Where i spliced the wires out of,

The wires im planing to connect to this battery read 12.26, so it will charge. But yeah does it need a regulator or anything?
I spliced some wires, i think they were for a 12v (cigarette lighter) and extended them into my bed, (ether way)multimeter reads 12+v
now I want to connect it to a deep cycle,
Do I need a one way diode ?
I'm going to put a 10 amp fuze on it. Is that correct to do?
Anything I should watch out for?
I don't want to fry my car!

 

You’ll want to put some sort of DC to DC battery charger between that battery and the power source you are using from the truck.

 

Yup in the bed,
I'm going to run a water pump and a small 12v fridge off of that deep cycle,
Maybe a small inverter, but that is later on,

 

You need Dual Battery Isolator:

 

To determine fuse size, you’ll need to determine the wire gauge and then google how much current it can carry. You’ll have to guess at the length of the truck’s wires, since the ratings are given based on length. I wouldn’t go any shorter than double the length from where you connect to them to where the truck battery is located.
Note, give yourself a wide margin of error and safety.

 

The truck already has an isolator, once the main power runs low it shuts off the internal 12v outlets, that is why I'm going that rout.
But still good point, wouldn't want to get stuck in the middle of nowhere,

 

Any way to measure that using a voltmeter, it runs through the car for an unknown length and connects to unknown circuitry. (I think it is just a 12v plug or "cigarette lighter" but it is a 2018 f150 stx I don't know what it was going on as far as computers)

 

No offense, but you need to stop and take a deep breath. You've done a ****-poor job of splicing into wires unknown. You've left the other end of the wire open to the weather... and what circuit did you cut in the first place?

A diode is one-way by definition.

What are you planning to fuse at 10A? Fuses protect wiring and should be positioned as close to the supply as possible... not to the destination.

Finally, you aren't going to run a fridge and a water pump for long off that little battery.

I would recommend you stop where you are, define your requirements, and solicit the help of someone who's been down this path before. You have all the makings of a vehicle fire on your hands...

 

I was going to type out pretty much this entire thing as well.

 

I was going to put it slightly differently.

Start by making a chart of your draw demands. IE - lights will be 12V and 1.2 amps (just an example don't use these numbers) and the fridge is 12V but 6 amps. SO when you run on battery alone you will need 12V and at least 8 amps over the period of time.

Now that time - say it's overnight so 8 hours. Now you are going to pull 8 amps over 8 hours so you will burn though 64 AH of battery capacity. Now this is a rough order estimate doesn't include line losses, temp losses etc.

At that point you have your big battery. Now charging it. in rough order that same you can work backwards to figure out how many amps you need for a time.

SO 64 AH battery, and you want it changed back up after say a 2 hour drive. would mean you need to provide it roughly 32 amps to charge. So one you need a bigger wire - a stronger source - and you'll need more voltage. Typically you want to charge a battery at least 1 volt more than it's base potential. IE a 12 Volt battery won't really charge unless you feed it at least 13 volts or more. Then there is more volts you feed it the faster and deeper it can charge but there is a limit also. SO the battery maker will have this lined out in a manual so you can see what the battery will tolerate.

This is a starting point - and from there you can see you'll need a fuse of __ size, and you want wires of __ size. it needs to hook to wires of equal or bigger size. Look up standard automotive wiring practices there is SAE handbook, and some other documents out there that are good references. Or as mentioned above look at what some others have done.

Good Luck and post updates.