Hooking Solar to Dual Batteries?

ScottG

Of late I have been revisiting the idea of carrying a spare battery. I'll detail my thoughts on that in a future post, but for now I have a question for you solar gurus... 

The basic set-up is diagrammed in the schematic below. To maximize the versatility of this system, I would run both batteries through a four-position switch that would allow connection to either battery individually, or to both at the same time in what would be a parallel configuration. In this scenario, the pigtail to the spare battery would be no longer than a few feet.

If the solar is only connected directly to the main battery, is this set-up adequate to charge both batteries when in dual battery mode? Stated differently, is there a compelling reason why I should ensure the solar is connected directly to both batteries? In terms of electrical continuity I can't see a difference, but I suspect solar may have some other considerations that are beyond my current understanding. 

Thanks in advance for any opinions or info.

CharlieRN

Your diagram shows the solar panel connected directly to the battery however, the output from the panel(s) to the battery needs to be via a controller, not directly. If the batteries are connected in parallel as the diagram indicates, I believe that the controller will see it simply as one large battery and charge both. That said, it also looks like your 4-way switch can isolate batteries. If this is the case, then the controller would not be able to charge both batteries in some switch positions. I'm sure others with way more experience will weigh in on this with more helpful information.

ScottG

I do have a controller--I just left it off the diagram for the sake of simplicity and laziness. Sorry for the confusion!  :-)

And you are right about the 4-way switch--if only the spare were connected to the camper, then the charge from the solar would not reach the spare in this arrangement. 

Ideally, I'd like to set the solar up on a similar switching arrangement, but that may be getting a little too elaborate for the number of times I expect to need to use the spare.

Denny16

And in lieu of the 4-way switch, all you need is a Marine A/B house battery switch, which has a charge source connection that by-passes the switch A/B position to keep both batteries on the charge circuit (solar controller). Yiu can get one with an off/A/B /Both positions also.  This is what I would use, and they are waterproof so could be installed in the front Tub with the batteries.
cheers

jkjenn

A little different but similar, Scott, I have used my GZ Lithoum battery to charge my T@b batteries via a smart charger. I already had a bus bar for my connections, so I added an the smart charger quick connect to the bus bars. Similarly, I added A Denson plug ls for my solar connection. When not using the Yeti, they were just left unplugged.there was no need for a switch.

rfuss928

The switch Denny16 describes is on the right in this photo.  In my setup the solar input switches with the battery selected.

DougH

@ScottG Looks good to me.  I have two batteries in back connected to the inverter, solar charger, battery monitor.  Then thick cables going through a switch and up to the battery in the tub that's connected to the shore power converter and 12V bus.  When batteries are bridged the shore power or solar charger can charge all three. Or I can isolate the solar system from the stock electronics. I think I had a reason for doing it at the time... maybe returning the T@B to stock slightly quicker for resale, or some such nonsense.

ScottG

Thanks, all. I have the switch in hand and it is is indeed like that described by @Denny16 and @rfuss. My question (perhaps not clearly stated) was less about the switch and more about the hook-up of the solar--specifically whether hooking the solar to the main battery would adequately charge the spare battery wired several feet away.

After a little more poking I think I have determined the answer is maybe. It seems the correct way to attach a charge (or load) to parallel batteries is to attach the leads on different batteries (see image below). Theoretically, this better balances the charge/load over both batteries.

 
Not that doing it the other way won't still work. @DougH, it sounds like that is how your set up is wired when your three batteries are bridged--the battery in the tub is some distance from the interior batteries where the solar is actually connected. Do you find that--practically speaking--all three batteries still charge just fine?

That is really the root of the question. There are a lot of things that are theoretically best practice, but in reality, the operational differences are negligible.  

DougH

@ScottG Yes, the more distant battery charges fine. I tested it both ways, covering up the solar and letting the stock shore power converter charge the front one, and after running through 8' of cable the rear set... and then with solar enabled the other way. Both ways always within 0.1V of each other when at rest. That let me know that the wiring was thick enough between the batteries.  I also tested separating the batteries and charging one set or another, then eventually bridging them to see how soon they'd equalize. I was concerned if I hooked up a 13.8V battery to a 11.9V battery that there would be a sudden rush of current that would blow the inline fuse between the two. Nope. Worked fine, and they reported the same voltage when both charged and reseparated the following day. This was with 125Ah AGMs. I was mildly surprised it worked so well... but I guess it takes more effort for Mr. and Mrs. Electron to charge a battery than to travel down a 4AWG cable.  But I do nonetheless use leads on different batteries for the rear pair, because I'd read that it's the better practice, even if it doesn't seem to make much difference on my 8' apart test case.

ScottG

Thanks, @DougH. I was also wondering about the effects of hooking two batteries at different states of charge together for charging purposes, but you seem to have real-world success in that are as well.

Though my knowledge of this topic is limited (but growing), I suspect the 4AWG was a factor for exactly the reasons you stated. I may consider making my pigtails a little heavier than planned!

DougH

@ScottG I was a bit nervous using 4AWG instead of 0000AWG or equally ridiculous thick stuff that would have added considerable weight to the camper. Consulted several AWG charts on amperage and distance for 12V, and for pulling 50A like running a couple major appliances, 4AWG allegedly would be insufficient. But I only charge at 10-20A with my solar controller, and don't see over 250W of solar from the roof, and I'd read that even when plugged into a 30A shore power circuit our stock WFCO converters never charge the batteries at anything close to 30A either. But I wouldn't feel safe rerunning the experiment with 10AWG or thinner, especially for eight feet distance. I suspect using the really thick stuff is more important for the battery to inverter very short connections, where there may be sudden 100A surges.

rfuss928

If the batteries are connected in parallel to the charge controller, the selector switch will not do anything.  The parallel connection will share both the charging current and the load.  The solar output needs to be connected to the convertor side of the selector switch to keep the batteries separated.  It is basically impossible to charge both batteries with a single source while drawing current from only one of the batteries.

ScottG

@rfuss928, my intent (albeit not well reflected in my diagrams) is that the charge current would only pass through to the spare battery when the two batteries were connected in parallel through the 4-way switch.

However, as I think it through, the simplest and most versatile option may be to just put separate, dedicated MC4 solar pigtails on both batteries. Since my current solar set-up is highly modular anyway, that would allow me to hook up either battery individually (regardless of which one was running the camper) or--if the batteries were joined--properly charge them both as shown in my second diagram.

I'm probably making it sound more complicated than it is. Ultimately it's a pretty low tech solution mostly controlled by simply plugging and unplugging and switching various connections as needed. Cumbersome, perhaps, but probably adequate for what I envision as more of a highly versatile back-up system that will probably get little actual use.

I appreciate all the input!   

Denny16

Doug, nothing in the TaB draws 50 amps of DC power, it is fused for 30 amps, so 4AWG is fine.
cheers

Jax0913

Late to the party on this... but love the idea. I have the same thing. 200 watt solar coming into a charge controller connected to the factory installed group 24 battery in my 400. I would love to add a separate battery and also use solar to charge that if necessary. @ScottG did you give up on the switch idea? 

Denny16

The factory nüCamp installed battery on a 2018 TaB 400 is a single 12 VDC group 31D, 224 amp hr AGM battery, and the later 2018 (after Feb 2018 builds) and 2019 onwards have two group 24 AGM 200+ amp, 6-VDC AGM batteries, wired in series. Which do you have?

We have a 2018 TaB400 with factory solar, and I was going to add a second battery to the single group 31D 200-Amp AGM battery.  But after camping with it, and one with three days off grid boondocking, I discovered the solar will keep the battery charged, even in the fall partial shade, getting mostly afternoon sun.  The battery never went below 35% discharge, so a second battery is not necessary for most camping situations.
cheers

ScottG

Jax0913 said:

Late to the party on this... but love the idea. I have the same thing. 200 watt solar coming into a charge controller connected to the factory installed group 24 battery in my 400. I would love to add a separate battery and also use solar to charge that if necessary. @ScottG did you give up on the switch idea? 

@Jax0913, nope. I've got the switch but beyond that I haven't given this project any more attention. I had to get my glycol change completed, and next on the agenda are my wheel bearings. I'm looking forward to making improvements again rather than just doing overdue maintenance!  :-)

Jax0913

@ScottG I hear you! If I go down this rabbit hole, I will certainly give you a tag to follow the process.

Jax0913

@Denny16 We have the single 12 VDC group 31D, 224 amp hr AGM battery and I have two flexible 100 watt panels installed on the back between the stargazer and the fan vents. In full sun, no problem. Those panels top us up perfectly running the 2 way fridge. We do a lot of Adirondack camping and boondocking and are not always in full sun, mostly shade, and had a trip last year where it rained two days and didn't get enough juice to top us off. This past weekend we camped in eighth lake, beautiful site, mostly shade. Ran the 2 way the whole time. Battery dropped to 67% by the time we packed up yesterday morning. I think we could have pulled enough to top up again but wasn't sure. I was debating adding another panel on the front above the widow before the vents to add a little more juice to the system. A suitcase might be a better option where I can move it around with another charge controller. I don't love the idea of a suitcase just because I really like pulling in and not having one more thing to unpack. 
 

Denny16

Yes, I hear you.  But in your situation (and sometimes mine also) camping in the woods gives little options.  I added two portable 100 watt foldable panels (not suitcase) which are more compact than a suitcase type, and added the second Victron controller to the SAE (Zamp type) plug installed next to the rear cargo door, where the battery is.  There is room in the 2018 for the controller also.  You can access this area by lifting up the Pax side rear bunk panel up.  If you do not have the (then optional) SAE plug, it is easy to install one.  

cheers

jrhamel

@Jax0913, Just got back from our first trip using suit case solar. 
We spent a week at Center Lake Campground in Custer SP in a mostly shaded site. The 190 watt roof top panel, alone, would never have been able to keep the battery charged enough but with the 200 wat Renogy suit case our batteries were almost always fully charged. Yes, it is a bit more work to have to move the panels around but it was nice to not worry about running my batteries low. The only downside to the 200 watt suit case is its a bit heavy. 

Sharon_is_SAM

Two 100 watt solar suitcases are easier to manage than one 200 watt solar suitcase.

ScottG

As a related aside...

I just replaced my still serviceable but aging battery (now serving as the spare) with a new group 31 wet cell. First trip was four days of clouds and rain so thick I didn't even bother deploying the solar panels.

The battery started the trip settled in at 12.9V. After four days/nights of using the fridge on propane, running the Alde occasionally for both HW and heat, and normal (judicious) use of lights and water pump, we broke camp with the battery still above 12.5V.

I'm not campaigning for one set-up over another, just pointing out that with a good (and relatively inexpensive) standard deep cycle battery and a three-way fridge, one can dry camp easily for a long weekend.

Jax0913

Nice! I had toyed with the idea of swapping to a 3 way for that reason, but the common insight was the 2 way is more efficient and I should be able to maintain the battery with solar. I did realize yesterday that I am having some sort of issue with my two 100 watt renogy panels. I haven't popped up on the roof yet to check connections and throw a multimeter on my panels, but in somewhat cloudy conditions on my driveway, which is wide open, my panels were registering 12.4 Volts 0 amps and 0 watts. Ugh! So now I have this as well. I like the idea of adding an SAE plug and controller in the storage bed and a suitcase, once I figure out my issue right now. 

ScottG

@Jax0913, though it's getting a little out of my pay grade, your findings may not be abnormal. I commonly see 12.xV from my panels in cloudy conditions. However, if you were measuring panel output and there was no power draw, it stands to reason you would see 0A (and therefore 0W) as you would have voltage but no current. I think it all depends on exactly how your were measuring things.

Jax0913

@ScottG Interesting thought... Although my battery was showing at 2/3 or around 72% according to my app so I assumed it would be showing some power coming in to charge the battery.

ScottG

Agreed. It wasn't clear to me that you were hooked to the battery, which should be drawing some current--however small--for charging purposes.

Under low light that current can be pretty small. After confirming all your connections are good, I'd check it again in full sun.

Jax0913

Thanks! Will do. Hopefully we will have some light this afternoon so I can dig in a little and check. 

Marceline

Denny16 said:

 I added two portable 100 watt foldable panels (not suitcase) which are more compact than a suitcase type

I'm in the market for new solar. What panels are these? Would you recommend them?

Denny16

Not sure Marceline, we got them off a Kick Starter campaign.  I will try to get your the name and source.  They were a good deal, and came with a case.
cheers

Jax0913

@Denny16 When you added your suitcase and Zamp plug did you put a fuse on the positive coming out of the charge controller to the battery? I don't have fuses on the battery side just the panel side and am trying to figure out if I need to add them.