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  • Battery selection advice requested

    I'm setting up a new system and currently have a brand new 105Ah Duracel AGM deep cycle battery. I need to bump up to 24v so I need a pair of matching batteries. The question is if I should buy a second 105Ah AGM or if I should instead go out and purchase two 105Ah FLA deep cycle batteries instead. Or if there's a third option I've not considered.

    The system I've got in mind (discussed in another thread) is as follows:

    3 x 265W panels
    ~93V output @ ~9A
    100' 10AGW copper wire to controller
    40A controller
    unobstructed southern exposure for roughly 10 hours a day
    summer months primary + tail end of spring and start of fall

    I expect to drain about 30Ah a day for two days in a row then shut down all draw on the batteries for the following five days. This means there's seven days of variable strength charge for two days of drain. As far as I can tell the planned system has what I need to charge either FLA or AGM option sufficiently for my needs.

    Thoughts about what I should do?

    If the advice is to ditch the AGM I'll do that, even though I don't have much use for it elsewhere. Sucks to think of the money in that battery going to waste, but if that's the way it winds up that's the way it winds up.

    Thanks!

    Steve

  • #2
    With a 24 volt system you will be putting around 30 amps into your battery, Two 105 Ah. 12 volt batteries are still only 105 Ah. @24 volts. Thirty amps is too much current for that size battery. You want to keep your current at 5 to 13% of your battery Ah. rating Four 220 Ah. 6 volt golf cart batteries would be a better match, although still at the top limit of charging power.
    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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    • #3
      Originally posted by SolarSteve View Post

      If the advice is to ditch the AGM I'll do that, even though I don't have much use for it elsewhere. Sucks to think of the money in that battery going to waste, but if that's the way it winds up that's the way it winds up.
      Since you already have one AGM battery, you might as well get another and use the pair. If you can truly stick to 720 Wh of discharge daily when you are there, and with occasional use, they might work out OK for you. AGM's will take a higher charge rate than FLA, and although your system is capable of producing C/3 at STC, which is about as much as you'd want to put into them, you aren't going to produce that charge rate often enough to worry about it. 2 days usage and 5 days of charging gives them the longer absorb/float period that they'll want.

      If you end up with bad weather on a four day weekend you could run into trouble, especially if that means you are stuck inside and using more power than normal.

      Edit: I saw you listed the Morningstar charge controller in your previous post. You might also want to look into the Victron 150/35, about $100 less.
      Last edited by sensij; 08-13-2017, 11:36 AM.
      CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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      • #4
        Thanks for the follow up advice.

        Here's a thought. Instead of upping the Ah of the batteries, how about lowering the charge going into the batteries? If I reduce my collecting capacity from ~93V @ 9A down to ~62V @ 9A I lose more in voltage drop as a percentage (roughly 3% vs. 2%), but I'm still at ~60V. As I understand it, that gives me sufficient voltage to maintain charging early/late in the day and suboptimal cloud conditions. It also puts the max charge at roughly 22A down from the previous 34A, which seems to be more inline with what a 105Ah system wants.

        Do I have that right? Is the lower amperage sufficient to keep two 105Ah AGMs happy over the 5 day charge period?

        What you can see me trying to do here is avoiding an overkill system for what amounts to keeping some beer cold This is why I'm not keen on upping the Ah storage capacity as there's a perpetual cost that comes with that. If what I've just proposed seems workable, it has the following cost benefits:

        1. I only have to purchase 2 panels instead of 3, which saves me ~$180
        2. I only have to spend ~$250 on a second AGM battery vs. a larger amount on any other battery solution (cost could be 2x or higher)
        3. I can knock my controller down from 40A to a 30A, which in the case of Victron is a ~$100 savings

        The savings "pay for" the second AGM battery, which isn't a bad thing in my mind.

        Of course the downside of this is I am truly sizing my system for a specific use purpose and pattern. Occasional problems, like extended stays with crap weather, can be overcome by the use of my existing 20A generator. However, should I significantly deviate from either what I'm running or how long I'm running it for, then I could run into trouble. I'm OK with running that risk.

        Steve

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        • #5
          Is this a Duracell MARINE "Deep Cycle" AGM Group 31M Battery?
          Absolutely, you can use two 12 V AGM batteries in series = 24 Volts.

          And if you have good SUN and your battery bank is 100% SOC then you can use way more than 30AH per day ...

          Energy generated by the PV Panels
          ============================
          795 Watts = 3 panels x 265 Watts
          4,000 Watt-Hours = 795 Watts x 5 Solar Hours
          166 AH per day = 4,000 Watt-Hours / 24 Volts

          or even 200 AH total consumption = 166 AH (PV) + 30 AH (Battery)

          If you have no sun then you would to need to reduce consumption dramatically that day.
          Just two days of no sun and your batteries are down to 40% SOC, which is quite low.
          We try not to go below 50% on a regular basis, but your batteries only cost $170 each.
          Let's see if you get 3 years, or more?

          Make sure you set your Charge Controller to AGM Type batteries.

          Specifically, what are your LOADS ?

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          • #6
            Originally posted by SolarSteve View Post
            Here's a thought. Instead of upping the Ah of the batteries, how about lowering the charge going into the batteries? If I reduce my collecting capacity from ~93V @ 9A down to ~62V @ 9A I lose more in voltage drop as a percentage (roughly 3% vs. 2%), but I'm still at ~60V. As I understand it, that gives me sufficient voltage to maintain charging early/late in the day and suboptimal cloud conditions. It also puts the max charge at roughly 22A down from the previous 34A, which seems to be more inline with what a 105Ah system wants.

            Do I have that right? Is the lower amperage sufficient to keep two 105Ah AGMs happy over the 5 day charge period?
            No wrong direction. You can charge AGM's as fast as you want. All slowing down charge is takes longer and less energy in a day.

            Kind of like saying you are cruising down the highway at 60 mph with 60 miles to go and say I will slow down so I can get there in less than an hour.
            Last edited by Sunking; 08-13-2017, 12:52 PM.
            MSEE, PE

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            • #7
              Originally posted by NEOH View Post
              Is this a Duracell MARINE "Deep Cycle" AGM Group 31M Battery?
              Absolutely, you can use two 12 V AGM batteries in series = 24 Volts.

              And if you have good SUN and your battery bank is 100% SOC then you can use way more than 30AH per day ...
              Yup, this is why I don't want to up the total Ah of storage. As long as my uses and patterns don't change dramatically, it's overkill to go with more storage. I already have an overkill system at home (4080Ah) and therefore understand the costs that come with it.

              If you have no sun then you would to need to reduce consumption dramatically that day.
              Just two days of no sun and your batteries are down to 40% SOC, which is quite low.
              We try not to go below 50% on a regular basis, but your batteries only cost $170 each.
              Let's see if you get 3 years, or more?
              The key thing is for the most part the system will only be in use for 2 days out of the week, therefore if I go into that two day period with a 100% charge I can just about have a blanket over the panels and come out OK on the other side. If the weather is that crap I might as well stay home

              BTW, the cost of the Duracell AGM is actually +$200 for the places I checked out. The one I bought was $220.

              Specifically, what are your LOADS ?
              You can see details in this thread:

              https://www.solarpaneltalk.com/forum...system-concept

              The primary drain comes from a refrigerator. The 30Ah draw is based on warm temps and 4+ people (two of which are teens). That's the worst case scenario. Normally it's 2 people who don't believe a fridge should be left open for 5 minutes every half hour

              Steve
              Last edited by SolarSteve; 08-13-2017, 01:34 PM.

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              • #8
                Originally posted by Sunking View Post

                No wrong direction. You can charge AGM's as fast as you want. All slowing down charge is takes longer and less energy in a day.

                Kind of like saying you are cruising down the highway at 60 mph with 60 miles to go and say I will slow down so I can get there in less than an hour.
                I guess I'm misunderstanding the concerns raised in the first two responses. Maybe you can break it down for me so that I can understand better? Although I've lived off grid for more than 10 years, I didn't design my system so this is the first stab I've made at understanding more than the basics. I'm learning a lot

                Steve

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                • #9
                  Steve you brought up the question or said something to the effect of lowering the charge rate on your batteries? While it is true you can charge a battery to fast, but not in your case. Flooded Lead Acid batteries you want to limit charge current to around C/6 to C/8 where C = Battery AH Capacity. Example on a 480 AH battery C/6 = 80 amps, and C/8 = 60 Amps.

                  That rule does not apply to AGM batteries because their internal Resistance is much lower, so they can be charged much faster. AGM can be charged as fast as C/2, or 240 amps in our 480 AH example.

                  With a fixed panel wattage can only generate up to X amount of watt hours everyday. Example say we have a 800 watt panel system and in winter you only have 3 Sun Hours. That only gives you 1600 watt hours of usable power in a day. Lower th epanel wattage and you lower your daily production form 1600 watt hours to something less.
                  MSEE, PE

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                  • #10
                    I agree so my answer to your original question would be yes to save $$$ by getting the matching 105 Ah AGM to wire in series.

                    Key is 3 x 265W = 795W max. 795W / 24V = 33.125A max. 105 Ah / 33.125A = C/3.17 which is a great rate for AGM.

                    If you want to save some money by going to a 30A charge controller go for it. That would make 105Ah / 30A = C/3.5 which is also great, maybe better.

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                    • #11
                      Thanks! That all makes sense to me. Since this system is going to be used periodically in the sunnier time of the year, I am fortunate enough to not worry about the darker winter (unlike with my home system). This gives me some flexibility that generally doesn't exist for an all year use system.

                      OK, looks like I can charge at a max of 52.5A into a 105Ah AGM battery. That right? If so, then it sounds like I should stick with the 3 panels and the 40A controller. That allows me to rapidly recharge the bank between uses, decreasing the chances that I'll start off a weekend with poor reserves. It also increases the ability to use the system longer, provided there is mediocre sun for some of the time. Also increases adding new draws on the batteries over time.

                      I think I'm getting close to having something on paper that fits our needs!

                      Steve

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                      • #12
                        Originally posted by AzRoute66 View Post
                        I agree so my answer to your original question would be yes to save $$$ by getting the matching 105 Ah AGM to wire in series.

                        Key is 3 x 265W = 795W max. 795W / 24V = 33.125A max. 105 Ah / 33.125A = C/3.17 which is a great rate for AGM.

                        If you want to save some money by going to a 30A charge controller go for it. That would make 105Ah / 30A = C/3.5 which is also great, maybe better.
                        Interesting thought. In theory that extra 3A charging sounds nice, but in reality... how often and long willl I get a rate that high? My guess is not all that often. Spending the extra on the 40A charger might not be worth it. Then again, at this point a cost variance of +/- $100 isn't by itself all that tempting to go after.

                        Steve
                        Last edited by SolarSteve; 08-13-2017, 04:24 PM.

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                        • #13
                          I think it depends on "What's next?" With the 40A controller, your panels are the limiting factor, and with anything less than 'ideal' conditions you would be charging at less than C/3.17 anyway. If your next upgrade is to the batteries, for more capacity, then consider staying with the 40A charge controller and let the panels be your throttle. If your next upgrade is more panels, to get more power from 'less than ideal' conditions, then the 30A charger (to provide the limit) will be crucial for limiting current to your current battery bank. (Or just set the current limit in your 40A charge controller to 30A if it is capable of that, but that is another rant for another thread...)
                          Last edited by AzRoute66; 08-13-2017, 05:27 PM. Reason: Wait. I just saw you looked up max charging current is 52A. Never mind about the 30A 'throttle'. 40A charger just adds flexibity so let your wallet and visions of the future decide.

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                          • #14
                            Originally posted by SolarSteve View Post
                            Thanks! That all makes sense to me. Since this system is going to be used periodically in the sunnier time of the year, I am fortunate enough to not worry about the darker winter (unlike with my home system). This gives me some flexibility that generally doesn't exist for an all year use system.
                            Yes, it certainly does. In my neck of the woods (100 miles from the Canadian border), there is about a 4:1 difference in solar power I can collect in summer vs. in Winter. That's huge.

                            Originally posted by SolarSteve View Post
                            OK, looks like I can charge at a max of 52.5A into a 105Ah AGM battery. That right? If so, then it sounds like I should stick with the 3 panels and the 40A controller. That allows me to rapidly recharge the bank between uses, decreasing the chances that I'll start off a weekend with poor reserves. It also increases the ability to use the system longer, provided there is mediocre sun for some of the time. Also increases adding new draws on the batteries over time.
                            Makes sense to me. (By the way, don't fall into the trap of specifying and thinking in terms of three significant digits given Sunking's general figure of "C/2". You can pump about 50 A into that battery, a little more perhaps or a lot less if you choose.) If you have any shading issues at all, three panels with an MPPT charge controller (the only kind you should consider) will give the bypass diodes in those panels more chances to allow full current, even with some substrings shaded. And dropping your maximum power point voltage from around 85 V to 55 V is significant. Given all the other costs involved, panels are cheap.

                            Also, if it's cloudy the whole week while you anticipate a break in the weather and look forward to a visit to your little retreat, you will be glad you've overpaneled a bit. PV output under cloudy skies is 10-25% of what it is when sunny, the top end being for a light haze where the sun almost still casts a shadow on the ground.

                            Originally posted by SolarSteve View Post
                            I think I'm getting close to having something on paper that fits our needs!
                            I think so, too.

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                            • #15
                              Originally posted by SolarSteve View Post
                              Thanks! That all makes sense to me.
                              You are welcome.

                              Originally posted by SolarSteve View Post
                              OK, looks like I can charge at a max of 52.5A into a 105Ah AGM battery. That right? If so, then it sounds like I should stick with the 3 panels and the 40A controller.
                              Steve you can get away with charging your AGM with C/2 or 50 amps, does not mean you should normally. Recharge is not going to be an issue for you because it is sized to fully recharge a completely discharged battery in a normal day. Charging at that rate and using AGM, I would be using Temperature Compensation. AGM are prone to Thermal Runaway, and one of the causes is Fast Charging. On Solar Thermal Runaway is far less likely to happen because they just not have the longevity of a AC charger. As the sun gets lower, power goes down along with the heating until sun sets and you cool off over night and begin the next day.

                              Anyway that is the risk you run charging that fast The real issue is how much you use at night and how deeply you discharge. As long as you do not over discharge you should be OK other than running Risk if Thermal Runaway. Easy to fix if you want or if it becomes a problem, loose a panel.
                              MSEE, PE

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