Occasional Use Battery System - Better with Large Ah or Deeper DOD?

[Xplode]

I am putting together a couple solar systems for my cabins. I started a thread about my setup in another part of the forum, but my thoughts keep coming back to the cost of the batteries.
I know, everyone agrees that you're better off to buy quality batteries with a large capacity, but I am only going to actually want to use the battery bank a handful of times per year.

Sooooooo.... I know if you run a 80% DOD on your batteries you'll get about half as many cycles out of the batteries, but if i'm only going to put 50-100 proper discharge cycles on the batteries over the next 5 years (occasional weekend visits to the cabin), where is the balance with cost benefit? How will the batteries' natural aging play in? Will my 10 year cost be any different if I I half the size of my bank and run it harder, vs keeping the size larger and being gentler on it?

[Mike90250]

I'd say go with the smaller size (cheaper) battery bank. Golf cart batteries will last you 2-5 years, their calendar will time out before you reach the cycle life

[littleharbor]

Be sure your charging system is adequate to get your batteries back to full charge in a full day. Being that you're using an undersized battery bank that shouldn't require too large an array.

[organic farmer]

I am using 24 Exide batteries [marine deep cycle] in a 48vdc 600h battery-bank. We cycle them down to 80% State-of-charge, and we are on our third year with this system.

A group of our neighbors are off-grid, and among them, the consensus has been to equalize once a year. I followed their advice, and this year I found that ten of my batteries had low cells. I had to pull those batteries out of the battery-bank and individually over-charge them to bring all the cells up. Basically 48 hours of overcharging on each battery separately.

I need to pay much closer attention to my equalize charges.

When I tell my charge-controller to 'equalize', it says it is 'starting EQ'. But I think it is delaying the process for some reason and it is not doing an honest equalize charge on the entire bank.

[Mike90250]

Many solar chargers have in internal counter to automatically start an EQ every 30 days. That seems excessive to me, but with flooded cells, you can check the SG of cells and see if you need to EQ. Choose two days where there is all sun forecast, extend your absorb on the first day, and 2nd day, start your genset and bulk the batteries early enough you can complete solar Absorb by noon, and then start your EQ at noon. EQ starts after batteries are fully charged, or else you are simply charging them.
But a year - as you discovered, is too long between EQ cycles.

[organic farmer]

I think that my charge-controller has one timer for bulk-charge, and another timer for float-charge. If today I can get full sun for 5 hours, these two charges [bulk and float] will consume the entire day. So even though I have told it manually to start EG, it refuses.

[NorthRick]

I'd say go with the smaller size (cheaper) battery bank. Golf cart batteries will last you 2-5 years, their calendar will time out before you reach the cycle life

That's not been my experience. I have Costco golf cart batteries at our cabin. Installed them in March 2010. They are still functioning just fine today.

This is a cabin that sees about 20 weekends a year of use. In between that the batteries are lounging around in float. It is also in Alaska so the batteries spend a lot of their time in temperatures below freezing, which I suspect adds to their calendar life span.

[Mike90250]

I think that my charge-controller has one timer for bulk-charge, and another timer for float-charge. If today I can get full sun for 5 hours, these two charges [bulk and float] will consume the entire day. So even though I have told it manually to start EG, it refuses.

That's why I said run your generator :
> ... start your genset and bulk the batteries early enough you can complete solar Absorb by noon, and then start your EQ at noon.

[organic farmer]

That's why I said run your generator :
> ... start your genset and bulk the batteries early enough you can complete solar Absorb by noon, and then start your EQ at noon.

I have access to grid power, whenever the grid is up.

I have only been starting EQ when the battery-bank is already at 100% SOC.

[Xplode]

So I've purchased a Tracer 3215AN charge controller and will be putting up 2x 265 or 280W panels. They are not in perfectly ideal orientation so during the summer I expect to get about 4 sun hours, and in the winter it'll be about 1. In the summer the panels will likely get the batteries back to full in a day, in the winter, not so much. I guess I'll have to consider running the genset before we leave in the winter visits.

I am planning to use Crown CR235 batteries if that makes a difference.
The tracer charge controller runs an EQ charge every 28 days, but given the random use, it may or may not be able to successfully complete the EQ charge every month.

[Sunking]

1. In the summer the panels will likely get the batteries back to full in a day, in the winter, not so much. I guess I'll have to consider running the genset before we leave in the winter visits.

Not sure how you come to that conclusion because you have not stated how much energy you will use in a day. I can only assume you are running 24 volt 235 AH batteries? I make that assumption because of your controller only being 30 amps with some 600 watts input maximum at 24 volts. At 12 volts power is limited to 300 watts

I am planning to use Crown CR235 batteries if that makes a difference.
The tracer charge controller runs an EQ charge every 28 days, but given the random use, it may or may not be able to successfully complete the EQ charge every month.

Do not get your hopes up of long battery life even with infrequent use. Batteries have two life cycles one being cycles vs DOD, and the other being calendar life. If you want to know how long they will likely last is to look at the warranty period. So you are looking at about 3 years.

As for DOD, you really do not want to go below 50% DOD. Below 50% DOD on a FLA battery formation and hardening of lead sulfate crystals accelerate rapidly. You made a false statement stating at 80% DOD cuts cycle life by 50% would be great if true but is far from fact. If you look below is graph from Trojan battery on their product line cycle life. Your battery is about the same as Trojan Signature series, the bottom gold line. At 20% DOD is around 1500 cycles, 50% @ 700 cycles, and 80% down to 300 cycles. So in reality is a 80% reduction in cycle life. As you can see, you get what you pay for. A battery that cost $100-Kwh capacity last 2 or 3 years., and one that cost $200-Kwh gets you up to 8 to 10 years. FWIW a lot better than lithium.

Now do not get your hopes up Trojans Cycle Life chart is anywhere close to reality because it is far from it. In Joe Public minds eye a day is a cycle and manufactures count on you thinking that. It is an easy lie to get away with. When they test their batteries, they do get the cycle life stated and can be proven. But what Joe Public does not know is the manufactures use accelerated testing under lab conditions. Where is the catch you ask? Answer is simple, the catch is in you head. You assumed a cycle is a day under real life conditions. That is not how they test.

The batteries are submerged in a water bath under tight temperature regulation running 5 to 10 cycles per day. They have removed Calendar Life from the test procedures. They have also removed environmental factors and human error. So how long will your battery last with TLC. The answer is your warranty period. Now you can use this info to gauge how far you can discharge. You have a 3 year battery which equates to roughly 1000 cycles. Look at the chart and you will discover 30% DOD for a daily use system. However as weekender you can go the max to 50% and still get your 3 years.

As for EQ, forget everything you have been told. You only EQ a FLA battery when your battery hydrometer tells you they need EQ. Here is the issue. EQ does damage. EQ is a controlled over charge, and over charging cause plate corrosion and shedding. On the flip side undercharging is more serious then over charging.

[Xplode]

Hi sunking - appreciate the detailed response. I am working with multiple posts/threads now. In another post in the off-grid forum I have a more detailed rundown of my consumption

If you refer to my original post up top, I am a weekender, and only a handful of times per year. So I am trying to decide between balancing an expensive battery with minimal use, or a cheap battery that is replaced more often. Somewhere along my internet travels I saw a chart that indicated the batteries get about half the cycles going to 80% instead of 50%. that might be where the confusion is coming from. Here's the chart from crown on the CR235's data sheets.
crown life cycles.jpg
It doesn't show 80% DOD, but you can extrapolate about 600 cycles (at 80% DOD) vs 1200 (at 50% DOD). Since I only use the system a handful of times a year, the calendar life of the batteries sounds like it will probably expire before i could possibly put any high cycle count on them. I'm aware of the "ideal test conditions" in the lab vs the real world. I do not expect to get exactly 600 cycles out of the batteries. that'll never happen. The system is sized to run down to 50% DOD after about 1.7 days of zero sun, and 24hrs/day of inverter drain... I expect to cycle the batteries to around 50% probably 20-30 times in the year, at most, with a few dips to 80% (mostly if it's someone else using the cabin since they won't be checking batteries and fussing with the system). So over the calendar life, it'll really only cycle a maximum of 120-150 times (I'm hoping for 5-6 usable years).

We use CR260 batteries in our scissor lifts at work, so I'm familiar with the wear on the batteries over 4-5 years, which is about what we use them for. they get heavier use than my solar system will (in terms of deep discharges per year) and I'm counting on the solar system keeping up at least a little in the summer. running down to an 80% DOD is not an intended regular event. It just doesn't seem worth it in my mind to spend significantly more money on "better" batteries when it's not a full time system. So that's why i created the original post, just to get a feel for what others might think. (I do realize i seem to have made up my mind, but i really am open to an educational discussion that might change my mind)


Now for the EQ - I do realize the hydrometer is your best friend, but again I won't be near the batteries all the time. Should I see about disabling the automatic EQ function (sadly I don't think that's an option, but I do have 2 more systems to build after my first one so learning and growing!)? My system is fairly undersized (Since it's not an everyday system) but the controller does support up to 2S2P 60 cell panels. I can connect 1120Watts or something like that... I'm going to install 560W to start (and hopefully that's all I need)... In the winter I will run the genset for a few hours before we leave to try and bulk the batteries back up above 80% before we depart...

[Sunking]

It doesn't show 80% DOD, but you can extrapolate about 600 cycles (at 80% DOD) vs 1200 (at 50% DOD).

As a battery manufacture competing in the Golf Cart Battery market of course I am going to say run 80% DOD as that is the norm for a golf course operation, enough for two rounds of golf per day using a commercial AC charger over night. Take that advice and you get to buy new batteries every 2 years like every golf course. Who does that benefit? You or the manufacturer? Does the manufacture make more money selling you new batteries every 2 years, or every 4 years? So heck yeah you can discharge to 80%, but there is a catch and the catch is great biz for the manufacture.

So here is another issue to be aware of. You can only charge a battery so fast. If you read manufacturers charging directions max charge rates are roughly the 8 hour rate. So for an 80 AH battery C/8 = 10 amps. That may not catch your attention at first, but there is no way to do that in a single day with solar. At best in June July you may get 5 Sun Hours and with 80% charge efficiency would require two full days with no use to recharge from 80% DOD. Not a problem for a golf course using commercial AC charger because they have 12 hours over night to fully recharge at a C/8 charge rate.

So if you plan on discharging 80% per day, you have to wait for two full days to recharge before you can turn back on the lights.

[Xplode]

So for my 235Ah battery bank, I will get a max charging rate of about 20-23A off my panels which is below the C/8 rate (C/8 would be 30A).
So I guess I may be looking at the addition of a couple panels if it does turn out to be in the 80% DOD range often.

But the question of "Who does it benefit" IS the exact question I've been answering. If the manufacturer says the batteries will do approx 600 cycles at 80% DOD, and I will only do 40 cycles to 80% DOD, and maybe 100ish cycles at 50% DOD over 5 years then the batteries should last that long without too much trouble.

I have looked into the surrette rolls and better quality batteries, but I'd be paying over double the price to get around the same capacity, albeit a better warranty. I'd expect they'd only last 7-8 years, even under my lighter use.

Sooo if I pay $800 for batteries and get 5 years, or I pay $1600+ for 8 years, I think I'm better off to go with the lower cost installation. This affords me some learning fudge factor so I don't roast an expensive battery bank, as well as lets the battery technology sector move forward for another 5 years. Hopefully costs and capacity will shift in my favor by then.

[Bozon]

I want to build a small system that can do one load of laundry per week. If it takes days to charg