Want to understand current between batteries in battery bank

[aerodyn]

Just as a matter of learning more about how batteries and charging works, I am curious about the current which flows between the batteries while they are being charged. I have 5 12V batteries connected in parallel. Let's say they are numbered 1,2,3,4,5 and the positive from the charger is connected to battery 1, the negative from the solar panels is connected to battery 5. A typical observation that I have made is this: the charger at the time is delivering 8 amps, on the positive side the amperage between 1 and 2 is 5.9, between 2 and 3 is 4.1, between 3 and 4 is 3.0, between 4 and 5 is 1.6. On the negative side, between 5 and 4 is -8, between 4 and 3 is -5, between 3 and 2 is -3.6, between 2 and 1 is -1.9.
So, given that the amperage between the batteries during charging reduces as the battery gets farther away from the source, are all the batteries actually receiving the same amount of charge? What is actually going on?

[inetdog]

Just as a matter of learning more about how batteries and charging works, I am curious about the current which flows between the batteries while they are being charged. I have 5 12V batteries connected in parallel. Let's say they are numbered 1,2,3,4,5 and the positive from the charger is connected to battery 1, the negative from the solar panels is connected to battery 5. A typical observation that I have made is this: the charger at the time is delivering 8 amps, on the positive side the amperage between 1 and 2 is 5.9, between 2 and 3 is 4.1, between 3 and 4 is 3.0, between 4 and 5 is 1.6. On the negative side, between 5 and 4 is -8, between 4 and 3 is -5, between 3 and 2 is -3.6, between 2 and 1 is -1.9.
So, given that the amperage between the batteries during charging reduces as the battery gets farther away from the source, are all the batteries actually receiving the same amount of charge? What is actually going on?

You will find the answer to that all in the reference linked by Mike90250 in the second message at this post: http://www.solarpaneltalk.com/showth...p-Why-Tutorial

[Sunking]

When batteries are in parallel there is no way to calculate how much current is flowing in any one string which is why you should not connect batteries in parallel. The current depends completely on internal battery resistance and connection/cable resistances all of which are dynamic and constantly changing.

[russ]

Another resource http://batteryuniversity.com/

[aerodyn]

You will find the answer to that all in the reference linked by Mike90250 in the second message at this post: http://www.solarpaneltalk.com/showth...p-Why-Tutorial

Inetdog, thanks much, that article is exactly what I needed to know.

[aerodyn]

When batteries are in parallel there is no way to calculate how much current is flowing in any one string which is why you should not connect batteries in parallel. The current depends completely on internal battery resistance and connection/cable resistances all of which are dynamic and constantly changing.

Sunking, thanks for the response. I wasn't trying to calculate anything, I was just concerned that the method I used to connect my batteries was not optimal. The article that Inetdog referred me to explained what I was asking. I had simply placed my clamp voltmeter around each battery connection and was surprised at how much difference the amperage was.

[inetdog]

Inetdog, thanks much, that article is exactly what I needed to know.

You are welcome. Just keep in mind what Sunking said, namely that all of the neat computer solutions and equations in the world will not match the real world if the batteries are not 100% identical. Not just same make and model but same age and maintenance status.
This is one reason that high voltage series strings of batteries are used for systems that do not make direct use of the low voltage DC.

[Sunking]

Sunking, thanks for the response. I wasn't trying to calculate anything, I was just concerned that the method I used to connect my batteries was not optimal. The article that Inetdog referred me to explained what I was asking. I had simply placed my clamp voltmeter around each battery connection and was surprised at how much difference the amperage was.

Even if you cross wire as the article says, you are still going to have unequal currents. You cannot control the resistances to any degree of accuracy. It changes second by second. If you cross wire like the article states it will help, but not solve the problem. When you place batteries in parallel you sacrifice cycle life.

[aerodyn]

Even if you cross wire as the article says, you are still going to have unequal currents. You cannot control the resistances to any degree of accuracy. It changes second by second. If you cross wire like the article states it will help, but not solve the problem. When you place batteries in parallel you sacrifice cycle life.

Sunking, since I am powering a 12V lighting system with these batteries, do I have an alternative to my parallel battery connection? I am just a novice, so I think that connecting them in series would produce a higher voltage than 12V. Would I then have to have some piece of equipment between the battery bank and the light system to step down the voltage?

[Sunking]

Sunking, since I am powering a 12V lighting system with these batteries, do I have an alternative to my parallel battery connection?

Yes but it is too late now to ask. Like most folks you get in a 12 volt box. For 12 volt batteries requires 6 cells, and capacity is directly linked to wight and size. So when you get stuck in a 12 volt box the largest capacity you can find is around 50 to 300 Amp Hours. Now there are some 12 volt batteries that are much larger like those made for railroads but not suitable for renewable energy application.

Once you get out of the 12 volt box you soon realize they make 2, 4, 6, and 8 volt batteries up to 4000 Amp Hours. For example let's say you need 12 volts @ 1200 Amp Hours. You would look for 2 volt 1200 amp hour cells like the Rolls S-1590 battery. You would buy 6 of them and wire in series to get 12 volts.

Here is the link to one of the best batteries you can buy for RE application. Take a look and see what is out there. They have two series. The 4000 and 5000 series. The 4000 series is a 7 year battery, the 5000 is a 10 year battery. Scroll down to the botton and look at the graph of cycle life vs discharge depth.

FWIW I hope you did not get Trojan T-105 batteries.

[woogerboy21]

So why such large gauge wiring in battery bank?

So I have been reading some of the items posted from links in this article (such as http://www.smartgauge.co.uk/batt_con.html) and noticed in there example they state some of the observed current draw being in the range of ~25amp between the batteries (different based on wiring configurations). I wouldn't consider myself an expert by far but from all the research I have done, many have always recommended that if you can use the largest possible wiring available between the batteries in the bank (even up to sizes such as 2/0 and even as large as 4/0) to handle sudden and large draws of current.

The question I have is, when wiring a battery bank up, will the current flow attempt to naturally even the current load out across wiring in the bank between batteries? What I mean by this is, say for example I have a battery bank consisting of 6 total 12v batteries run in parallel (throw all the discussion of going higher voltage aside for the time being). If I have a 150amp current draw on the bank itself and its in an ideal wiring configuration allowing for even current draw between the batteries (im dreaming i know), wont there only be a total of 25amps being drawn from each battery only requiring a very small gauge of wiring between the batteries relative to the size of wire going from the bank to the load? And if so, say for example we take and up the 150amp current draw to 300 amps. Could you then run a second set of cables (in parallel to the first set of cables) on the +/- posts of the same gauge,same size as the first set of wires to handle the additional load?

I understand that its simpler to just buy a gauge of wire that will handle the current load required, I am just trying to understand how the current in a bank works when wired in different fashions.

[Sunking]

I am just trying to understand how the current in a bank works when wired in different fashions.

It boils down to one simple Law of Physics Ohm's Law and Resistance. The current in in parallel path is dictated by the Resistance and Resistance only.

[inetdog]

So I have been reading some of the items posted from links in this article (such as http://www.smartgauge.co.uk/batt_con.html) and noticed in there example they state some of the observed current draw being in the range of ~25amp between the batteries (different based on wiring configurations). I wouldn't consider myself an expert by far but from all the research I have done, many have always recommended that if you can use the largest possible wiring available between the batteries in the bank (even up to sizes such as 2/0 and even as large as 4/0) to handle sudden and large draws of current.

The question I have is, when wiring a battery bank up, will the current flow attempt to naturally even the current load out across wiring in the bank between batteries? What I mean by this is, say for example I have a battery bank consisting of 6 total 12v batteries run in parallel (throw all the discussion of going higher voltage aside for the time being). If I have a 150amp current draw on the bank itself and its in an ideal wiring configuration allowing for even current draw between the batteries (im dreaming i know), wont there only be a total of 25amps being drawn from each battery only requiring a very small gauge of wiring between the batteries relative to the size of wire going from the bank to the load? And if so, say for example we take and up the 150amp current draw to 300 amps. Could you then run a second set of cables (in parallel to the first set of cables) on the +/- posts of the same gauge,same size as the first set of wires to handle the additional load?

I understand that its simpler to just buy a gauge of wire that will handle the current load required, I am just trying to understand how the current in a bank works when wired in different fashions.

If you look closely at the information on the smartguage site, you will see some examples of the different current flows for different wiring methods. The simplest is when you use a very large bus bar to connect the load and individual wires from the bus bar to each battery. In that case you are right that the individual battery wires only need to be large enough for their proportional share of the load...As long as the resistances of the bus to battery wires and their interconnections are extremely closely matched. You would be better off to assume that each battery wire might have to carry 1/3 of the total current at some point instead of just 1/6.
If you use the multiple level interconnect wiring shown at smartguage (which will only work for 2, 4, 8, 16 etc. batteries, not for 6) then you can determine the differing amounts of current at each level by looking at how many batteries connect to the loads through the same piece of wire.
The simple diagonal method with 6 batteries would have fairly uneven current flow between the end and center batteries and the different interconnect wire pieces.

[woogerboy21]

If you look closely at the information on the smartguage site, you will see some examples of the different current flows for different wiring methods. The simplest is when you use a very large bus bar to connect the load and individual wires from the bus bar to each battery. In that case you are right that the individual battery wires only need to be large enough for their proportional share of the load...As long as the resistances of the bus to battery wires and their interconnections are extremely closely matched. You would be better off to assume that each battery wire might have to carry 1/3 of the total current at some point instead of just 1/6.
If you use the multiple level interconnect wiring shown at smartguage (which will only work for 2, 4, 8, 16 etc. batteries, not for 6) then you can determine the differing amounts of current at each level by looking at how many batteries connect to the loads through the same piece of wire.
The simple diagonal method with 6 batteries would have fairly uneven current flow between the end and center batteries and the different interconnect wire pieces.

I can see were when using multiple level interconnection wiring method you could not use 6 batteries. But using this type of wiring method, using the smallest amount of gauge required, wouldn't you need larger and larger gauges as each level progressed to the load on the bank? (Basically between 2 batteries you may be only pulling 25 amps, but when those two are ran in parallel with another 2 you have a 50amp total current draw for that level?)

It boils down to one simple Law of Physics Ohm's Law and Resistance. The current in in parallel path is dictated by the Resistance and Resistance only.

So then yes, assuming all things are equal (knowing though getting things equal is most likely not possible), you can make up for the extra current draw by simply adding additional channels of wiring to the bank rather than upgrading the entire wiring set. No?

[billvon]

What I mean by this is, say for example I have a battery bank consisting of 6 total 12v batteries run in parallel (throw all the discussion of going higher voltage aside for the time being). If I have a 150amp current draw on the bank itself and its in an ideal wiring configuration allowing for even current draw between the batteries (im dreaming i know), wont there only be a total of 25amps being drawn from each battery only requiring a very small gauge of wiring between the batteries relative to the size of wire going from the bank to the load?

Yes. Under ideal circumstances.

And if so, say for example we take and up the 150amp current draw to 300 amps. Could you then run a second set of cables (in parallel to the first set of cables) on the +/- posts of the same gauge,same size as the first set of wires to handle the additional load?

Yes. Under ideal circumstances. Since often things that are not ideal happen (motor starting surges, reductions in temperature, cable failures, bad batteries etc) cables are typically sized larger than you need them to be in a perfectly balanced system that always sees a certain nominal load. Going bigger on wire gauge is one of the simpler things you can do to increase overall reliability and performance of a low voltage DC power system.