Battery Bank Wiring

Hi everyone,

I have a simple battery wiring question.
I am planning to install a 48v bank made up of 16 six volt batteries. I am hooking it all up to two inverters as part of the Flexware-500 system.

My questions is, should I run the pos and neg of each series to the respective shunts/bus bars of the DC enclosure?

Or can I connect the two series at the positive with two crossover cables, and then run 1 cable to the shunt/bus bar, and do the same for the negative?

I will be using 4/0 wire, I am using larger wire than necessary for safety reasons.

I appreciate the help.

Island Mon

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Bank Wiring

    The thing that's missing from your info is the expected current draw.

    Otherwise it doesn't really matter so long as the wiring is sufficient. You can put the two banks of eight together in parallel and treat it as one big bank, running one set of wires from it to the inverter set-up, or you can have bank 'A' and bank 'B' with a switch. Depends on your usage requirements.

    Generally speaking, the fewer wires, shorter lengths, larger gauge, and less connections the better.
  • BB.BB. Super Moderators, Administrators Posts: 29,697 admin
    Re: Battery Bank Wiring

    Balancing the current through your batteries will really help extend their life...

    Here is a website that show how NOT to connect your batteries in parallel (they do not know this fact). Note that the current from the closest battery goes through less wiring than current from the "bottom" battery of the string (the top battery will supply/absorb more current--the battery at the bottom of the string will supply/absorb the least amount of current because of the extra resistance/voltage drop of the wiring).

    Here is a website that shows how parallel and series-parallel connections should be made (so that the current has to flow through the exact same length of wire--no matter which path is taken).

    What ever wiring method you choose... The total length of wire (and even number of connections) should be balanced through all of the parallel current paths.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • homan7homan7 Registered Users Posts: 7
    Re: Battery Bank Wiring
    BB. wrote: »
    Here is a website that shows how parallel and series-parallel connections should be made (so that the current has to flow through the exact same length of wire--no matter which path is taken).

    -Bill

    Please seperate this into another thread if you feel that I'm hijacking. In the link above the site mentions that they don't recommend more than 4 batteries in parallel. What is the reason for this?

    I ask because I was given 10 100Ah batteries and was planning to connect them in parallel. I don't want to overlook a safety issue though.

    Thanks,
    TD
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Bank Wiring

    It's about keeping the charge/discharge even across all batteries so that one isn't handling all the load while another just sits there. Often it is recommended that you wire no more than two banks in parallel, and then "diagonal wire"* the inverter and charge controller to try and ensure even current flow in and out of the banks. Otherwise the batteries don't last as long.


    *Diagonal wiring:

    Inverter (+) to Bank 'A' (+), Inverter (-) to Bank 'B' (-)
    Charge Controller (+) to Bank 'B' (+), Charge Controller (-) to Bank 'A' (-)
    Bank 'A' and 'B' tied (+) to (+) and (-) to (-) of course.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: Battery Bank Wiring

    Make sure EACH battery has its own fuse/breaker for over-current protection when connected in parallel
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Battery Bank Wiring
    homan7 wrote: »
    Please seperate this into another thread if you feel that I'm hijacking. In the link above the site mentions that they don't recommend more than 4 batteries in parallel. What is the reason for this?

    I ask because I was given 10 100Ah batteries and was planning to connect them in parallel. I don't want to overlook a safety issue though.

    Thanks,
    TD


    Actually it says they don't recommend more than 4 parallel sets of batteries...which doesn't make much sense anyway and they don't bother to explain their reasoning.

    Probably the reason is due to the fact that the larger the battery bank, the more difficult it becomes to make sure that every battery has equal charge and load.

    But that's just their preference and IS NOT a rule. "Difficult" doesn't mean "you must not do it".


    Personally, I don't see that it matters as long as you are wiring properly to equalize the path of the current - but that IS more difficult to achieve with a bigger battery bank.

    With a larger bank, even having connecting wires of slightly different lengths (different resistances) can throw off the balancing act. "Pretty close" to the same length might not be good enough.

    Here is another tutorial on the subject:

    http://www.smartgauge.co.uk/batt_con.html

    Quote:

    "Finally, if you only have 2 batteries, then simply linking them together and taking the main feeds from diagonally opposite corners cannot be improved upon.

    Once the number of batteries gets to 3 or more then these other methods have to be looked at.

    With a large number of batteries it may be necessary to go to the 3rd method shown above.

    Even with 8 batteries it is possible to get reasonable balancing by placing the main "take off" feeds from somewhere down the chain instead of from the end batteries. Remember, count the number of links each battery needs to run through to reach the final loads and get these as equal as possible."
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery Bank Wiring

    actually wiring example #3 with equal lengths of wire going to the posts from a common buss works best, but you could go with example #2 and just move the batteries around from time to time in addition to some eqs. example #4 does try to equalize the lengths to each battery in its own way. definitely keep wires larger to keep interbattery connections lower in resistance thus helping to minimize any differences. for example if you use #4 to your inverter then at least #3 between the batteries, but #2 is more commonly available. to cut the resistance in half by going at least 3 gauges would be even better, but can get more costly and difficult to work with.
  • homan7homan7 Registered Users Posts: 7
    Re: Battery Bank Wiring

    Thanks for the replies. I apologize to Island Mon for jacking the thread, though it's somewhat related.

    I've been to the smartgauge website before and have been following Method 3 for my first bank of batteries. The plan for the new bank is exactly as niel describes with method 3 going to a common buss. It doesn't look the neatest but I'm not sure method 4 would look any neater (that is if I could figure out the configuration for 10 batteries).

    Thanks,
    TD
  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: Battery Bank Wiring

    Consider the scenario... two batteries in parallel, tapped diagonally. The resistance path to the charger on both the pos and neg legs must be equal. Using wire of equal gauge and length should accomplish this goal. However, the pos leg may contain a disconnect and a fuse. The neg leg may contain a shunt. The resistance of these devices is not expressed in ohms per foot. For example, is the resistance per foot between the terminals of a 300A fuse equal to that of the wire that it is connected to? Some may consider these differences negligible. But no less so than a given length of wire. The length of which depends on gauge. We may be talking inches of fractions thereof.

    A solution... connect the disconnect, fuse and shunt as close to the bank as possible. Then determine the length of the longest run (may be either the pos or neg leg) to the charger. Disconnect from the bank. Use a Wheatstone bridge (General Radio 1650?) to determine the resistance of the run, measured through the fuse or shunt or whatever. Do the same on the other side of the bank but use an over length piece of cable. Measure the resistance of this leg in the same way as before (including disconnects, fuses or whatever) and trim the wire length until the miliohms are equal.

    Fine tuning for sure. Not sure it is worth it. But I do have a couple of the 1650's lying around. They're not calibrated, but this is a relative test.

    K
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery Bank Wiring

    you may be misunderstanding what it is to be done with the equal lengths and what we are trying to accomplish. this is not to make the positive resistance connections equal to the negative resistance connections, but to make all positives equal to each other and all negatives equal to each other. a fuse or disconnect is irrelevant as it applies the same resistance to the whole positive or negative circuit thus keeping equality. you could be accomplishing this equality with all positive connecting wires at say 5ft each and all negative connecting wires at 3ft each. if in my example 1 battery out of the bunch had a 5ft positive and a 4ft negative this is more resistance for that battery than the others would see and that battery won't deliver or charge with as much current as the others. if it was shorter than the others with less resistance then that battery would deliver and charge with more current than the others would. that's the imbalance and you are right that it is very small resistance values.
  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: Battery Bank Wiring

    Yep, I was misunderstanding. Thanks.
  • interd0ginterd0g Registered Users Posts: 7
    Re: Battery Bank Wiring

    I just like to add that for the last 15 years we have been replacing and rewiring large battery banks in megayachts here in Antigua and have had the chance to observe how they behave.
    Simply put, unless the charge and discharge currents are balanced among the parallel branches of the bank, then the imbalance escalates quickly resulting in early failure and dangerous levels of gassing as cells fail.
    I would like to endorse the equal resistance methods advocated above and add that since cable resistance in a parallel branch probably exceeds battery internal resistance ( which is variable over lifetime), this swamps the individual variations between branches and greatly improves equality of current distribution throughout battery life.

    We try to monitor each branch current separately during charging and discharging to confirm the desired equal distribution,

    My own alternate energy home has 16 T105's ( i can just about lift those) with equal total branch lengths and batteries arranged relative to the buss connections so that it looks really tidy.
    They keep going and going and going.........
  • Island MonIsland Mon Solar Expert Posts: 29
    Re: Battery Bank Wiring

    Relating to the battery wiring methods....

    I am running 2 series of 8 AGM 6V batteries for 48vdc @ 800 AH (20 hr)

    Once I have chosen how I want to wire them, I will have two or more positve and two or more negative leads coming from each series of batteries.

    All of the crossover cables for each series will be 4/0. I will be drawing roughly 20 amps.

    Would it be acceptable to tie those leads onto positive and negative 4/0 cables to the inverter by bolting them together.

    For example, I will have two positive cables (one coming from each series of batteries) and two negative cables (coming from each series of batteries). And then will have those connected to positive and negative 4/0 cables running to the DC distribution box. I would connect the two negative and two positive series cables to the longer pos and neg wires by bolting the three pos and three neg cables together.

    Or would it be better to just run each pos and neg from each series directly to the pos and neg connectiong of the DC dist. box. I hope this makes sense.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery Bank Wiring

    ya got me confused some. think of each series of batteries as acting like a single battery. to do this each string would need identical everything and thus equal in every way to each other. once that's done then apply what you know of paralleling two batteries together (in this case strings).
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Battery Bank Wiring

    Yes, it makes sense.

    Yes, you can bolt them together.

    I think that you should have 3 fuses though; one per hot from the battery bank, and one between "where they bolt together" and the inverter.

    [Also, are you are saying 4/0 (which is rated for 300 amps load) when you really mean #4 (which is rated for 125 amps)?]


    Anyway, here's why I would recommend the third fuse:

    If your are using 4/0, and protecting each hot from a battery bank with a 300a fuse - then it is possible to pull as much as 600 amps from the batteries *without blowing either 300 amp fuse*. That's enough to (enormously) overload the shared 4/0 between the inverter and the 3-way connection.

    So there should be another 300a fuse after the 3-way connection to protect the inverter leads.

    If you sized the fuses at the battery ends at 150a, so that the combined load *could not* be enough to overload the 4/0 wire between the 3-way connection and the inverter, then you could omit the 3rd fuse.
  • _OS__OS_ Solar Expert Posts: 207 ✭✭✭
    Re: Battery Bank Wiring

    There is one more thing you must consider if your batteries have very low internal resistance. That is to use equal torque when fastening the cables. I am not kidding:

    A friend of mine ordered four of the biggest SunXtender batteries for parallel connection a few months ago and the distributor told him that due to the very low internal resistance in these batteries even a slight difference in cable resistance would shorten the life span of the battery bank. If he wanted to connect more than three batteries he had to use a torque wrench and a "bus bar" for all connections. You should contact the manufacturer of your batteries to get the correct torque to use. You can borrow a wrench at your local car repair.

    JMTC,
    Ole
  • Ralph DayRalph Day Solar Expert Posts: 890 ✭✭✭✭
    Re: Battery Bank Wiring

    Better to buy the torque wrench outright. Battery terminals being lead will need to be tightened/re-torqued occasionally (yearly?) so why not have the tool for the job. This from the man with almost all the tools he'll never need.:p (Just ask my wife)

    Ralph
  • BB.BB. Super Moderators, Administrators Posts: 29,697 admin
    Re: Battery Bank Wiring
    Ralph Day wrote: »
    ...This from the man with almost all the tools he'll never need.:p (Just ask my wife)

    Almost being the key word here. :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Ralph DayRalph Day Solar Expert Posts: 890 ✭✭✭✭
    Re: Battery Bank Wiring

    Almost...because there's still a heartbeat, and there's still a Mastercard.

    Ralph
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery Bank Wiring
    Ralph Day wrote: »
    This from the man with almost all the tools he'll never need.:p (Just ask my wife)

    Ralph

    now why would i ask her for she probably doesn't even know what each tool is let alone what it's for? keep in mind she only needs 1 tool to hit you with too.:cry::p
  • Island MonIsland Mon Solar Expert Posts: 29
    Re: Battery Bank Wiring
    _OS_ wrote: »
    There is one more thing you must consider if your batteries have very low internal resistance. That is to use equal torque when fastening the cables. I am not kidding:

    What would be considered low internal resistance? I am using Full River AGM DC-400 6 volt batteries with an internal resistance of 1.6 milliohms fully charged.

    Regardless, I will be using a torque wrench.

    Thanks for your input!
  • mike95490mike95490 Solar Expert Posts: 8,463 ✭✭✭✭✭
    Re: Battery Bank Wiring

    What about the low torque, conductive greases (silver filled)
    http://www.mgchemicals.com/products/8463.html
    a tad pricey, but if they help lower the electrical contact resistance....
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Ralph DayRalph Day Solar Expert Posts: 890 ✭✭✭✭
    Re: Battery Bank Wiring
    niel wrote: »
    now why would i ask her for she probably doesn't even know what each tool is let alone what it's for? keep in mind she only needs 1 tool to hit you with too.:cry::p

    I won't show her that post...sometimes I don't even know what some of the tools I have are used for (in the shop!)

    Ralph
  • Island MonIsland Mon Solar Expert Posts: 29
    Re: Battery Bank Wiring

    I thought I would follow up on my wiring situation.

    I have decided to run two cables (pos and neg) from each end of each series string, to the DC combiner box before the inverters.

    I am still curious about torqueing of battery terminal studs. I am aware that they should all be torqued the same because of the batteries internal resistance, but I am not sure of what torque to use.

    I will be calling the manufacturer in the AM.

    They are AGM Full River dc-400's, 6 volt 400AH.
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