I have a 2003 ford e350 7.3 diesel. 850k+ miles on it. Over the winter I parked it and would start once a week...at first it was fine, but after a couple times, the battery would drain. The battery was bought in 2021, so last month I ended up taking the battery to get warranty replacement. Ended up getting a interstate battery. After installing the battery, the van would drain the battery overnight. Tested alternator and it charges good and amp draws are good.
I traced the ground to going to the starter and the plastic coating peeled off and you can see the wires, it was also full of dirt and oil from it motor leaking. I was going to replace the ground, but then noticed that I have a second battery mounted on the passenger side below the rear door. I did not know about it. I've only had this van since last may.
now my questioquestiif the secondary battery is shot, would that cause my starting battery to drain? Is there any way to only use the starting battery and disconnect the second battery all together.i use this van to haul my food truck and honestly don't know when it'll go down, it's got over 850k miles on it. But she still fires up and hauls great! Only issue is the battery draining overnight now...

 

You said you start it once a week in winter, do you mean stop-n-go short trip (how many miles combined?) or a long trip like 100miles or just idling?
When you don't drive long enough, your battery really isn't getting charged. It's better off to buy a noco/ctek charger (make sure with 2 banks like noco genpro10x2) and slow charge them perhaps once or twice a month, don't need to hook them up 24/7 as a trickle charger.
If you park your van on the street all the time and no wall outlet of course, then you need to go for solar chargers, they are so cheap now no reason not to buy unless you live up north with very cold weather all the time. Get around 20W-30W per charger (the ones below 20W are useless), make sure it comes with regulator, if you can hook a single solar charger to both batteries that's fine but I recommend one charger per battery.
To go along with that you also need a battery monitor (actually x2) like this:
As far as your question regarding 2nd battery draining your starting battery, both batteries actually are starting battery.
They are linked only in RUN/START position so that you have dual battery power to start the car. Then the 2nd battery can also do other things the main battery can't or not appropriate to.
Your alternator will charge them both when your car is running (RUN position). All other positions they are isolated so they won't be draining each others.

If your 2nd battery is dead, buy another one preferably the exact same one as your primary battery. In the meantime go to battery junction box (BJB 14A003, pdf attached below) and pull out F1.10 (60A) to unlink them entirely instead of disconnecting 86 (trigger from ignition, hot in run), 30 (always hot) and 87 (always hot if 2nd battery is not dead) one by one because they are always hot you don't want them to touch something and short.

Your very last line saying "Only issue is the battery draining overnight now..." which is not so true when the current state of charge is already too low to begin with. Get the battery monitor on Amazon you can check the realtime voltage easily. All batteries will drain when it's not hooked up to a charger or being charged, it's just a matter of how much. That's why to compensate that you need to charge the battery and if you think your alternator can't do the job because you don't drive this van a lot then you need the slow chargers.

 

i dont drive this van much miles. Around town with the trailer only.
is there a way to hook up the secondary battery to a charger or would i have to lower the battery to get to the terminals?
if I take out the f1.10, that would unlink them. Does that mean I can run the van on just one battery, or would that not be recommended?
is the battery junction box for the f1.10 that one to the right of the motor, or is it located else where?

 

The 2 batteries are always connected together . (In parallel)

If one is defective it will drain the other. If there is any question of battery condition, BOTH need to be replaced at the same time.
The charging system only sees the system voltage and if it thinks the voltage is low, it will attempt to correct the problem by addl charging and OVERCHARGE the good battery and ruin it.

If there is no way to take the batteries out of the circuits, with non use, they will eventually lose their charge and NO charger will correct the problem of a defective battery.
You can, however , reduce the small drain every system has, by removing the negative battery cables that completes any circuit. Even if nothing is on there is a small amount of leakage through the alternator and that is why removing the negative cables will prevent that.

 

So, I can disconnect the negative on the second battery and it would be fine to run just 1 battery?

 

NO !
That second battery needs to be active to help start a cold engine. Don't forget those glow plugs while waiting for the wait lamp to go out , are consuming plenty of battery power to assist starting the cold engine plus when connected it gets the power taken ,replenished..
Both ground cable terminals should be disconnected.
Unless there was some type isolation system, both batteries when connected help get the engine started.

 

Thanks Hanky for pointing out that both batteries are always connected in parallel.
I just realized that the very 1st and 2nd diagrams above there are some inconsistent info. After going thru more diagrams in alldata I found that quite a few put different variant and configuration into the same or different diagrams.
The 2nd diagram above that aux relay is not present, this incomplete simplified diagram is to illustrate only the starting circuit or it's how it should be when connected in parallel.
The 1st diagram you can see 4,5,6,7 are pointing to aux relay 95-1.

Kwm2011 please go to your E350 and see if you can locate aux relay 95-1 so we can confirm your batteries are connected in parallel or your batteryII's B+ cable is going thru aux relay 95-1. BTW, that should be called solenoid instead of relay.
It's possible that the 95-1 is for non-diesel only. Here's the diagram of your battery-II, follow C1249a (B+ cable) and trace it to near or under where your main battery tray is, if 95-1 is at all present.


That 95-1 looks more like a starter solenoid like :
If yours doesn't come with 95-1 and you still want to disable your battery-ii of course it's more than possible if your main battery can do its job, even if it can't, you can always hook up a solar charger and have it gets charged unattended or using a noco charger like genius10, not genpro10x2 anymore.