Two 6 volt vs two 12 volt run time

It can be very misleading for RV'ers reading about batteries in these forums in which conversation centers around comparison of two 6 volt golf cart batteries to two 12 volt deep cycle marine batteries. That's a wrong comparison in that it does not represent the right type of 12 volt batteries to be considered for RV use.

This link is typical of the right type of 12 volt battery for RV use. Compare 12 volt batteries like this to 6 volt golf cart batteries:

http://www.trojanbattery.com/product/scs225/

"a dual purpose battery can only (safely) be taken down to 80% state of charge."

Total and complete bull if you have true deep cycle 12 V batteries!

I have been drawing down my 12V group 31 deep cycle marine batteries I have been using (always have been purchased from Meijer) for the last 30+ years to 50% and sometimes even lower for the 8-10 years I have each set (bank) of 5 to 7 of the batteries in our RV's and they always recharge to 99% to 100% on my meter (12.6 to 12.8 volts after sitting for atleast 24 hours. In all these years I have only had one battery fail as it was easy to know which one it was as it got real hot to the touch. Simply disconnected that battery as it was off gassing and sure didn't want it to blow. One batyery disconnected made little difference. Had 6 still working fine and replaced the bad one later.

I do wait for a sale to buy 5 to 7 new batteries when I have replaced them and never have paid over $100 each for them. Haven't looked at 6 volt golf cart batteries for years now as my experience with them was not good as it take 2 of them to have 12 volts and our Allego MH had 4 in the battery bank and when the one failed had to disconnect it's mate as there'd only be 6V on that leg with just one 6 V battery. Our present I'd ordered new 13 year old diesel truck has dual alternators which recharge the battery banks real fast when driving as there's #2 gauge fine strand copper welder for both + and - going directly from the alternator junction block to the RV battery bank using stainless wingnuts on 3/8" brass studs to connect the truck's 12 V alternator current to the trailer battery banks thru an 80 amp marine slow blow fuses at each end of the + cable and a marine either 300 amp or a 500 amp barrel switch (depending on which RV) to turn on or off. Has never blown one of the fuses in the 13 years from the present truck.

Sure, and ofcourse, most RV owners just use the small wire in the OEM harness on the tow vehicle going to the 7 pin plug which doesn't carry squat for recharging amps tp the coach batteries/battery bank and is why they seldom truely fully recharge their coach's batteries.

Had the same system I'd designed and installed in my previous 2 trucks also but they just had a single largest alternator offered I could order on them.

Some of this stuff is very easy to think thru, design, and build when you've been an engineer for over 50 years now. I have helped over a dozen other friends with a similar system for their RV rigs over the last roughly 12 years.

For me, it's 2 volt batteries only in and RV rig setup as it's proven to be excellent and I could care less what others want to do!

I almost exclusively boondock....Well I can tell you, I switched from 2-12v. deep cycle marine batteries to 2-6v. golf cart batteries. There is a big difference...Where I would only get about 3 days with the 12 volts, I now get 4-5 days with the 6 volts. You can state all the reserve capacities and discharge rates you want, but the proof is actually using the 2 6volts. They do much better than 2-12 volt batteries.
Try it you'll like it.

Bob Vaughn wrote:
I was just informed that two 6 volt batteries would run the residential refrigerator many times longer than two 12 volt batteries. I wonder if this is just hype?

MUCH speculation on on my part coming !

6V lead acid batteries are typically deep discharge golf cart batteries. 12V batteries are likely either automotive starting batteries or "dual purpose" marine batteries.

2 6V golf cart batteries have about 400-450 minutes of reserve capacity (RC) at a 25A discharge rate. A dual purpose 12V maine battery only has about 190 minutes of reserve capacity at the same discharge rate. The big difference is that true deep cycle batteries can safely be repeatedly taken down to 50% state of charge while a dual purpose battery can only (safely) be taken down to 80% state of charge. Eve if yo have 2 marine batteries, your RC is only 380.

It is also likely that 2 golf cart batteries will cost less than 2 marine batteries.

wa8yxm wrote:
What battery gives me the longest run time per DOLLAR COST. .. That is the GC-2 Golf Car DEEP CYCLE in pairs.

Same conclusion I had.

Bob Vaughn wrote:
I was just informed that two 6 volt batteries would run the residential refrigerator many times longer than two 12 volt batteries. I wonder if this is just hype?

Yes. and No. Six volt batteries are normally GC-2. For Run time estimates you may assume they are 220 amp hours give or take 10, for the pair... You may use half of that if your inverter will let you.

On most 12 volt batteries you should not use more than a quarter of their capacity

Some numbers
Group 24 75 ah
Groups 27 and 29,, Just below and above 100
Group 31 130

But here is the kicker. a 4D or and 8D Well now we are back up to 200 to 230 amp hours. AND,,, You can get them in DEEP CYCLE (Like the GC-2) as opposed to MARINE/deep cycle (like the more common Group 27/29)

and I've used 12 volt batteries that made the 4D look like a Group 24 capacity wise.. So you see your question, as ask, can not be answered

HOWEVER this one can be: What battery gives me the longest run time per DOLLAR COST. .. That is the GC-2 Golf Car DEEP CYCLE in pairs.

I only use group 31 12V deep cycle batteries with a 225 minute draw of 23 amps and they last between 8 and 10 years before I replace them. If one of the 5 or 7 should go bad early, Just disconnect it form the bank and replace when the same battery is on sale as it's no big deal because I only lose 1/5 (20%) or 1/7 (14.3%) of my 12 volt amps available and why not save some money?

However, when I had a 4 battery bank of 6V batteries on one of our big Allegro MH, and 1 battery bit the dust, I had to disconnect 2 of the 4 6V batteries to maintain 12 volts (paired 6V batteries to make 12 volts) and it cut our stored available battery amps right in half (50%)! Made the inverter nearly useless for 110 AC. Will never have a battery bank of 6 volt batteries again!!! Learned a real lesson!

Two 6V batteries in series.......12V with SAME amp hour

Two 12V batteries in parallel----12V with double amp hour


So depends on WHICH 6V/12V and their amp hour ratings

MrWizard wrote:

this is where (2) 6v will out last (live longer) than (2) avg 12v car batteries

whoa, we aren't talking about car batteries for our 12 volters.
bumpy

I guess that about covers it...

an amphr at 12v is 12wH

does not make any difference, if its (2) 6v in series or (2) 12v in parallel

total watt hours or amp hours will be the same

the difference becomes noticeable in useable lifetime
when you regularly cycle them to 50% SOC

this is where (2) 6v will out last (live longer) than (2) avg 12v car batteries

this all changes if you are getting quality 12v AGM batteries designed to be used, and your keeping them above 70%, not down to 50% regularly

NOW whaat was it the the dealer said ? His words ?
don't paraphrase
did he say run longer (for your fridge)
or LAST longer, meaning longer life

for the same ampHrs, NO they will NOT run the fridge a lot longer (aka hours or days camping)

they will most likely 'out live' a OEM 12v batteries that would normally come with the TT

unless that is a pair of NICE AGM batteries