I recently bought a brand new battery for my 78 F150. It is a dual battery system. Once I hooked up the radio, the battery has been draining and dying making the truck not able to start. I have the 12 V constant from the radio hooked up to the positive terminal of the battery, this is how it Said to do it on the manual does anybody know why this is happening and what I can do to fix it?

 

Aftermarket radio or factory?
Main radio 12v power should be keyed.
If aftermarket the keep alive memory should be a constant 12v.

 

And if you are not already doing so, disconnect the battery when not in use until you find the problem.
You don’t want to ruin a perfectly good new battery by draining it too often.

Which wire are you connecting to the battery? Are they still using yellow as the color for the keep alive, memory power?

 

im connecting the yellow wire from the aftermarket radio to the 12v constant that goes straight to the battery.

 

Where did you connect the 12v keyed power? Original harness, cigar lighter, or elsewhere?

 

it’s going into the original harness

 

Ive heard stories of this b4. they say Sometimes a dual-battery system is THE DRAIN ITSELF. meaNING a 12.7 v new battery will try to equalize with a 11.9 v battery and drain itself doin so.
SO the system should have a diode in there somewhere to block this effect.
-regardless of why, do like suggested and get a $5 neg terminal twist off disconnect to save you ruining a new battery unless/until you find the real problem

 

Cigar lighter is constant a on.

 

12v keep alive uses current too.

Do you need two batteries? One will be fine.

 

To see if it is the radio or something else I would disconnect the radio, both sides and see what happens.

I was also thinking the other battery pulling down the good one.
Had that happen on my 81 F100 when hooked to a trailer that has a battery on board for the electric jack.
Battery would only charge to about 75% after being charged over night.
Pulled the trailer to a show and left it hooked up to truck and 4 hours later could not start truck, dead battery.
I have replaced the battery and added a disconnect out front and disconnect the trailer when at shows just to make sure I dont end up with a dead battery again.
Dave ----

 

I used to run two batteries when we were camping, but I only had one in the start circuit with big fat wires, etc. The other was on the driver side apron, and only had smaller (16 ga.) wires run to bed to power lights, a fan, or small TV. To recharge the extra battery, had a small (16 ga.) wire running from the positive post to positive post with a toggle switch and light bulb in the line. The light bulb limited charging current amps as well as told me when was connected for charging ... the toggle switch was so I could just easily connect to charge as we rode along, or flip off at campsite to save my main battery. Both batteries were grounded and secured like stock.

 

'
I added a second battery parrilla to my 1972 F-250 Ranger XLT Camper Special.
The voltage source remained the same as if there was only one battery, I just doubled the capacity.
Batteries should really be the same brand, CCA rating and age or you could shorten the life of both of them.
I have no idea how yours batteries are connected or what they connect to.
You might disconnect the second battery to eliminate the possibility of one draining the other like ck1984 mentioned.
Or completely disconnect the radio for a few days like FuzzFace2 suggested.

Are you checking key on and off voltage with a test light or volt meter?
These trucks are old and I would definitely verify voltage rather than just trusting to color of the wires.

 

What is the reason for two batteries??
Just curious.

 

Even today I plan dual battery setups in all my pickups. I don't always do it, and I don't always need them. But I've used them often enough in the last fifty years (Broncos and pickups) that my mind is still in that mode.
I've used isolators, relays, and specialty relays with switches to jump start the main battery with the aux battery. All with great results.

So, needed or not, how do you have yours connected lucashoopes?
Do what the others have said. It's a natural start to the diagnosis process. Whether your batteries are isolated or not, disconnect the aux battery and see if the problem persists.
Since you say it started with the new radio, and maybe you already had your dual battery setup(?), then certainly disconnect the radio to see if the problem persists.

Check your alternator as well. Does it stay warm long after the engine has stopped running? Does it output about 14.5v or so when running? Still an externally regulated stock-ish alternator, or something other than stock?
If external, you can disconnect the regulator and see if the drain is still that.
Lots of things can drain a battery. Including itself!

Paul

 

On my '72 I was going to install a CRS that was key on switched but then decided to just connect the 2 batteries together.
This if for battery power to my travel trailer on my 1976.



The original plan for my '76 was to add a second battery but when I couldn't find the right battery tray for the drivers side I opted to add an ammo can to carry a tow chain and a recovery strap.