Battery Bank/ Inverter Question

[howardq99]

My goal is to build a semi-portable system to transfer between my house and my trailer. I have twelve 285W Silevo solar panels four of which I will be mounting on the trailer and eight for the house. I will be using a Midnite Solar Classic 150 charge controller, four 6V Fullriver DC250 AGM Batteries and I have one Xantrex Prowatt SW2000 12V Inverter.

What I am hoping to do is to configure the four batteries in series for 24V to be able to maximize the power harvest of the eight solar panels at my house (the Midnite string sizing tool will allow this 24V battery configuration with four parallel strings of two panels). If I drop the battery bank down to 12V I can only use four panels at a time.

My question is can I configure the 12 volt inverter to be used with a Blue Sea 9001e dual bank battery switch to pull power from each 12V set of two batteries (so to speak a sub-bank 1 and sub-bank 2) while keeping all four wired in series? The Blue Sea switch has a setting for bank 1, bank 2 or banks 1+2.

Thanks for your input!

[howardq99]

CA Traveler wrote:
Yup, AZ is a solar issue these days IMHO. Ironic don't you think with all of the sun shine available!

Living in one of the sunniest cities in the US, solar costs plummeting and you can't save any money by going solar, very sad day in AmeriKa.

[CA_Traveler]

Yup, AZ is a solar issue these days IMHO. Ironic don't you think with all of the sun shine available!

Have you considered just using the RV during an emergency situation at home? If the power is out no one is going to deny your TH in the community. In fact they will begging you for some power.

[howardq99]

time2roll wrote:
With 2.2 kW on the house you really want a grid tie inverter to maximize solar harvest. All 8 can easily be in series and the Midnite 150 can just stay on the RV. Just the four on the RV is still a lot for just 4 batteries.

Sunny Boy 3.0

I've looked at those Sunny Boy transformerless grid tie inverters and really like them with the secure power supply feature. Unfortunately our local power company here in Phoenix has restructured their rate plans to make it so if you generate any power you'll never recoup your costs and save money if you sign on with them and interconnect to their grid.

[CA_Traveler]

Given what you've now posted your best short term option is to use 4 panels at home and use both home and RV as 12V systems and live with that limitation if you ever have to as an emergency home backup. Not what you want to hear but...

And more: Don't buy those batteries and here's why. Go to some of the off grid forums and really study what they tell newbies like you. They will tell you to buy the cheapest possible batteries because you will ruin them while learning how to maintain them. ie It is what it is. And tell them you want to switch between 12V and 24V - who know they might have some good advice for you. Not what you want to hear but...

[howardq99]

Thank you everyone for your input, advice and suggestions!

I’ve had family obligations so I’ve just now been able to read the replies...

So for the short version:
Mr. Wizard, now that I’ve looked at the wiring diagram of the Blue Sea 9001e and I see what you are saying about only the positive leg.

Do you think I could use something like a DC combiner in place of that battery switch? Use equal lengths of wire from both banks of 12V jars (two 6V batts in tool box to make 12V bank) in parallel to a DC combiner and then on that output connect the 12V inverter? If I keep all the parallel cable lengths equal would this keep the batteries balanced by drawing from both jars equally?

Long version:
What I am trying to do is make a portable system that I can use in the trailer when we go camping (always boondocking, many weekend trips with a 2 week trip every year) and then…

Its MAIN use and purpose will be as an emergency backup at home. For emergency backup I’ve done an energy audit and the four Fullriver batts are the right size to run my critical systems. Combined with the Midnite charge controller and the 8 panels I can easily juice the AGMs at 20% of their C/20 Ah rate to keep them happy. I know this is overkill but it will allow me to run additional loads while the sun is shining and charge the batteries too.

I’ve already purchased the 12 panels and the 12V inverter, I'm about to pull the trigger and order the Midnite Charge Controller and the four Fullriver batts. Unfortunately, that just about blows the solar budget for this year so a 24V inverter (looking at a Magnum Hybrid) will have to wait until next Fall.

I’ve got no problem moving the gear back and forth as I am building this out in portable cases. I will have the batts in two separate case, each a big 12V battery (130lbs!). They will be on caster wheels with equal length 2/0 cables used to interconnect (along with proper fusing and circuit breakers). My trailer has a toyhauler ramp in the back so I can easily wheel the batts and gear in and out of the trailer when it’s campfest time. The trailer is too long to fit at my house so I can only really keep it overnight one night maybe two nights in a row at most.

If you look at these pictures

https://www.amazon.com/photos/share/MCon3qJOmwoCuIHPWqvCKVcMZdfjXVIuZHJ4Ibw2nB0

you can see I’ve built a test unit with Rigid tool boxes, two T-105s, a Tracer 4215BN charge controller and the Xantrex 12V SW2000 inverter. I’ve used 4/0 wire from the batts to inverter, 2/0 between the two 6V batts and 4AWG from the charge controller to the batts.

This set-up works great but it is not enough to keep up with all the critical loads at my house. I also want to dump the maintenance and gassing of the FLAs and go with AGM batteries.

So until I can spring for a 24V Magnum inverter next year my hope WAS to put two 6V AGMs into one Rigid case to make on large 12V jar. I would make two of these big 12V jars (let’s call them jarA and jarB). I would connect jarA and jarB in series to take advantage of the Midnite charge controller and the 8 solar panels . Then connect (+) and (-) cables from jarA to the Blue Sea battery switch bank1 and connect (+) and (-) cables from jarB to Blue Sea Battery switch bank2. I would connect this to the inverter and always keep the battery switch on bank 1+2 so I would always be using each bank equally. I was hoping this would let me use the current 12V inverter until next year.

[BFL13]

"What I am hoping to do is to configure the four batteries in series for 24V to be able to maximize the power harvest of the eight solar panels at my house (the Midnite string sizing tool will allow this 24V battery configuration with four parallel strings of two panels). If I drop the battery bank down to 12V I can only use four panels at a time."

I think I finally figured this out-- ?

OK, I get that the controller can do twice the watts with 24 than with 12. This is because the amps out is the same, and the controller has the same amps limit either way.

However, the Voc limit stays the same too, so the max number of panels you can use in series doesn't change between 24 and 12. The OP got me going by saying the number of panels changes.

So if he can only use four panels in series with 12 but he can use eight with 24, it is the power limit for 12v there and perhaps the Voc limit of the controller; he goes series/parallel to stay inside the same Voc limit when using eight with 24 at twice the power.

EDIT--I see he is using two- panel strings, so the Voc limit is with two, so his four on the RV must be also series /parallel.

The effective AH of the four batteries doesn't change so you could recharge them faster with the same amps at 24. But they won't act as storage any better for the four or eight panels. No idea how much storage is needed for the stick house. Just making it a 24v bank won't increase the storage capacity. It does allow you to have twice the power to use while it is coming in even if there is no place to store it all.

You need a certain amount of AH size bank to run the inverter in the RV. You need enough battery storage to get through the night. You need enough solar to run whatever during the day plus store some more for overnight.

[MEXICOWANDERER]

24 volts can be readily bucked to 12 volts for powering rig electricals. If you had a motorhome you would lose alternator charging but it's a trailer. Generator battery charging would entail some planning. But only you can decide whether 24 volt configuration has more advantages than disadvantages. Automatic voltage reconfiguration is extremely complicated and costly.

[time2roll]

With 2.2 kW on the house you really want a grid tie inverter to maximize solar harvest. All 8 can easily be in series and the Midnite 150 can just stay on the RV. Just the four on the RV is still a lot for just 4 batteries.

Sunny Boy 3.0

[CA_Traveler]

OP, Not clear to me what you want to do: Is it 8 panels and 24V at home and 4 panels and 12V in the rig both with a 12V inverter? If so I don't see any good options.

But you could have a 12V and a 24V inverter. You could use 24V in the rig allowing for a single 24V inverter and a 24-12 converter for the rest of the rig. A 24-12 converter for a 2K inverter would be a lot of amps and maybe not a good option.

Others are correct that it's a big job to move the equipment and 4 panels at 1200W is large for 500AH (at 12V) of batteries.

[MrWizard]

Not with that switch
It's either/or/Both for only one leg, the positive

You have to switch both wires positive and negative back and forth between from one pair to the other

[time2roll]

BTW that is a lot of solar on just 4 batteries.

[KD4UPL]

You are really making things complicated for yourself. I'd just use a 24v inverter in the RV. Trying to pull 12v out of a 24v bank is asking for trouble. The bank will get out of balance very quickly, shortening it's life.
I take it you're going to move the charge controller, batteries, and inverter back and forth between the RV and house? I hope not often. That's a lot of work to do it once. I can't imagine doing it several times. Why not leave it all in the RV and fix up a plug to connect the additional 8 panels on the house when the RV is parked there. You can then plug in whatever home load you were going to power to the RV system as it sits there.

[BFL13]

Brain trouble here. 😞 I know that the controller allows twice the wattage of panels when batts are 24 vs 12.

I know that half the amps charge 24v batts the same as twice the amps do 12s. So if you do 24-24 at 8 amps DC output that is as good as doing 16 amps output with 24-12.

But where does it say you can have twice the wattage on the same set of batts by going 24 instead of 12, so you get the same amps---which is worth twice as much for recharging?

Please explain. I know I am having a problem here! 🙂

[2oldman]

howardq99 wrote:
can I configure the 12 volt inverter to be used with a Blue Sea 9001e dual bank battery switch to pull power from each 12V set of two batteries (so to speak a sub-bank 1 and sub-bank 2) while keeping all four wired in series?

In theory it can work, but it will be a real challenge to keep each set of two in balance to be properly charged at 24v.

I've done something similar..drawing off 1 12v battery in a series string of 36v. Things get out of balance very quickly, and series charging is not very forgiving.