Adding 2 new 6v: what config?

If the old batteries are 4 years old expect to buy all new in 6 years.
Just get 2 now and call it good enough. Especially if solar is on the roof.

So, ... bank switching.
When one bank is depleted, switch to the other, fully-charged bank, using an "Off-Bat1-Both-Bat2" switch (as in Roy Ken's diagram) in the "loads" circuit.

But I would want my solar charger to charge the now off-line, depleted bank. So I guess a second switch in the "charge" line, in addition to the switch in the "loads" circuit would do that; allowing the solar to charge either bank independent of which one is used for loads.

And storage without shore power? I won't be around to monitor and switch banks. So hopefully then at least I could charge the old+new banks in parallel ("both" position on the "charge" switch, "off" position in "loads" switch), without destroying the new batts.

And whaddya do if neither bank has enough juice to get you through the night, but both together do? Get up at 2am and switch?

Dunno, but maybe buying four new batteries would be worth it after all. Certainly would be a lot easier.

Am going to have to think about this for awhile.

Many thanks for all the good advice.

Not a good idea since it looks like you have a cell going bad, 1.262 with another not far behind at 1.265. All should read within plus or minus 0.005 of each other.
A manual equalize may bring the one back but hard to tell.
Look here for proper wiring.Smartgauge if you go with two banks.

Bank switch your 6 volts as Roy Ken suggested ... no different than if one had an existing 12 volt battery but wanted to add a new second 12 volt - better to run them sequentially rather than in parallel.

New in series paralled with the old in series.

Only time to separate old/new would be if stored without float.

The best option certainly is to have all the batteries be the same, of course.

If that's not possible, having the two new ones in series and the two old ones in series, and paralleling them together, is much preferable to having one of each in both of the series-connected pairs. Having a new battery in series with an old battery will cause trouble for one or both (probably both) of them and will limit your available charge to the lower-capacity battery in the pair.

Only do it if you can bank switch. I use simple perko switches for my twin 566 amp-hour 12 volt jar AGM battery bank even though all were purchased at the same time.

You might want to consider a BLUE SEA four position switch.

Then you can add the two new ones in series for bank one and the two old ones in series as bank 2.

This battery switch will select OFF-BANK1-BOTH-BANK2

To be real safe just use one bank at a time. When it starts getting low then switch to the other bank.

When you are recharging the batteries they will just re-charge like you are doing now depending on which bank you have switched in.

Just a thought...

Wiring diagram would looks something like this...



Roy Ken

It's best to never place batteries in parallel in any configuration. Never ever new ones in parallel with old ones. When you do a replacement, replace them all at one time.

It is an interesting question- the ramifications of using one old and one new in series, thus the 2 parallel banks would be matched. What are the effects of an old and new in series?