Battery use and lifespan

Wbuffetjr1

I have recently read some threads about battery life that have me a little confused.  Some folks on the threads were saying that it is better for your batteries to draw them down to say 75% occasionally.  I have a 48V bank of golf cart batteries. Three strings for 690AH.  I recently got my system hooked up to MyMidnite and have been remote monitoring it for a little while now (since 11.08.17). System is 1,600 miles away so not easy to access. I have a 1,000W motor running a compressor set up on a timer that runs 5 hours per day, everyday. Just about everyday the system is in float by 10:00am and does not make it below 97%.  I have actually seen it go below 90% only one time so far and it went to 74%.  The very next day it was back to 100%. Average battery temp over the last few weeks has been in the mid 50's F. I have not seen the batteries below 50F or over 62F. I topped all the batteries off with water on 7.31.17. The next time I was back at the system was 11.08.17 and all the batteries combined only needed 1 gallon of distilled.  I thought this was all great until I read some of those threads.  Am I not drawing my batteries down enough? Will that actually lead to a shorter life span? I am a newb and this is my first system so plz bear with me! 

I know three strings isn't ideal, but I wanted to start with golf cart batteries first assuming I would screw them up. 

Estragon

There are a couple of issues with shallow cycling.

One is there are a finite number of discharge/recharge cycles (mainly grid corrosion), so in theory a bank will last longer cycled every second day than if cycled every day. This is at least partly mitigated by it sitting in a partly discharged state, and by the fact that it will eventually die of old age anyway.

The bigger issue IMHO is potential stratification. Sitting at float voltage for long periods can let the stonger, heavier acid settle at the bottom, and weaker acid at the top. This can lead to sulfation in the upper part of the plates. A gallon of water use over ~3mos sounds about right to me though, which suggests the charging routine is likely enough to mix electrolyte regularly.

You didn't mention taking SG readings. If you haven't done them, I'd highly recommend doing so regularly. If stratification is a problem, it will show up in the SG readings, which will be low because you're pulling weak acid from the upper part of the cells. My guess is the charging is okay, but checking SGs would confirm.

jonr

I'd review what Vic has written.  Maybe implement 1 skip day (to reduce plate corrosion without increasing permanent sulfation)?   

bsolar

i of course dont know your reasons for the compressor, but if its not needed daily then why run it daily ... and why charge daily .. imo ideally you run the batts down with your load once a week to 80% or so and charge once a week then equalize once a month or so .. they can sit inbetween it wont hurt them .. if your load occurs during daylight the only time theyre cycling at all is in cloud or rain and thats definitely not good to just have voltage on them pretty much at the top everyday .. i'd lower the values for absord and float several 10ths if that situation is unavoidable and be sure to equalize once a month or so ..

Wbuffetjr1

Thanks for all the input. I constantly read pretty much all I can find on the subject. Unfortunately, the load is needed every day. The site is at high elevation with a lot of snow.  I worry about doing skip days because of the possible weather up there. I am worried that I will run the batteries down with a skip day or two and then get hit with a big snow of 2',3' or 4' leaving the panels covered for an unknown amount of days and I will damage the batteries that way. With this being the first year with the system up there I have a lot of unknowns.  Also, the weather (temps, amount of snow and timing of snow) is wildly unpredictable from year to year.  More reasons I started with golf cart batteries. 

Estragon

Skip days doesn't stop charging entirely, it just skips absorb (if voltage is over rebulk setting) and charges to float voltage. If you hit rebulk voltage, it does a bulk/absorb cycle irrespective of skip days setting.

I have a similar issue with snow. Making matters worse, the low winter sun means I get shade issues for ~a month around winter solstice. The arrays are as steep as the racks allow (~65°), so the cold, light, fluffy sort of snow we should normally get at this time of year slides off. The last couple of years though, we got a mild spell in mid-Dec with wet, sticky snow. This formed a layer 1-2" thick of frozen crust, which later cold snow also stuck to.

A thin layer of snow will melt off pretty quickly if even a bit of the panel can get warmed by the sun. By the time I got there in Jan though, there was 6-10" cover. Without manually clearing, the snow would have stayed there until spring.

Does the load HAVE to run every day? With enough snow cover, it will run the bank down to low battery cut-off, and the load won't run. Even without the load running, controller/inverter tare loads will eventually run bank down to whatever min operating voltage those devices need, and they may not come back on properly when the snow melts.

The good news is if the bank is cold, the slower reactions mean it takes longer for sulfation and self-discharge to permanently damage it. The bad news is a discharged bank is more prone to freezing damage. Also, if the bank is discharged below device operating voltage, you'll need a way to charge up to that voltage. I've used a small 6/12v automotive charger hooked up to a segment of the 48v bank to get controllers and inverters to wake up and start proper charging.

softdown

I have seen "solar batteries" that were identical to the golf cart batteries offered by the same manufacturer. Other than possibly size, I am not presently convinced that solar batteries are automatically significantly different than golf cart batteries. 

Sometimes it seems as if a product costing 3 times more while lasting twice as much, the economics may be up for debate. 

The general idea is to have a large deep cycle battery bank with a quality cabling and charging arrangement. No need to feel "inadequate" about golf cart batteries. 

Considering the fact that major solar battery manufacturers are constantly massaging their suggestions, I think that battery "science" is still a subject of considerable controversy. Though it seems like we all agree that cooler batteries last much longer. 

Wbuffetjr1

Thanks Softdown, that makes me feel even better about them. 

Estragon - I currently have the panels tilted to 55 degrees.  I could go much steeper if you think that would be better. They are mounted on an MT Solar mount and it will go almost vertical IIRC.  I was initially concerned about limiting production, but that doesn't seem like a problem now. We SHOULD not have any mild spells like you describe until Spring. However, if we did have an issue up there with snow/ice on the panels that lasted for longer than I am comfortable with, I could get a buddy to run up in his snow cat and knock the snow off. I have the low voltage disconnect set at ~47.5 volts. 

Wbuffetjr1

Also, is skip day still as important if I have the Wbjr and use end amps? 

Estragon

Personally, I would go steeper for winter. At, say 40°N latitude, the winter sun is only ~ 20-25° above the horizon, so somewhere around 65-70° from horizontal would be about perpendicular to noon sun. You lose a bit of morning & afternoon production in longer fall/spring days, but the really short solstice period days should be a bit better, as well as shedding snow better.

At the margin, I suppose using WBjr end amps makes skip days less important, since in theory you're better matching absorb time to SOC. I could argue that fewer, deeper cycles are still better, but in your application (daily cycling), skip days are likely to be less of a factor anyway. I use both, but my system (hopefully) just floats most of the winter, so skip days prevents the short daily absorb cycles being done for no reason.

softdown

Here you can plug in your latitude and find the best angle for seasonal production: http://pvwatts.nrel.gov/pvwatts.php

I am at 37N so ~52 degrees is my "ideal" winter time angle while ~22 degrees is best for summer. I used 45 degrees year round and would repeat that angle if I did things over again. 

Some of our northern friends dedicate a 90 degree panel to help keep the batteries alive during their long, cold winters. With your 1000 watt compressor running so much, that is not a viable option. 

If faced with a lot of cloudy winter weather, this challenge could be difficult indeed. All of us eskimo's have found that snow and ice can cling tenaciously to even vertical surfaces unless the sun is bright. 

littleharbor2

There's a cool phone app called Solar Tilt. It will give you the perfect tilt from your exact location, or any latitude, today or any day of the year. Also has a tilt gauge built in so you set your phone on the face of your panels and it gives the exact tilt.

Horsefly

I'm not sure why this site doesn't get more mention here: http://www.solarpaneltilt.com/  Although I don't think it has been updated in quite a while, it seems to be well grounded in the physics. Maybe updating it hasn't been needed since the physics hasn't changed.  

Since Chad's array is completely tiltable via his cool MT Solar mount (I really wish we could have made that system work for us), it seems to me like he should plan on setting his tilt to something 65 deg or steeper. The steep southerly angle of the sun in the winter combined with the chance of snow could argue for a near vertical tilt. He can easily set it back to something closer to 40 to 45 degrees in the other three seasons.

Edit to add: I realize it is too late for this winter, I guess I'm suggesting 65+ degree tilt in all future winter seasons.

westbranch

littleharbor2 said:

There's a cool phone app called Solar Tilt. It will give you the perfect tilt from your exact location, or any latitude, today

Just loaded it up and I like it.... but I did notice one discrepancy: I looked at the seasonal tilt  for Winter and it gave me  70.4* as the OPTIMAL,then I did the daily (today, November 26) and it gives me a higher angle 73.6*.... So I checked  Dec 1, and got 75.0*,   the Winter Solstice on Dec 21 got 76.3*,  , Nov 1, got 68.4*, Oct 22, got 65.2*...

Moral of the story is you have to decide which season you want to optimize for... up here I would (did) choose as steep as I can get so i can max out in the depths of winter..

Wbuffetjr1

I am at 38* North and used one of those sites for my current "Winter" tilt. I think I messed up by using the "adjust twice a year" table. It is not too late for me this year to get it cranked up some.  I have a buddy going up tomorrow and can get him to adjust. Luckily, we are behind on snow up there this year and a pickup can still make it up there. 

So, I will have fixed one issue that I wasn't worried about, but I am still concerned about not drawing the batteries down often enough.  So far, it looks like they might get drawn down into the 70's once a month. Is that even close to enough?  Of course I am sure things will change this Winter once the snow starts to really fly. 

I thought that with the compressor running 6 hours per day I was exercising the batteries pretty well daily??

I will NOT be able to EQ until sometime in the Spring. How bad is that?

Estragon

A couple of questions:
- Are you running the compressor to exercise the batteries, or it it essential for some other purpose.

- Are the batteries in an unheated space, and if so, any idea what sort of high and low temps might be reasonable to expect over the winter?

Wbuffetjr1

The compressor is aerating a lake to prevent winterkill of fish so it is essential. Acyually one of the biggest reasons to finally install a solar system up there. 

The system is in an ICF structure.  VERY well insulated.  Compressor throws off quite a bit of heat.  So far it seems the inside temp stays 30*-40*F above outside ambient.

Estragon

Having the batteries in a well insulated space heated by the compressor, the warm batteries will sulfate faster than they would in cooler temps. My guess is you aren't likely to have significant stratification problems so probably not a big deal though. I wouldn't worry about only getting to 70%SOC once a month. The main thing is they get a full absorb at least once or twice a week. That should keep things mixed and prevent sulfation.

Being in an insulated space at least they shouldn't freeze if something goes wrong and it takes a couple of days for someone to get there. Even 50% discharged, it has to get pretty cold to freeze them hard.

softdown

Hmmm, very interesting application. I wonder how long the lake could go if power was lost. Clean, cold water should be pretty good about retaining oxygen levels if not overstocked. Any dead fish would be very undesirable from a water quality perspective. I've done professional aquarium work for 24 years now.

mike95490

Wbuffetjr1 said:

The compressor is aerating a lake to prevent winterkill of fish so it is essential. Acyually one of the biggest reasons to finally install a solar system up there. 

The system is in an ICF structure.  VERY well insulated.  Compressor throws off quite a bit of heat.  So far it seems the inside temp stays 30*-40*F above outside ambient.

We use a windmill air pump for pond aeration, mostly to prevent stagnation in summer

Wbuffetjr1

softdown said:

Hmmm, very interesting application. I wonder how long the lake could go if power was lost. Clean, cold water should be pretty good about retaining oxygen levels if not overstocked. Any dead fish would be very undesirable from a water quality perspective. I've done professional aquarium work for 24 years now.

Softdown - I have no idea and have never talked to anyone that could tell me. I have a thread on this lake going in a pond forum with 200k views.  In our 20 year history with the lake it has never over wintered fish.  We are 5 years into trying to reverse that. We had ~6 acres of eutrophic water that combined with all the ice and snow at 10,000' made things tough. The solar aeration this Summer alone completely changed our dissolved oxygen profile. Needless to say I am very excited to see what happens this Spring!

Mike - we have TWO windmills! They just do not produce enough consistent air to cut through all the ice and snow for the entire Winter. 

Here's a pic a buddy sent a couple days ago. 

jonr

I wonder how the efficiency of moving up water from the bottom of the pond (just above freezing) compares to pumping in well water (perhaps 50F).   Maybe more btus moved per kWh?

softdown

How deep is the lake? Wondering it it almost freezes solid without intervention. I find this experiment pretty fascinating. Have you checked dissolved oxygen levels? Such meters are readily available, last I checked, though the membrane may need periodic replacement...which was easy on my unit.

What type of fish are inhabiting the lake? A threat may be too much aquatic life though doubtfully a present problem with a newish ecosystem. 

Decaying plant life, flora, will remove considerable oxygen. Same with dead insects and animals, fauna, though the fish would hopefully devour such things. 

What altitude are you at? It would be easy to find expected oxygen levels based on temperature, water movement, and altitude. Different fish species have different oxygen requirements. Trout need plentiful oxygen for example.

EDIT: the lake picture is finally displaying. I have a suspicion that freezing may be the predominant problem. Most of the lake is quite shallow and freezes solid pretty fast. I would consider using a backhoe to increase the depth of part of the lake. 

I'm sure you considered all this and decided that aeration/water movement was the best solution.

softdown

jonr said:

I wonder how the efficiency of moving up water from the bottom of the pond (just above freezing) compares to pumping in well water (perhaps 50F).   Maybe more btus moved per kWh?

I'm not seeing a source that adds and removes excess water though one may easily exist.

Wbuffetjr1

The lake is at 10,000' and stocked with Brook Trout. The incoming water sources all freeze up during the Winter (as far as I know). There is a spillway that spills water from Spring through late Fall. 

Jon -  Interesting idea. There are tons of people aerating lakes just no one that I have met in my conditions (altitude).  The general consensus seems to be moving air is the best and cheapest route.  Typically well water has very low or possibly even zero dissolved oxygen so it would have to be aerated first (not impossible). Second issue is the lake needs to "vent" gasses that develop from decomposition. It does this through the holes in the ice. If the gases do not vent they can become toxic.  As the water vents the gases it also absorbs oxygen.  I think it would be hard to make the well water produce the vents around the lake. PLUS, I don't have a well! Water system for the cabin is all gravity fed and any excess already flows to the lake. 

Deepest part of the lake is 22' deep.  Average depth is 11'-12'. I have a DO meter and our levels have changed drastically since using the solar to aerate.  When we began this process 5 years ago we had zero DO below 6'. The fish were living in the top 4.5-5' of water and were doomed in the Winter. After three years with two windmills we had zero DO below 12', fish were probably only living in the top 8' of water and the fish were still dying during Winter. Last year the two windmills failed to keep a hole open in the ice for 40 days in a row and once again all the fish died.  Installed this solar system in July and as of early Novmeber we had 8.3ppm at 21' deep. This solar ran compressor changed the game in the lake and I now expect the fish to live. 

Back to batteries! Lol
As far as battery use, life, mixing, etc. Maybe I should add some details that could possibly help you guys help me. The system regularly makes over 3,000 watts and up to 4,000 watts. Average production is 7KWH per day. Best production day so far is 16KWH. Is this kind of use not thoroughly mixing the batteries even though my SOC rarely goes below 95% ?

Estragon

You're probably fine in terms of stratification IMHO. I assume there are times when you get a run of 2-3 cloudy days in a row taking SOC lower. You just need a long enough absorb once in a while to get some bubbling to mix electrolyte.

jonr

Just to be clear, I was thinking of well water discharged upwards from somehwere below the lake surface as a heat source to melt holes in the ice.  Just as bottom water (moved by bubbles) does, but well water is much warmer.  Agreed, well water doesn't directly do anything for fish.  But it's such a small quantity (as a % of the entire lake) that I wouldn't bother aerating it.

softdown

Alrighty, you are doing great from appearances. 8.3 ppm DO is pretty incredible at 10,000'.  Hard to beat simple air bubbles for maximizing DO levels.

Since you have so much solar, strong surplus from appearances...you could likely operate your batteries at a lower temperature though I would wait and see how this year goes first. 

It seems the current threat is snow covering the panels for a long time. How sunny is the climate? I seem to have missed the location.

Cool experiment...looks successful.

Wbuffetjr1

Thanks guys. Location is Colorado. IMHO the biggest threat is definitely a big snow covering the panels for an extended time.  I just don't know what to expect with this being the first year. The sun is definitely intense at that altitude so it will be interesting to see how quickly the panels shed snow.  It is like a giant science experiment for me up there! Lots of firsts. So far the mountain has been kicking my butt. 

softdown

Was thinking this could easily be Colorado. Summit county area by chance? I'm in southern Colorado in an alpine desert....the San Luis Valley. The panels have never been covered for too long though I always get impatient and scrape them it seems. 

A freak storm and cloudy weather is a threat though I suspect the lake may be able to handle a couple weeks without power if need be.

Are you living in Colorado? If so, you may notice unusual snow and clouds. Couple options are 4WD with chains to get you close then snowshoe the rest. Or snowmobile. 

I would guess that you could monitor real time images of the closest mountain pass. If the snow doesn't melt from the black road for a couple weeks then you may have a real threat. My gut feeling is that a week or two is not a life threatening problem. Your batteries may be able to power through a few days though you could lower the shutdown level a bit. I'd encourage research but I would guess the very cold lake could go a few weeks without supplemental oxygen. That is a GUESS of course. 

I am not an expert on this particular subject. I might try a wildlife biologist specializing in high altitude trout. They would find your experiment fascinating. Just be aware that anyone can innocently break one of ~75,000 rules without even knowing it. So exercise due caution there. 

I hope you keep us up to date on this thrilling experiment. And tell us about where in Colorado...

westbranch

If you are using the MYMIDNITE you will know if the panels are ''snowed under'', couple that low input  with  the Weather service IR image of the area which gives you the cloud cover.... easy to know without having a camera using precious power