12v and 6v hookup

I sometimes get flamed for saying "There are now six volt batteries in RVs"

And yes, I know the do put six volt batteries in RV's. I mean I have two of 'em myself, but when you istall them something magic (not really) happens.

The two six volt batteries, wired in series, become, a single 12 volt battery.

So you said you are putting in 4 Trojan T-105s.. I would say you are putting in 2 Pairs of Trojan T-105s..

When you see the difference, you will have your answer

If you can not see it:

Each pair, in series, is a 12 volt battery

So you are asking: Can I parallel a 12 volt battery with a 12 volt battery?

And clearly the answer is yes.

The "Dangers" are mostly myth but there is one that's real.. IF the cells in the older battery fail SHORTED,, it can cause the bigger and newer Trojan's to dump charge (current) into them far faster than they can accept, this can lead to the older battery "Exploadig" (Low level explosion but it sprays acid all over so it's a pain to clean up)

I would expect the new batteries to age out similar to the existing.
So if you milk the old batteries three more years then expect the new batteries to last about the same. Otherwise the four new Trojans with good care should last 7 to 12 years.

Whoah!. I just skimmed through all those other threads and now have no idea what to believe! But some people sure are sure of their positions. Maybe I'll just bite the bullet and trade in my two 12v batteries and buy four Trojans.

Thanks for your answers guys!

Probably something useful in here.

Add 12v to 2 6v
12 and 2 6v 2010

Add 2 new 6 to 2 old 6 (2009)

Yes, the performance of the Trojans won't be the best they can be but will work and it wont damage them.
You'd be better off installing a Perko switch and switching between the two banks.