Battery bank 12 or 6 volts using solar....

I did read all the topics I could find in here on this subject. Also have YouTubed myself into a coma on this subject. Wanted to ask you guys. Boondocking, with a suitcase solar set at first. Meaning, I pull the panels out, clip them to battery and set in the sun. Not mounting panels or a control panel at the start. May later on down the road. If you had the choice which would you do? 2- 6 volts or 2 12 volts? I know the 6 volts must be wired in pairs. Can you wire up 12 volt in configuration of 3? I am looking at AGM batteries. Planning on ripping out the ugly and useless dinette in my Dolphin and doing something to store batteries there. As it is now there is a teeny little compartment on the side with just enough clearance for 1 12 volt. There is a lead acid battery in there now. I have never used it or depended on it. Just got my RV late last fall.  It is heavy, messy and I want to move it if I can to allow me, a single female Rver, a friendlier way to deal with batteries and also get the most juice I can. Just looking for suggestions or opinions on what you would do if you were me?

Hi CiCi!

Yes, 6 volt batteries have to be installed in pairs.  You connect each pair in series by connecting the (-) on one battery to the (+) on the other and connect the load wires to the remaining two terminals.  This makes the pair into a single 12 volt battery.

12 volt batteries are connected in parallel, (+) to (+) and (-) to (-).  You can connect any number of 12 volt batteries this way, one or two or three batteries are fine.

If you're looking at using AGM batteries I'd go with as many 12 volt batteries as you want, connected in parallel.

Make sure you protect the battery terminals against something accidentally falling on them and shorting them out.  Even a single battery contains enough power to arc-weld a wrench to the terminals and turn it red hot.

The main advantage to using 6 volt golf cart batteries is cost vs capacity.  If you're using wet cell batteries the 6 volt golf cart batteries are true deep cycle batteries at a good price.  12 volt wet cells are usually a compromise between starting and deep cycle and don't do either duty as well as a battery specifically made for that use.

AGM batteries provide a wider range of power, they aren't as sharply divided between deep cycle and starting types.

Since you are planning on putting the batteries inside I would go with a couple 12V AGM as Lou suggests, rather than 6V golf cart. Although the 6V would give you the most amp hours per dollar, the flooded batteries need to be contained in a battery box with two vents (an "in" and an "out") to vent explosive gasses.

Definitely need AGM type batteries for that interior location, and AGMs are also the most "friendly" to deal with. Essentially zero maintenance or worries as long as they are connected to a decent charging system. Unfortunately, the original 12v converter/charger in that Dolphin was crap, so I'd plan on replacing that as part of the battery upgrade. Another couple hundred bucks...

6v flooded cells give the most amp-hours per dollar, simply because they are produced in high volume and there are several competitive manufacturers. 12v deep cycle batteries are more of a specialty item and priced accordingly.  AGM types, are also higher priced but have benefits to go with that.

I've written a non-technical article on battery choices for RVs. It's in the RVForum Library at http://www.rvforum.net/miscfiles/Choosing_right_battery.pdf

First. 2 six volt in series = ONE BIG 12 VOLT..
And you can put any number of 12 volts in parallel

The major advantage of the six volts
A 6 Volt GC 2 is 220 amp hours. two of them in series also 220 amp hours but at 12 volts.

IT is about all I care to lift and in fact the DEKA G-20 (230 amp hours) is too heavy for me to lift onto the battery tray in my RV. Had to have help to do it.

They are DEEP CYCLE  Meaning they do not mind running down to half full. and may evern recover from an OH S&&&! level discharge (mine have once).

Most 12 volt batteries (Exception is the GC-12 which like the GC-2 (Golf Car) is a deep cycle) are MARINE/Deep cycle 75% State of charge (25% discharge) is ... pushing it.

In all of the discussions about connecting multiple batteries in series and parallel, I haven't seen any mention of differences in internal resistance and other properties (even smaller differences between the same make/model/lot of batteries).  I'm curious about how they affect charging rate, achieving full charge in all batteries connected in a group, and accurate measurement of charge state of each battery.  It's impossible to intentionally mass produce parts to have precisely the same characteristics - there's always a statistical distribution of property values, including internal resistance, as well as variations in conductivity of terminal clamps, connecting cables, etc.

Does anyone have any hard data on the distribution of lifespans of batteries of the same make/model/lot (assuming batteries are bought at the time from the same lot)?  Ideally, it would be best to pair batteries with internal resistances that are as close as possible.

Thanks and All the Best,
Jim

I use 6 volt batteries because they are designed for golf cart use and handle deep discharge a little better though I can't even think of the day I have ever took them below 50%  discharge.


I'm going 4 years on my 3 pairs of EGC2 golf cart batteries and they are going strong.  Each bank is individually switched so I can condition each battery bank individually for longer life.

It is all about capacity.. and one other thing
Batteries come in different "Use" types.. Starting batteries are designed to crank out a lot of amps (Compared to their total capacity) for a very short period (30 seconds)  But like to be 80% full or fuller

DEEP CYCLE can not crank out as many amps (For the same capacity) but can crank them out for a long time  and do not mind being down to 50% state of charge.

So my starting battery is less than 80 amp hours but delivers a couple hundred if ask
The GC-2's are 220 amp horus (Give or take) and also crank out about 200 amps max.
But as a percentage of their capacity.. that's much less.

MARINE/deep cycle.. well they are mostly starting batteries.

MOST 12 volts (The GC-12 is an exception) are MARINE/deep cycle. I really do not like them 

Thus I'd go with a pair of GC-2's for your install.


Someone ask about internal resistance.. Comparing a pair of GC-2's with good clean tight battery to battery connection to a 4D (Same capacity) 12 volt .. The tiny (Very tiny) difference will be very very hard to measure. (It will be the resistance in the two additional connections and jumper. Ideally you want that to be zero)

jim_manley said:

In all of the discussions about connecting multiple batteries in series and parallel, I haven't seen any mention of differences in internal resistance and other properties (even smaller differences between the same make/model/lot of batteries).  I'm curious about how they affect charging rate, achieving full charge in all batteries connected in a group, and accurate measurement of charge state of each battery.  It's impossible to intentionally mass produce parts to have precisely the same characteristics - there's always a statistical distribution of property values, including internal resistance, as well as variations in conductivity of terminal clamps, connecting cables, etc.

Does anyone have any hard data on the distribution of lifespans of batteries of the same make/model/lot (assuming batteries are bought at the time from the same lot)?  Ideally, it would be best to pair batteries with internal resistances that are as close as possible.

Thanks and All the Best,
Jim

Click to expand...

I hate to be negative, iv'e not had my meds this morning..but you can't beat a dead horse. !!

ideally, it would be best to NEVER parallel FLA's, there are many good technical reasons, however, we are not talking about 99% uptime reliability systems here, we
are in the DIY world of 12 Volts only and due to the nature of the beast, most people will parallel as may batteries as they can to avoid doing the job correctly.
why.. here are some reasons:

1. because it doesn't really matter for typical RV use patterns.
2. RV makers really don't give a crap how they build these things, so any poor engineering practice is acceptable.
3. Cheap.. yes, El cheapo.. batteries are cheap, 12 V systems have been in place for years and therefore 12 V stuff is cheap, why change ?
4. some people are intimidated by the very thought of using 24 or higher voltages.
5. most people really don't know or care about cell resistance or charging mismatch


having said that, there are those who need/require much higher kW/hr capacity and those exceptions to the rule will move up to 24 or 48 Volts.
the industry as a whole is slowly moving to higher voltages, there will be a time when cars/trucks etc will move to 24 or 48 Volts
just as vehicles moved away from the limitations of 6 volt systems, but that time is quite a few years away.

note to OP. PM me if you require more technical info.

Good reply from solarman.  Sometimes the internet provides so much info on trivial details that RVers can't see the forest through the trees!  Factors that can be measured in a lab often have little or no importance in real life, or they get submerged by much more important factors.  Cabling for 1, 2, 4 or even 6 batteries is one such, where the difference between "ideal" and "practical" is of little import.  As long as the connections are clean and tight, other differences are pretty much irrelevant in the overall 12v system.  Battery age, temperature, and condition is going to far outweigh anything else.

If you are building a battery bank of dozens of 2v L16 batteries in series/parallel, I would probably sing a slightly different tune, but that sort of system is not common in RVs.

Higher voltage systems are more efficient, but the wide variety and low cost of 12v equipment makes it the obvious choice for consumer-grade systems.

The biggest advantage of using a 12 volt house system is maintaining compatibility with the vehicle's automotive system.

If you go with a higher voltage system you give up the ability to have the engine recharge the house batteries while driving, unless you install a second alternator or a DC-DC voltage converter.  Both add cost and complexity to the system.

And in a mororhome you lose the ability to use the house batteries to boost a dead starting battery.

Lou Schneider said:

The biggest advantage of using a 12 volt house system is maintaining compatibility with the vehicle's automotive system.

If you go with a higher voltage system you give up the ability to have the engine recharge the house batteries while driving, unless you install a second alternator or a DC-DC voltage converter.  Both add cost and complexity to the system.

And in a mororhome you lose the ability to use the house batteries to boost a dead starting battery.

Click to expand...

i would agree for a motorhome, but not for a towed trailer, there one has "carte blanche" and no direct dependency on the TV systems.