Adding batteries to existing array

RoySalisbury
RoySalisbury Solar Expert Posts: 84 ✭✭✭
in Off Grid Solar & Battery Systems #1
I know I have asked this before, and have gotten good advice that I have been following. However, I now have some specific numbers that I would like to throw into the mix to see what I can/should do.

Here is the reason for the question. I originally sized my panels and battery based on a consumption of about a 50 watt/hour load (95% of the time). It was also sized so that I would not go below ~70% SOC after 3 days of no charge. I felt this was a good place to start even though most recommendations were to go ahead and account for 50% after 3 days. But 70% gave me room to work with the other 5% of the time when I really used lots of power. Plus I wanted my batteries to last longer by not going so deep.

Now, after about 15 months, the power usage has crept up to about 90 watt/hour, and I have lost my 3 day buffer. I'd like to add more battery capacity to the system (along with the necessary panels), but am afraid of the issues with adding new batteries with old ones. I have some very specific SOC numbers from my batteries over the last year that I will add at the end, and they tell the % of time that the batteries have reached a specific SOC. Only at one time did my batteries ever get really hammered and that was after 5 days of really bad weather where the system could just not keep up. Most of the time they only get to ~90% overnight and then charge back up within 2 or 3 hours.

So my question is, how bad would it be to add more batteries? I currently have 4 ea 6v Trojan 420a/h batteries in a 24v configuration. Is it too late to add 4 more?

Here are the stats for the SOC of the batteries over the last 15 months or so (basically 89% of the time that have stayed at or above 80% SOC).

SOC PercentSOC
100 17.3484746162100
99 9.1931689166100
95 7.2675818345000
94 6.7523912530200
96 6.5282760225600
97 6.3926797799900
98 6.3891545814400
93 6.3626503108800
92 5.6168723090200
91 4.3100095308200
90 3.8075501652600
89 2.5619054041800
88 2.4118486532800
87 2.0096003173500
85 1.7653597470400
86 1.6858522644400
84 1.2102329026400
82 1.0934963390800
83 1.0744634643800
81 0.8529141947700
80 0.7772596502700
79 0.5997260683500
78 0.5394033397600
77 0.4469488127500
75 0.3831994739300
76 0.3672654699100
74 0.2110775913700
73 0.1577972662300
72 0.1379330521800
68 0.1303151061200
70 0.1294464630900
69 0.1242212821700
71 0.1158679205300
67 0.0964193761600
66 0.0876130411600
65 0.0791983948900
49 0.0751536092600
63 0.0720547385700
64 0.0702774842800
42 0.0678394217900
45 0.0671279871600
58 0.0616603322700
43 0.0574396863900
44 0.0508555854000
46 0.0374502385400
40 0.0274635082600
57 0.0268027000700
61 0.0264456504800
52 0.0259447152400
53 0.0259313924900
41 0.0258061586800
60 0.0257821777400
50 0.0256729312200
59 0.0256116465900
54 0.0252865715900
56 0.0250574203600
47 0.0249508384000
51 0.0249188638100
48 0.0240742017200
39 0.0223315865700
62 0.0209673373900
55 0.0190009001100
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Adding batteries to existing array

    What Trojan 6 Volt ha a 420 Amp hour rating? :confused: The old L16 'A' batteries were 320 Amp hours.

    Your batteries are less than 1.5 years old. They've been kept up and not heavily discharged. Check the specific gravity on all cells and the resting Voltage. See if that is consistent across all batteries with an SG within 0.005 of new. Chances are pretty good you can add the new set without any trouble.

    Are you sure you mean you only use 50 Watt hours per day? or is that 50 Watts 95% of the day (which would be 1140 Watt hours)?

    Let's see ... 420 Amp hours @ 50% DOD is 210 Amp hours * 24 Volts nominal = approximately 5040 Watt hours / 3 days = 1680 Watt hours per day.

    Can't get the numbers to work out. Maybe I've had another stroke. :blush:
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Adding batteries to existing array

    you would essentially be doubling everything and if your batteries aren't too old then go for it. watch that you also don't reuse the same wire as doubling a system will double the currents and that will double the voltage drop with the same pv and battery voltages.

    i think the best would be a small generator to take up the slack of those prolonged rare periods of rain and clouds, keep the present battery bank the same, and maybe add a few more pvs to bring the battery bank up quicker and more inline with your usage.
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  • RoySalisbury
    RoySalisbury Solar Expert Posts: 84 ✭✭✭
    #4
    Re: Adding batteries to existing array
    Cariboocoot wrote: »
    What Trojan 6 Volt ha a 420 Amp hour rating? :confused: The old L16 'A' batteries were 320 Amp hours.

    These are the batteries:

    Trojan L16P-AC 6V 420Ah Flooded Lead Acid Deep Cycle Battery
    http://www.batteriesinaflash.com/deep-cycle-lead-acid/6v/trojan-l16p-ac-6v-420ah-flooded-lead-acid-deep-cycle-battery

    I have 4 of them.

    The initial plan was sized at 50 watt/hr (1200 watt/day) .. 3 day was 3600 watt (somewhere between 65-70% SOC depending on actual use and other factors (30-35% DOD).

    Now I am around 90 watt/hr (2160 watt/day) .. 3 day would put me at 6480 watt. Well over even the 50% mark. so adding another 4 batteries would put me back into my 65-70% SOC range again.

    At least that is what my basic calculations come out with.

    Roy
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  • RoySalisbury
    RoySalisbury Solar Expert Posts: 84 ✭✭✭
    #5
    Re: Adding batteries to existing array
    niel wrote: »
    you would essentially be doubling everything and if your batteries aren't too old then go for it. watch that you also don't reuse the same wire as doubling a system will double the currents and that will double the voltage drop with the same pv and battery voltages.

    i think the best would be a small generator to take up the slack of those prolonged rare periods of rain and clouds, keep the present battery bank the same, and maybe add a few more pvs to bring the battery bank up quicker and more inline with your usage.

    I have a small (and large) generator, but not an autostart. And since this is a remote site, I can only use the generator when I am actually at the location. Usually when I am at the site its always going to be sunny (I use it for astronomy, so if there are clouds, I won't be there anyway). However, when I am on site, I regular pull 200 watt/hr during the night (2400 watt or about 75-78% SOC), and the current array can get that back up to 100% the next day with no problem.

    However, I do plan on adding 2 more panels with the increased battery bank (from 3 to 5 - 210watt panels .. that will put me to about 7kw generation per day).

    The big concern was the ability to add the new batteries. But it sounds like that should be ok. I'll check the SG for each cell next time I am at the site to make sure they are ok.

    Roy
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Adding batteries to existing array

    Okay, so you've gone from 50 Watts average (1200 Watt hours per day) to 90 Watts average (2160 Watt hours per day). A change from 50 Amp hours per day (150 for 3 days) to 90 Amp hours per day (270 for 3 days). Minimum battery bank size shifts from 300 Amp hours to 540 Amp hours.

    You're going to go from 420 Amp hours (L16P-AC - not in my renewables list!) to 840 Amp hours.
    You should have about 1600 Watts of panel right now and will double to 3200 Watts.
    I'm going to guess a new charge controller gets added on too, unless you already have an FM80 or MidNite Solar 150. :D
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  • RoySalisbury
    RoySalisbury Solar Expert Posts: 84 ✭✭✭
    #7
    Re: Adding batteries to existing array
    Cariboocoot wrote: »
    Okay, so you've gone from 50 Watts average (1200 Watt hours per day) to 90 Watts average (2160 Watt hours per day). A change from 50 Amp hours per day (150 for 3 days) to 90 Amp hours per day (270 for 3 days). Minimum battery bank size shifts from 300 Amp hours to 540 Amp hours.

    You're going to go from 420 Amp hours (L16P-AC - not in my renewables list!) to 840 Amp hours.
    You should have about 1600 Watts of panel right now and will double to 3200 Watts.
    I'm going to guess a new charge controller gets added on too, unless you already have an FM80 or MidNite Solar 150. :D

    Right now I have 3 210 watt panels (630 watt) .. ~7.5 hours of sun per day. I can get about 4000-4200 watts out of the array on a sunny day. But I have NOT been using 50% as my DOD, but rather keeping it much safer (30-35% DOD, or 65-70% SOC). So...

    Old
    ===
    6*4* 420=10080 / 3 = 3360 / 3 = 1120 watt / 24 = 46.6 watt/hr .. close enough

    New
    ===
    6*8*420=20160 / 3 = 6720 / 3 = 2240 watt / 24 = 93.3 watt/hr .. close enough


    Panels Old (210 rated, 185 in real world)
    ======
    3 * 185watt * 7.5hrs = 4162 watt ... will recharge a 65-70% SOC in one day.


    Panels New
    ======
    5 * 185watt * 7.5hrs = 6937 watt ... will recharge 65-70% SOC in one day.

    The old system is also sized so that when fully utilized (at night) the system can pull about 225-230 watt for 12 hours and still fully charge the next day. The new system will allow me to double the nighttime usage as well, which will be a BIG plus for me (right now I am always have ing to turn monitors off on the systems and can't run some other equipment that I would like to).

    Roy

    Oh.. and I have an FM60 CC ... 24v system so its good for up to 1500 watt combined panel output.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Adding batteries to existing array

    Roy;

    The only flaw with your system is that those tall case batteries won't be getting much current and may not be going through a long enough Absorb cycle to stave off sulphation. Do you ever run an equalization cycle on them via generator? I see shortened battery life occurring here. These batteries really do want to be run down and charged up every now and then. And when recharging, they want to be recharged quickly; at a heavy current rate (Trojan recommends 10% minimum on these - 42 Amps). You can't see sulphation; it just crops up one day a severely depleted capacity.
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  • RoySalisbury
    RoySalisbury Solar Expert Posts: 84 ✭✭✭
    #9
    Re: Adding batteries to existing array
    Cariboocoot wrote: »
    Roy;

    The only flaw with your system is that those tall case batteries won't be getting much current and may not be going through a long enough Absorb cycle to stave off sulphation. Do you ever run an equalization cycle on them via generator? I see shortened battery life occurring here. These batteries really do want to be run down and charged up every now and then. And when recharging, they want to be recharged quickly; at a heavy current rate (Trojan recommends 10% minimum on these - 42 Amps). You can't see sulphation; it just crops up one day a severely depleted capacity.

    The absorb cycle is about an hour each day, and I have an equalization cycle setup for every 30 days. Unfortunately, as you have noticed, that is where my system is not setup right. I don't have the generator setup of the equalization, but just the solar array. It gets the voltage high enough, but not the amps (no where near it). My next addition (before the batteries actually) is to replace my 250 watt inverter with an outback VFX3524. Then I can start using the generator for equalization. I should have that replaced by the end of the month.

    A bit off topic, but what would you recommend I do once I get the VFX3524 (to asset in "reconditioning" the batteries) due to any sulphation that has already occurred?

    Roy
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Adding batteries to existing array

    You can't reverse sulphation, really. You can equalize to a point but - the damage done remains.

    Couple of things about the VFX3524 Inverter (which i have). One, they suck up power. 20 Watts all the time unless in "search" mode. Keep that in mind because it figures into your power consumption. Heck it's 20+%! You might want to keep that small inverter (or a Morningstar 300 if that would handle the draw) and get a stand-alone charger to do some EQ while you're there. Another thing about the Outback is - you really do need one of the MATE programming interfaces. Although they work without it, you can't change any of the parameters (and there are lots of parameters) and you'll go mad trying to use it with the default settings.

    Something to consider: keep your existing batteries, allow 25% DOD (100 Amp hours = 2400 Watt hours roughly) daily, keep 25% "in reserve" for day 2 (total 50% DOD) and seriously add panel so you can push the batteries back up rapidly when the sun does shine. Right now with 630 Watts of panel you're not making the minimum 5% charge rate recommended for "regular" size batteries. Roughly:

    630 Watts of panel @ 77% efficiency = 485 Watts / 29.6 Volts charging = 16 Amps.
    50 Watts used @ 24 Volts = 2 Amps. 16 - 2 = 14 Amps net charge rate, or 3.3%
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  • RoySalisbury
    RoySalisbury Solar Expert Posts: 84 ✭✭✭
    #11
    Re: Adding batteries to existing array
    Cariboocoot wrote: »
    You can't reverse sulphation, really. You can equalize to a point but - the damage done remains.

    Couple of things about the VFX3524 Inverter (which i have). One, they suck up power. 20 Watts all the time unless in "search" mode. Keep that in mind because it figures into your power consumption. Heck it's 20+%! You might want to keep that small inverter (or a Morningstar 300 if that would handle the draw) and get a stand-alone charger to do some EQ while you're there. Another thing about the Outback is - you really do need one of the MATE programming interfaces. Although they work without it, you can't change any of the parameters (and there are lots of parameters) and you'll go mad trying to use it with the default settings.

    Something to consider: keep your existing batteries, allow 25% DOD (100 Amp hours = 2400 Watt hours roughly) daily, keep 25% "in reserve" for day 2 (total 50% DOD) and seriously add panel so you can push the batteries back up rapidly when the sun does shine. Right now with 630 Watts of panel you're not making the minimum 5% charge rate recommended for "regular" size batteries. Roughly:

    630 Watts of panel @ 77% efficiency = 485 Watts / 29.6 Volts charging = 16 Amps.
    50 Watts used @ 24 Volts = 2 Amps. 16 - 2 = 14 Amps net charge rate, or 3.3%

    I had saw that 20 watt search mode stuff, but really was not sure what that meant. I have the MATE already (and FN-DC). Really the main reason I decided on the VFX3524 was because I had the MATE and could monitor the system fully. I'm not against a different inverter/charger as long as I can fully monitor it like I can with the MATE. The inverter I currently have is a 250 watt Exceltech (6 watt no load, 87% eff). So basically at 100 watt, the Exceltech is costing me 19 watt extra already .. and the Outback is costing 23 watt (20 watt + 8watt (92% eff)). So it comes out close between the two (an extra 8-9 watt). Plus the added benifit of the auto transfer switch is nice.

    As for the minimum charge rate... That is something that I have not considered. I will add the other 2 panels into the system, but that still only gives me ~27 amps. Closer for what I need on the 50 watt draw, but not for the 100 watt.

    What might be best is to put the new panels and inverter in place for a month, see how the new panels do and ow the different inverter effiencies work out. Then revisit the batteries again. I'd just really like to not have to keep pushing out the battery additions. They just keep getting older and don't want to get to a point where I can add new ones any longer.

    Roy
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Adding batteries to existing array

    Curiously, if you had gone with two sets of "golf cart" T105's you'd have 450 Amp hour @ 24 Volts, and batteries that would be perfectly happy with 5% peak recharge current. If you then targeted your panels for 10% rate (1730 Watt array) you'd have that on the sunny days, and might still make the 5% rate on cloudy days.

    The OB inverter can be set to "search" mode, in which it uses about 6 Watts - until enough of a load comes on to bring it in to power mode. Ironically, that mean it might be able to use less power over-all than the Exeltech, depending on what your loads are and how they operate.

    The Morningstar SureSine 300 draws 450 mA no load and is 92% efficient: http://lib.store.yahoo.net/lib/wind-sun/SureSine.pdf

    No sense buying a $2,000 inverter when a $200 one will do, right? :D
    Of course you'd need to add a $150 charger: http://www.solar-electric.com/dls-45.html

    It's 12 Volt, though, so you'd have to reconfigure the system which might not be desirable.

    Still, it's a possibility.
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  • RoySalisbury
    RoySalisbury Solar Expert Posts: 84 ✭✭✭
    #13
    Re: Adding batteries to existing array
    Cariboocoot wrote: »
    Curiously, if you had gone with two sets of "golf cart" T105's you'd have 450 Amp hour @ 24 Volts, and batteries that would be perfectly happy with 5% peak recharge current. If you then targeted your panels for 10% rate (1730 Watt array) you'd have that on the sunny days, and might still make the 5% rate on cloudy days.

    Yea.. when I drew up the systemI was looking for something that I could expand capacity with and the T105's just did not seem to fit the bill (let alone the space I had for storage). I really should have done a bit more research about what batteries were better for the load and not just for capacity.
    The OB inverter can be set to "search" mode, in which it uses about 6 Watts - until enough of a load comes on to bring it in to power mode. Ironically, that mean it might be able to use less power over-all than the Exeltech, depending on what your loads are and how they operate.

    It might just be better sine it will always have a load of at least 100 watt. I'll have to do a bit more research on this.

    Roy
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