 5.8 liter (351) Modified engine. It has a Dual battery set up also. It was like when I purchased it and I had no clue about how the deal was hooked up. What i discovered though is that the second battery does nothing for engine start ups, It wont play the raidio and hooks up to abslutly nothing except the alternator and the Ligtine and brakeing system for the Outlet on the rear Bumper. Funny hu? LOl I thought it would help out too!

I am not sure if My truck was made this way or it was installed after it left the factory, It appears factory though.

I have two batteries in my truck at this time but It does nothing but add a little wieght on my drivers side. Unless i have a trailer that uses a Big Plug, and I assume electric brakes.

I imagine if you wanted you could route the power however you like or put a switch ( Heavy Duty) in the cab that would let you switch your power from one battery to the other.

 

I have no personal experience with dual battery setups, but I've heard that you shouldn't add a new battery to a system with an old battery. You should always replace them together. If someone else can't help out more than I with your post, may I suggest you try posting this question to the diesel list.
Jason Kendrick
1970 F100 Custom 390/C6
1978 F150 Ranger Lariat Supercab 460/C6

 

Unless your two batteries are directly connected together by cables, then there is no need to change both batteries if one is dead. An example of this would be two 12V batteries wired in parallel (both positives conected together and to starter; both negatives hooked together and to ground) similar to a jump start hookup, to give more amperage & amp-hours. Another method would be in series such as two 6V batteries to give 12V total (positive on battery 1 connected to negative on battery 2; negative on battery 1 to ground; positive on battery 2 to starter) similar to a flashlight, portable radio, or most anything using multiple batteries to give more voltage.

In the typical dual battery setup; hovever, the only common is the ground and what is called a battery isolator. The isolator is a box with cooling fins and three terminals, two output and one input. The input is connected to the alternator, and each output goes to the positive of a battery. The isolator's job is to direct power from the alternator to the batteries without having the batteries positive side connected together. It has been my experience that only the weaker battery will get the charge until it is equal to the charge of the stronger battery. The isolater will switch back and fourth between batteries until both batteries are fully charged. I can tell in my truck when the auxilary battery is charging because the vehicle's accessories will drop slightly in voltage (lights dim, wipers slow down, ect.) My isolator is at least 15 years old, so there may be some better ones on the market that don't act this way.

Having dual batteries is great for RV's because you can run all of your lights in the camper off of the auxilary battery without worrying about draining the main vehicle battery to the point of not being able to start the engine. Also great for revovery winches in off-road vehicles.

 

my 68 f250 had the same setup, and i'm wondering if you have a camper special.

i'm no expert, but i came to the conclusion that the second battery was feeding the camper hookups - ie the wires went under the cab to the trailer plug in on the rear bumper - and not the main electrical path. I also had the idea that both charge and use were controlled by some switches in the cab. But there were a few cut wires, notably two large guage lines coming through the firewall dead center over the tranny hub in the cab. I eventually removed the related wires to clean-up the engine compartment and simplify my limited abilities to troubleshoot electical.