12v and 6v hookup

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.

Probably something useful in here.

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

Add 2 new 6 to 2 old 6 (2009)

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!

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.

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 have to agree (almost) with XYM's last paragraph.

His contention that the Exploadig is not all that bad is less than factual. They can be bad. I have done boat work in this area up until the depression shut it off. In that time and in my small business area (~300 non-trailerables), I got to deal with at least three and often many more (the high being about eight) battery explosions a season.

Yes, some were little. They only blew the caps or top off one battery in the set. Some were not so fun. This was so regular that the motorhome that was my office/tool crib/meeting room/electronics shop and occasional motel had an inventory of baking soda.

Explosions of single batteries are very rare.
Explosions of identical batteries in parallel are pretty common.
Explosions of batteries in parallel that are in any way different are no surprise.
Explosions of batteries in parallel that are vastly different can be expected.

Simply, Lead Acid batteries do not play well in parallel.

Paralleled batteries of any sort will be very difficult to charge to full density. - Just the way that they are.

Matt

Matt_Colie wrote:
I have to agree (almost) with XYM's last paragraph.

Explosions of single batteries are very rare.
Explosions of identical batteries in parallel are pretty common.
Explosions of batteries in parallel that are in any way different are no surprise.
Explosions of batteries in parallel that are vastly different can be expected.

Simply, Lead Acid batteries do not play well in parallel.

Paralleled batteries of any sort will be very difficult to charge to full density. - Just the way that they are.

Matt

By "not that bad" I meant it is not likely to blow a hole in your motor home, But it WILL spray acid all over the place (I said that) and that is a ... witch of a job,, to fix (Fill a 1.5 Gallon garden sprayer with water and baking soda,, Spray everything, Do it again, Keep doing it till you get no more fizz,, Then hose it down with a garden hose and sprayer,, Hose well).

I often hear that line about parallel batteries being hard to charge to full density.. Since State of Charge depends on voltage or rather determines voltage.. I would think if one of the batteries is a tad hungery, it would continue to eat till full.. and once that happened they will forever be voltage locked till one battery fails. When it fails one of two conditions will happen:

High resistance and/or open cell, Battery basically dissappears from the bank.

Shorted cell: See above

Note: Though rare,, one possible failure mode with parallel batteries involves a fire.

wa8yxm wrote:

Matt_Colie wrote:
I have to agree (almost) with XYM's last paragraph.

Explosions of single batteries are very rare.
Explosions of identical batteries in parallel are pretty common.
Explosions of batteries in parallel that are in any way different are no surprise.
Explosions of batteries in parallel that are vastly different can be expected.

Simply, Lead Acid batteries do not play well in parallel.

Paralleled batteries of any sort will be very difficult to charge to full density. - Just the way that they are.

Matt

By "not that bad" I meant it is not likely to blow a hole in your motor home, But it WILL spray acid all over the place (I said that) and that is a ... witch of a job,, to fix (Fill a 1.5 Gallon garden sprayer with water and baking soda,, Spray everything, Do it again, Keep doing it till you get no more fizz,, Then hose it down with a garden hose and sprayer,, Hose well).

I often hear that line about parallel batteries being hard to charge to full density.. Since State of Charge depends on voltage or rather determines voltage.. I would think if one of the batteries is a tad hungery, it would continue to eat till full.. and once that happened they will forever be voltage locked till one battery fails. When it fails one of two conditions will happen:

High resistance and/or open cell, Battery basically dissappears from the bank.

Shorted cell: See above

Note: Though rare,, one possible failure mode with parallel batteries involves a fire.

The problem with paralleling batteries is that the weaker, lower terminal voltage, battery will continually drain the "stronger", higher voltage battery.

Think about the extreme case, for instance, one of the batteries has a partially shorted cell.

If you do a quick internet search you'll find huge banks of batteries in homes storing solar and wind power. Seems to be common practice. You'd think if explosions were commonplace then there'd be some warnings or at least some mention of them.

Not to contredict anyone, but is it really COMMON ?

RussellBerge wrote:
If you do a quick internet search you'll find huge banks of batteries in homes storing solar and wind power. Seems to be common practice. You'd think if explosions were commonplace then there'd be some warnings or at least some mention of them.

Not to contredict anyone, but is it really COMMON ?

Russell,
You are correct, but you have missed two important facts.
Those big banks don't get shaken or even rocked slowly. (Except in California.)
Most of those bank have huge protective fuses included. I mean like a 200~300 amp fuse.
And, They are usually cared for more intently than any boat or RV bank.

Matt