What is the voltage on each battery ? If there is even a small difference in voltage on each battery, the lower voltage battery will drain the higher voltage one, even with no load. You can prove this with you ammeter !
lawrosa wrote:
Drain to what extent? AFAIK they attempt to match voltage right? If they are close like .1 volt off will they not just equal in voltage? Maybe a few amps from one to the other?
It doesn't work that way ! The lower voltage battery will always "pull down" drain the higher voltage battery when connected in parallel.
When using 6V batteries in series this does not happen !
(Hint - Switch to 6V golf cart batteries)lawrosa wrote:
On my truck because its ECM controlled I always see 14.4-15.5 volts.
I find that statement hard to believe ! Maybe for the first few minutes after starting, but every vehicle I have ever seen with "smart" charging will regulate the voltage to the battery to between 13.2V and 13.8V.
Find a CPAP machine that runs off of 12VDC. You are losing between 10-20% of you energy in conversion losses.
