BATTERIES

Which is better two 12 volt batteries or two 6 volt batteries. We were up camping for four days and it was cold in the mornings so the furnace was used a little during the early morning hours, The batteries had a complete charge when we arrived, and drained the batteries during about a six hour period. That morning I hooked up the generator and charged the batteries for about six hours, the next morning the same thing. Any suggestions.

You didn't say which kind of 12V batteries you have.

6V "golf cart" batteries are designed for continuous/heavy daily use and daily/nightly recharging, thus the term "deep cycle". Some 12V batteries are deep cycle, but many are not.

Marlin said:

Which is better two 12 volt batteries or two 6 volt batteries.

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This is a 'how far is up' question...it's really all about the total amp hours of your battery bank - think of your battery bank as an 'electron' fuel tank - the larger the tank (more amp hours), the longer you can run your 12 volt stuff...

If you have two Group 24 DC batteries, you probably have about 160 total amp hours capacity (80 AH x 2 (in PARALLEL) = 160 AHs)

If you were using two 6 volt, golf cart batteries, you'd have about 225 total amp hours capacity (225 AH, 6 volts in SERIES connection = 225 AHs @ 12 volts).

Your furnace has an Amp hungry squirrel cage blower that uses lots of AH's when running, and can deplete your batteries charge overnight if you have a smaller AH battery bank in your RV...

The other factor that many overlook, is their battery bank's state of charge - you batteries must be fully charged in order to perform up to snuff, when you need those AH's in colder weather...it's best to have a regular routine to recharge you batteries EVERY day for an hour or two to insure they are fully charged before the next 'USE' cycle - many never get their batteries up to a full charge before the next use, and then it's a case of diminishing battery charge that you are faced with...

If you allow your batteries to run down into the 11 volt range, it takes HOURS and HOURS to fully recharge the battery bank (with the genny running), which many RV'ers don't realize that what you 'take out' of your battery bank, must be replaced to get back to a fully charged condition...there's just no 'Free Lunch' when it comes to battery charging management...!

Ray

Some folks are like politicians they circle the question. I would like to know as well, which will last the longest under the same load while boondocking, 2-12V deep cell or 2-6V deep cell?

lostagain said:

Some folks are like politicians they circle the question. I would like to know as well, which will last the longest under the same load while boondocking, 2-12V deep cell or 2-6V deep cell?

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That is nearly impossible to answer.  Two GP 27 batteries will have more power available than two GP24 batteries.  In some cases the GP27 deep cycle batteries could have more power than two 6volt batteries.  It all comes down to amps/load ratings.  Two 12 volt batteries with 500 amps ratings each will have more power available than two 6 volt batteries with the same 500 amp rating.

Easy answer is the two six-volt batteries.  How much longer they last is dependant on the condition and type of the 12V ones.  I'm surprised that two 12V batteries did not last through a night when you used them a little in the early morning.  Sounds like one or more of your current batteries may not be adequate.  But if you are replacing batteries and want to be able to boondock, the 6-volt batteries are better designed for that use and will outperform the 12V ones.

...2-12V deep cell or 2-6V deep cell?

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That's not a easy answer because you didn't specify a size of the 12v battery. The 6v's we can pretty much assume the standard golf cart battery, since theer are few other v available these days. But 12v's come in a variety of sizes and amp-hour ratings, so there is no clear cut answer.

However, to make things easy, I'll side with COMer on the short answer. The two 6v GC2 batteries will equal or exceed two 12v G24, G27 or G29 batteries in amp-hours.  And in most cases will be a more robust battery with alonger useful life. I suggest the Interstate U2200 or Trojan T105 brands.

If you don't want to change your battery wiring, then get two 12V Group 29 batteries for about the same amp-hours as the 6v. Or two Group 27, for just a slight bit less. Walmart will have a decent G27 12v deep cycle, labeled as a "trolling motor battery".

It is all numbers.. As Tom said most six volt batteries today are the Golf Car type, these are designed to be Deep Cycle, they give up their power over time and can survive the occasional "OPPS, I ran them down too far" far better than most starting batteries.

And due to the tremendous quantity of electric golf cars. They make thousands of this one size battery.. Where as with the 12 volt sizes they are constantly re-tooling for different sizes and shapes.. This may result in lower cost per amp hour.

Speaking of amp hours that's what it's all about.

A pair of GC-2 size six volts is about 220 amp hours

A pair of (I THINK) Group 29, is about the same amp hours.

But 12 volt DEEP CYCLE, are still more expensive since they are produced in smaller runs

This is changing though... Because the latest golf cars use 12 volt batteries.

But as the question is ask

Is a pair of six volt Golf car batteries better than a pair of 12 volt Group 24: Clearly yes

Is a pair of six volt Golf car batteries better than a pair of 12 volt,  Clearly NO,

(One 8-D is roughly the equal of a pair of Golf Car batteries.. Though you may wish to have the dealer install it... (The battery is also as HEAVY as a pair of six volts)

I believe we're making this more complicated than needs to be...

When Boondocking, the MORE Amp Hours you have available from your battery system - the LONGER you will be able to run all your 12 volt stuff...

Add up the advertised AMP HOUR ratings of your existing battery system to get the TOTAL AH's...

Now add up the AMP HOUR ratings of any proposed battery system modifications...

The HIGHEST AH number will give you a system that will run your 12 volt stuff LONGER...

This DATA will of course be dependent on how you manage the recharge cycles of your bookdocking battery system, and assumes you begin each USE cycle with fully charged batteries -

in my battery sales experience, may RV'ers DO NOT allow enough time during charging to fully recharge their systems - relying on the VOLTMETER is only a guide, and many times is not the true indicator of the entire systems actual state of charge...

In most cases, the battery system's charge is depleted with hours and hours of use - the time to fully recharge varies with the capacity of the recharging equipment AND the time it is running - and again, may take hours to FULLY recharge the battery system!

I stress the proper recharging as I've experienced many who run down their battery systems - run the genny for an hour, and the volt meter shows an elevated number - then shut down the genny thinking all is well - when in reality they've only achieved maybe a 60% recharge...the next USE cycle then becomes shorter as there's less to draw upon - then the genny running time becomes even greater, etc., etc till the RV'er believes he's got a bad battery system...

Believe me, I see this scenario all too often, and it's difficult to make part-time RV'ers aware of how their battery systems MUST be maintained in order to perform properly - it's NOT always the battery's fault!  It's how they are used, in many, many cases...

Ray

You're right Ray, it's being made way too complicated.

[quote author=Mexray]Add up the advertised AMP HOUR ratings of your existing battery system to get the TOTAL AH's...
Now add up the AMP HOUR ratings of any proposed battery system modifications...
The HIGHEST AH number will give you a system that will run your 12 volt stuff LONGER...[/quote]

Just a minor clarification for the benefit of the OP - don't add the amp hours of two 6V batteries connected in series to provide 12V. Two 6V batteries, each rated at 220AH, connected in series provides a rated capacity of 220AH at 12V.

It's almost that simple.  Experienced boondockers don't try to recharge their batteries to 100% on every cycle.  That last 10% or so can take a disproportionate amount of time once the charger goes into float mode.  What happens in reality is the batteries are discharged to about 50%, then recharged until the charger goes into float mode.  At that point, the batteries are charged to 80-90% of full with a minimum amount of generator run time.  If there is solar power available, it will often finish the charging, but most don't have enough solar power to keep up with even minimal battery usage.  The end result is the batteries are cycled between 50% and 80-90%, using only 30-40% of the AH capacity each cycle.

I certainly agree with all the statistics provided and think this paints a clear picture of how to determine what to buy.  But I still think that running his batteries down with "a little use of the furnace in the early morning" suggests that his current batteries are not being very productive.  He could buy almost anything and expect far better service than what he is getting now.  He should also remember that the set much be matched.  Don't buy one and replace what you feel is the worst current battery. 

Ned said:

It's almost that simple.  Experienced boondockers don't try to recharge their batteries to 100% on every cycle.  That last 10% or so can take a disproportionate amount of time once the charger goes into float mode.  What happens in reality is the batteries are discharged to about 50%, then recharged until the charger goes into float mode.  At that point, the batteries are charged to 80-90% of full with a minimum amount of generator run time.  If there is solar power available, it will often finish the charging, but most don't have enough solar power to keep up with even minimal battery usage.  The end result is the batteries are cycled between 50% and 80-90%, using only 30-40% of the AH capacity each cycle.

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OMG - Ned,  all the "battery bigots" over on RV.NET must be suffering apoplexy right now at your suggestion that most experienced boondockers would allow their batteries to only be charged to the 90% level.  Don't you know that the effects of  plate sulfation  are even worse than the dreaded stigma of "bad breath" or "dandruff" of the past. 

Lou, ask me if I care

Ned said:

Lou, ask me if I care

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Drat! you just ruined all chances of this becoming a 14 page thread.  ;D

But I still think that running his batteries down with "a little use of the furnace in the early morning" suggests that his current batteries are not being very productive.

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Agree. They should not have died that fast if they were fully charged and in decent condition. One wonders if the water level is up, how old they are, and things like that.

lostagain said:

Some folks are like politicians they circle the question. I would like to know as well, which will last the longest under the same load while boondocking, 2-12V deep cell or 2-6V deep cell?

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If you mean time to recharge requried.. 200 amp hours of battery is 200 amp hours of battery, So long as they are both deep cycle (Can go down to about 1/2 state of charge) it makes no difference.  Even if the 12 volts are starting batteries (Should keep them fuller than that) Time to 50 percent is the same for the same amount of battery, Makes no difference.

Now, Here are the differences.

DEEP CYCLE batteries are more forgiving of a deep discharge.. And the common wisdom is keep 'em at 50 percent or higher (V/s 75 to 80 for starting batteries)

Second, the CG-2 Six volt is one of the most common batteries made and there is something I call economy of scale

That is ... The more you make in one run, the lower the re-tooling costs for making it and the less expensive the result.

Which i$ why I favor six volt batterie$

Oh, what happens if you run a battery down below the minimum state of charge level too often?

REPLACEMENT

My Golf Car batteries (Interstate U-2200's) have been run down a few times more than I really like to admit.. I've never had a starting battery come back from being that low or that many times.

They still show great on a load test.

lostagain said:

Some folks are like politicians they circle the question. I would like to know as well, which will last the longest under the same load while boondocking, 2-12V deep cell or 2-6V deep cell?

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Well, the simplest answer is that all batteries with the same amp hour rating should last just as long as the other. An amp is an amp is an amp. If you have a 12v package rated at 200 amps and a 6v package rated at 200 amps they'll both provide 200 amps of power. If you run them that low you can probably thro either package out. If you discharge both to 100 amps and recharge repeatedly, the 6v would probably last longer and recharge faster.

If anyone's interested...a little Battery 102....

The Amp Hour ratings we're tossing around here, let's use the 200 AH example, is actually called a '20 Hour Rating' (for the batteries we mostly use in RV's)...there are other AH ratings for industrial batteries, but we're dealing with the 20 Hr rating for our RV stuff...

You take the 200 AH number, divide it by 20 (hour rating) = 10 Amps, which is the MAXIMUM amperage load you can draw from the batteries for it last for 20 hours (till voltage drops to 10.5 volts), at which point it will be completely discharged.

This '20 Hr rating' isn't really a 'usable' bit of info in our battery's everyday life for computing running time, etc - IT IS MEANT FOR PURPOSES TO COMPARE BATTERY SYSTEMS - it allows one to decide on what battery is better for which purpose, etc...

When it comes to recharging your 200 AH system that's discharged 50%, and now requiring 100AH to be returned to a fully charged condition;
1. an 'AMP HOUR' is ONE Amp charging for ONE hour...
2. so you could charge ONE amp for 100 hours = 100 AH
3. you could charge TEN amps for Ten hours = 100 AH
4. you could charge 50 amps for TWO hours = 100 AH

Anyway, you get the idea...the problem comes in that your charger doesn't produce constant Amp ratings as the battery charge progresses, and the battery system's internal resistance changes as the battery approaches a full charge status - the Amp charge rating drops off (3-stage chargers accommodate this process)...

The long and short of all this is that the 'literal' AH charge rates shown above can't be considered an accurate estimate of time it takes to recharge - I always advise a factor of 150% when trying to figure a 'realistic' recharge time for most situations...this is a 'general' rule, and can be refined to fit your particular situtation, type and rating of the charger, etc.

In other words, using the #4. example, I'd amend it to 50 amps for THREE hours (50 x 1.5 /150%\ = 100 AH) to fully charge the battery system...

Without getting too technical, there are other factors that effect your battery system as well...in some non-technical terms...such as; 6 volt deep cycle golf cart batteries use much thicker internal plates than their 12 volt deep cycle 'cousins' - causing a longer charge time for the recharge to 'soak' from the surface of the plates all the way through...

When replacing batteries for the 'coach' side of your RV, always remember to specify DEEP CYCLE constructed batteries...
1. Deep cycle batteries have thicker plates and more dense separators between plates - this helps keep the plates in place during the many discharge/charge 'cycles' when the plates expand and contract with changes in temperature...

2. Deep cycle batteries are designed for a slower discharge rate over a long period of time - repeated many times during their active life in an RV, Boat, Inverter system, etc...

3. STARTING type batteries won't hold up the the abuse if used for the 'coach' 12 volt system - they use thinner plates, and a more porous separator - this allows more plates inside the the battery container, for a higher, quick discharge needed for operation of a hi-amp starter motor...

4. You will see deep cycle batteries rated in AMP HOURS...

5. Starting batteries will be rated in COLD CRANKING AMPS (@0 degrees f); CRANKING AMPS (@32 degrees f); or MARINE CRANKING AMPS (also @32 degrees f)...just for your info, these ratings are advertising 'gimmicks', as all lead-acid batteries will perform better as they are warmer - hence the the CA & MCA rating will ALWAYS be higher than the CCA rating, making a prospective buyer think he's getting a 'better' performing battery...The battery industry has always used the CCA ratings to compare batteries till some Ad genus devised the CA & MCA ratings to confuse the issue!

To recap, I'd say that what ever level of 'charge' you take out of your battery system MUST be replaced to get back to a FULL charge - and it takes longer that many RV'ers realize - no 'free lunch' here - what goes out has to be replaced - if the batteries have been used for many hours, it may take MANY additional hours to recharge fully, depending on the size of the batteries and the capacity of the converter/charger...

Happy boondocking...

Ra

edit: Replaced a few abbreviations with full descriptions to avoid possible confusion.

Thanks Ray, appreciate you taking the time to write that. Although the technical folks among us knew/understood that stuff, it might be a good addition to our forum library for the less technical folks and for people coming here in the future. Maybe combining that with your earlier writing.

Hope you don't mind that I took the liberty of changing a couple of minor things to avoid possible confusion, e.g. replaced 'DC batteries' with 'deep cycle batteries', and 'GC batteries' with 'golf cart batteries'.