This is driving me nuts!!!

billygoatninja88billygoatninja88 Registered Users Posts: 10
So I have been doing way too much searching and I am having a hard time grasping this.

I want to set up a 48v battery bank and been doing mind numbing searches on 12v batteries and 6v (making 48 volts) it seems to me that 6v batteries will provide much more capacity in a single string (again 48 volts) but that's twice the connections twice the maintenance.

I guess it's weight and size because the biggest 12v I have seen is 225ah but 6v 420ah but same weight and a 420ah in 12v would be huge. This is comparing Trojan batteries.

Correct me if I'm wrong.

My guess is its worth the extra connections and and maintenance since its twice the capacity.

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    edited November 2015 #2
    No correction is possible since you are right! 
    The amount of energy in a lead acid battery is pretty much the same for a given size and design whether that battery is 12V, 6V or even 2V.
    By using twice the number of batteries you get twice the energy storage because you are getting the same 48V but at twice the AH capacity.

    Note, however, that it is also twice the cost!!!
    What is almost undisputed is that for a given energy capacity you will have a longer lived more stable battery bank requiring less maintenance if you have a series string of 6V batteries rather than two parallel strings of 12V batteries.
    SMA SB 3000, old BP panels.
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    To be honest it dawned on me as I was writing the post but posted anyway because someone else might have the same question. But also to make sure I had it right (after the light bulb turned on........)
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    Vary much indeed.

    Well I also decided that I'm just gonna bite the bullet and save up for the system I want rather waste time and money with buying lower end stuff then replacing with higher end stuff later and just do the extra research to make sure I set it up right and further midigating waste.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    Depending upon the battery bank capacity needed some folks use 4 volt batteries or even 2 volt. 
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    2v might be a bit much but if I have the need im sure I could integrate 48/2v string and make the transition. I also want a backup string but more then likely it will be a second string but identical setup charged ready to go if need be. Or maybe just some standby batteries and to replace if one goes bad or whatever.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    ............I also want a backup string but more then likely it will be a second string but identical setup charged ready to go if need be. ....... 
    Or maybe just some standby batteries and to replace if one goes bad or whatever.
    If it is a lead acid string sitting there charged, but not being used or being charged, the charge will slowly dissipate through self discharge. In turn that makes for more maintenance for little benefit, IMO.  Same thing if you have one or two "spare" batteries sitting there just in case one of the operating batteries goes bad. If you look after the working set of batteries; don't abuse them with deep discharges, keep the electrolyte level up, EQ when needed, etc, you should not have a problem. My GC-2's are now 76 months old and working well. If you do have a problem that requires a battery replacement a few years down the road I would buy a replacement at that time and insert it. The new one will quickly learn to act about the same as the older batteries. That will probably not be any worse than inserting a battery that has been sitting around since the start up date. And it would save the initial expense of a spare or two and the charge maintenance routine required.  Just my opinion. Keep it simple if possible. 
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    Makes sense. I have read that if one does go bad it won't deadline the whole bank till you can switch it out.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    The main thing is if you use flooded batteries and keep a log of the sp gr readings you can see trouble coming before the 2x4 smacks you in the forehead. 
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    Another thing that I have been frustrated about is amp hours vs watt hours 48v vs the other common voltages. In order to match the ah of one string of 8 6v I would have to have two sets of 4 12v batteries but ether way its only 400 and some change ah but its like 20kwh. 400ah seems low aspesailly this dude on YouTube made a 12v 1000ah bank with 6v 100ah batteries. I don't remember how many he has but he made a cart for them so it can't be that much if it fits in a cart so how is he getting 1000ah when you have to do series parallel unless he has more then when it looks like on YouTube. And as everyone knows series it ups the voltage but the capacity stays and opposite with parallel. What matters most? Watt hours or amp hours?

    Yes I have searched and searched and all the stuff I have read hasn't answered my question.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    A 12V 1000AH bank is a very poor design. To use a corresponding amount of power at 12V will require excessive current and to keep that bank charged properly will require at least two CCs which would have a total amp capacity between 80A and 120A.
    Of course it might be a low power system (e.g. 500W inverter) with a grossly oversized battery bank, but I doubt it.
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 32,656 admin
    The general answer is:

    Amp*Hours * Voltage = Watt*Hours

    The specific issue about why AH vs WH... When working with battery systems, everything is 12 volts (or whatever)--So you assume that amps*time is always referenced to a fixed system voltage.

    When working with 120 VAC and 12 VDC (mixed) voltage systems--You have to pay attention to the actual voltage... 1 amp at 120 VAC would take 10 Amps at 12 VDC:

    120 VAC * 1 Amp = 12 VDC * 10 Amps = 120 Watts

    So to keep things straight with off grid power systems, we tend to work in Watts and Watt*Hours, and then do the final conversions when need to Amps and Amp*Hours (wiring size, fusing, battery bank capacity).

    Lead Acid batteries are actually pretty close to 100% efficient regarding current/AH flow... You draw 100 AH from a lead acid battery, you recharge ~100 AH back into the battery (during final charging/floating when batteries are gassing, the AH "efficiency" does drop--gassing/generating hydrogen+oxygen is a 100% loss).

    Since lead acid batteries have a change in voltage (14.5 volts charging, 12.7 volts 100% SOC resting, 12.0 volts 50% SOC, 10.5 volts at near 0% SOC and/or heavy surge loads)--The Amp*Voltage or actual power output varies:

    14.5 volts * 10 amps = 145 Watts
    10;5 volts * 10 amps = 105 Watts

    One reason (I believe) reason why battery systems tended to use Amp*Hours vs Watt hours... Many of the old DC loads were kind of fixed at current (i.e. loads would draw 5 amps at 12 volts or 5 amps at 14.5 volts--roughly). Filament lamps, motors, etc. tended to be self regulating (current wise)...

    However, when we throw an AC inverter into the mix--These inverters are "constant power" loads on the DC system side... For example, a 120 VAC @ 1 amp load looks on the DC side like:
    • 120 VAC * 1 amp = 120 Watts
    • 120 Watts * 1/0.85 AC inverter eff * 1/14.5 volts battery (charging) = 9.7 amps @ 12 VDC (charging)
    • 120 Watts * 1/0.85 AC inverter eff * 1/12.0 volts battery (charging) = 11.8 amps @ 12 VDC (discharging)
    • 120 Watts * 1/0.85 AC inverter eff * 1/10.5 volts battery (charging) = 13.4 amps @ 12 VDC (near dead battery)
    So--A fixed "AH" rating for a battery over a wide range of voltages and states of charge is not really very easy to "understand" the amount of available energy in the battery.

    You will now see some batteries (generally those designed for UPS systems) not only have an AH rating, but also have a Watt*Hour Rating--Which better matches how an AC inverter actually draws power/current from the battery bank.

    In the end--Use AH for battery rating, and I suggest "nominal voltage" (like 12 VDC) to estimate battery energy capacity. Use Watt*Hours for all of your other power/energy calculations--And just convert to Amps and Amp*Hours when needed (fusing/breakers/wiring at 120 VAC or 12 VDC as an example).

    For us, when helping... We get people say they have a 10 amp load--But we don't know if that is 10 amps @ 12 volts (120 watts) or 10 amps @ 120 Volts (1,200 Watts) or even some other voltage (12/24/36/48/120/230/240 volts etc.)--So we trip over each other trying to clear things up.

    We stay with Watts and Watt*Hours for initial system design, and do the voltage conversions when we select components (charge controller, battery bus voltage, battery sizes, etc.).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    edited November 2015 #13
    It was a 3000 watt inverter.
  • BB.BB. Super Moderators, Administrators Posts: 32,656 admin
    48 volt input inverter?

    How much Power do you need (peak watts at 120 VAC)?
    How much Energy do you need (Average Watts * Hours per day--1,000 Watts * 5 hours per day = 5,000 WH per day)?
    Is this an emergency backup system, weekend/seasonal cabin, or a full time off grid system 9+ months of the year?
    Where (roughly) will the system be installed (near what major city)?
    Do you plan on having a genset? Will there be grid power?
    What is your expectation of they system (how do you want to use it, install yourself, are you prepared to perform maintenance)?

    Define your needs first--Then design a system that will support those needs. Remember that a 6 volt @ 200 AH battery is basically the same size/weight as a 12 volt @ 100 AH battery:

    6 volts * 200 AH = 12 volts * 100 AH = 1,200 WH of storage

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    Yep but now I'm thinking 24 volt might be better in my situation because I can get close to the same price as but again I would like to stay with one string. Screw it I'm going 2v jk idk I guess I'll just have to wait till I'm about to cross that bridge. Thanks for the info have given me a lot to think about.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    It was a 3000 watt inverter.
    Look at the current that will be required for 3000W of inverter output at 12V: about 300A. Even more to support the surge capability of the inverter. That is not easy to handle safely and the cables are expensive and hard to work with.
    At 24V you are only looking at 150A plus surge.
    A 48V battery bank will only require about 75A plus surge factor.
    SMA SB 3000, old BP panels.
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    edited November 2015 #17
    Well inetdog I can't figure out how to to quote so I'll improvise.

    That's why I wanted to go with 48v 75A is wayyyyyyh lower than 300A even 150A and its more efficient even if my needs don't require a 48v system I don't think it will hurt having more then I need. Sure more expensive but that's OK I will have a better system from the get-go rather than waste money upgrading later. Some batteries I was looking at have a 10 year warrantee so they are expected to at least last that long as long as I do my part. They only upgrade I can think of is going to bigger 2v batteries some day.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Well inetdog I can't figure out how to to quote so I'll improvise.
    Second button from the left in the row of tools below the text of the post that you want to quote.
    You have to use that before you open/select the post you want to quote.
    SMA SB 3000, old BP panels.
  • billygoatninja88billygoatninja88 Registered Users Posts: 10
    Oh OK I see I didn't select the text to quote.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Oh OK I see I didn't select the text to quote.
    You cannot preselect, but you can edit what has been selected.
    If you really want to cut and paste and make use of the Quote indentation (but not the labeling of the person quoted) you can select the text and apply the Quote format to it using the drop down from the paragraph sign in the upper formatting bar.

    The Spoiler format is particularly useful for telling riddles.
    Isn't it!

    SMA SB 3000, old BP panels.
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