Fusing battery strings

[SDC]

Hey guys, I need some advice here. I've recently changed all the batteries on a system and I want to fuse each string for extra piece of mind. Here are the stats:

System voltage: 24
Bank: 16 us2200 232Ah@6v wired in 4 24V strings. All strings are paralleled together on a bus bar using the exact same length of 2/0 wire.
Power is then fed to a magnum 4024 inverter via a fuse.

Maximum draw from inverter is about 170A but the real maximum draw in real life is 130A. So, correct me if I'm wrong here but normally, the current flowing into each one of the four string shouldn't rise above 42A (170/4). Let's add the 1,25 factor and that brings us to a maximum string current of 52A. Right?

First question: The local shop where I usually buy my stuff gives me a choice between a 50 or 80 amps fuse. I ordered the 50A considering that maximum output of the inverter is never used (I know I shouldn't say never but...). Does that sounds right to you or I should have gone with the 80?

Second question: All the wires paralleling the 4 strings are exact equal length to achieve a proper balance and I want to keep it that way. So, since the fuses will add extra resistance, I figure that, in order to keep that balance, I would need to cut a but of the positive wires (I'm fusing the positive side) to equal the resistance of the negative wires. Right? So, my question is: how am I suppose to figure out what length of wire to cut in order to subtract the resistance of the fuse on each positive line?

Thank you!

[Sunking]

Maximum draw from inverter is about 170A but the real maximum draw in real life is 130A. So, correct me if I'm wrong here but normally, the current flowing into each one of the four string shouldn't rise above 42A (170/4). Let's add the 1,25 factor and that brings us to a maximum string current of 52A. Right?

Wrong. It would be nice if it worked that way, but it doesn't. On paper and in theory is a true statement. But in real applications does not work that way. What you are going to learn here in the next year or two when you replace the batteries is the reason you should not ever parallel battery strings unless necessary, and with solar it is never necessary. What you are about to discover is the strongest battery string does all the work. Initially when power is turned on the strongest string provide just about all the current until is discharge down to the next strongest battery and then they carry they load until they reach the level of the 3rd strongest string. Even when all four strings voltage equalize the stronger battery still does most of the work because it has the lowest internal resistance of all four strings. There is noting you can do to make all the battery internal resistance equal, except add resistance to the strongest battry string which is a No-No

Anyway after just a couple of short years you end working the batteries to a premature death.

Second question: All the wires paralleling the 4 strings are exact equal length to achieve a proper balance and I want to keep it that way. So, since the fuses will add extra resistance, I figure that, in order to keep that balance, I would need to cut a but of the positive wires (I'm fusing the positive side) to equal the resistance of the negative wires. Right? So, my question is: how am I suppose to figure out what length of wire to cut in order to subtract the resistance of the fuse on each positive line?

Waste of time and money. You do not need or want to balance the lines. It takes both lines to complete a circuit (supply and return aka negative and positive polarity), not one. All you would be doing is adding unnecessary resistance, spending more money for no benefit. Just a waste of your time and money.

[inetdog]

Second question: All the wires paralleling the 4 strings are exact equal length to achieve a proper balance and I want to keep it that way. So, since the fuses will add extra resistance, I figure that, in order to keep that balance, I would need to cut a but of the positive wires (I'm fusing the positive side) to equal the resistance of the negative wires. Right? So, my question is: how am I suppose to figure out what length of wire to cut in order to subtract the resistance of the fuse on each positive line?

Basic principle of electricity: The current in each circuit will be proportional to the total voltage divided by the total resistance. As long as the sum of the + and - lead lengths is the same, the wire contribution to the battery string resistance will be the same.
Since you are not running cross wires between intermediate points of the multiple strings (if you know what is good for you), you do not even have to keep the lengths of all of the positives the same or the length of all of the negatives the same.
That said, making the length of all corresponding wires in the strings the same is a simple way to make the total lengths equal.

[SDC]

Wrong. It would be nice if it worked that way, but it doesn't. On paper and in theory is a true statement. But in real applications does not work that way. What you are going to learn here in the next year or two when you replace the batteries is the reason you should not ever parallel battery strings unless necessary, and with solar it is never necessary. What you are about to discover is the strongest battery string does all the work. Initially when power is turned on the strongest string provide just about all the current until is discharge down to the next strongest battery and then they carry they load until they reach the level of the 3rd strongest string. Even when all four strings voltage equalize the stronger battery still does most of the work because it has the lowest internal resistance of all four strings. There is noting you can do to make all the battery internal resistance equal, except add resistance to the strongest battry string which is a No-No

Anyway after just a couple of short years you end working the batteries to a premature death.



Waste of time and money. You do not need or want to balance the lines. It takes both lines to complete a circuit (supply and return aka negative and positive polarity), not one. All you would be doing is adding unnecessary resistance, spending more money for no benefit. Just a waste of your time and money.

Well, thanks for the answer! Ok, first of all, I know that paralleling multiple strings is never a good idea. But the point is that there is very often in life a huge gap between what we should do and what we can do given the circumstances... Trust me, that particular system is a mess from the very beginning. There is multiple reasons for that (I could write a book about it!) and those reasons led to a rather premature failure of the previous batteries (batteries were constantly undercharged, over-discharged, did not get proper watering, etc. a real mess... not mentioning that the bank was approx. 2 times undersize given the loads... ) Anyways, my point here is that I want to configure that system in the best manner I can to extend the batteries life as far as possible. I managed to adjust charge rates and generator run time to give this new bank a better chance of survival.

So, I don't want to start a debate here but the reason why I came up with the idea of fusing each string individually is that it is obvious to me that in a parallel configuration with four strings, the current in one string is far lower than the total current. Therefore, I don't think (correct me if I'm wrong) that the main fuse protecting the inverter will prevent a fire if one battery goes nuts on one string. Now, (and this is where I don't want to start a national debate), pretty much all the information I read on the subject strongly advise fusing individual strings when batteries are paralleled to prevent hazards from one battery or wire screwing up. As I said, I know paralleling is a waste, but since it is already done, I at least want to make sure it won't turn into an even bigger mess. So, why (I mean electrically speaking), do you suggest Sunking that fusing each string is a waste of time and money. Thank you.

[SDC]

Basic principle of electricity: The current in each circuit will be proportional to the total voltage divided by the total resistance. As long as the sum of the + and - lead lengths is the same, the wire contribution to the battery string resistance will be the same.
Since you are not running cross wires between intermediate points of the multiple strings (if you know what is good for you), you do not even have to keep the lengths of all of the positives the same or the length of all of the negatives the same.
That said, making the length of all corresponding wires in the strings the same is a simple way to make the total lengths equal.

Thank you Inetdog for the reply. Yes, it is indeed more than logical (basic Ohm's law...). Ok, I was going to reply with another question when I got what you mean... You say that even if my positive and negative leads on one string are not equal in resistance, it doesn't matter as far as the resistance of every string is the same. Right? That actually makes a lot of sense!

You said: "Since you are not running cross wires between intermediate points of the multiple strings (if you know what is good for you)". Are you suggesting that paralleling strings on a common bus isn't the best way to go? I honestly knew it would be quite a challenge to keep these batteries balanced as Sunking suggested and this is therefore the reason why I decided to go that way (because, at least, I was a 100% sure that the power from each string would "travel" the same distance/resistance). What do you think of this?

Thank you

[Mike90250]

I'll do it.

Fuses protect the wires.

Each string SHOULD be able to run the inverter alone, so that wire needs a suitable fuse.

The inverter manual should state a recommended fuse size too, so each battery string wire should be rated for that amperage.

PS apply copper conductive grease to the fuse holder contacts, so they don't cold weld themselves in place.

PPS the price of fuses, you may want to use DC rated ckt breakers.

[inetdog]

You said: "Since you are not running cross wires between intermediate points of the multiple strings (if you know what is good for you)". Are you suggesting that paralleling strings on a common bus isn't the best way to go? I honestly knew it would be quite a challenge to keep these batteries balanced as Sunking suggested and this is therefore the reason why I decided to go that way (because, at least, I was a 100% sure that the power from each string would "travel" the same distance/resistance). What do you think of this?

I was saying that the strings should only be connected at the two ends, rather than like a ladder with connections across the strings at intermediate points. Some people start out with the idea that that would somehow be better. It is not.

[SDC]

I was saying that the strings should only be connected at the two ends, rather than like a ladder with connections across the strings at intermediate points. Some people start out with the idea that that would somehow be better. It is not.

So how does that look to you?

IMG_0458.jpg

Thanks

[inetdog]

So how does that look to you?

[ATTACH=CONFIG]4804[/ATTACH]

Thanks

It looks nice.

Per string battery fuses are optional, depending mostly on the distance between the batteries and the bus bar.
But you certainly need fuses in the + lead (or both + and - in an ungrounded system, which would be possible if lower than 48V nominal) of both the CC and the feeds to inverter and DC loads.
The inverter fuse will typically be the largest one.

[SDC]

It looks nice.

Per string battery fuses are optional, depending mostly on the distance between the batteries and the bus bar.
But you certainly need fuses in the + lead (or both + and - in an ungrounded system, which would be possible if lower than 48V nominal) of both the CC and the feeds to inverter and DC loads.
The inverter fuse will typically be the largest one.

Oh, sorry, my mistake! I forgot to draw the inverter fuse as it seemed so obvious to me! There is indeed a 250A fuse on the inverter feed. The system however lacks a fuse between the CC and bus. Coming soon!

One last question: Sunking seamed rather far from optimistic about the chances of such a bank to stay balanced over time. What is your feeling on this? Also, is there anything I can do to help in that manner?

Thank you!

[inetdog]

One last question: Sunking seamed rather far from optimistic about the chances of such a bank to stay balanced over time. What is your feeling on this? Also, is there anything I can do to help in that manner?

I agree with him on that one, but some people seem to get away with it.
The best thing that you can do, IMHO, is watch the SG of one cell per battery in each string as well as checking the battery voltages precisely (not necessarily accurately) to a few hundredths of a volt to see problems coming. At least once a week.
A clamp-on DC ammeter can give you some confidence that the charge and discharge currents are sharing equally and thereby show up interconnect problems.

[Sunking]

The best thing that you can do, IMHO, is watch the SG of one cell per battery in each string as well as checking the battery voltages precisely (not necessarily accurately) to a few hundredths of a volt to see problems coming. .

That would be what we call a Pilot Cell. Just about every telephone and data center keeps a Thermometer and electronic SG meter in one of the cells in each string.

[SDC]

Thank you all, now I just cross my fingers then! I will definitely look very closely at the SG of pilot cells and consider buying a dc clampmeter (that gives me one more reason to buy one...)

Thank you all, I appreciate the inputs.