6 batteries in parallel is bad

snokidsnokid Posts: 6Registered Users ✭✭
Wanted to start a new thread so this information is easy to find for the next guy that needs help!!!

I have a sailboat and am going to be away from land or at least the ability to buy things for long periods of time.

I have 6 agm batteries wired in parallel...

this is going to be hard to explain how they are wired but try to follow along...lol

The batteries and in two different spots on the boat, (space is always a problem on a boat)  So 2 batteries are forward and they also are on a A/B/Both switch the output of that switch goes to midship to the fuse panel.  The other set of batteries are aft thru just an on/off switch.  They are wired in parallel first battery has the positive output going to the switch then both the positive and negative go to the next battery and so on till the last battery where the negative of that battery goes to the fuse panel negative buss bar.  The solar comes in on the same output terminals on those batteries.

I has come to my attention that wired like this might not be the best way to do this, I'm not sure I can change a lot.  It's about 15' between the forward 2 batteries and the rest of the bank.

Ok so knowing this might not be the best, what happens?  why isn't it done this way?



  • PhotowhitPhotowhit Posts: 4,706Solar Expert ✭✭✭✭
    snokid said:
    Ok so knowing this might not be the best, what happens?  why isn't it done this way?
    They don't share the charging or the load evenly.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • BB.BB. Posts: 27,905Super Moderators, Administrators admin
    edited April 10 #3
    A suggestion--Makes for easier debugging (you have what you have--It can work OK, you just have to keep on top of the maintenance and monitoring).

    Instead of (for example) 6x 12 volt @ 100 AH batteries (guess on size), if you can find 6x 6 volt @ 200 AH batteries (i.e., "golf cart size")--Or other cell types/configurations (there are 2 volt and sometimes 4 volt cells available in larger AH ratings).

    The idea is you would only have 3 parallel strings (better than 6, in my humble opinion) and now you can do a quick voltage sanity check across each 6 volt (or other 2/4 volt cell/) battery. This lets you monitor the health of each battery (i.e., 6.5 volts + 6.5 volts = 13.0 volts -- Both batteries are "balanced"). If you find something bad (like  6.0 volt and 7.0 volts) you know immediately if something is wrong.

    Monitoring/logging battery voltage over time and conditions (discharging, charging, resting, etc.) and you can get a good idea how everything is working.

    Also, get an AC/DC Current Clamp DMM (DC specifically, there are lots of AC current clamp meters, and they are cheaper--AC&DC are a bit more expensive, but work with your DC power system).

    http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P ($60 from sears, good enough for our needs)
    https://www.amazon.com/gp/product/B019CY4FB4/ref=ox_sc_sfl_title_5?ie=UTF8&psc=1&smid=ATVPDKIKX0DER ($136 meter, nicer)

    Set up your battery banks so you can monitor each string's charging/discharging current easily:


    Smart Gauge #3 is nice (three/six parallel connections back to master bus bar). Or #2 or #3 and if you use 6 volt batteries, you can clamp on the jumper between the two batteries in the common string.

    You are looking for differences. Any immediate current reading may show less than perfect current sharing... I would suggest you "worry" if two strings are at 10 amps charging/load and one string is OVER 20 amps. In general, because the one set 15 feet away--You will not have perfect sharing. The added resistance of the longer wire run just kills that.

    With more detail, perhaps we can come up with a balanced resistance solution... Say your 2 strings have 3 feet of cables at 4 awg and 30 amps per string):


    4 AWG
    Voltage drop: 0.075
    Voltage drop percentage: 0.62%
    Voltage at the end: 11.925

    And then the 15 foot (one way run) with heavier cable:

    1/0 cable, 15 feet:


    Voltage drop: 0.088
    Voltage drop percentage: 0.74%
    Voltage at the end: 11.912

    You have pretty closely matching voltage drop between all three strings (2x 6 volt batteries in series, 2 strings midships, and 1 string forward (or do this with 12 volt batteries with above issues).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Posts: 2,212Solar Expert ✭✭✭✭✭
    edited April 10 #4
    Having 6 batteries  paralleled in close proximity to each other where all positive/negative terminals are connected with equal length cables to a common terminal or bus bar would be ideal if such a configuration were to be used, but not the best way of achieving capacity, constant monitoring would be required. Two batteries in parallel would be much less problematic, as long as they are connected diagonally as outlined in the smartgauge link, since the batteries are separated perhaps it would be better to charge/discharge  each group of 2 as separate systems and distribute loads, essential loads such as navigation aids, gps, on a separate system, this adds redundancy, should  one fail, there is a backup,  just some thoughts to ponder.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • mike95490mike95490 Posts: 7,915Solar Expert ✭✭✭✭
    Maybe an upgrade, to pairs of co-located, Series 6V batteries, can be done at the next battery change.  That would reduce it to 3 parallel strings
    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 ,

  • EstragonEstragon Posts: 2,843Registered Users ✭✭✭✭✭
    If I'm understanding the wiring correctly, it sounds like the aft batteries are not so bad ("diagonal" wired). They might arguably be better if connected with short, heavy, equal length wire to heavy positive and negative buss bars, but space may not allow for this.

    The forward batteries may be of more concern. 15' is quite long for a parallel battery connection. On a boat, with the difficulty fishing heavy cable, I'm guessing the wire is relatively light, and routing makes wire length longer than 15'. The problem is these may be prone to chronic undercharging because of the extra wire resistance and voltage drop. As the charger will only read average voltage in parallel with the aft bank, this also has a risk of overcharging the aft batteries while undercharging the forward. AGMs are sensitive to overcharging, with potential overheating and venting permanently damaging them.

    If the aft batteries are near the engine, they may also be in much different ambient temperature than forward. Hot batteries need a lower charging voltage than cool/cold ones, and it's tough to get it right with batteries in different ambient temps.

    It may make sense to not normally parallel the forward and aft batteries, and instead to treat them as separate banks in normal charge/discharge operation. Maybe the forward bank could be reserved for winlass use and/or backup house-starting "emergency" reserve?
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • ScoobyMikeScoobyMike Posts: 27Registered Users ✭✭
    I think mcgivor described a great solution to the battery bank separation issue.  I would add that you might consider using the A/B switch between these 2 battery banks and hard wire the 2 forward batteries together.  You could split up your loads between the 2 banks however on the sailboat you will be closely monitoring the battery status and switch banks when you deem appropriate.  Of course this solution requires 2 separate PV systems. I'm not sure what controller you have but you could add an inexpensive PWM controller on the forward bank and make your aft bank the workhorse bank.

    1.2KW off grid system; 2 strings of 2ea 305W 60 cell panels on a redneck ground mount;  MNPV3 combiner feeds a MN Classic 150 located 100' away;  12V 460AH FLA battery bank powers a cabin-wide 12V DC system as well as a Cotek 700W PSW inverter; Honda EU2000i  and IOTA 55A charger bridge cloudy days and a Champion 3800W generator for short duration, power hungry appliances.

  • snokidsnokid Posts: 6Registered Users ✭✭
    Wow who would of thought this could be so complex!!!!

    I guess I need to fill in some blanks.  
    I have right now 2 36v 180 watt panels, I will be adding 1 more panel as soon as the weather permits getting the boat back in the water, I have just 1 mppt controller right now, and I found out it's too small.  So what I'm going to do is have 3 separate charge controllers 1 per panel.  

    This should solve the forward 2 batteries being undercharged.  I will just wire 1 charge controller to that bank and the other 2 chargers to the aft bank.  

    The golf cart batteries wont fit up I don't think, and these batteries are only 1 year old.

    The forward bank is split into bank 1 (engine start)  bank 2 (house) but to charge them the switch needs to be in the both position.

    This is a sailboat so I don't run the engine often, but here's the catch, I have a watermaker (makes drinking water from salt water)  That's a high current draw when running.  I was planning on just running the engine while I make water...  

    Problem is the watermaker is wired to the rear 4 batteries and the engine alternator is wired to the front 2 batteries but they are connected with that 15' of wire so they would get charged but not full....  I think I can solve that problem running another wire off the alternator to the back bank.

  • EstragonEstragon Posts: 2,843Registered Users ✭✭✭✭✭
    If I'm understanding this correctly, I'm not sure running an extra wire from the alternator is going to make much difference. Running an extra wire (or replacing with a single heavier wire) from forward to aft batteries might help minimize differing resistances some.

    If the batteries are close to full, the alternator will output at regulated voltage of ~14.4v with enough current to hold the batteries at that voltage (minus drop from wire loss) and run the watermaker. Adding another wire from alternator to watermaker might reduce voltage drop to the watermaker a bit, but batteries are still going to rise to alternator voltage (minus drop) as the batteries and watermaker are all parallel loads. I'm assuming the alternator outputs enough current to run the watermaker with some room to spare?

    Having a separate charge controller and panel for the forward batteries should help. Are the controllers mppt or pwm type?

    One other thing worth mentioning - each battery should be fused close to or at the positive post of each of the six batteries. This is because a short failure in one battery could otherwise get the combined current of the remaining five flowing through the short.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • snokidsnokid Posts: 6Registered Users ✭✭
    Thank you estragon
    the alternator I believe is 60amp, and the water maker draws about 40 amp while running.

    My controllers are Morningstar mppt.

    I have never seen a fuse at each battery post, is that a common thing?  What amp fuse?

  • littleharbor2littleharbor2 Posts: 1,032Solar Expert ✭✭✭✭
    Technically, an ungrounded system should have fusing at both positive AND negative battery posts

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • EstragonEstragon Posts: 2,843Registered Users ✭✭✭✭✭
    Assuming the watermaker will be run while underway, the alternator is likely okay. If run at idle RPMs though, maybe not.

    Mppt should work well with "36v" panels and your 12v system.

    It's a bit unusual to have so many parallel battery connections. Two in parallel is okay (though the bank wiring as whole should be protected in any case with breaker/fuse on each device connection ie controller, inverter, etc). It's just with more than two parallel connections that the battery fusing becomes necessary. More than two in parallel is uncommon, so the need to individually fuse is also.

    There are a few different types of fuses and holders depending in post type, bolt size etc. Something like this might work:

    I'm not sure about size. Large enough to handle expected loads, but not so large that the wiring melts before the fuse. There should be ABYC spec for your specific wiring.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mcgivormcgivor Posts: 2,212Solar Expert ✭✭✭✭✭
    Sorry to open this kettle of fish, it was my suggestion to start a new thread, the original thread was a question regarding controllers, here it is  http://forum.solar-electric.com/discussion/353181/newbie-mppt-controller-sizing-question#latest  I thought it better to ask questions related to parrallel battery  connections, to get feedback specifically regarding the battery situation, but in retrospect it would have been better to have combined both, because they are interrelated. Without knowing both sides of the coin, it would become confusing to interpret  information biased on one side of the equation... so my bad, but it is important to understand the dynamics none the less.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • snokidsnokid Posts: 6Registered Users ✭✭
    thanks everyone I got some work to do..

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