Odd or Even Battery Strings

rp3703 · May 2018

I have a quick question about battery banks wired in series/ parallel. I currently have a 24V 440AH battery bank consisting of 8 Deka 6V 220AH batteries. I need to up my AH to at least 660 to cover my projected loads. My brother-in-law remembers reading somewhere that charge controllers like even numbers vs odd. So I'm wondering if adding another string of 4 batteries will be fine or should I add 8 to keep an even number of strings.

Estragon · May 2018

I don't see how a charge controller could possibly "know" whether a bank has an even or odd number of strings.

Estragon · May 2018

Adding more strings in general can be problematic. Going to 3 or more strings will need careful wiring and monitoring to avoid unbalanced charging and discharging. Adding new strings to old can also result in the new batteries aging to the same state as the old. If the existing strings are older, or have been worked hard (eg deep discharges), it may be wiser to reconsider adding strings, and instead go for larger batteries in fewer strings.

You should also fuse each individual string with more than 2 strings.

mcgivor · May 2018

Without details on what the increased capacity is actually for, perhaps it would be better to build a new second system to provide for the additional loads, mixing old batteries with new often leads to the older batteries dragging the newer batteries down to thier level and beating them with experience, read shorter life. The more strings, the more difficult it is to ballance, odd or even, 2 in parallel is probably the best, if parrallel is the only option, I'm suspecting growing pains, a dilemma many have experienced. Please furnish us with more information as to the rationale behind the proposed expansion.

rp3703 · May 2018

We will be adding 1 new string to two strings that were installed last year and used for 30 days at best. Then they were left on trickle charge all winter so not real old or over worked.
I am more afraid they could have suffered some unforeseen damage while on trickle charge during the winter but assuming that has not happened, I think they will be pretty close to new still.

mcgivor · May 2018

Assuming they are close to new adding a new string may be possible, but I would read these guidelines http://www.smartgauge.co.uk/batt_con.html and perform an equalization charge, if flooded batteries are used.
Note. Although four batteries are used in the example the same applies to three.

rp3703 · May 2018

Thanks for the info Mcgivor. So for three series strings wired in parallel, it sounds like I should run from the CC to the middle string and then run two sets of equal length parallel connections to the other two strings. Does that sound right? Something like this.

mcgivor · May 2018

All links to the positive and negative battery terminals should be of equal length to the common point. In other words don't use the center battery terminals as the common, additionally use a fuse on each positive in order to prevent the other two batteries back feeding a short in one string, although rare, there is always the possibility.

rp3703 · May 2018

How do I size the fuses?

littleharbor2 · May 2018

Provided your cabling is correctly sized your fuse sizes are based on the ampacity of the cables. Their purpose is to protect the wires/cables they are inline with. Too big, you will risk frying your wires. Too small, isn't dangerous but could be a nuisance.

rp3703 · May 2018

Well the wire from my CC to the bats is 4 AWG copper. It's the max diameter the Midnite Classic will take. The positive side runs through a 100 AMP breaker. I used 4/0 copper for series and parallel wires on the battery bank and to my inverter.

mcgivor · May 2018

Size according to the current capacity of the conductors used to interconnect, this would be the maximum allowable, or take the maximum current the inverter would draw, divided by 3 then add 25%, but the value should never exceed the conductors maximum capacity. The maximum allowable current for conductors I'd dependent on specifics, insulation type, temperature free air and so forth, tables in attached pdf. A DC rated circuit breaker or fused disconnect should also be used on both sets of conductors connected to the inverter and controller.

rp3703 · May 2018

I already have 100 amp breaker on the 4 AWG copper positive wire between the CC and the batteries and my inverter has a 200 amp fused connection coming from the batteries through a 4/0 copper wire. So I’m guessing I need to go with 200 amp fuses on the connections to the positive terminals for each string.

mcgivor · May 2018

Since the total current is shared equally by each string a smaller fuse on each battery could be used, 100A perhaps, but always use quality fuses such as these, https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A

rp3703 · May 2018

Ok, so I re-did the battery connection drawing. Please let me know if it is correct this time. I also have one more question about my wire size for my series and parallel connections. Since my inverter can pull up to 200 amps, my wires from the battery bank terminal strips to the inverter should remain 4/0 copper but if that 200 amps will be evenly divided amongst all the batteries due to this improved wiring scheme, would the max wire size needed from the terminal block to the batteries and for the series connections from battery to battery only need to be 4 AWG copper?

Estragon · May 2018

IMHO, this wiring looks better. In theory, the inverter should draw about the same current (200a ÷ 3 = ~70a) from each string, so you could use smaller wire (with fuse sized for the smaller wire). Of course, if in practise the current isn't equal (loose/corroded connection, string taken out due to a failing battery, etc.) you'll want to have a supply of spare fuses on hand ;-)

rp3703 · May 2018

Ok, thanks. Sounds like I should be fine with the 100 Amp fused 4 AWG wires then. Assuming one string goes bad, the remaining two strings will be properly sized and fused to supply same load. If more than one string goes down, it'll be time to replace everything.

mike95490 · May 2018

I would suggest using 1/O wire for the battery-block cables, and then the 4/O from the block to inverter.

What size fuses are you using for each battery ?

Where is the fuse in the 4/O inverter cable or is the fuse IN the inverter?

mcgivor · May 2018

@rp3703
I like the improvements, yes the conductors interconnecting the batteries could be of a lighter gauge but larger is always better, it's great to see you are making improvements it will pay off in the long run. If you don't already have one, invest in a DC clamp on ammeter, it will be the most useful tool to have, especially with parrallel batteries but not limited to, being able to measure the charge /discharge currents of each battery. Measured often will detect if imbalance is occurring, it's also useful in testing panel output and so forth as well, a mid range model will set you back around $50, well worth it.

rp3703 · May 2018

Mike, I was planning to use 100 amp fuses on the battery strings. The 200 amp fuse is inside the inverter.

Mcgivor, I do have an amp meter I bought to use on my HVAC unit when it was melting wires a few years back. Now I'll have another use for it.

BB. · May 2018

You are correct to "over design" the fuses/battery string cables. As you say, if one string fails you want to still run without popping fuses.

Also, remember that even if everything is working correctly, you could have times (charging/discharging) where one string is carrying more current than its other... I would humbly suggest that, for example, 2 stings into a 200 Amp load, or 100 Amps per string, 1.5x current flow ~ 150 Amp fuse per string (and wire AWG to safely carry the current).

-Bill

mike95490 · May 2018

mike95490 said:

I would suggest using 1/O wire for the battery-block cables, and then the 4/O from the block to inverter.

Consider the scenario where a battery fails, blows the fuse, and the remaining cables have to carry the load. you want to use cable larger than it's expected share of the load.

rp3703 · May 2018

Ok guys, I need to buy parts soon, so we need to come to a consensus. So I will keep using the 4/0 series cables I am currently using to wire my strings together as well as the 4 AWG cable from my charge controller and the 4/0 cables to the inverter. I will be purchasing 6 exact same length 1/0 copper cables and three 150 AMP terminal fuses. I plan to run all my parallel cabling off of a terminal block like the one pictured below. CC and Inverter will tie into the outer studs and batteries to inside. One stud will just have to have two cables tied to it.

Image: https://us.v-cdn.net/6024911/uploads/editor/bo/r9er5d130des.jpg

Organic Farmer · May 2018

I have six strings of 12vdc batteries, all of the cabling is 4/0 welding cable. My buss-bars are strips of 3/4" copper tubing hammered flat. All of my negative cables are the same exact length, and all of my positive cables are the same exact length.

My battery-bank is 48vdc, 600ah.

Odd or Even Battery Strings

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