How to Wire 32 Batteries (12v each) to make a 48V Bank.

TucsonAZTucsonAZ Solar Expert Posts: 139 ✭✭
I understand parallel connections are less ideal however, this isn't an 8 year solution as I need need to get things up and working so 24-36 months would be fine for me.  I'm willing to accept slightly shortening their life in a less than ideal application until I can get a Lifepo4 bank going.  They're AGM batteries and I of course know that I will end up with 8 banks consisting of 4 batteries in series paralleled. 

My question is really if I can I wire this the same way I would if I were making the bank up with 8 batteries and just use the negative from the first bank and positive from the last?  Is there anything else I should factor in when making it?

Additionally, do you have thoughts on connecting the cells?  I had a 1" wide 1/4" thick copper bar so I used pieces from that for my current bank with 4 cells.  But is solid bar better than wire for something like this?  Any other protections I will want like a fuse on each bank or anything like that?

Thanks for the help, I have done a lot with solar but never anything this large and given the massive amount of energy in this bank I want to make sure it's done well and as safely as possible.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    In theory, is it simple. 4x 12 volt batteries in series = 48 volts. And 8x series strings connected in parallel = 8xAH rating for bank.

    The details:
    • What is the AH rating of the batteries? These are Flooded Cell Lead Acid Batteries?
    • What is the maximum continuous current you expect to draw (loads) and charge the bank?
    • Need the above to figure out the AWG of the wiring and size of bus bar.
    Then there is the safety question. When you put 3 or more strings in parallel--You should look at a fuse/breaker per string to prevent a short in one string from being fed by the other 7 parallel connected strings (i.e., 200 AH batteries that can output way more more than 200 amps in a short--7 parallel strings feeding >> 1,400 Amps into a shorted string--Not good).

    200 AH @ 480 Volt bank (40x 6 volt @ 200 AH batteries in series) ~ 3,000+ amps into dead short before breaker popped (3,000 amps from 200 AH battery):

    https://www.sbsbattery.com/PDFs/VRLAshortCurrentsStorageBatterySystems.pdf

    Example of simple bus bar current rating chart:

    https://www.copper.org/applications/electrical/busbar/bus_table3.html

    Simplified AWG vs Wiring chart:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    AWG vs fusing current chart (very rough numbers). Not suggesting to use wire as fuse (i.e., bunch of 4 AWG wire with one 8 AWG series interconnect link per string):

    https://www.powerstream.com/wire-fusing-currents.htm

    Actual fusible link/wire:

    https://www.awcwire.com/automotive-wire/fusible-links/j156

    Breakers and fuses are not cheap. 8x sets of breakers and fuses are not cheap.

    Ideally, one Over Current protective device per string. And from the bus bar, one OCPD device per wire leaving the bus. A grounded negative battery bank would only need OCPD per positive lead.

    Obviously, protecting wiring and terminals from shorts/sharp edges, etc. is important. 

    Example of a very compact fuse/battery/bus bar fuse holder/MRBF terminal blocks (1 and 2 circuit). Very compact, high current ratings, not cheap set:

    https://www.bluesea.com/products/category/16/72/Terminal_Fuse_Blocks
    https://www.amazon.com/Single-Terminal-Fuse-Block-Battery/dp/B075ZJ3MRV ($20.55 per holder--Just quick search)
    https://www.amazon.com/Blue-Sea-Systems-5190-Terminal/dp/B0016HTXAY/ (around $16.50 per fuse)

    Some various circuit breakers. DIN rail/box mount. 60 amps or less ~$15 each:

    https://www.solar-electric.com/search/?q=Midnite+Solar+MNEPV

    I would also highly suggest a DC Current Clamp DMM (really AC+DC Current Clamp DMM). Allows you to measure charging/discharging/resting current per string (find open/shorted cells, bad/high resistance wiring/connections, etc.):

    https://www.amazon.com/gp/product/B00O1Q2HOQ (cheap meter, up to 100 Amp AC+DC capacity)
    https://www.amazon.com/gp/product/B019CY4FB4 (mid cost meter, up to 600 Amp AC+DC capacity)

    Note that fuses and breakers do have a maximum AIC (ampere interrupt current) rating... AIC should be >> several thousand Amps for a larger FLA battery bank.

    Longer discussion about picking the proper fuse/breaker for your system:

    http://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

    Would I keep a fire ax/Bolt Cutter handy to cut a shorted wire--Maybe...

    Regarding a fuse/breaker per parallel string of batteries... "In Theory", the correct thing to do. If location is relatively fire resistant (not on wood floor, out in concrete block building, locked access--no kids/unauthorized access, etc.)--Probably not many DIY folks put OCPD per string (I don't know what is common--Just an observation). The old question of Solar Panel Combiner Boxes vs not "writ" large.

    DIY Solutions vs Engineered $olution$ -- Large unattended battery bank--Things can go wrong. Will it happen to you? Probably not.

    Your thoughts/questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TucsonAZTucsonAZ Solar Expert Posts: 139 ✭✭
    Thank you for the really awesome response!  Bill, you have been such a huge help to so many for a very, very long time!  I started using the forum for a trip I was gearing up for 7 years ago and I learned so much.  Anyway, just wanted to say think you!

    • What is the AH rating of the batteries? These are Flooded Cell Lead Acid Batteries?
    • What is the maximum continuous current you expect to draw (loads) and charge the bank?
    • Need the above to figure out the AWG of the wiring and size of bus bar.

    They're 150ah AGM batteries hooked up to the MPP Solar LV6548 which is a 6,500 Kwh Hybrid inverter that's currently hooked up to 20 275 watt panels.  This will likely be expanded in the near future adding an additional LV6548 and 20 more panels.  Caution will be taken to be certain the bank doesn't fall below a 50% SOC and so on. 

    For each grouping of batteries the plan was to make up the 48V by connecting the 4 batteries with 3/4" flattened copper pipe.  Then 8 equal leads going to a central point to try to make sure resistance is as equal as possible among all 8 groups of batteries.  The BlueSea terminal fuse blocks would be fine, can  you suggest an amp rating on those?

    Also, I know I want resistance from each bank to be the same but can I have the positive and negative leads be different, for example 8 leads that are 4' each for the negative and then 8 that are 2' for the positives? 

    Want to make sure it's done right and as safely as possible so advice is more than welcomed!  Thank you again!

  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    You are very welcome TAZ,

    The "total wire" run for each series string matching is what is important... If you have 6" negative and 12" positive on one string and 12" negative and 6" positive on the other--Does not make any difference in total resistance.

    Have you already purchased the batteries? My first suggestion would be to get 2 volt AGM cells:
    • 8x 150 AH @ 48 volt strings = 1,200 AH @ 48 volts (and 32x 12 volt @ 150 AH batteries in series parallel connection
    • 24x 1,150 AH @ 2 volt batteries in series = 1,150 AH @ 48 volt battery bank (24x 2 volt batteries total) 
    https://www.solar-electric.com/search/?q=agm+battery (search for 2 volt cells--1,150 AH seems to be "popular" with multiple vendors)

    The single series string--No parallel connections/cables/fuses/etc... Makes wiring much easier and cheaper (2 volt cells may be more expensive bank--So savings in wiring costs/labor may help break even on overall costs).

    With COVID/Trade/Tariff issues--Check lead time on the batteries you may be purchasing... Have read that some are >32 week lead time (I think--I am not in the business).

    Back to your bank of 12 volt @ 150 AH batteries... I assume a 6,500 Watt AC inverter @ 48 VDC input? And 20-40 @ 275 Watt solar panels? Assuming inverter is running near 6,500 Watts of AC loads (and 2x surge rating for starting induction motors/whatever)... Need to figure out the maximum current from battery bank. Some guesses:
    • 1,200 AH (48 volt) * C/8 hour discharge rate = 150 Amps from bank (8 hour discharge rate, 4 hours until 50% state of charge)
    • 1,200 AH * C/5 discharge rate = 240 Amps from bank (5 hour discharge rate)
    • 6,500 Watt inverter * 1/0.85 inverter eff * 1/42.0 volts low battery cutoff = 182 Amps max continuous AC inverter load
    • 20 panels * 275 Watts * 0.77 panel+charger eff * 1/59 volts charging = 93 Amps approximate max continuous solar charging
    • 40 panels * 275 Watts * 0.77 panel+charger eff * 1/59 volts charging = 186 Amps approximate max continuous solar charging
    For now, lets pick 186 Amps as the maximum bank current (charging from 40 panel array--Adjust if your needs are different). You have 8 parallel strings--But assuming variations in wiring/connections/failed connections/failed battery(ies), I would suggest that we assume 4 working strings sharing the current (worst case design).

    And for battery charging/discharging I like to use the NEC continuous current derating of 1.25x or x0.80 (i.e., a 20 amp wire/breaker will carry 0.8*20=16 amps and never pop the fuse/breaker, and at 20 amps will eventually minutes to hours trip).
    • 186 amps * 1.25 NEC derating = 232.5 Amp minimum suggested branch circuit wiring/fuse/breaker rating (total current)
    • 232.5 Amps total / 4 parallel (worst case) battery strings = 58 Amp ~60 amp minimum per battery string wiring/fuse ratings
    And here is a detailed thread on understanding/picking fuses and breakers for DC power systems:

    https://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

    Your 20/40 panel array charging a 1,200 AH @ 48 volt battery bank:
    • 20 panels * 275 Watt * 0.77 panel+controller derating * 1/59 volts charging = 93 Amps typical best case charging current
    • 93 amps / 1,200 AH capacity = 0.0775 = ~8% rate of charge
    • 40 panels * 275 Watt * 0.77 panel+controller derating * 1/59 volts charging = 186 Amps typical best case charging current
    • 186 amps / 1,200 AH capacity = 0.155 ~ 16% rate of charge
    8% rate of charge--The question is always what are your loads (WH per day? Every day?), Base Loads (must run every day) vs optional loads (those that only need to run when the sun is out), by season, at location (hours of sun per day/by season), and backup genset--If any...

    Anyway, the numbers look pretty well "balanced" (bank size, array size, guess at load/charging current, etc.)...

    Lastly, highly suggest you get some sort of AC+DC Current Clamp DMM so you can measure and monitor the per string current (if you go with parallel battery strings). Strings not properly sharing current can be an early indicator of upcoming problems (loose/dirty wiring connections, weak cell/battery, etc.):

    https://www.amazon.com/gp/product/B019CY4FB4 (mid priced AC+DC clamp meter)

    Note there are AC Current Clamp meters and AC+DC clamp meters--We want the AC+DC version. There can be confusion, you can get an AC current clamp DMM that will measure AC+DC voltages--But only do AC Current Clamp readings--We don't want that. (AC only clamp meters are great devices--But do not measure DC current in battery wiring).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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