On-Board Charger

[chumbucket]

chipde, good info!
I know it's not related to reelapeelin's question, but for my own knowledge; When batteries are tied together in parallel I know just a common negative won't drain the stronger battery, but if the positive lead selector switch is in the "All" position, couldn't this potentially be a problem without an isolator? I'm just curious because I ran into that problem in the past running both batts on all while running to charge them both. One battery was quite a bit older than the other and ended up eventually draining and shortening the life of the newer batt. I'm a loy more careful now to maintain the two batts equally. Isolators are pricey and I would rather not add too much more to the boat in that respect.
Just curious and thought you'd be able to shed some light on that as well.
Thanks.