Best way to parallel 3 battery strings? - Paid gig!

Chris Miller

Hi all,

I've seen the link that is often shared out here for the best way to series batteries but I am having a hard time wrapping my head around how best to parallel 3 series strings so that I am pulling from them all equally.

I am running a 48v system with 24 T-105-RE's. So 3 strings of 8 batteries each. Each string will also have one of these fuse holders.

If someone could be so kind as to draw me up an example of how best to tie these strings together so that I am pulling from the middle of the load on each string and have equal resistance across the board, i'd be happy to drop you $20 over paypal. This is just really giving me a hard time. No matter how I lay it out I am getting the same result - pulling from one end of 8 batteries.

Thanks in advance!

Cariboocoot

Re: Best way to parallel 3 battery strings? - Paid gig!

Pretty sure I have a ready-made diagram of this.
It shows two strings of two in parallel but the principle is the same: equal length wires between batteries in the strings, and equal length wires from the strings to the common connection point. The idea is that the wiring resistance for each string is the same, thus as little variation in the resistance as possible.

BB.

Re: Best way to parallel 3 battery strings? - Paid gig!

Also, remember that is a FUSE HOLDER... You still need to purchase fuses (and spare fuses) too.

-Bill

Chris Miller

Re: Best way to parallel 3 battery strings? - Paid gig!

Thanks again for the help! So I had my wife draw this up really quick. The top string shows how I already have things now. (Rows of batteries back to back for easier, more consistent connections) Because of my configuration and space constraints, my only real option is to continue to add strings outward away from the wall like this. I'd like to go 8 batteries wide but just don't have the space.

Do you think this configuration will suffice? I guess I would need to track down a bus bar that would work with the battery lugs.

If this is of any use, anyone is free to use it. If this isn't the ideal way though, let's delete it as to not steer people wrong.

Attachment not found.

Blackcherry04

Re: Best way to parallel 3 battery strings? - Paid gig!

Chris Miller wrote: »

Thanks again for the help! So I had my wife draw this up really quick. The top string shows how I already have things now. (Rows of batteries back to back for easier, more consistent connections) Because of my configuration and space constraints, my only real option is to continue to add strings outward away from the wall like this. I'd like to go 8 batteries wide but just don't have the space.

Do you think this configuration will suffice? I guess I would need to track down a bus bar that would work with the battery lugs.

If this is of any use, anyone is free to use it. If this isn't the ideal way though, let's delete it as to not steer people wrong.

Attachment not found.

That works, BUT the 6 connecting cables to the buss + & - need to be all the same length. Your drawing does not show them that way, the other wise you'll have unequal resistance. I know you say 2 ft, but not sure thats enough.

BB.

Re: Best way to parallel 3 battery strings? - Paid gig!

In the drawing text, it says all cables are 2' (I assume from battery post to bus bar).

-Bill

Cariboocoot

Re: Best way to parallel 3 battery strings? - Paid gig!

How you configure the batteries mechanically is not important. It's the electrical configuration that matters.

Your diagram appears correct.
1). Wires between individual batteries all the same length and gauge.
2). Wires between battery strings and common connection point the same length and gauge.

A note on #2: the negatives can be all of one length and the positives another; they do not have to be equal on each polarity. To be specific, the total length of positive and negative wire per battery string needs to be equal. As such you could have 2' positive and 1' negative on one string (3 feet total), 1.5' positive and 1.5' negative on the "middle" string (3 feet total), and 1' positive and 2' negative on the third string (3 feet total).

You will have some difficulty with placing a remote temperature sensor with three strings. If you can pack the cases together closely enough and place the sensor in the middle of the middle string that should work. But this sort of thing is why we sometimes see discrepancies in batteries in large banks; there are actual thermal differences between the outermost and innermost cells.

NorthGuy

Re: Best way to parallel 3 battery strings? - Paid gig!

Chris Miller wrote: »

Do you think this configuration will suffice?

Looks like it'll be real hard to reach the back row for watering and SG measurement.

Chris Miller

Re: Best way to parallel 3 battery strings? - Paid gig!

BB. wrote: »

In the drawing text, it says all cables are 2' (I assume from battery post to bus bar).

-Bill

Yes - that was the thought. My interconnects are all exactly 12".

Cariboocoot

Re: Best way to parallel 3 battery strings? - Paid gig!

The OP's drawing is labeled that the wires are equal from battery posts to common connection point. Just like this diagram only with the strings 'U' shaped.

Chris Miller

Re: Best way to parallel 3 battery strings? - Paid gig!

NorthGuy wrote: »

Looks like it'll be real hard to reach the back row for watering and SG measurement.

I'm running a single point watering system across all 24 batteries so watering will be OK. Measuring SG will be a stretch back there maybe...

Chris Miller

Re: Best way to parallel 3 battery strings? - Paid gig!

Cariboocoot wrote: »

How you configure the batteries mechanically is not important. It's the electrical configuration that matters.

Your diagram appears correct.
1). Wires between individual batteries all the same length and gauge.
2). Wires between battery strings and common connection point the same length and gauge.

A note on #2: the negatives can be all of one length and the positives another; they do not have to be equal on each polarity. To be specific, the total length of positive and negative wire per battery string needs to be equal. As such you could have 2' positive and 1' negative on one string (3 feet total), 1.5' positive and 1.5' negative on the "middle" string (3 feet total), and 1' positive and 2' negative on the third string (3 feet total).

You will have some difficulty with placing a remote temperature sensor with three strings. If you can pack the cases together closely enough and place the sensor in the middle of the middle string that should work. But this sort of thing is why we sometimes see discrepancies in batteries in large banks; there are actual thermal differences between the outermost and innermost cells.

All good information to have. I have 2 temp sensors too. One from the inverter that mounts on a battery terminal, and another that comes from the charge controller that sticks to the side of a battery. I guess I'll try to get it in the most common place possible.

inetdog

Re: Best way to parallel 3 battery strings? - Paid gig!

Chris Miller wrote: »

All good information to have. I have 2 temp sensors too. One from the inverter that mounts on a battery terminal, and another that comes from the charge controller that sticks to the side of a battery. I guess I'll try to get it in the most common place possible.

If you are concerned about reducing the CC voltage protectively when the batteries get hot, the worst case will be in the middle of the pile where the heat transfer to the air and rack are worst.
If you are concerned about raising the CC voltage to get good charging when the batteries are cold, put it on one of the outside end batteries.
If you worry about both, put the thermometer halfway between the center and the edge. :-)

YehoshuaAgapao

Re: Best way to parallel 3 battery strings? - Paid gig!

Chris Miller wrote: »

Thanks again for the help! So I had my wife draw this up really quick. The top string shows how I already have things now. (Rows of batteries back to back for easier, more consistent connections) Because of my configuration and space constraints, my only real option is to continue to add strings outward away from the wall like this. I'd like to go 8 batteries wide but just don't have the space.

Do you think this configuration will suffice? I guess I would need to track down a bus bar that would work with the battery lugs.

If this is of any use, anyone is free to use it. If this isn't the ideal way though, let's delete it as to not steer people wrong.

Attachment not found.

I'm using the DC busbars in my PDP to parallel my batteries (Up to 3 inverters and 4 charge controllers connect to these busbars also). Strings are within 0.2 amps of each other during bulk charge (9:00 AM solar, about 61 amps total, 30.5 amps each string). It requires a separate set of inverter cables for each string of batteries. The PDP will handle up to 3 sets of inverter cables going out to battery strings - they have it shown that way in their 3-phase 3-inverter configuration.

Concorde (SunXtender AGM) shows a wiring diagram like yours, using busbars to parallel the strings.

YehoshuaAgapao

Re: Best way to parallel 3 battery strings? - Paid gig!

You will have some difficulty with placing a remote temperature sensor with three strings. If you can pack the cases together closely enough and place the sensor in the middle of the middle string that should work. But this sort of thing is why we sometimes see discrepancies in batteries in large banks; there are actual thermal differences between the outermost and innermost cells.

Even though they say it isn't recommended, but since I have each string in their own box, I put a sensor into each string. Xantrex says when there are multiple sensors, it compensates based on the highest reading and I believe the system acts as documented. My left box is usually 0.5C warmer than the right box in winter and up to 2C warmer in summer. Proximity to swamp coolor probably reason for higher summer difference - it blows into house (cools bedrooms; Port-a-Cool 16 inch vertical tank - stands 5' tall obstructs 2/3 of doorway height), but humid air blows back into garage (and keeps the garage from getting too far over 90 degrees) and vents out through open man-door (reason for security door) with broken dryer blocking the bottom half so the warmer air vents out (There is a 2nd unit at the patio sliding door that cools the main house).

niel

Re: Best way to parallel 3 battery strings? - Paid gig!

i'm almost scared to start commenting here as i might write a book much like bb likes to do.:roll: i don't like doing that as it can cause a reader to become confused or disinterested midstream. the simplified version would be to have all using the same gauge of wiring with the string end connections equal to each other in length. now i can go off and say one polarity might be of one length and the other polarity a different length, but as long as overall the length for each string is the same overall length with each other that it works to keep the resistance the same between the strings. same thing holds true for the battery interconnects also being of the same gauge as the end connections that they must be the same overall lengths with each other, but they don't have to be the same lengths as the end wires.

wire gauge used in battery strings should be at least as large as the largest gauge wire you'll use anywhere else in the system. usually this is the same as the inverter wire size. i have often said it is better to go one size wire better for battery wires as you are trying to make it appear as 1 battery without differences showing up between the strings. some have argued that because it is separate strings that it could have a smaller wire based on the fact that it is divided by the number of strings into the main wires beyond the bus tie points. works in theory, but i don't advise it and i probably should not have even brought this aspect up. think of this this way, if you had a single string would you justify having a weak link in it with a smaller length of small gauge wire? the bank would be made up of individual strings like that each with their own built in handicap and tied together at the bus.

bus bars should be as heavy as possible as this is a tie point and must be kept as low in resistance as is possible. it is also possible to have all wires with lug connections going to a single large bolt, but other problems could arise here that i won't go into at this point.

in the diagram coot posted in post #10 the lengths of the end wires appears uneven and one would probably want to show a bus large enough to show each wire is equal in length and would probably appear as a rectangular box on end so these wires could stretch out horizontally and equally to the bus. the real reason buses aren't depicted that way is because it should represent a single low resistance point of connection. as such the middle wire appears shorter on the middle string, but ideally it should have a wire seen as a serpentine or sideways s indicating there is more length to the wire than what appears. that s shaped wire connection would probably confuse people. there's no easy way to express in a diagram for all to fully comprehend without confusion in some cases and this is partly the reasoning behind the confusion. just keep in mind that the end wires are to be equal in length inspite of the diafram appearances.

gww1

Re: Best way to parallel 3 battery strings? - Paid gig!

Niel

it is also possible to have all wires with lug connections going to a single large bolt, but other problems could arise here that i won't go into at this point.

I hate to butt in but could you "go into" this a bit more?

Coots drawing does state in writing that all wire is same length.

Also if bolts where used would you use brass or stainless steel?

Thanks
gww

Blackcherry04

Re: Best way to parallel 3 battery strings? - Paid gig!

niel wrote: »

i'm almost scared to start commenting here as i might write a book much like bb likes to do.:roll: i don't like doing that as it can cause a reader to become confused or disinterested midstream. the simplified version would be to have all using the same gauge of wiring with the string end connections equal to each other in length. now i can go off and say one polarity might be of one length and the other polarity a different length, but as long as overall the length for each string is the same overall length with each other that it works to keep the resistance the same between the strings. same thing holds true for the battery interconnects also being of the same gauge as the end connections that they must be the same overall lengths with each other, but they don't have to be the same lengths as the end wires.

wire gauge used in battery strings should be at least as large as the largest gauge wire you'll use anywhere else in the system. usually this is the same as the inverter wire size. i have often said it is better to go one size wire better for battery wires as you are trying to make it appear as 1 battery without differences showing up between the strings. some have argued that because it is separate strings that it could have a smaller wire based on the fact that it is divided by the number of strings into the main wires beyond the bus tie points. works in theory, but i don't advise it and i probably should not have even brought this aspect up. think of this this way, if you had a single string would you justify having a weak link in it with a smaller length of small gauge wire? the bank would be made up of individual strings like that each with their own built in handicap and tied together at the bus.

bus bars should be as heavy as possible as this is a tie point and must be kept as low in resistance as is possible. it is also possible to have all wires with lug connections going to a single large bolt, but other problems could arise here that i won't go into at this point.

in the diagram coot posted in post #10 the lengths of the end wires appears uneven and one would probably want to show a bus large enough to show each wire is equal in length and would probably appear as a rectangular box on end so these wires could stretch out horizontally and equally to the bus. the real reason buses aren't depicted that way is because it should represent a single low resistance point of connection. as such the middle wire appears shorter on the middle string, but ideally it should have a wire seen as a serpentine or sideways s indicating there is more length to the wire than what appears. that s shaped wire connection would probably confuse people. there's no easy way to express in a diagram for all to fully comprehend without confusion in some cases and this is partly the reasoning behind the confusion. just keep in mind that the end wires are to be equal in length inspite of the diafram appearances.

Then you add all the internal resistance differences of each cell in each Battery and then you figure you'v pretty much wasted your time and money.

Cariboocoot

Re: Best way to parallel 3 battery strings? - Paid gig!

Blackcherry04 wrote: »

Then you add all the internal resistance differences of each cell in each Battery and then you figure you'v pretty much wasted your time and money.

That is the variable you have no control over. Which is the reason for trying to control the other variables: wire lengths, gauges, and connections.

The single bolt connection Niel mentioned (poor man's bus bar) needs to be kept insulated; it can't just flap around free at the end of the wires. The easiest way is to use a long, stainless (reduce corrosion problems) bolt through a fixed piece of wood. Bolt it solidly to the wood first, then connect the wires; do not rely on the wire connection pressure to keep the bolt stable in the wood!

inetdog

Re: Best way to parallel 3 battery strings? - Paid gig!

Cariboocoot wrote: »

do not rely on the wire connection pressure to keep the bolt stable in the wood!

Not because the bolt will be unstable but because relying on the wire connection pressure to hold in the wood also means relying on the wood pressure to hold the wire connection. And that is a very bad idea which will lead to bad connections as the wood swells and shrinks.

gww1

Re: Best way to parallel 3 battery strings? - Paid gig!

So my copper terminals would work better on a stainless steel bolt then on a brass one. I guess it will need anti sieze paste so the nut doesn't meld to the bolt. Have I got it right? Would most of you prefer a copper bar tapped and threaded over the bolt? Thanks for expanding your answer to my earlier bolt question.

gww

BB.

Re: Best way to parallel 3 battery strings? - Paid gig!

I would recommend brass/bronze bolts... Both for the fact they don't seize, and because stainless is a fairly poor conductor of both heat and electricity.

Copper is pretty soft... It would have to be pretty thick for threads to hold (i.e., no smashed copper water pipe bus bars). Pure Copper may be difficult to drill/tap too (sort of flows).

Copper drilling advice - Practical Machinist

-Bill

Cariboocoot

Re: Best way to parallel 3 battery strings? - Paid gig!

My suggestion of stainless steel is because it's fairly easy to get such a bolt and yes use some anti-seize on it. The conduction issue Bill mentions are not significant because the bolt itself is not really the conductor: the copper lugs are flattened together and transfer power between themselves.

Brass or bronze bolts are harder to come by and more expensive, especially in large sizes. Around here anyway. Likewise copper is a premium product and in a bar big enough to be used for this ... well that's why I said "poor man's bus bar" for the stainless bolt. Cheap, easy, and it works.

gww1

Re: Best way to parallel 3 battery strings? - Paid gig!

Bill and coot,

Can't thank you enough.
gww

niel

Re: Best way to parallel 3 battery strings? - Paid gig!

wow, it got pretty well covered here on the bolt. it may also need copper or brass washers between the lugs and the bolt is, as coot said, just for compression so use something stronger for that like the stainless and add antiseize if you wish to get it apart again. i would not use steel as this would set up galvanic reactions, but zinc or tin coated steel may have some possibility if liberally coated to prevent reactions with the lugs or washers. this would need periodically checked for integrity, but is a poor man's way it could get done.

as to the inconsistencies from each battery string, this is real and one reason i always emphasized large wires in the interconnections and end battery wires to help minimize these inconsistencies from another angle. it helps.

inetdog

Re: Best way to parallel 3 battery strings? - Paid gig!

niel wrote: »

as to the inconsistencies from each battery string, this is real and one reason i always emphasized large wires in the interconnections and end battery wires to help minimize these inconsistencies from another angle. it helps.

On the other hand,

If you have three batteries, with internal resistances .01, .02 and .02 ohms and the interconnecting wires associated with each one are .03 ohms, the individual currents into or out of the three batteries, assuming they start with equal SOC and capacity, will be in the proportion 2.5, 2.0 and 2.0, with the actual total current determined mainly by the load or charger.
But if you do a totally awesome job of interconnecting and end up with .00 in the wires, the battery currents will be in the ratio 10, 5, 5. A lot greater imbalance.

The best strategy for balancing will change as the batteries develop small differences. One reason that it is easier to use one series string than fight all the possible effects of paralleling.

Several factors still support your argument though:

The smaller the total wiring resistance, the smaller the variations are going to be.

With a really small total resistance in each leg, the differences in battery voltages as the SOC changes will soon swamp out the differences in resistance, leading to an equalizing effect in terms of keeping the batteries tracking in terms of SOC (or at least voltage). Higher resistances will tend to equalize the current, whether a particular battery needs more or not.

niel

Re: Best way to parallel 3 battery strings? - Paid gig!

inetdog,
you still have inconsistencies and i do follow what you're saying here and it might just work out slightly better that way if you don't need large currents. most with large battery banks and multiple strings do want and need large currents so one would need to analyze the results either way to determine what works best for someone.

inetdog

Re: Best way to parallel 3 battery strings? - Paid gig!

niel wrote: »

inetdog,
you still have inconsistencies and i do follow what you're saying here and it might just work out slightly better that way if you don't need large currents. most with large battery banks and multiple strings do want and need large currents so one would need to analyze the results either way to determine what works best for someone.

Quite right. For reasons other than just paralleling, you always want to minimize resistance to the point that it is economical to do so.
And I think that I will just fall back on the summary that figuring out exactly what would be best for balancing in any given situation is probably not worth the effort. :-)

NorthGuy

Re: Best way to parallel 3 battery strings? - Paid gig!

These calculations, which show huge current imbalances, assume that all batteries are on the same SOC.

During discharge, the string giving higher current will discharge faster and will have slightly lower SOC, which will decrease the current through the string. Because of the process, current will be approximtely equal between strings (unless there's a shorted cell or similar disaster somewhere). SOCs will differ slightly, but currents will be about the same.

During charge, the string receiving higher current will charge faster and will have slightly higher SOC, which will decrease the current through thes string. This will equalize currents quickly.

As a result, the batteries "close" to the inverter will charge and discharge slightly more. I don't think it would impair normal operation, espcially if you "rotate" strings from time to time.

Similarly, a battery is a set of plates connected in parallel. They do have imbalances between panels, or wose yet between parts of the same plate, which also work in parallel. These imbalances do not prevent battery functioning.

However, if a single cell shorts in parallel strings or strings become uneven for whatever reason, the difference between SOCs of different strings may become far greater, which may become really bad.

solar_dave

Re: Best way to parallel 3 battery strings? - Paid gig!

Does someone produce bus bars for this purpose? I see a cottage industry opportunity here.

How about using a brass stud with jam nut (custom) for the connection points.

Cariboocoot

Re: Best way to parallel 3 battery strings? - Paid gig!

solar_dave wrote: »

Does someone produce bus bars for this purpose? I see a cottage industry opportunity here.

Might be a bit difficult due to the individualized nature of the installations. Any 'standard' bus bar could be too large/small/long/short et cetera.

How about using a brass stud with jam nut (custom) for the connection points.

This is the poor man's bus bar we were discussing, along with the problems of keeping it insulated and the price/availability differences of stainless vs. brass or bronze.

Since it's best to avoid multiple strings anyway and only two won't require such connections ...