2 12v Batteries vs 2 6v Batteries

[Silverbeard]

I was actually thinking of wiring them thru a marine switch: Batt 1, 2, both or none, tho I'm not sure exactly how to wire them this way...

What size and kind of battery wire should I use??

The marine battery switches I'm familiar with have four positions: off, 1,2, all. If using one battery at a time, just use battery 1 on odd days and 2 on even days to keep track of what you doing.

[jbailey]

Here is a good article about wiring batteries. Hopefully it helps.

https://www.batterystuff.com/kb/arti...-tutorial.html

[TheSpareTire]

I was actually thinking of wiring them thru a marine switch: Batt 1, 2, both or none, tho I'm not sure exactly how to wire them this way...

What size and kind of battery wire should I use??

The size and type of wire you should use is based on the length of wire from the battery and switch to the location you need to run it. Even at that I would upsize the wire to give you room to grow in the future if you are going to take the time to replace them. With higher capacity chargers you and the proliferation of lithium you may in the near future want to charger much faster then you do today. I would suggest that the added cost of going from a automotive grade alternator cable to welding wire makes it easier to work with. I typically take my longest run, do my calculation for the number of amps I want to carry with the length and go up to the next larger amp size. If I can upsize and it will be compatible with the converter, switch, etc, I will do that as well.

Blue Sea who makes a variety of isolaters and switch has a fairly easy to read chart that I have used in the past, Wire Size for a DC Circuit.

One thought that I have about isolating the battery, it does not seem like a good idea from my understanding. You will constantly be depleting one battery (regardless of which one) to a lower level then if you had both batteries hooked up in parallel. So if you constantly depleted battery one 40% night after night vs battery one and two 20% you are not putting as much wear on the battery's lifecycle.

It seems the go to product for most people to monitor their batteries is a Bogart Engineering Trimetric battery monitors. If you set them up in parallel and once they are charged and the Trimetric has been calibrated you will know the exact state of each battery. If you have for example two 50ah 12 volt batteries in parallel (100ah total) and use 20ah overnight, then you know each battery has been drained 10ah.

On my old rig I used two 6 volt US 145 XC2/ batteries which I was able to purchase for less then the Trojan T-105's. The claim is the 145 XC2's used thicker plates which should have given you more cycles thus longer life. While I was happy, if I were to do it again I may choose a cheeper alternative but I would still go with dual 6 volt batteries.

Caution if you do go with 6 volt batteries most manufactures require a equalization charge which the normal stock converter on most RV's today won't do. Normally this is known as a four-stage charger and if you have a cheep WFCO converter to the best of my knowledge they only make three-stage chargers. If you have solar and are using a better quality solar controller it may take care of this operation for you.

[WhittleBurner]

We have 2 6 volt batteries that we got a deal on we couldn't pass up. Put in a battery box next to the one already in place for the 12 volt. I leave the 12 volt in place fully charged not hooked up. Now if for some reason we have a problem with the 6 volts it's a simple matter to go back to the 12 volt sitting their ready to go.

[J&G garage]

In determining which type batteries to use you first should do an energy audit of your trailer. For this you will need some sort of battery monitor so you can check the current draw of each 12 volt device in the trailer. With this information you can figure how many hours (or partial hours) a day you use that device. Add those numbers up and you have your amp hour requirements for a day. If your Interstate battery is like ours it's probably between 75 and 100 amp hours. So two in parallel will give you the same 75 to 100 amp hours of useable power. Trojan T-105s are rated at 225 amp hours so two of those in series will in theory give you 225 useable amp hours. So a little more out of the Trojans. The stock battery box will not support the Trojan T-105. It's too small. So a new box would be in order with the Trojans. With two 6 volt batteries you lose your 12 volt system if one battery goes south on you where as you can run some of your stuff on a single 12 volt battery. You have decide the likelyhood of a battery failure for yourself. As far as the 1,2,both switch it is my opinion that you are asking for grief installing one of those. You have to be mindful of the position of the switch at all times or risk under charging or over discharging a battery. Remember you can only use half the amp hour capacity of a lead acid battery before recharging so you would need to monitor their use very carefully if you start switching between them. No problem with leaving them connected together all the time so there is really no advantage to the switch. If you use 12 volt batteries and have a failure of one of them you can simply move the trailer cables to the good battery as you will probably be going into the battery box to troubleshoot the situation anyway. Hope this help and as usual, YMMV.

[avetraveler]

This is a really good and timely thread. I basically have the same plan and will be going with two 12 volt Interstate batteries. I did a basic energy audit (on paper) and it's a little discouraging. Based on what I've read elsewhere I can expect the refrigerator controls to be my biggest energy user. I have those controls listed as consuming about 36 amps per day when running the fridge in propane mode. That's almost half of my available amps.

I want to monitor the batteries very closely the first time out. Is there a thread or does someone know for two 12-volt interstate OEM batteries. What the voltage should read at the following intervals:
Full Charged = xx.x volts
75% = xx.x volts
50% = xx.x volts

This summer is really a proof of concept run (3-month test) and then I'll likely upgrade to a more robust system this fall based on the results.

[Wizbangdoodle]

I've read that 12.7 is full charge and 12.2 is 50%.

Sent from my XT1650 using Tapatalk

[avetraveler]

Thanks Wiz, that got me started and helped me wade through the many discussions I found with many variables. The best I've come up with is:

resting voltage
Full Charged = 12.65 volts
75% = 12.45 volts
50% = 12.24 volts
25% = 12.06 volts
0% = 11.89 volts

Does anyone know of ways to mitigate that 36 amps the fridge controls want to consume?

[TheSpareTire]

12 volt deep cycle batteries are not close to 6 volt batteries. The plates in 6 volt batteries are far thicker and far better at handeling deep cycles. I get the convienence and cost is a factor.

Speaking of I would consider getting a less expensive white label Interstste battery such as Sears Dihard, Autozone Duralast, Walmart Everstart, Costco Kirkland (and more). They are all made in the same factory by Johnson Controls. Find the best warranty to price.

How many amp hours are the batteries? 36AH is a fairly large amount, I don't know for sure but I would have expected on LP for it to be closer to 15-20AH. You may have better data, I know mine is in a slide and apparently has two cooling fans which if true adds parasitic load as well.

The only advice I can give you is voltage will never give you the true measure of battery charge level. If you pull a large load the volts will dip. I suggest you get a hydrometer from the local auto parts store. That will be very accurate to give you the state of charge.

As far as mitigation a fridge fan has always been toted as a way to circulate the cool air faster and keep the fridge from running so often (lowering the AH). I have no proof of this, possibly someone could jump in with their experiences. It would be a fantastic experiment. Filling your freezer and fridge with as much solid material as possible will also helps, no so much you block the airflow. If you have a roof vent, a solar powered fan put into the vent compartment can make a big difference in efficiency, I believe Camco has one readily available.

[SrAirwick]

The most common 2200 series of 6 volts has a capacity of 220-230 amp hours a 6 volts. When put into series, you are increasing the voltage to 12 volts and the amperage stays the same. So the two batteries become one 12 Cell basically. The footprint of the two 6 volts is exactly the same as two RV24 batteries. The 6 volts are taller and heavier than the 24 series giving them more capacity, almost three RV24s and just over 2 of the larger RV27s. The 6volt plates are larger, being more resilient to charging and discharging it isn't uncommon for people to get 8-10 years out of a pair. But YMMV, you have to do maintenance on them. Keep a jug of distilled water nearby and check with a hydrometer on a regular basis. Try not to kill your batteries completely dead also. That will reduce the number of effective life cycles that you battery go through.