New Battery Bank

stmar
stmar Solar Expert Posts: 370 ✭✭✭
My new battery bank has been functioning for about a month now and I have a few questions/observations:
It seems that the "load" is slightly higher than it was, an amp or two, it is 3 and 4 amps average where in the past it was 1 or 2 amps, and I have not altered my usage or added any appliances. Could the batteries be charging more thereby increasing the load? I did lower the charge rate to 12 amps, my bank is 400ah. Also the voltage seems to stay at float more than with the old bank, it does go up during the day but I think the load is keeping it at float more. This bank is one string 4 X 6 volt = 24 VDC, where as the old bank was 2 strings @ 12 volt x 2 = 24 VDC. I have not gotten a grid power bill yet so cannot say for sure that I am using more or less grid power, I guess that will be the telling factor.

I know I will get a lot of input on this one: You are not supposed to mix old and new batteries. Why would this be detrimental? New batteries are 4 X 6 volt 400ah AGMs, old batteries are 12 volt 256ah AGMs. What would happen if I added two of the 12 volts to my bank, even if they are in good condition? I am not going to do it but just was curious as to the theory behind the rule not to.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: New Battery Bank

    AGM batteries have much less self discharge--And can be upwards of 98% efficient during charging/cycling (we use 90% efficiency to be conservative).

    I am a little confused, why/how did you reduce the charging current to 12 amps? AGM batteries can actually take very high charging current when they are significantly discharged (say below ~80% state of charge).

    Nominally, we recommend 5% to 13% rate of charge and a 400 AH battery bank should be very happy with 40 amps charging current.

    12 panels * 64 Watt * 0.77 panel+controller derating * 1/29 volts charging = 20 amps charging current

    Your array (if I understand it correctly) is capable of 5% rate of charge--Certainly at the "floor" of what we would recommend for minimum charging current.

    So, from a design point of view, adding another 256 AH of battery to your existing batteries would, in my humble opinion, really require a lot more solar array than you currently have for a daily cycling array. If you are just floating the batteries and only using them for emergency power (and have a backup genset), you can go down towards a 1% array just for float charging.

    Mixing old and new batteries on the same string. I would give a couple of answers of why to avoid this:

    First, lead acid batteries are very sensitive to voltages when charging/discharging. The voltage is really set by state of charge, and age/condition of battery bank. When you put old+new batteries in parallel, the two banks may not sure charging/loading current equally. What can happen is the new batteries (with lower internal resistance) will take more load/charging current than the older string--And can cycle age more than the older bank--And the newer bank will sort of age faster.

    Basically, batteries ideally should be in "matched sets" for optimal operation.

    Second, mixing old an new batteries. You need to watch your battery bank like a hawk anyway--A shorted or open cell in a parallel connected battery bank can be missed pretty easily unless you go out once a week to once a month and check battery voltages with a good DMM looking that all batteries are equal voltage, and using a DC current clamp to ensure parallel battery connections are sharing loads reasonably closely together.

    When you have an old string, there is a possibility that old string will fail years before the new string... And if it fails with a shorted cell, it can drain/kill the new bank--So there is an elevated risk the old string may take out the new string as time goes on.

    There is also the issue of paralleling mixed batteries (capacity, brand, age) that makes proper current sharing a bit dicey too...

    Will your battery bank end up in a puddle of acid fumes and melted lead tomorrow if you parallel the old and new strings in your case... No, they will not.

    Will the mixed battery bank perform OK... Probably--But you should monitor them closely to ensure that they do.

    Will there be more maintenance/monitoring and eventual "early replacement" or removal of the older string... Yea.

    My two cents... If the new string is capable of supplying your loads, I would not add the old bank (if it is many years old).

    If the old sting is only a couple years old, personally, I would try adding it--Especially if the 50% more AH capacity is needed to supply my loads.

    In any case, if you are daily cycling the batteries--Your current single string could easily use 2x larger solar array and be very happy. If you add the old battery string, you would need to make your array 50% larger just to support the larger battery bank at 5% minimum rate of charge.

    Running at 5% or less rate of charge usually means you have to watch the weather (sun) and loads like a hawk to ensure your battery bank gets charged >90% at least a couple times a week.

    You have grid tied power? So, cycling/charging of the battery bank is pretty much at your choice anyway (at least until the power goes out).

    Solar panels have never been cheaper, and batteries have never been more expensive... Now is a very good time to add more panels to your array and get your charging current up anyway. Running at with a smaller array is a recipe for "deficit charging".

    AGM's are a bit difficult to determine how well they are being charged (you cannot measure specific gravity, only measure resting voltage and monitor charging voltage/current over the day). You might want to look at a battery monitor of some sort (Victron is another good brand). It can make monitoring of your AGMs a lot easier (day to day). (battery monitors are not perfect either--so keep track of cell/battery voltage, charging current, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: New Battery Bank

    I set the charge amps to 12 to match my EU2000 generator and since I am on the grid I thought the lesser charge was okay. My SW4024 can charge up to 30 amps AC. I will be starting the generator manually so I can change the charge amps at will so don't really need to leave it at 12. Could reducing the charge amps, like I did, have an effect on the load and grid input values? I thought that 30 amps AC charge was too much for a 400 ah bank at 24 VDC. I found this formula in my inverter manual: 1/5 X 400 = 80 amps DC, 80/4 = 20 amps AC. I used my bank's values, 400 ah @ 24 VDC. I will increase the charge amps, is 20 the correct value or am I missing something? Was your 40 amp suggested value AC or DC?

    I will not be combining the old batteries, I was just trying to gain more knowledge as to the whys and hows of this complex discipline. My array could use expansion, even for the 400 ah bank. Is there a rule of thumb for choosing which panels to add to my existing array? These are older panels, smaller than what I have seen advertised so I want to make sure I can incorporate without issues.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: New Battery Bank

    Converting AC Amps @ 120 Volts to DC Amps @ 24 Volts is approximately X5 factor. So 30 Amps AC would be 150 Amps DC. The charger probably does not go up to that level, rather the 30 Amps is the maximum AC IN which would be divided for both charger current and load current.

    Working in the other direction we want 40 Amps on the DC side which is only 8 Amps AC. But if you set the AC input limit to that there will be no allowance for load. Check your SW menu listings: I'm pretty sure that somewhere in there are the two different settings: one for maximum AC in and one for maximum charge rate in AC Amps.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: New Battery Bank

    Setting your AC max current (charging/breaker) to 12 amps @ 120 VAC is perfectly fine (as Marc typed). You might actually set the maximum input breaker to ~10 amps @ 120 VAC... They eu2000i is good for about 13 amps maximum, but I would pull it back to about 10 amps--More than enough energy to charge your battery bank (when needed), and avoids putting >80% of rated load on the genset (they are nice generators, but most non-commercial gensets really do not like running near 100% rated output--which is easy to do with an AC battery that is bulking from 50% to 80% (10% rate of charge, 3+ hours of run time).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: New Battery Bank

    Ah the ol' percentages game. :D

    So if you set the maximum AC IN to 12 Amps to prevent overloading the EU2000i then your 8 Amps for the charger becomes about 66% (if that's an allowed value). You could go 70% for AGM's (8.4 Amps or 42 Amps DC). Not totally accurate but close enough for government work.
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: New Battery Bank

    Max AC in is 60 amps, I have a 60 amp breaker that feeds the inverter from my grid main panel. So the lower max charge amp setting is okay at 12 or even 10 amps AC? If I take the generator out of the equation, since it is a manual operation and it's charging value can be set at the time it starts, then should I set the max charge amps to 20 AC per the formula which would be 80 amps DC or 10 amps AC to get the 40 amps DC. I guess my confusion is what does the change in max charge amps do to the batteries; too low? too high? just right?? Keeping in mind that I am on the grid and so they are constantly charged or charging the majority of the time. Would a higher charge rate be more efficient and use less energy?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: New Battery Bank

    In theory a higher charge rate will charge batteries quicker. More efficient? For a generator yes, as it means it will be 'better loaded' - hopefully to a point where it operates at maximum efficiency. For grid power it isn't going to make any difference as there is no change in generation availability or cost.

    For the batteries there is an optimum point where they receive the most current they can accept. Above that point an ever greater percentage of the charge current goes to heat and less to actual charging. For AGM's that point is fairly high; consult the maker's specs. For flooded cells things start to get pretty warm around 15% capacity and 20% should not be sustained.

    Most of the efficiency gain is to be had from best use of the generator, not from increased charge current per se.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: New Battery Bank

    You could very well end up reprogramming between Grid and backup genset (unless this inverter has both an AC1 and an AC2 input).

    Gensets tend to be most fuel efficient >50% loading of their rated output.

    Over ~80%, you run the risk of overheating (and/or motor stalling sometimes--depends on genset).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: New Battery Bank

    It does have AC-1 and AC-2 but only one max charge amps AC so I assume that whatever AC input is charging, AC1 or AC2, will have a max of that menu item. I do have a Set Gen (AC2) amps AC menu 11 AC Inputs. I think that regulates the generator input so it would not matter what the max charge amps are, it would max out at the AC2 value, I think I set it to 12 when I was programming the inverter, to match the EU2000.
    So I am going to leave my Max charge amps on 12 since there is no benefit of increasing or decreasing it. I will do some testing as time goes by, disconnecting from grid and see how the system recovers, maybe even crank up the generator on a cloudy day. Array is pumping out about 15 amps give or take and batteries are in float and load is about 4 amps, have the crock pot cooking. Grid input is 0 so no $$ going out right now, lol.