Adding Batteries

RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
First off.. I know your not supposed to add new batteries to an existing bank. However, that is exactly what I want to discuss.

I have had a system up and running for the last 8 months that consists of 4-420ah 6v Trojan batteries and is wired as 24v. I also have 3-210watt panels attached to an Outback charge controller. On a good day in the summer I can generate about 4kw per day, and in the winter, about 3.4kw. This is great.

I use this power for a remote astronomical observatory in the AZ desert (battery box temps range from 20F to 100F during the year, but usually stay between 40F-85F). A typical day of power use is about 1200 watts for the computer monitoring systems, security cameras, and the like. This usually brings the battery bank down to about 92-93%. I get that back in about 2-3 hours of sun in the morning. However, when I am on-site, I usually pull about 3800-4200 per day. If the sun is out the next day (and it usually will be if I am on-site), this will bring me back to about 98-100% by the end of the day.

Basically, the bank is usually only discharged to ~90% per day, and 3 or 4 days a month is gets down to 75% .. I think once or twice I got it down to about 68% (in the last 8 months). (I have charts on my website that show this - http://www.hualapaivalleyobservatory.org/power/ ) .

Now, with all that background, here is what is happening next. I am expanding the observatory for a larger computer system when I am on site and am going to be pulling more power from the system (5200-5400). While I could get another panel to recharge the bank faster, I am worried that I will start discharging the bank below 50%. So I wanted to double my bank size and leave the panels alone (since I will have plenty of "down time" to recharge a larger bank). So here are my questions:

* Am I being to conservative with my existing bank and am trying to solve a problem that does not (or won't) exist?

* When is it too late to add more batteries to existing bank. Its only 8 months old, and as I have stated, most of the time in has never been below 90% charged.

* If I did double the bank size, should I mix the batteries (2 old & 2 new per per 24v string)?

* What are my other options other than a generator? I already have one, but would like to avoid running it in the middle of the night.

Thanks for your feedback!

Roy

Comments

  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Adding Batteries

    Question is, How much are you on site and what are your loads?

    I would suggest that a $600 Eu 1000 honda generator at 29# might make better sense than buying another $400-600 (L 16s?) worth of batteries.

    That said, with your current loading, adding the extra batteries will be mostly fine,, not an ideal, but they are new enough and soft used enough that the new ones won't be much compromised.

    All that said, drawing them down 3 or 4 days a month to 75% soc is no big deal, so maybe your are in search of a solution to a problem that doesn't really exist. On the other hand if you will indeed routinely draw to 50%+ I think your solution a good one.

    You do however want to keep the balance of charge current to between 5-13% of Ah capacity, so to much battery and not enough Pv might be a problem. Do the calcs using your known PV current input average.

    Your current PV is able to put what ~ 25 amps into the batteries. So your current bank is ~420 ah, and will become ~ 840 ah. 5% of 420 is 21 amps,(about in the ball park) 840 is of course 42 amps. My guess is by doubling the size of the bank you run the risk of not enough charge current

    Tony
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Adding Batteries

    Going right into "quote mode" :D

    * Am I being to conservative with my existing bank and am trying to solve a problem that does not (or won't) exist?

    Sounds like the system is running pretty good. Normally the maximum DOD is 50%, with 25% (75% SOC) favoured. You're not running close to this so far. But your panels are already a bit low for a 420 Amp hour battery bank. What sort of charge rate do you see? I would guestimate 18-20 Amps? It ought to be at least 21 to keep the batteries from sulphating and make sure the electrolyte is mixed.
    * When is it too late to add more batteries to existing bank. Its only 8 months old, and as I have stated, most of the time in has never been below 90% charged.

    A bank that's less than 1 year old and well maintained with relatively light-average use will not be a problem with new batteries. The problems develop after years/abuse that "ages" the batteries, decreasing their actual performance; any new batteries will get "dragged down" right away, shortening their lifespan.
    * If I did double the bank size, should I mix the batteries (2 old & 2 new per per 24v string)?

    Nope. Just build a new string of the new batteries. However, you shouldn't be adding batteries. Even if your existing PV array was big enough for the existing bank, adding more battery capacity means you must add more PV. Unless the array was already large enough to recharge a bigger bank.
    * What are my other options other than a generator? I already have one, but would like to avoid running it in the middle of the night.

    You wouldn't necessarily be running it at night, and the new inverter generators are very, very quiet and economical. Always a good idea to have a back-up generator at a remote site, just in case.

    My suggestion would be to add more panel. Your 420 Amp hour bank could supply you with up to
    4.8 kW "over night", so the issue is when do you use the most power? One of the "tricks" with off-grid is to try and use power when the batteries are full and the sun is shining; you get better "harvest" out of panels that otherwise would be not producing to full capacity because the batteries don't need it once they reach "Float" stage.

    If you really need to hit that 6 kW hour capacity mark you would need to increase beyond 420 Amp hours, and add appropriate panel capacity to recharge. That would mean either duplicating your existing bank (@ 840 Amp hours - need lots of panel) or replacing the existing bank with one that fits the minimum need. (Can't add different capacity battery bank to the existing one.)
    Thanks for your feedback!

    Roy

    You're welcome. I'm sure some others will have a few observations and suggestions too. :D
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries

    Actually, it's more like $1200 for the batteries, but I understand your point. And I already have a generator for large loads, just nothing hooked directly into the system.

    If charge current is the issue on the current panels, then it won't be for long. I plan to add another 2 panels within the next 6 months as well. For now I was more concerned about the large loads at night and pulling more power from a smaller bank.. And also trying to increase the bank size before it got too late.

    Roy
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Adding Batteries

    Increase the bank now, add the PV later, monitor the battery performance in the mean time, and charge with the gennie as needed. Sounds like a perfect solution. Like I said before and 'Coot agrees, the batteries aren't old enough to make that much difference.

    The biggest issue that I see is people with too much battery and not enough PV, leading to constant over draw and not enough charge, but it sounds like you are on top of the issues,

    T
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Adding Batteries

    i think more pv is in order regardless just to be sure the electrolyte gets mixed while charging. now you don't draw down on the battery bank too far now as you state it averages about 90% soc. doubling your average draw will still be a very good 80% soc. you just want to be sure on those rare occasions to not go lower than 50% soc. i think you would be able to determine your power usage well enough to know if it may go too far and even if it's the middle of the night a good inverter generator may prevent excess usages by supplementing for a short time period.
    another battery bank will necessitate a doubling of the pvs needed unless one would have separate switched battery banks. use one bank while the other one charges. in a pickle you could drain a bit from the other battery bank, but this only means the charging will be needed by both banks the next day and the pvs may not bring both banks to full charge without using a charger from a generator.
    i don't believe you should go with more batteries unless you are sure you need them so try your present system with 2 more pvs. normally we'd say for you to get at least as much in pv as you will drain out of the system with the efficiency derating and the pvs producing for about 4hrs. for instance as an example 1000wh of extra power needed / 4hrs = 250w in pv. at 50% overall efficiency factoring this would look like 500w stc of pv to be added. it may be overkill for you doing it this way, but you can also try adding a pv at a time to account for your extra loads if it becomes needed after you have added 2 more pvs now to your present system. the 2 extra pvs now i figure will properly mix your electrolyte and just may satisfy your electrical needs too.
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries

    I did a quick query of my database and from what I can tell it's peeks at about 21-24 amps during the day, but have records going as high as 29 (but that could be an equalizing charge).

    As for what I would run at night .. I did a quick calculation and it would be about 3kw total in about 12 hours (6pm to 6 am). One of the things that makes it easy to figure out is that I only have a 250 watt inverter on the system. I want to get a larger outback inverter/charger, but was thinking that the batteries/pv panels were a better place to spend the money right now.

    So if I understand your recommendation, it is to get more PV and let the batteries go down to to 50% DOD if needed. With more panels I can fully recover from the 50% during the day? If that is the case then perhaps getting the newer inverter/charger would be better so I can just use my generator (it's an 8kw) to make sure the batteries actually do get charged during the day.

    So many possibilities..

    Roy
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Adding Batteries

    Just a note, A 8 KW gennie is very big for routine running. From a noise/dollars point of view a little honda Eu 1000 is very quite, very fuel efficient and for the 4 nights a month might be a pretty good alternative. (I know you have rejected the idea of a genny due to noise, but if your experience has only been with a an 8 kw rig, you might be very surprised to hear how quiet the hondas really are)

    Thats the last I will mention it.

    t
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries

    Ok.. So another vote for no extra batteries. Not because it's too late to add them, but because they are not necessary or the correct option (at least that is what I understood you as saying).

    Doing simple math .. I have 3 panels now that will generate about ~3800 watts during the day. So adding another panel would give me another ~1200 watts which would put me at my anticipated daily use. And since at least 1/3 of that use is during the daylight, my SOC should never go below ~60% (right now it is ~80% and I am not actually going to use double, but it will be close).

    Roy
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Adding Batteries

    "So if I understand your recommendation, it is to get more PV and let the batteries go down to to 50% DOD if needed. With more panels I can fully recover from the 50% during the day?"

    yes, that is pretty much what i was saying. to reach 50% dod would be 420ah x 50% = 210ah. 210ah x 24v = 5040wh. if you use less than this you are fine with the batteries. you still should add the 2 pvs.

    "If that is the case then perhaps getting the newer inverter/charger would be better so I can just use my generator (it's an 8kw) to make sure the batteries actually do get charged during the day."

    i'm not sure if you are following me or the other way around. an inverter/charger is a device that will tie to the grid and is not the same as an inverter generator going through a charger. inverter generators are just more efficient and quieter and still need a battery charger to be connected to facilitate charging. it will be less expensive to add the 2 pvs than to add an inverter/charger
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries
    icarus wrote: »
    Just a note, A 8 KW gennie is very big for routine running. From a noise/dollars point of view a little honda Eu 1000 is very quite, very fuel efficient and for the 4 nights a month might be a pretty good alternative. (I know you have rejected the idea of a genny due to noise, but if your experience has only been with a an 8 kw rig, you might be very surprised to hear how quiet the hondas really are)

    Thats the last I will mention it.

    t

    I actually have 2 generators. A small 1.8kw that is pretty quite, but not as quite as others, and the larger 8kw one. The 8kw was purchased during construction because I really needed the power for all the equipment. Plus I use it during the summer for the A/C unit during the day. The reason I was not wanting to use the generator at night was not just the noise factor but that it was not one that is tied into the system, so I would need to power things down to switch over the power feed. If i had one that was tied into my inverter it would be a possibility, but for now it is not.

    Roy

    Roy
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries

    Its probably just me using the wrong terminology. What I meant was an inverter like the Outback VFX series. One that has a charger built into it that I can then hook up to a generator. It would both supply power to the system plus charge the batteries.

    Perhaps I don't need a new Outback inverter and only a simple 24v charger that I can hook up to my generator and then wire into the bank (going through a shunt of course so my usage was properly tracked).

    Roy
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Adding Batteries

    Roy. I love this topic. (I use the same batteries)
    I'm a n00b so don't put any weight behind what i have to say.

    SOMEONE STOP ME IF THIS IS WRONG:
    I was told that adding new batteries to an old battery array/bank will create an odd
    situation where the new batteries that can be charged to 100% will get drained
    by the older batteries that can't be brought back to the 100% charge. THUS
    making your new batteries only as good as your old batteries.

    AND SO.... are you willing to buy these new batteries as 8-month old
    used batteries?



    *OFF TOPIC*
    BTW, how did you get those stats of your system on your site? Those are AWESOME! I would love to put something similar to that on my site.
  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Adding Batteries

    It is probably even more complex--New batteries (at least flooded cell) have a higher resistance/less capacity vs same batteries with ~50+ cycles on them.

    Ideally, you want the batteries in matched sets so they charge and discharge "in sync" with each other.

    However, if you need more capacity--You need more capacity. You could use a DC Clamp Amp Meter to monitor charging and discharging current to make sure that loads are substantially similar.

    The warning (at least from my limited experience) is really more towards the issue of an old bank (over 1/2 its life used???), and adding a new set of batteries. There, the concern is the new batteries will cycle deeper than the old set and you run the risk that the new set will carry most of the load/cycling and not last nearly as long as they would in a matched set.

    Perhaps one of the guys here who has much more experience than I has some good advise to add.

    What we are also trying to avoid is replacing one battery at a time as a bank ages/cycles to failure vs replacing the entire set if the bank is substantially impaired.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Adding Batteries

    gloves,
    that is basically true, but if the batteries are nearly new with no abuse or problems then it is close enough to add some news ones to the mix. it may be stretching it to go a year and a half or more, but i would consider it depending on the battery and i'm speaking primarily of quality batteries. batteries that are made to typically last 3yrs would not qualify imho for even a year, but some may argue that point too because of variances in the manufacture dates some battery banks may have in them already. adding new batteries to a bank of batteries fairly new with no problems or abuse will not degrade because the older ones have not degraded yet or very much that would make any difference. make sense?:confused:
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Adding Batteries
    Gloves wrote: »
    AND SO.... are you willing to buy these new batteries as 8-month old
    used batteries?

    If it were me, I would do it. Don't base your multi-hundred dollar decision on
    my thoughts. i'm still messing up my first pair of Trojan T-105s
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Adding Batteries
    Gloves wrote: »
    SOMEONE STOP ME IF THIS IS WRONG:
    I was told that adding new batteries to an old battery array/bank will create an odd
    situation where the new batteries that can be charged to 100% will get drained
    by the older batteries that can't be brought back to the 100% charge. THUS
    making your new batteries only as good as your old batteries.

    AND SO.... are you willing to buy these new batteries as 8-month old
    used batteries?

    As explained in the previous posts, it is not a problem in this situation.

    Batteries are funny things. As Bill mentioned, when first installed they don't have full capacity. After a few cycles they actually go up in Amp hours. Then they start to go down. How far down they go and how fast is a function of:
    1). How long they've been sitting in the warehouse before being sold.
    2). How they're constructed, as some batteries simply last longer than others (all other factors being equal, which they rarely are).
    3). How deeply and how often they're discharged.
    4). How well they're recharged and maintained.
    5). Sheer bloody luck, as things can always go wrong.

    So in this instance where you have batteries less than a year old and lightly cycled/well charged with good SG the capacity is probably pretty close to 100% of what you'd expect. New ones would be close enough to. They'd never be exactly the same. Even batteries from the same batch aren't exactly the same!

    The problems arise when you hook new batteries on to batteries that have already lost significant capacity through age/cycling/misuse. Then you're connecting new "100 Amp hours" to old "86 Amp hours". It's then the same problem as you get with paralleling dissimilar capacity batteries; the "smaller" ones will drain faster and charge harder than the "larger" ones. They will try to pull power from the "bigger" bank and will keep it from being properly charged (as the system reacts to the smaller capacity).

    So as with matching up different solar panels it all comes down to how much percentage variance can you tolerate? The difference being panels don't degrade each other; batteries do.
  • zeuspaulzeuspaul Solar Expert Posts: 59 ✭✭✭✭
    Re: Adding Batteries

    Another vote for more PV and no more battery capacity.

    You have enough capacity to go over night with less than 50 percent discharge. You will probably get more bang per battery buck with your 420 amp hr battery with occasional 60 or 70 percent discharges than you would by going to 840 amp hrs. I doubt you will get double the life and you have more battery at risk.

    With more PV you have a better chance of getting your 420 amp hr battery to full charge. With even more PV you can fully charge your battery even on a bad day minimizing the need for the generator.

    If 29 amps is your current max output you have room for another 25 amps of PV using the 13 percent charge rate rule of thumb.

    $1200 of batteries translates to +/- $5000 in PV because PV has a longer life.

    Get a 24 volt battery charger that you can plug into your generator. The more PV you get the less you will need the generator.

    Zeuspaul
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Adding Batteries

    Just an anecdote. When we built the new house, we did this calculation. The question was were we better off to wire 4 L-16s to provide ~650 ah and charge them at 25 amps (our capacity) or 4 t-105s to provide 450 ah. The cost of the t-105 was ~ 1/2 of the cost of the l-16s. We decided that if the t-105s lasted 5 years and the l-16s 10 we were money ahead to stay with the smaller batteries. Now if it turns out that the t-105s last 10 like our last set then we would be way ahead. Now it is hard to predict how much longer either set would last.

    Since we draw ours ~10-20% max on a daily basis, they are very well balanced between charging current and average loading. We are now in year 4 and they seem to be as good as new. My conclusion is that if we had gone with the bigger batteries and hadn't increased the Pv we would be chronically undercharging them, and running the risk of letting the loads grow since we would have the capacity.

    Tony
  • backroadbackroad Solar Expert Posts: 185 ✭✭
    Re: Adding Batteries

    I'm an old adult ADD person and my head is truly running around in circles!

    .........but I still luv this stuff......
  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Adding Batteries

    And they told me it was hamsters that made the generators "go-round":

    15KW Perkins 4cyl generator...image.php?u=139&type=sigpic&dateline=1265597876

    Now I see is Dalmatian puppies! That explains a lot about "diesel fumes". :p

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Adding Batteries
    BB. wrote: »
    And they told me it was hamsters that made the generators "go-round":

    15KW Perkins 4cyl generator...image.php?u=139&type=sigpic&dateline=1265597876

    Now I see is Dalmatian puppies! That explains a lot about "diesel fumes". :p

    -Bill

    the hamster run models were a later development. remember fred flintstone and everything run by his big feet?:roll::p
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries

    Sorry this is a late reply ..

    The stats on my site were gathered using the Outback Mate device connected to my equipment. I record all the data (1 record per second) into a database and then generate the charts and information. All the software was custom written by myself for the monitoring and the reporting.

    The site also has pretty much the same type of data collected for my weather station that is on site.

    Roy
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Adding Batteries

    Roy, you are way too gifted! The Outback Mate System probably won't work for me
    as I think it pairs with the Outback Mate inverter. I really want to see some stats
    on my setup, I want to count kilowatt hours stored and used. and Amp hours would
    also be a plus.

    Then writing custom JSP/ASP/PHP scrips that upload my data to some MySQL
    database and are displayed via more custom AJAX scripts on my personal website is
    an ENTIRELY different project for people who are generally 10 times more computer
    savvy than I.

    Hell-of-a setup you go there. I did bookmark your page for inspiration.

    I still want your setup :D
  • help!help! Registered Users Posts: 9
    Re: Adding Batteries

    Can someone explain to me how a 250 watt inverter (as stated on his website) can handle Roy's system? That will barely power a laptop by itself, when nothing else is running, won't it?

    Have I asked a stupid question?
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Adding Batteries

    I have 2 Acer laptops and each lists this as a rating:

    the Ms' laptop:
    19V :: 3.42A = 65W

    and Mine:
    19V :: 3.42A = 65W

    I know for a fact that mine uses much less energy than hers. She has a 15.5"
    screen and I have a 14". Mine is rated to last 6hrs on the same battery
    (6-Cel Li-ion) that hers will last 3 hours on. I think it's mostly the difference in
    processors.
  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Adding Batteries

    Roy has a series of photos documenting consruction:

    http://www.hualapaivalleyobservatory.org/construction/default.aspx

    The last date has a picture of one of the telescopes...

    Power wise, a good sized laptop will only take 30-50 watts of power (screen turned off can be even less). Don't know about what he uses for a comlink (satallite up link? Cell Modem?)...

    1,200 to 4,200 WH per day (unmanned/manned):
    • 1,200 WH / 24 hours = 50 watts average (unmanned)
    • (4,200 - 1,200) WH / 12 hours = 250 watts (manned 12 hours per day--pure guess).
    If I recall correctly from earlier posts--Roy also has some direct DC loads (perhaps even the computers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dagr51dagr51 Solar Expert Posts: 72 ✭✭✭✭
    Re: Adding Batteries
    help! wrote: »
    Can someone explain to me how a 250 watt inverter (as stated on his website) can handle Roy's system? That will barely power a laptop by itself, when nothing else is running, won't it?

    My 15" laptop, DSL modem and wireless router pull 45 watts.
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Adding Batteries

    My sat modem, wireless router each draw ~ 12-15 watts. The lap top draws under 50 depending on state of charge of it's on board battery.

    T.
  • RoySalisburyRoySalisbury Solar Expert Posts: 82 ✭✭✭
    Re: Adding Batteries

    The system at the observatory use the following:

    24/7 Equipment -
    Sony Viao Micro PC (uses about 15 watts of power)
    Cellular Modem w/Wireless Access Point (about 8 watts)
    Security Cameras (about 15-20 watts).

    Basically about 40-45 watts

    When Manned
    Apple Mac Mini (about 35 watts)
    21" Widescreen monitor (about 50 watts)
    Telescope and cameras (about 30 watts)

    Not much at all. Just now I have the 24/7 stuff running and the Mac Mini running so I can do some stuff from home (I shut it on/off remotely via X10). Its pulling 65 watts of power.

    I only use LOW POWER stuff.. :)

    Roy
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