I keep reading about using two 6-volt batteries in series for RVs. Why? It seems like it would be simpler to have one 12-volt or two 12-volt batteries in parallel. Sorry, still a newbie trying to figure all this stuff out. —Richard, 2023 Geo Pro 19FDS
This has been ongoing discussion for years! Back when I was at Winnebago in the mid-1980s we used 12-volt deep cycle batteries in all the units, while Fleetwood used 6-volt hooked in series. They advertised that the 6-volt batteries were more industrial and provided more power—which was not true.
However, the benefit of 6-volt batteries is they have more plates and therefore provide more cycles—which is draining down to 50% and recharging. The disadvantage that we pushed is they gas more when being recharged. That meant loss of fluid, and the average RVer does not check the fluid level. Therefore, what was an advantage was no longer the case. It was kind of like the Chevy versus Ford argument.
Since then, lead acid batteries have not changed much other than quality of materials and workmanship available at cheap box stores. However, battery technology has changed with GEL, AGM, and now Lithium batteries.
Here is my take on the difference between 6-volt deep cycle and 12-volt deep cycle.
6-volt deep cycle
6-volt batteries have 3 cells each that produce 2.1 volts each. They require two batteries connected in series, which means positive to negative, to create a 12-volt power supply. They come in amp-hour sizes of Group 24, 27, and 31, providing 80-140 amp hours. When they are connected in series, the voltage doubles from 6-12; however, the amp hours do not. 6-volt batteries have been very popular in the golf cart industry. Many manufacturers such as Trojan Batteries have produced a quality battery with superior lead plates, solid connections or welds, and an overall better quality battery that lasts longer.
The disadvantages of 6-volt batteries is you need two to make the 12-volt power supply. That means more expense and more space. If you want to add more amp hours for boondocking, it means two more batteries, for a bank of four—or more and more room. However, if they are maintained properly, meaning checking and adding fluid when needed and not draining them past 50%, they will cycle more and last longer.
12-volt deep cycle
While at Winnebago, one of the “pitches” we used was 6-volt batteries were harder to find. They were mostly available through a dealership or golf course—neither of which were open at night or on the weekend, when batteries seemed to go bad! Today I don’t believe that is the case as more auto stores and other outlets are carrying them.
If you have a small trailer and don’t do a lot of dry camping, you can use only one 12-volt battery and save money, weight, and space. If you want to add more amp hours, you can add just one more battery and get the same amount of power as four 6-volt batteries.
However, if you have an RV that has the house batteries under the steps or in a tray on the tongue, there is only room for two batteries typically. No, they do not last as long in cycles. But if they are maintained properly with water levels and desulfation, they are a good choice for the average RVer.
If you are running a residential refrigerator and other items in one of the “Big Boys” and doing a lot of dry camping, I would go with four 6-volt batteries and an inverter. But do the math: Four 6-volt batteries will give you about 500 amp hours of storage power. However, you can only use 50%. That means 200-250 amp hours at a price of $600 versus the same amp hours in two 12-volt batteries at $300. So it depends on how often you dry camp and how long you plan to RV.
That is also why so many RVers are going to lithium as the price is coming down to a more reasonable range. Some are below $800 for 100 amp hours, which can be used at 100%. There are cheaper ones on the market; however, I would be very cautious of those. For more information, read the article on choosing a quality lithium battery here.
You might also enjoy this from Dave
My RV’s 6-volt FLA batteries are dry, but service manager says they should be good
In the fall I purchased a slightly used one-year-old motorhome from a dealer. The original owners made one 350-mile trip in it, decided it was too much for them, and parked it for a year before trading it in on something smaller. Knowing the RV had sat unused for a year, I insisted all batteries be checked (one 12V engine battery and six 6V house batteries). I also had a 340-amp solar system installed and wired to charge the batteries even with the battery disconnect off. The dealer said they confirmed the batteries were good.
Read the rest of Jim’s question and Dave’s answer.
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
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My real world experience has been : started out with 2/12 volt batteries,when camping without shore power and not really using much power to run lights and water a minimal amount. We were going dead in just about a day . ( 2am & the propane/carbon monoxide/voltage detector goes off ) before this happened I had both batteries tested & said to be good . Ended up through trial & error had 1 bad battery and that lead to stress on good battery that also soon after failed. Currently 2/6 volts going on 6+ years. Personally I won’t run 2/12’s based on my experience, but may go to a larger single such as an a industrial battery.
not also sure the amps on Dave’s 4/6 calculation is accurate?
Were you running your furnace or something? When we were without hookups using 2/12 volts we would still be well above 50% SOC after a 3 day weekend. We even played the radio most of the time.
I used two 12 volt for about 8 years when one of them went bad. I was able to get another 2 years out of the other. I had always intended on replacing them with a pair of 6 volts but got to thinking if one those goes bad there is no redundancy like I had with the two 12s.
During this period many of the campgrounds we go to have added power so we were not dry camping as much as we used to. So going so long with a single battery was not really a problem for us.
Math on 2x12V vs 4x6V is wrong. Basically, power will be a function of weight.
Yes, math is incorrect. Need to adjust 12V by 50 pct. for apples to apples comparison.
“… maintained properly with water levels and desulfation, ….” I understand the water level part by keeping the water at the proper level but what is the desulfataion part and what do I have to do to the battery as a result? Thanks.
As power is drained from a FLA battery, sulfation occurs on the plates. A typical converter/charger applies 13.6-volts until the battery reaches 12.6-volts, then drops to a 13.2-volt maintenance charge which does nothing for sulfation. To accomplish that, the battery needs a multi-stage charger that starts with a bulk or desulfation stage that is 14-16-volts and boils the liquid breaking up the sulphur on the plates, then goes to equalizing and float. Battery Minder from Northern Tool is a good alternative rather than replacing the converter. It sends high impact waves rather than boiling.
Dave, thats a very informative suggestion that you’ve made by using “high impact waves rather than boiling the water. I would assume that using the HIW you don’t gas as much as you would with using the high 14-16 volts needed to brake down the sulfation that has accrued.
I myself have AGM batteries that have hardly any gas released!
HEY Dave it would be great if you could do a full write up on the battery minder from Northern tool, it has some awesome advantages!
My two 6 volt Trojan AGM batteries have been awesome. Maintenance free and lose very little charge while in storage for months on end.
“High Impact Waves” are snake oil.
Long, long ago, someone built a desulfator with a half-wave rectifier – because they were too cheap for a full-wave rectifier – and theorized that the pulsing would help. It doesn’t. It certainly doesn’t physically knock the sulfur off of the plate for it to dissolve the way the snake-oil guys tell you. It just drives an electrolytic process that converts lead sulfate back to sulfuric acid in the electrolyte and elemental lead on the plates.
Desulfation also doesn’t “boil the electrolyte”. That would take a whole lot of power and make your battery really hot, and ruin it. What you think is boiling is that electrolyte emits hydrogen gas, again as part of the electrolytic process.