Originally Posted by Travelwrench
My batteries are draining I used to turn coach batteries off and turn chassis batteries off and disconnect inverter. Everything was fine. Then I started having issue so I thought I can take negative of chassis batteries. This is where the question is. I have 2 chassis batteries but I have negative connections to coach from both. Do I need to take both off. And why are the 2 negatives connected together
If I understand your situation correctly: Your negative posts are tied together, which means your positive terminals are also tied together. You should have a positive cable coming off one of your positive terminals and connecting either to the starter solenoid, the starter directly, or the alternator directly. The negative terminals also should have a cable connecting to the frame, the alternator housing or both. If this is your situation, you have a standard parallel battery connection. Tying two batteries together doubles the capacity and gives you much more starting capacity - important for diesel engines with glow plugs.
When you describe having connections from the start batteries negative terminals to the "coach", that's too vague. Do you mean to say that you have cable(s) tying the negatives of the house batteries to the negatives of the start batteries? If this is the case, it too is not abnormal. I personally would not wire them this way, but coach manufacturers do all sorts of things to cut corners and costs, and as long as the various batteries are connected to "ground" (read: the FRAME) they could indeed be considered grounded. Why that might not be a good idea is due in part to the cable or cables that are connected to the frame, and their respective ampacity. So without knowing which situation you actually have, and what size wiring you have connected to the ground and power sources, it's almost impossible to advise you further.
If ALL of your batteries are disconnected from each other, AND they are disconnected from the vehicle, they will still lose their charge at about 10% per month for flooded lead acid and or AGM batteries, and less than 1% for lithium batteries.
Its always fun telling folks that conventional batteries don't like sitting for a month without a charge, much less sitting for 4-6 months with no charge source.
So if you are looking for the "short" that's killing your batteries it might actually be in the bottom of each of your batteries. The sulfated lead in the bottom of every conventional battery will build up over time and can build an internal bridge to create a positive to negative discharge conduit inside the battery case. If this condition is present, the battery will discharge to dead unless the condition is corrected. The method to correct it involves draining all of the acid out of the battery and filtering out the oxidized lead, then pouring the acid back in and repeating the process until you remove all of the black nastiness out of the bottom of the battery. Most people choose to replace the batteries to avoid the hazard of dealing with all of that acid, and a small percentage of us choose to use this opportunity to upgrade our house batteries to lithium batteries to eliminate this hassle.
Hope this helps!