Batteries 6V vs 12V, Battery Guru needed

[Rob Dillon]

Hello All,
This will be my first post and question, as I'm new to airforums. I purchased a 2001 27' Safari, I bought it used 4 years ago. I have struggled with the 12v battery that is mounted on the tongue ever since we purchased it. It doesn't seem to have enough juice to hold up through the night. The previous owner had installed a solar system on the roof to recharge the battery, and it works fine. Shore power, generator, solar system all work great, for keeping it charged and use during the day. However, in cold weather I run the heater at night, and keep thermostat at about 62/63.
After waking up in the morning with a dead battery for the first year or two, I purchased a new 12v deep cycle interstate battery about 2 years ago, that is now doing the same thing, drains completely dead by morning.
There is limited space on the tongue between the propane tanks and trailer, and I have about 13" of space. I want to add a second battery, and was recently told by my local RV guy the best option with this limited space is that I could get 2 6v batteries, and that this would also create more amp hours.
Finally getting to my question, has anyone with this model done this, or what options are best for this space? Is there any reason not to go with 2 6v batteries? I'm told 6v are better, with thicker cells, will give me double or tripple hour time, and have longer life span as far as years, compared to 12v. Any truth to this? There might be an argument for it either way, but looking for some truth and battery knowledge or experience as far as what will produce or hold more hours.

[DKB_SATX]

Hopefully idroba and/or Mexray will see this thread and respond with some more authoritative information. The 6V golf-cart batteries are tough, have high amp-hour capacity and live through more charge/discharge cycles, but there's no free lunch here. A pair of 12v batteries will roughly double your amp-hour capacity (compared to the single 12v of the same class), moving to 2 golf-cart batteries will do better than that but cost more, at least in the short run. idroba swears by pairs of 6v golf cart batteries in series, and he boondocks almost all the time he's camping.

Are you sure that you're starting the night with a truly full charge? If you add capacity but aren't putting back as much charge in the daytime as you're using at night, over the course of a few days you'll eventually run a bigger bank of batteries flat as well.

[arodriguez60]

How many watts do you have of solar? I have found that you want 100 watts per battery. This is a rough approx but it works for me.

[Mcdenny]

You might want to first figure out how much power you need for your loads and desired length of time. Then you would know how much battery to buy.

You want deep cycle batteries. Congenital wet lead acid should not be repeatedly drawn down much below 50%. AGM lead batteries cost 2x but can treats more deep discharge cycles. Energy storage is measured in amp hours or watt hours.

Lithium Iron Phosphate batteries are coming dow in price. They are 1/3 the weight and volume of lead. Can be deeply discharged. Cost 3x lead and require a special battery charger. Best performance and highest cost.

[SeeMore]

I suspect from your wording that you have a single 12v battery and the problem appears when you are not on shorepower. Adding a second battery will add more hours of use between recharging and ensure some power remaining after the first night off-grid. My Safari's biggest single 12v hog is the heater blower motor. It can drain a single battery overnight in cooler/cold weather.

I suspect your battery box is quite similar in size to mine. Two 12v batteries will fit tightly. 6v batteries are taller and my box required a lid modification because 6v batteries are typically taller in the same amp hour capacity. Always match battery size, type, and age when using multiples.

Without writing a dissertation on 12v versus 6v, be aware that two 12v batteries double the amp hours of a single 12v, and two 6v batteries double the voltage to 12v but will not increase amp hours over a single 6v. 6v golf cart style batteries are designed to be drawn down frequently without shortening lifespan whereas the 12v are designed for greater initial starting loads followed by replenishment shortly thereafter. Hence, the preference for 6v by frequent boondockers. You will get more years from a pair of 6v batteries if you maintain them properly.

You didn't mention a very important ingredient to the overall equation - the output capacity (wattage) of your solar panels. Solar recharging has a combination of factors which determine the time frame to fully recharge. Most importantly, the slightest shadow on even a small portion of the panel greatly reduces output which increases the time to recharge. Time of year (angle of the sun) and hours of exposure are also important. You'll need to factor recharge time into your expectations if you intend to off-grid for multiple days. The trick is keeping daily consumption below daily recharge inputs.

[wayneskid]

Originally Posted by SeeMore

....Without writing a dissertation on 12v versus 6v, be aware that two 12v batteries double the amp hours of a single 12v, and two 6v batteries double the voltage to 12v but will not increase amp hours over a single 6v. 6v golf cart style batteries are designed to be drawn down frequently without shortening lifespan whereas the 12v are designed for greater initial starting loads followed by replenishment shortly thereafter. Hence, the preference for 6v by frequent boondockers. You will get more years from a pair of 6v batteries if you maintain them properly.

For clarification, FWIW:

• Batteries wired in PARALLEL multiply Ah and batteries wired in SERIES multiply voltage. So, two 12v 100Ah batteries connected in PARALLEL will give you 12v and 200Ah. Two 6v 100Ah batteries connected in SERIES will give you 12v and 100Ah.
• For maximum battery life it is best to avoid drawing down your batteries to less than 50% State of Charge; 85% SOC is even better. i.e. shallow discharge cycles are better. Trojan suggests never drawing a battery more than 80% (20% SOC).
• Deeply discharging your batteries will reduce their capacity. Even doing this once will permanently diminish battery capacity and useful life. So figure out your load and charging scenarios before you replace batteries.
• Battery voltage is at best a very crude and unreliable indicator of SOC. A decent battery monitor will give you information that is actually meaningful and allow you to get the most from your batteries.

[wayneskid]

The questions posted in this thread are asked, in varying ways and, to put it gently, frequently! There is a lot of information available via the internet and like everything on the web it's sometimes hard to separate the facts from the myths. I'd like to suggest a couple of good sources of factual information that can be downloaded as pdf files for reading at your leisure and for reference as you trouble shoot the problems that will come up from time to time:

• Trojan Battery Users Guide: http://trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf
• Lifeline Technical Manual: http://www.lifelinebatteries.com/manual.pdf

Both Trojan and Lifeline make top quality batteries; Trojan makes FLA, Gel, and AGM batteries while Lifeline is focused primarily on AGM batteries. There is more technical information on the Trojan and Lifeline websites.

[RickDavis]

I changed to 2 6 volt batteries in 2003 after being less than happy with the 12 volt deep cycle batteries.
In my case it has been a good move.
The batteries (exide) that I installed are now into year 11 and still doing fine.
The one thing I do NOT do is leave them float on charge for extended periods when on shore power. Not sure if that really made a difference in their life or not.

[DKB_SATX]

Originally Posted by SeeMore

~~
Without writing a dissertation on 12v versus 6v, be aware that two 12v batteries double the amp hours of a single 12v, and two 6v batteries double the voltage to 12v but will not increase amp hours over a single 6v. 6v golf cart style batteries are designed to be drawn down frequently without shortening lifespan whereas the 12v are designed for greater initial starting loads followed by replenishment shortly thereafter. Hence, the preference for 6v by frequent boondockers. You will get more years from a pair of 6v batteries if you maintain them properly.
~~

An additional detail that's useful to include to what you wrote above is that, for a similar footprint, the 6V battery usually has a much higher amp-hour rating. So while 2 6V batteries in series have the same amp-hour rating at 12V as one of them has at 6V, the pair of 6V batteries you can squeeze into the same battery box as a pair of 12V batteries will usually deliver as many or more amp-hours as the pair of 12Vs, along with the extra longevity and durability of the heavy 6V. The extra height of the 6V has to be dealt with, though.

[gtc6452]

The 12volt Side of Life (Part 1)

[wayneskid]

Originally Posted by DKB_SATX

An additional detail that's useful to include to what you wrote above is that, for a similar footprint, the 6V battery usually has a much higher amp-hour rating. So while 2 6V batteries in series have the same amp-hour rating at 12V as one of them has at 6V, the pair of 6V batteries you can squeeze into the same battery box as a pair of 12V batteries will usually deliver as many or more amp-hours as the pair of 12Vs, along with the extra longevity and durability of the heavy 6V. The extra height of the 6V has to be dealt with, though.

The upgrade I did last year is a typical example:

• Original batts; installed by Airstream: 2 Discover AGMs; 12v, 84Ah each. Parallel connected for 168Ah total.
• New batts: 2 Trojan T-125 Plus FLAs; 6v, 240Ah each. Series connected for 12v, 240Ah total.

So I got a nice 43% bump in capacity. The Trojans are taller but fit nicely in the OEM battery box. Subsequent to this upgrade I installed a Trimetric battery monitor. Now that I can see what is really happening with my battery bank I wouldn't try to operate an RV without a good battery monitor.

[SeeMore]

One of the nice things about this site is the willingness of others to finish the dissertation. Thanks wayneskid & DKB_SATX

You will find much of what you seek here and battery box modification pics:
http://www.airforums.com/forums/f449...lts-64324.html

[ventport]

Originally Posted by wayneskid

For clarification, FWIW:

• Batteries wired in PARALLEL multiply Ah and batteries wired in SERIES multiply voltage. So, two 12v 100Ah batteries connected in PARALLEL will give you 12v and 200Ah. Two 6v 100Ah batteries connected in SERIES will give you 12v and 100Ah.
  

I do not get this. How can two 6 volt batts with 100 Ah total, give me more useable power then two 12 volt batts with 200 Ah total. I know many people do this and it must work, but it does not make sense from the amp hour numbers. Another words, how can 100 Ah out last 200 Ah? Can someone smarter then me, (which should not be hard to find) please explain how this is possible? Thanks Brian

[HiHoAgRV]

Originally Posted by ventport

I do not get this. How can two 6 volt batts with 100 Ah total, give me more useable power then two 12 volt batts with 200 Ah total. I know many people do this and it must work, but it does not make sense from the amp hour numbers. Another words, how can 100 Ah out last 200 Ah? Can someone smarter then me, (which should not be hard to find) please explain how this is possible? Thanks Brian

Because the example is kinda odd. Typically a 12V battery might have 100Ah and a 6V about 200Ah. Parallel the 100Ah's and you get 200Ah's(for 12V batteries). Series the 200Ah's and you still have 200Ah's (for the 6V batteries)

[daveswenson]

A type 24 will give you about 76 Ah. Wired in parallel. 12v 152 AH.

T-125 is 240 Ah. Wired in series. 12v 240 Ah.

[wayneskid]

Originally Posted by ventport

I do not get this. How can two 6 volt batts with 100 Ah total, give me more useable power then two 12 volt batts with 200 Ah total. I know many people do this and it must work, but it does not make sense from the amp hour numbers. Another words, how can 100 Ah out last 200 Ah? Can someone smarter then me, (which should not be hard to find) please explain how this is possible? Thanks Brian

Sorry to confuse; my example was just to illustrate the math involved. HiHoAgRV is correct, typically 6v FLAs have significantly greater capacity (Ah). See my earlier post for a real life example.

[idroba]

I have been using two 6 volt golf cart type batteries (Trojan or Costco) for at least 12 years now in various trailers. They are, in my opinion, the very best solution for an RV in terms of capacity, life of battery, and cost. These days at Costco, or Sam's club, golf cart batteries are in the range of $90 each, and each weighs about 62 to 65 #. The capacity of most golf cart batteries is in the area of 220 to 240 Amp hours each, so a pair, in series has a total capacity of 220 to 240 Amp hours at 12 volts. This is considerably more than the pair of paralleled type 24 batteries Airstream supplies, which are in the range of 75 to 80 amp hours each, two totaling then 150 to 160 amp hours. Some Airstreams will hold type 27 batteries, which have approximately 100 amp hours each, or 200 amp hours for a pair.

I do think that the Trojan brand of golf cart batteries is somewhat superior to the run of the mill Costco or Sam's club, but unfortunately the cost of Trojans has increased greatly in recent years, to the range of $140 - $160 each, making them considerably more expensive that they have been in the past.

So, golf cart batteries in series will give you more capacity than either a pair of #24's or #27's, will cost about the same or maybe a little more (Costco or Sam's) and in my experience, will last considerably longer. They seem to last longer do to being more ruggedly built, have thicker plates, and since they are higher capacity, they are not run down as easily. When batteries are not as deeply discharged, they last longer. A single pair of 6 volt batteries in series takes a charge better and more evenly than a set of 12 volt batteries in parallel.

The only disadvantage to the golf cart type batteries is the extra inch of height, making it difficult to put them into many Airstreams and Argosy's without modifying the battery box. And of course you need two of them and many older units were only equipped with space for one battery.

I boondock almost exclusively, and the set of Trojan's that I put in my '74 Argosy were still useable after 10 years. I did replace them finally, as I felt the capacity was decreasing.

I have a relatively small solar system on the Argosy, in the range of 100 watts. All fluorescent lighting in that rig, with a replacement furnace which draws about 3.2 amps when running. I camp in cold weather and am one who likes to stay warm, so I do not freeze in the dark when I camp. My laptop computer is often on for several hours each evening. I move farily often, so there is engine charge plus solar going into my rig's batteries, but in all the years I have had the Argosy, I have never run them down to the point that I could not have light, heat and water (pump).

In any event, a modern 3 stage converter/charger is necessary for good battery health. I have used Progressive Dynamics 9200 series in 5 rigs now, and find them to be trouble free and do a very good job of keeping the batteries in good condition. There are other good brands, I simply have no experience with anything but PD's.

On my new 2014 Flying Cloud 20' one of the first modifications I made was to remove the factory supplied single stage Intellapower converter/charger and replace it with a 3 stage Progressive Dynamics unit (4600 series for that change out). Next I put a Tri Metric metering system on it for best battery monitoring. Then I put two, 100 watt solar panels on the roof, and am using a Blue Sky Maximum Power Point Tracking charge controller. In the spring, I will change the two #24 batteries for a pair of Golf Cart type ones, most likely from Costco. I will have to deepen the battery box about an inch. Actually I plan to move the cover up and inch, not difficult to do on my rig. The new FC already has LED lighting, and the furnace on my 20' one only draws about 3.4 amps when running.

So, that is a long winded comment on the original question. I think you will find that two 6 volt golf cart batteries in series will be a great improvement over your present system, with very few disadvantages, height being the main one.

[wayneskid]

Good comments idroba. I would add; "Golf Cart" batteries are usually FLA design vs VRLAs or the AGMs that come with many modern RVs. In addition to the extra height of typical 6v batteries, FLAs generally have these disadvantages:

• Periodic maintenance is required. This can be a major consideration if the battery box is not easily accessible.
• Can only be used in an upright position.
• Produce gas (oxygen and hydrogen) when charged.
• May emit acid spray if overcharged abusively.
• Require ventilation.
• Higher self-discharge rate than deep-cycle VRLA batteries.
• Cannot be shipped by air.

My battery box is easily accessible so for me the 6v Trojan FLAs are the battery of choice. At least until I feel like spending the $$$ for a set of LifeLine AGMs.

[Mexray]

DRAT.. Raining here, & I was typing away a great dissertation on this subject when our local power Co pulled the plug..."Hey Honey, did we pay the bill this month?"...

Being in the Battery Biz... I usually suggest the 6 volt, Golf Cart type batt's for RV's when you can fit them into your rig...

Typical GC Batt's have a 225 AMP HOUR rating, with robust internal parts to withstand the rigors of bouncing around a golf course every day, and can withstand daily discharge/recharge cycles in that kind of service...

There are several other popular 6 volt DC batt's available, but they get heavier and larger as the AH's increase - those 140 AH Trojans mentioned above are taller than typical GC Batt's...

One add'l note - the more AMP HOURS you have on board, the longer you can run your 12 Volt stuff - HOWEVER - if you discharge em' too deeply, if will take MUCH longer to recharge em' back fully, again...

When boondocking, we top off our batt's EVERY day with the Honda 2K genny - when the converter/charger's voltage drops to the 'FLOAT' setting, we turn off the genny...

There's nothing wrong with running 12 Volt Deep Cycles, and many of us don't have the option of using the GC's without major modifications (in older AS's)... BUT if you have the choice, I'd go with the GC Batt's...

Happy Trails...

[HiHoAgRV]

And one other item for the statistical geeks who like to look at failure mode analysis- a pair of 6v batteries have 50% less 'parts (cells)' than a pair of 12V's. 1/2 as many parts can equate to 1/2 the chance for a part to fail.

Of course the same argument can be said for single engine aircraft vs twins... "But I have a spare engine(battery) in case one fails" IMHO that doesn't mean life is good, it simply implys a slight level of control over WHERE the pile of debris ends up.

you milage may vary...