Adding 2 more 6 v

Add your 2 6v batteries and don't worry about the age of the other ones. I see no problem at all. Though not 1000% theoretically ideal, you will not notice any difference. Great set-up, go for it.

Thanks fellas. Do the amp hours on the new batteries need to match the old ones?
Thanks
Bill

Bill, do a good top charge on those existing 6's, and an EQ if necessary, after checking SG levels. If things look good, simply parallel the old pair with the new pair, and you should not have any worries.

You'll need to do the same on the new ones... both before putting them into service, and after a few good cycles on their own. (as a 12v string, that is).

You may find each string's ocv (resting open circuit voltage, i.e; no loads whatsoever) must be very, very close, to get them to balance out nice, at first. I split my bank for a while, and found it a little tricky getting the two strings to play nice with each other, afterwards. But a few tries, involving short bursts of charge followed by a good rest, eventually brought things close enough to not make one string leach off the other when paralleled.

Assuming you have not tortured your batteries during the first six months you are fine to add a second pair.

200 to 500 watts of solar will make an amazing difference.

4 is nice. My A was a few months old when I added 2 more. No problems. I have a 150 watt portable solar unit. (suit case type) Although it doesn't furnish enough without the generator it helps out during the day. I kick the generator on to top off the batteries every morning. Works for me.

Second try on image