MegaCap Batteries for a Cabin install?

Steven Lake

Howdy y'all!  Long time, no chat!!  Probably because all my setups have been running without issue since we last talked, or the issues have been easy to figure out.  Anyhow, it's getting to be time here at the end of the year to replace the batteries in our cabin up in the Upper Peninsula of Michigan (nice place, middle of nowhere, built mid 50's or so) and I'm wondering your thoughts on something.  Right now we're running a set of 4 (24v setup, 6v * 4) UB 380AH AGM's for the cabin.  On the upside, they're awesome and are more than enough for our 4-5 stays at the cabin during the spring/summer/fall (about 2 weeks each stay).  However, they leave me with a rather unfortunate situation that, in order to keep them from freezing in the winter, I have to leave them on 24/7/365 even if we're not there.  In the summer, not so much, as I can just throw the main cutoff and walk away, then spin them back up when we get back, and they seem to do pretty good operating like that over the summer, only needing about a 10% top off with the genny when we get in.

But, when we leave for the last time in the fall, we need to leave the switch turned on, so that they're getting at least some charge over the winter, to keep them from freezing.  So far it's worked pretty good.  (the idea to leave them charge over the winter to avoid freezing was suggested by several people I was reading online)  However, to me that just induces unnecessary wear on a low usage battery bank. (ie, 9-10 weeks out of 52, and only light usage even then, and mainly for lights and electronics as stove and fridge are gas)  So, after doing some research, I noticed that, while not as high a capacity, it looks like the big megacap batteries might be a better option for me.  Advantages I've seen are, they don't freeze, they don't need to have a maintaining charge (ie, they can be allowed to go dead with no negative side effects), they're shockingly reliable (ie, they take abuse well), they store well without needing any special attention, aside from keeping moisture off them, no worries about off gassing (ie, you can store them in a sealed box with no issues or negative side effects), and they seem to charge well.

The disadvantages I've seen are small AH size (40ah average, which results in a much bigger battery bank size wize), costlier per AH, and only about a 3-5 year operational life.  However, the last item isn't so much of a disadvantage in my case as I'm only using them about 10 weeks a year max.  So, if they have a 3 year cycle life, assuming I don't succeed in squeezing the 5 year max out of them, that's 15 years of operation rather than the 6 I'm getting now with my UB's.  So, in the longer term picture, they almost seem like the better choice.  However, I'm merely going on my own research and experience, so I'm wondering what your thoughts are.  Is there any possible gotchas to this setup that I'm not aware of?  Or would I be better going this route vs replacing my current UB's with an identical set?  I'm only replacing them this year because, when I finally got to go to the cabin and gave them a load test, they noped out on me part way through the test, despite being 13.1v after a full day of charging.  So that tell me they're ready to be replaced.

So, what's your thoughts?  From what I can see (at least without actually live testing them), the charge profile *should* be pretty similar to AGM from what I can see, so I shouldn't need to change charge controllers.  But, again, that's just from my own research.  Your millage may vary, thus why I'm seeking your expert advice.

PS, for those who remember my fiasco with our field monitors, I'm happy to announce we got that sorted finally.  Turns out it was user issue, not equipment fault.  I was setting them up wrong. ^_^;;  Thankfully a friend got me sorted out.

mcgivor

What you are referring to are capacitors, which are technically not batteries despite having some similarities, they are commonly used  to smooth rectified AC, or used to supply large amounts of energy for short durations, which is the opposite of what a battery is designed to do.

BB.

I have the same confusion as McGivor. Are you talking about "Mega Capacitors" (high Farad value), or a brand of batteries called Magacap Batteries?

I am also wondering about your AGM batteries... AGMs can "freeze" but be OK because they won't get plate or case damage. Of course, they have to be defrosted to charge/discharge.

Another question is--AGM are lead acid chemistry, and as long as they are charged, they should not freeze until they get very cold.


If you assume a 75% state of charge lead acid battery is >~1.200 sg, then freezing is around -30C (-22F).

And most room temperature AGMs should last 6 months between charges and stay >~75% SoC (and cold batteries have much lower state of discharge...). And you could always put a 1% rate of charge (solar panel+simple/small solar charge controller) to float them during this time and get below -40C/-40F pretty easily.

So, I am not quite sure why AGMs are not working for you... I believe you had a security system you ran when you are away--Is that causing you issues with your batteries/winter conditions?

-Bill

Steven Lake

Yeah, I call them MegaCaps, but yeah, they're Super Capacitor batteries.  People around here are using them as replacements for car batteries and such to pretty good success.  As for the freeze level, you're saying they should be okay even if frozen?  My only concern is I don't want them to freeze and be no good the next year when we roll in.  I want to just throw the disconnect on the system, and walk away until spring and be able to just flip the switch on and pick up where I left off with no issues.  As for a security system, nope, the only thing I'm doing when we're not at the cabin, at least over the winter, is just keeping the batteries topped off so they don't freeze.  But if freezing won't impact them, then I can probably switch them off in the fall, come back in the spring, turn things back on and be fine then I take it?

BB.

Generally, Super Caps do not store that much energy... For example, a 500 Farad battery replacement:

https://www.amazon.com/Maxwell-Durablue-Capacitor-ultracapacitor-Amplifier/dp/B07SGJR6S5
https://www.maxwell.com/products/esm/

• E = 1/2 * C * V^2
• E = 1/w * 500F * (14.4v^2 = 10.0v^) = 27,200 Watt*Seconds
• 27,200 WS / 3,600 seconds per hour = 75.6 Watt*Hours
• 75.6 WH / 12 volts = 6.3 Amp*Hours

Say you want to start a car engine... Crank for 20 seconds.

• I = C*dV/Dt
• I = 500F * (14-10 volts) * 1/20 seconds = 100 Amps current flow

At least this is how I remember the math from college electronics class... It is possible I messed something up.

I am not sure, but it looks like they may be putting the Capacitor bank in parallel with the existing battery(ies). Use the capacitor bank for starting, and the standard batteries for other stuff (lights when engine not running, etc.).

-Bill

mike95490

What would be wrong with leaving your solar charger running in the winter, and switching the inverter off ?

If batteries do freeze, they must be thawed before they can be charged.   AGM batteries are generally not damaged from freezing, but flooded batteries are usually destroyed, with acid spilling out from cracked cases. (but usually, they froze because they were dead, and dead batteries only have very weak acid)
 Lithium batteries get real lazy below 35F and cannot be recharged below 33F. Getting cold without usage, does not hurt them, AFIK.

Steven Lake

Okay, so basically I was originally told wrong about having to keep charging the batteries over the winter to keep them from freezing as freezing would destroy them.  Then again, since you mention about lead acid dying in those conditions, their advice was probably for those, not thinking AGM is different.  Okay, so if what you guys say is the case, then I can just flip the main cutoff, power everything down, and walk away until the next time we get in, and not have to worry about the batteries being killed/dying over the winter since they'll be disconnected from everything else, as we have a master cutoff between the panels and the charger, and the charger and the batteries.  Might be one more, like to the inverter, but I don't remember.  But, that's fine as the inverter has its own switch.  So, okay, if I'm understanding this correctly, if I flip the cutoffs, isolating the batteries, and just walk away, even over the winter, all will be fine, and I can spin them up the next year without any worry.  If that's the case, then I'll start doing that, which should help my batteries last longer, which is what I need.

BB.

Again, details matter... Roughly, for every 10C (18F) below 25C (75F), besides the batteries aging 1/2x as fast... They also have (roughly) 1/2 the self discharge... So, say your battery bank is at 0C (corrected, was 5C, but for the 1/2*1/2 needs to be 20C below room temperature) or less (20C below room temperature).

That means that the battery will last longer (during winter, age 1/4 as fast), and if the self discharge was 6 months (for AGM), then, in it would go 4x6 months or 24 months to self discharge from 100% to 75% (the typical State of Charge when a Lead Acid battery should be recharged).

For FLA batteries, 1 month between charges, becomes 4 months between charging.

And, the other option is to get a panel or two (something like 1-2% rate of charge is more than enough, with a simple charge controller) which are frequently hung on the southern exposure of building, out of snow drifts as much as practical, and supply float charge during the winter. And if it does "get really cold", and the panel(s) do drift over, then it really does not matter if there is no floating for a month or two anyway).

People have found that leaving a larger/full featured MPPT controller on charge in "snow climate" on float--And the array gets iced/snowed over for a few months, the 2-6 Watts or so tare losses (to run the controller's computer and networking) and some other loads (alarm, security lights, Internet connection, etc.) can discharge the battery bank too much (below 75% state of charge). Say worst case of 3 months of ice/snow over array with large MPPT controller left running "in float" (worst case assumptions--Your mileage may vary):

• 6 watts * 24 hours per day * 30 days per month * 3 months * 1/12 volt battery bank = 1,080 AH @ 12 volts

A simple PWM controller with a matched panel (or several, depending on bank voltage), may be a better choice.

Some other folks here can probably give you some more suggestions/ideas... The closest I come to snow is grinding ice from the freezer for (non-alcoholic) drinks. So take my suggestions as just a starting point for more research.

-Bill

Steven Lake

Huh, very interesting information!  This is why I like asking you guys stuff like this as my research only gets me so far.  So, basically, if I switch off the inverter, and switch the charge controller to float, or just let it do its normal thing (it's an outback, so I think it does float anyways when the battery is full) and then come back in the spring and pick up where we left off without reducing battery life very much, as that's more of what I'm worried about is burning out the batteries too quickly.  Of course, your comment about turning off the inverter would probably be the best thing I could do as I forget those things draw current even if you have nothing pulling current in the house.  So I probably wore the batteries out not realizing that I was putting load on them over the winter even when I wasn't using any actual power in the cabin.  So, aha!  Mystery solved.  Thanks guys!!  I always learn such cool stuff from y'all.

BB.

If you were leaving an inverter ticking away with no, or very light loads (say alarm system), and the array got drifted, yes that can kill your bank (larger inverters can easily take 20+ Watts tare losses).

An FM 80 from Outback Tare loss is not big:

https://www.solar-electric.com/outback-power-flexmax-fm80-150-mppt-charge-controller.html

• Standby power consumption: typically < 1 Watt
  

But any "Trivial" load with dark days/snowed over/winter shading can be hard on the battery bank during that time.

Does your array drift/ice over during parts of the winter?

-Bill

Steven Lake

Um, never been there when its drifted over, but I'm pretty sure it does given the just absolutely stupid amounts of snow that area gets during the winter.  I mean, I'd be shocked if it didn't at least part of the winter.  Usually when we button it up in October, we don't see it again until Aprilish or so on a normal year.  This year we didn't get to it until late, late May because of all the covid bs.  That's when I loaded tested them for the season and got the "uh-oh" from the tester.   Last year's load test was a pass, but not by much.  This year, nope.  I mean, they'll hold us through the summer, but are gonna need replacing by fall, or sooner.  So when I replace them I'll have to make sure to turn off the inverter when we leave and watch them the next 5 or so years and see what kind of impact that makes.  Knowing what I've learned hear, I'm hoping it makes a big improvement on longevity.

mike95490

Why replace them in the fall when they are not going to be used for several months ?

Leave the current set there, on float charge (check the nightime power drain of the charge controller, and see if it will deplete the bank with 2 months of snow on it) and see what shape your "tired" batteries are in come spring.  Then you will know if the No inverter, Float all winter is a viable option for your new bank .

InCogKneeToe

I leave my system (International Falls MN) in an Unheated Cabin 365 days a year.  With FLA Batteries. PWM Controller On, Inverter Off. My Panels are Roof Mounted, and sometimes go months without getting cleaned off in -35/-45F temps. So 1-3' of snow covering. I have yet to have any issues (4 years). Get there in the Afternoon, the Controller is in Float, mornings and it's charging.

My shed has 2 40w panels, with a 7.5a Controller and 6 AGM Car batteries. I turn on a light to look for something, Skil Saw to cut a board, otherwise it's always off. I got the AGMs because, working at a Car Dealership, Car batteries die. AGM's can freeze, but take a long slow charge to bring back to life, with an AGM compatible charger. After they have thawed of course.

Steven Lake

Mike, I'm replacing my batteries here in a month or two (maybe as late as this fall the way things are going) because they failed the load test this spring, meaning they're kinda EOL.  I mean, they'll get me through summer, but come fall they'll need to be replaced.  Or, if I don't replace them this summer, then probably spring next year.  But yeah, they're pretty much getting ready to tap out at this point.

InCog, you mention FLA batteries in your setup.  I'm AGM right now.  What's the benefits of FLA over AGM?  I think someone told me once, but I can't remember.  Supposedly they were more durable or something if I remember right.  I'm running exclusively AGM at the farm and the cabin, and once we got things sorted out (and I got solar de-noobed), we've been having no troubles with our setups anymore.  Well, except the cabin, obviously.  Since I'm changing batteries anyways, my looking into a better battery setup than what I have now is on the table.  I mean, may as well.  Probably not for this setup, as my replacements are already ordered.  But I have other setups I might be able to use them on if they're better than AGM.  Obviously not until those need replacing, but foreknowledge is always good to have as it gives me time to plan.

mike95490

the general rule of thumb I've used for FLA vs AGM is FLA is half the $ of AGM, and 2x the life of AGM

otherwise,watt hours are still watt hours, with AGM being more expensive and never needing water (adding water is what allows Flooded to last much longer)

InCogKneeToe

Yes FLA cost less, and Last longer. AGM can Freeze (from low-discharged) and survive without Case/Internal damage.

My Cabin is unheated (when not in use) Late Nov.-May, many sub zero temps and often -40's. AGM sounds beneficial, but my system hasn't drained enough to Freeze while unattended. I find (from Car Batteries) that a FLA battery must be nearly 0% before it will freeze from -40 temps. AGM's can freeze even if there is some life left. AGM batteries also take more effort to recharge from Dead.

So, just shut off your output, leave the array/controller on and you should be fine. Most times I have made it to the Cabin (pm hours) the Charge is in Float Mode, even with 1' of snow on the panels

Steven Lake

Awesome, thank you everyone for your input!  This has been really super helpful.