Opinions on 2021 T@B 320S Boondock Alternate Battery Configuration ... for Early Learning

mntrails

I have a new 320S and it came home with the standard Interstate Group 24 SRM-24 RV/Marine battery purported to be ~80 Ah (?).  I happen to have a nearly new good quality Group 27 Marine Deep Cycle battery with a capacity of ~90 Ah.  

I understand the best solution is lithium batteries, followed by 2 6V Golf Cart batteries (either glass mat or wet cell), etc... but we're just learning how this T@B works and don't know what our usage pattern is, and  my thinking is that while I'm in learning mode, I could add in the Group 27 with a Marine 4 position battery selection switch (Off, Batt 1, Batt 1&2, Batt 2) and use each battery separately (connected to the switch in parallel obviously).  

This really only makes sense because I have the Group 27 in hand but I see it having the following advantages:
1) If we make a mistake and use a lot of battery power by accident, I have a backup (switch to the alternate battery)
2) If plugged into shore power or are around a 120V outlet (we're thinking this is infrequent as we initially intend to spend short trips of 1 or 2 nights boondocking), I have a small compact smart charger that I can hook to the disconnected battery and charge it.  The 'battery in use' would be charging via solar or the converter.
3) I don't have to spend $$$ to get more battery capacity starting out, I just use what I already have.  (After a year or two, if we boondock a lot, then upgrading to a better battery solution would be the natural progression)
4) I don't have to cart a generator around as a backup - I'm really not a fan of the noise.  Even the quietest ones don't fit with my idea of being in the outdoors.  I've used a Honda during rural power outages and I know this from experience.

When switching between batteries, I would cycle the switch from Batt1, through Batt 1&2, then Batt 2, and thus avoid a 12V power outage.

Does anyone see a problem with doing this or some issue that the T@B electrical systems wouldn't like?  As an electrical engineer, I don't see a technical problem but ... there's a lot of smart systems onboard the T@B that I'm not familiar with so figured it was best to ask. 
Thank you

Sharon_is_SAM

My humble opinion.  If you are going to spend only 1 or 2 nights out boondocking, it would be the perfect time to assess your usage and determine your future needs.  Sounds like you have solar already.  I say try it on your original battery and make changes later.  We boondocked out west and found we used only about 16 - 18 amps a day.  We use Lucci solar lights inside at night and in our side tent.  Of course our MaxxAir fan And ARB frig draws less, so that accounts for limited usage.

Dalehelman

For for years I have used two 12 volt batteries as you suggest. The boating industry has done this as long as I can remember and I am 75. When on shore power I can charge 1 battery at a time or both at the same time. By the way the batteries are different sized. This has worked for over 5 years.

db_cooper

Sounds like a good idea to me. If you find you need more juice for your type of camping, then you'll have an idea of what will work for you.

ScottG

Sounds like you have a good system. No additional investment and a solid backup plan (extra battery). I agree with you regarding generators!

I've dry camped up to nine days with a single group 27 wet cell and a 100W solar panel. Others have gone much longer. As long as you get some reliable sunshine every so often you can probably go indefinitely with this simple set-up.

I'm not an electrical engineer but see no problem with your proposed configuration. As long as the two batteries are wired properly, they will still only deliver 12V as you cycle through the both of them. There might be slight fluctuation in voltage as you do so, but no more than you would have with turning on a charger, illuminating a solar panel, or turning a battery on or off. I can't see where any of the T@B components would be unable to handle that. Of course, if in doubt, you could always turn things off before making the switch-over. Though I guess that does defeat the purpose of wanting to maintain uninterrupted power...

mntrails

Thanks to all for the feedback and real-world experiences.  Very helpful.  I'm going to go ahead and put in the switch and box for the second battery and try that configuration.  I really like the 'backup' idea ... just my personal preference :-) .

As part of this reliance on solar, I found the video for the Victron BMV-712 Battery monitoring system that gives one a much better idea of battery capacity status.  I had to laugh at the author's comment that said he likes to mount the monitor with the batteries rather than in the RV living space to avoid the tendancy to fixate on running a solar power station - good words for me to remember!  Oh, and I don't have any plans to install a BMV-712, just want to enjoy the T@B and learn how to use it to go where we want to.

ScottG

@mntrais, I had forgotten about this discussion until I followed your link from another recent thread. I know you have since upgraded to dual 6V batteries, but I'm wondering how your spare battery set-up worked for you last season, and how often you ended up needing to use it.

mntrails

@ScottG, the backup battery worked well for me - I took 6 trips for 2-3 days each and ended up using the backup 4 times.  The 2 times I didn't use it, I was staying in a driveway area at friends (but was not plugged in). The driveways were fairly sunny and on those occassions, the solar recharging was all I needed.  The other 4 trips were all in state forest or church campgrounds and heavily wooded.  The stock Group 24 Interstate battery would last between 24-36 hours for me before dropping below 12.2V and I'd switch to the backup.  

I think with care, I could certainly keep using this setup. After living with it for a season and knowing that I will take longer trips and continue to boondock for the majority of them this summer, I decided to go with the golf cart batteries and have a bigger reserve. 

ScottG

Thanks, @mntrails --good to know that it worked for you. It does sound like in your case stepping up to the dual 6V GC batteries was a logical next step.

I've pondered GC batteries as well, but in my case it would require a major upgrade to the front tub/tongue storage. Weighed against the fact that for multiple trips of 4-8 nights over five seasons my g27 wet cell and solar panel have always been adequate, such an expensive mod seems overkill for my needs. A little insurance would be nice on longer trips, though, and since I plan to replace my aging-but-still-serviceable battery this year, I'll already have a spare on hand.

Did you find that you were able to switch from one battery to the other without power interruption? For me, that would only be an issue for the fridge running on propane. If I cut the 12V supply to the circuitry, I need to manually relight the fridge--always a mild source of stress!  :-)

mntrails

@ScottG, the Marine switch I used worked well and didn't produce any power spikes that caused anything in the T@B to stop working.  Based on a good digital voltmeter, it was a seamless switch when going from Batt1 to Batt 1&2 to Batt2.  Now I didn't go find a scope to put on it and look at the exact voltage transitions as the switch went through the postions...  <mostly joking but if I'd had one, I may have looked> but from everything I observed, it worked well. 

Your comment on the fridge propane gave me a chuckle - from camping with my parents many years prior, I recall it was always a source of angst to make sure the fridge got properly started up on propane when unplugging from electric.  Sounds like things haven't changed all that much with the pre-2021 T@Bs.

But back to the topic, as others have noted, the single biggest 12V consumption for me is the refrigerator. I was practicing many of the better power management techniques last year like pre-cooling on 120V, putting in all pre-chilled food along with a small cold pack however I still found I was in and out of it a lot.  This year I'm going to try being just a lot more careful with the access. Hopefully that will help.

ScottG

Thanks again, @mntrails --it makes sense that the 4-way switch should work without power interruption. I was planning to test mine with a multimeter just like you did.

And yeah, the propane fridge is great for saving battery power, but I swear it actually senses your angst when you are trying to light. I've developed an uneasy truce with mine and for the last several years it has been pretty cooperative. At least once it is on, it stays on.

Denny16

Marine battery switches are a make/brake (power on 1st position stays on until position 2 is engaged) type switch, so the power load is not dropped when switching.
cheers