Batteries: Two-6V In Series, or Two-12V In Parallel ???

[ron_dittmer]

Before spring I will be replacing our two 12V Walmart coach batteries. Our 7 year old rig has it's original Tripplite 2000 watt inverter/charger similar to THIS ONE. I was thinking of replacing the two 12V batteries with two 6V batteries as these forums seem to favor 6V in series compared to 12V in parallel. I called the maker of my inverter. The technical person there told me two 6V batteries in series will work just fine with the inverter.

But I have a concern switching from two 12V to two 6V batteries, this in regards to the drop in amp hours. Assumed identical amp-hour ratings, two-6V will be 1/2 that of two-12V. Currently our inverter adequately supplies power to make a pot of coffee and watch TV with a sound system. Will two-6V batteries in series be able to meet those demands the same as the two-12V?


In General....

With a rig like ours that has a 2000 watt inverter and no solar panels, what benefits and drawbacks might there be in using two 6V batteries in series? Keep in mind that we almost never hookup to shore power. Battery charging is done while driving and when parked for days at a time, running the generator.

Will it take more time or less time to charge 6V batteries?

How does the physical size of a 6V battery compare to a 12V? I hear they are taller. Do they take up the same footprint in our battery tray?

I am especially interested in hearing from people who made the switch from 12V batteries to 6V. Also, I'd appreciate your input on the best 6V batteries to buy, where to buy them, and what I could expect to pay per battery.

Thanks!

[DiploStrat]

pnichols wrote:
… No matter what the source, whenever the coach's 12V system voltmeter indicates that voltages greater than about 12.8 volts are present and the ammeter indicates coach battery current is zero or only slightly positive ... then the coach batteries are fully charged. This is a very simple and full-proof battery condition monitoring method that has worked well for us for years on AGM coach batteries.

With the addition of the ammeter, I agree completely.

But, as noted, a voltmeter alone will not give a great state of charge in an active system. It is fine for a resting battery, but batteries installed in an RV are rarely, if ever, at rest.

An hour counter is useful in giving you an estimate of how far down you are and how much longer your charge will take. (But even this is only a guestimate depending on the algorithms your meter uses.) An hour counter is also useful in assessing the impact of a given appliance, like a microwave.

At the end of the day, the most important thing is that your charger(s) have a reasonable profile and that you charge your batteries fully and often.

[time2roll]

With a bit of practice the voltmeter is just fine.

I owned a car with no fuel gauge.... I would watch the odometer.
250 miles and you fill up. Not a big deal.

Ultimately I would prefer a larger fuel tank over a more accurate gauge.
JMHO

[pnichols]

DiploStrat wrote:
-- A volt meter is interesting, but very limited. When on charge, it tells you the voltage of your charge source, not your battery. And the rest of the time, it gives an artificially low reading as most RV's are discharging all the time. Your refrigerator is probably you biggest load.

What you say is why I have a permanently wired ammeter in our RV that constantly monitors the current into (positive ammeter readings) or out of (negative ammeter readings) the coach batteries .... along with a couple of voltmeters that monitor both the voltage on the engine's 12V system from it's alternator and the voltage on the coach's 12V system.

The voltage on the coach'es 12V system can represent that from the batteries only, or represent that from the engine alternator, or represent that from the converter powered by the generator, or represent that from the converter powered by hookups.

No matter what the source, whenever the coach's 12V system voltmeter indicates that voltages greater than about 12.8 volts are present and the ammeter indicates coach battery current is zero or only slightly positive ... then the coach batteries are fully charged. This is a very simple and full-proof battery condition monitoring method that has worked well for us for years on AGM coach batteries.

[DiploStrat]

Having a camper with an electric stove and no genset, I tend to obsess a bit about this stuff.

Two comments:

-- When should I charge? Whenever you can and as fast as you can. Lead Acid batteries do not like to be discharged and leaving them discharged is an easy way to convert them into paper weights.

-- A volt meter is interesting, but very limited. When on charge, it tells you the voltage of your charge source, not your battery. And the rest of the time, it gives an artificially low reading as most RV's are discharging all the time. Your refrigerator is probably you biggest load.

A hour counter, such as those made by Bogart, Victron, or Xantrex is a really, really useful investment, even with all of their failings. Otherwise it is as if you are trying to determine how much fuel you have left by looking at the speedometer.

[ron_dittmer]

Okay, I measured 12.74V at the pair of 6V batteries using a good quality volt meter. The volt meter I installed on the kitchen stove hood read 12.70V. So it is reading a tad bit less which is to my benefit, indicating the batteries are 0.04V less than they actually are.

I also bought a 12V plug-in volt meter that reads to only one decimal place. That meter read 12.8V. The 12V socket happens to be located in another area of the rig which may have changed the dynamics a little differently.

On our next trip, I will see how it all goes with loads and such.

I think all is well and I am good to go, ready for the next adventure, and a little better prepared in this regard.

I am really pleased with the change-over from two 12V lead acid batteries to two 6V sealed AGM batteries. And now I can monitor their drain better, charging them before any damage is done.

[ron_dittmer]

I hear you DrewE. I will take your comments into account. Thank you!

I really wanted the green LED over the other colors for we have other green LED indicators from the various detectors mounted around the rig, which provide a green glow at night. This new display will increase the green night light effect. Fortunately it is mounted around the corner from our bed, out of direct view from either pillow. With our dark cherry cabinetry, there should be minimal light reflectivity. If we later determine the light is unbearable at night, then I will add an on/off switch for it in the upper right corner of the control panel. Given the clock is powered by a button battery, no worries over resetting the clock every morning.

[DrewE]

I have the same exact model of voltmeter (well, nearly the same--mine uses blue LEDs for the display, because that was a buck cheaper at the time) waiting to be installed. When I compared mine with my Fluke, the two agreed quite closely, with about a .03 V difference as I recall. Depending on where you're measuring the voltage at in the system, you could very easily get a larger difference and variation just from wiring losses, particularly if there's some nontrivial load on the particular 12V circuit. I'll be running a separate (properly fused) 12V sense line and a dedicated ground for the meter to avoid this.

(Obviously, with these cheap meters, the calibration may not be consistent between units. The advice to check individually is good.)

[ron_dittmer]

Thanks for the advise Dakzuki. I'll be following your instruction to get familiar with the reading, variation, & accuracy.

[Dakzuki]

Ron, One thing you will want to do it double check your new voltmeter against a trusted hand held unit right at the battery when you are on your next trip. I used three points to validate mine....charging ( the high number), at rest (mid number), and discharged (low number). If the numbers match between meters, you're good. If there is an offset (hopefully consistent between the three numbers), just keep that in mind when interpreting the numbers on the new meter. Also remember resolution to two decimals does not necessarily mean accuracy to two decimals. Two decimal resolution can be helpful interpreting trends as voltage goes up and down. A bit nerdy but test instrumentation is my day job.

[tenbear]

Thanks Ron. I have had some thoughts of adding water pump switches to the one on the hood but could never get the panel off. I tried prying it but was afraid of breaking it.

[ron_dittmer]

tenbear wrote:
Very nice job Ron. Really professional!

One question. How is the panel attached and how did you get it off? I don't see any instructions under the hood in my kitchen.

Thank you for the compliment tenbear.

To remove the panel without damage to it, you need only one tool, a plastic putty knife or equivalent.
1) take off the exhaust fan control knob.
2) Using the plastic putty knife, Pry at the left side of the black plastic panel. It will unsnap there.
3) Once that end is unsnapped and away, slide the entire panel to the left and it is released.

[tenbear]

Very nice job Ron. Really professional!

One question. How is the panel attached and how did you get it off? I don't see any instructions under the hood in my kitchen.

[ron_dittmer]

O.P. Here,

I installed a voltmeter just the way I wanted. Here are the details I posted on the Phoenix Cruiser forum.

On our 2007 2350 with a new pair of 6V AGM batteries, I was inspired to add a volt meter to better monitor them.  The simplest way would have been to plug a nice little round one LIKE THIS ONE into the 12V outlet located inside the cabinet to the right of the main TV.  But I wanted something "In My Face" and the control panel in the stove hood seemed the ideal location.  I removed the panel following the instructions on a sticker located under the hood.  Doing anything different would have broken something.  On the back side of the blank space were knock-outs for features.  So I did a little Ebay shopping and found something to utilize those knock-outs.

Here is my control panel before.  Note the large blank space available to the right.


Here is the back side.  Note the knock-outs for switches and a display.


THIS is what I installed.  It has a 3-function display.
- Voltage to two decimal places
- 12 hour clock that can display the seconds (backup battery retains the time)
- Thermometer in Degrees F
It comes with a 2-switch micro-switch panel to operate the display, but I bought separate spring-back switches on Ebay HERE to match the others which fit right into the available positions.


Here is the finished product










I was surprised how quick and easy the project went.  I had to be careful when cutting for the display because it was a little smaller than the knock-out provided.  The switches were easy, just cut to the plastic frame structure size.  One thing caught me off-guard.  I assumed the primary control panel wires had red as power and black as ground, but that was not the case.  When tapping into power and ground, first ohm out which is which.  Doing so the first time would have saved me an hour's time of re-work.

I did buy one of those little round plug-in volt meters and it is quite nice.  I will be plugging it into my dash board cigarette lighter and see how I like it on our next excursion.  It will be interesting to watch the voltage changes when the alternator switches over to charge the house batteries.  It seems it will not get in the way of shifting gears because it does not stick out that far.

If you like the idea of having a volt meter to monitor your batteries, the simple plug-in one located to the right of the main TV will do the job just as well as the fancy one I installed.  Just keep a close eye on it.  As I understand it, the general rule monitoring voltage is when it reads 12.1V with all things turned off, you better get your batteries charged right away or they will soon get damaged.  Fully charged batteries are around 12.7V.  I will know better how to interpret the voltage readings by the end of our next trip.

[Dakzuki]

tenbear wrote:
Let's talk real life.

It's 3 PM, your voltmeter reads 12.2v. You have minimal electrical loads, just the refrig and the CO and propane monitor. Do you recharge now, disconnect the battery for 4 hours and check the voltage (and hope the ice cream is still not melted), or wait until morning?

In 4 hours maybe the limit on generator noise will start. In the morning the voltage will be still lower. worry, worry, worry.

I guess you know my answer.

12.2 volts and 4 hours to go? I charge. When I'm dry camping in the winter I start charging as soon as we are up and about.