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OT: Keeping stationary diesel engine batteries in parallel charged

Scottl...Yes I think that would work to use two chargers. It would also mean that I would need to separate the two in order to charge them. I was really hoping to discover if someone had this problem and said "well all you got to do is throw in some celery salt" or something like that. I think that having two batteries connected to a "smart" charger regardless of their initial condition may not be the best practice. I could add a switch like I have in my boat that lets you select no connection, bat1, bat2 and both. Pricey and kind of a pain in the ass to wire however. Also not automatic. Another option may be to use a "stupid" charger with a plug in time clock. Have it slam both batteries for a while every 24hours. Again, not automatic and not very elegant. Another option is to just use one battery as was suggested. This is a military surplus genset and may have had two batteries for redundancy. It was probably designed to be used in sub zero climates and with the glow plugs going and the oil as thick as peanut butter, it may very well have been designed to need two batteries. Maybe I can get away nicely with just one.
Not sure this applies but from my short time playing with lipos in rc stuff and the fact that you mentioned smart charger. I believe those are cc/cv so the amps stay consistent until voltage threshold is hit then goes to constant voltage dropping the charging amps to maintain voltage.

In parallel the charger only sees the highest voltage(newer bat) and could careless about the older one. If the older battery requires more amps/time to be completely charged then the newer one the older battery just won't get to full charge. Also if the older battery drains faster than the newer one the imbalance between the two batteries will get more and more every charging cycle.

Might be completely off but that's my understanding.
Ideally one starts with two new batteries and charge in series then switch over to parallel for service. OTOH I've had good luck with parallel charging RV batteries with a solar panel. With demand being intermittent it seems to work well. Clearly YMMV.