Odd Battery Questions

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Solar Expert Posts: 336 ✭✭✭
HI Forum

I have some questions about a battery set up as i'm about to add 6 kyocera panels and regulator to an existing system.

The existing system has 5 parallel strings of 2x200Ah IEB 12-200 12V gel batteries- YES, I know, it's a recipe for disaster, but what's done is done.

My questions are:

1. The old installer has connected some of the positive wires to the positive terminal of the 1st battery string, but he has connected the positive wire of the PV array to the positive terminal of the 5th battery string! A similar pattern is followed with the negatives. I always thought all the positives should be connected to the same positive terminal (1st string), and all the negatives too (last string). But maybe it doesnt matter as they are all connected in parrellel anyway? what are the consequences of what he has done?

2. All the strings were giving the same voltage of 24.2V at the time of measurement. However, in the 3rd string the two battery voltages were split 11.1V and 13.1V (in all other strings the individual battery voltages were at 12.1V)- What are the reasons for the voltage splitting like this between the two batteries in this 3rd string??

3. Final question, nothing to do with batteries actually. We are putting in 3 strings of 2x12V Kyocera 120W panels, hence the system is at 24V. The new panels come with one cable of 10mm² (#8 AWG), yet the cable size needed is only 4mm² (#12 AWG) -at least until the junction box. I know you are supposed to keep the cable sizes the same, but is there any harm in using the existing short 10mm2 cable for the connection between -ve of panel 1 and +ve of panel 2 of each string (to make the string), but using 4mm2 for the cable runs from the +ve and -ve of each string to the junction box????

Hope the questions are clear... .I really appreciate any pointers

Thanks
Larry

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Re: Odd Battery Questions
lazza wrote: »
HI Forum

I have some questions about a battery set up as i'm about to add 6 kyocera panels and regulator to an existing system.

The existing system has 5 parallel strings of 2x200Ah IEB 12-200 12V gel batteries- YES, I know, it's a recipe for disaster, but what's done is done.
As long as you keep the maximum charging current to around 5% of the total bank AH capacity--You should not be causing problems.
1. The old installer has connected some of the positive wires to the positive terminal of the 1st battery string, but he has connected the positive wire of the PV array to the positive terminal of the 5th battery string! A similar pattern is followed with the negatives. I always thought all the positives should be connected to the same positive terminal (1st string), and all the negatives too (last string). But maybe it doesnt matter as they are all connected in parrellel anyway? what are the consequences of what he has done?
As long as the current paths are balanced, you can connect to multiple locations... However, most configurations that I can think of only have two different "balanced" location points--So random connections probably are not great.

If the current requirement is low--it may not matter too much. If you have high sustained current, then the unbalance becomes worse (i.e., you draw/charge more current to the closely connected batteries, and the others are not).
2. All the strings were giving the same voltage of 24.2V at the time of measurement. However, in the 3rd string the two battery voltages were split 11.1V and 13.1V (in all other strings the individual battery voltages were at 12.1V)- What are the reasons for the voltage splitting like this between the two batteries in this 3rd string??

First off, the 24.2 volts is probably around 50% discharged--So you should be looking at getting the resting voltage closer to 12.7 volts instead.

Second, the 11.1 volt battery may have a shorted cell or the 13.1 volt battery may have a high resistance cell (or one battery is dead and the other is overcharged). Probably one, if not both, are toast.

I always suggest getting an inexpensive DC current clamp meter for folks with paralleled batteries so they can monitor load/charging current to find problems before they damage the battery bank.
3. Final question, nothing to do with batteries actually. We are putting in 3 strings of 2x12V Kyocera 120W panels, hence the system is at 24V. The new panels come with one cable of 10mm² (#8 AWG), yet the cable size needed is only 4mm² (#12 AWG) -at least until the junction box. I know you are supposed to keep the cable sizes the same, but is there any harm in using the existing short 10mm2 cable for the connection between -ve of panel 1 and +ve of panel 2 of each string (to make the string), but using 4mm2 for the cable runs from the +ve and -ve of each string to the junction box????
Nope, not a problem. Since solar cells are really current sources (unlike batteries which are voltage sources), they do not usually have a problem with "current sharing" because of slight differences in voltage drop/wire resistance with paralleled connections (unlike batteries which can be killed by such problems).

Hope this helps.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 336 ✭✭✭
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Re: Odd Battery Questions

HI Bill

Thanks very much for the replies, a great help.

Very interesting about the current source/voltage source information. So I understand therefore that batteries create current due to the voltage difference between the terminals- and this is very much affected by the resistance of the wires/apparatus connected. Yet PV panels create current through the photovoltaic effect and are not so dependent on what is connected.... but i dont understand therefore where the cell voltage comes from??

apologies for such basic questions

Larry
• Solar Expert Posts: 1,571 ✭✭
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Re: Odd Battery Questions

Hi Larry,

Regarding your first question, here's an in depth look at parallel wiring: http://www.smartgauge.co.uk/batt_con.html
• Solar Expert Posts: 1,973 ✭✭✭
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Re: Odd Battery Questions
lazza wrote: »
HI Bill

Thanks very much for the replies, a great help.

Very interesting about the current source/voltage source information. So I understand therefore that batteries create current due to the voltage difference between the terminals- and this is very much affected by the resistance of the wires/apparatus connected. Yet PV panels create current through the photovoltaic effect and are not so dependent on what is connected.... but i dont understand therefore where the cell voltage comes from??

apologies for such basic questions

Larry
Batteries and PV modules both create a current flow through a voltage differential between their + and - terminals which is proportional to the load, but a battery can generally cause a lot more current to flow into a low resistance than can a PV module. Batteries have a much lower internal resistance, and PV current is limited by the amount of carrier generation going on due to insolation, which is on the order of 10 to 15% of 1000 Watts per square meter of module area. That is why shorting a PV module or string does not damage the modules or wiring while a battery short can be a catastrophic event.
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Re: Odd Battery Questions

Larry,

A battery tries to keep (for example) 12 volts on its terminals... The lower the resistance, the more current can flow. If you have two batteries in parallel and there is different resistance in the connection wiring, the battery with the lowest resistance wiring will supply/take the most current.

A solar panel on the other hand between ~Vmp and 0 volts on its terminal will simply output rated current (Imp). So if you have several panels in parallel, and the output voltage is Vmp or less, any resistance in the current path does not matter. The panel with the lowest resistance path will supply Imp and the panel with the highest resistance path will also supply Imp.

Of course there are limits. A 12 volt battery's output voltage will collapse if you load it with too much current. And a solar panel will supply less current if its output voltage is over Vmp--And supply zero current if its output voltage is Voc (voltage open circuit).

And, the output current of a solar panel is proportional to the amount of sun hitting it... 1/2 full sun (~500 Watts per sq. meter), it will output 1/2 rated Imp (still as a current source).

Technically, an ideal battery has zero output resistance and and ideal current source has infinite output resistance.

Kind of mind blowing that your solar panel has a very high output resistance--Isn't it. But it make sense mathematically. One way of determining resistance is to measure the change in voltage divided by the change in current (R=Delta V / Delta I). An example:
So, in the case of a smaller battery, a 0.1 ohm difference in wire resistance is very significant. For the solar panel a 1 ohm difference will affect sharing very little.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 336 ✭✭✭
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Re: Odd Battery Questions

thanks for your replies... not sure if i understand less or more now

I dont understand very clearly the difference between voltage source and current source. I know that electrons knocked free from the depletion zone in the solar cells, allow current to flow.... but doesnt that create voltage as well?

just trying to get my head round it- maybe i should go back to school and pay more attention in physics classes... ah lost youth

Larry
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Re: Odd Battery Questions

For a battery... Basically any load (or charging source) connected, the battery will remain around 12-14 volts (to its chemical and physical limits).

For a solar panel, full noon time sun, 1,000 watts per sq. meter, it will, between zero volts and Vmp, output about Imp worth of amps (with Imp being roughly proportional to the amount of sunlight hitting the panel. Shine 500 w/sm and it will output roughly 1/2*Imp, etc.).

If you connect a solar panel (correct polarity) to a battery. The Battery will set the "system voltage" (12 volts or whatever). And the Solar Panel will set the charging current (~Imp).

We are so used to using a volt meter to measure line voltage (12 vdc, 120 vac) to see that everything is OK--We really don't get the idea that a random solar panel powering a random load will not have a set voltage output but will have a set current output (as long as the output voltage is between zero and Vmp volts).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Odd Battery Questions
lazza wrote: »
thanks for your replies... not sure if i understand less or more now

I dont understand very clearly the difference between voltage source and current source. I know that electrons knocked free from the depletion zone in the solar cells, allow current to flow.... but doesnt that create voltage as well?

just trying to get my head round it- maybe i should go back to school and pay more attention in physics classes... ah lost youth

Larry
Ideal voltage sources and current sources are merely theoretical concepts; nether exist in the real world. Every source of electricity falls somewhere in the area between them.

A battery is more like a voltage source because for a wide range of load resistance, the battery will deliver whatever current it takes to keep the voltage at the terminals relatively constant. A PV module is more like a current source because for a wide range of load resistance, the current stays about the same and the voltage at the terminals varies.

The difference is internal series resistance. A battery has very little and a PV module has a lot. That's what makes large batteries so dangerous when it comes to short circuits. Here's that 4800A you ordered with your .01 ohm short circuit! ;^)
• Solar Expert Posts: 336 ✭✭✭
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Re: Odd Battery Questions

Ok, thanks.. things clearer now
• Solar Expert Posts: 336 ✭✭✭
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Re: Odd Battery Questions

Hi Forum

Some more odd battery stuff- looking at the same installation of 5 parallel strings of 2 12V gel batteries (IEB 200Ah at C20).

With a clamp meter I measured the current whilst the battery charger was on powered by a generator. The total current was about 20A. This I suppose would presume 4A per series ideally.

Whilst in the first, middle and last string showed the expected current of about 4A, the 2nd and 4th string showed very peculiar outputs. The 2nd showed half the expected current of about 2A, and the 4th, double the expected current of 8A!!!

Can anyone help explain such an peculiar distribution of charging current??

Many thanks
Larry
• Solar Expert Posts: 1,973 ✭✭✭
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Re: Odd Battery Questions
lazza wrote: »
Hi Forum

Some more odd battery stuff- looking at the same installation of 5 parallel strings of 2 12V gel batteries (IEB 200Ah at C20).

With a clamp meter I measured the current whilst the battery charger was on powered by a generator. The total current was about 20A. This I suppose would presume 4A per series ideally.

Whilst in the first, middle and last string showed the expected current of about 4A, the 2nd and 4th string showed very peculiar outputs. The 2nd showed half the expected current of about 2A, and the 4th, double the expected current of 8A!!!

Can anyone help explain such an peculiar distribution of charging current??

Many thanks
Larry
Because of the low internal resistance of batteries, minor differences in cable resistance can make significant differences in charging currents in parallel connected batteries.
• Solar Expert Posts: 2,490 ✭✭✭
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Re: Odd Battery Questions

Welcome to the world of Paralleled Batteries. As was said " Resistance " is the issue.

""The resistance of lead acid goes up with discharge. This change is caused by the decrease of the specific gravity, a depletion of the electrolyte as it becomes more watery. The resistance increase is almost linear with the decrease of the specific gravity. A rest of a few hours will partially restore the battery as the sulphate ions can replenish themselves. The resistance change between full charge and discharge is about 40%. Cold temperature increases the internal resistance on all batteries and adds about 50% between +30°C and -18°C to lead acid batteries.""

Everyone with paralleled banks has the same problem you are seeing. Yours will be more critical with gel batteries. The temperature in the 2 getting the 8 amps will be more and shorten their life over time and as they get worse they'll drag down the rest.

The EV ( electric vehicle ) guys have all kinds of different Distributive charging schemes with switches, relays and combiners. If you had a system that measured the amps going into each string, it seems you could change the resistance and make them balance out automatically . ( come on you EE guys, we need help )

What I do with the 10 batteries in mine is, I have them numbered and I shuffle them around a couple times a year so they will get more equal treatment as they discharge unequally also.

Check your wiring and measure it's resistance, clean and tighten all your connections.
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Re: Odd Battery Questions

You can measure the voltage at each battery and see if you are getting too much spread in voltage (0.1 volt is probably too much spread--0.01 to 0.05 maximum is better).

It would be interesting to see the difference between the 8 amp and 2 amp batteries.

If the voltage is the same on each battery (for the parallel connection points)--then those low amperage batteries may be going dead.

Also, monitor the current spread over time--It is possible that they will balance out somewhat as the high amperage batteries become fully charged.

It could also get worse--if the high amperage batteries get significantly hotter than the low current. High battery temperature drops the charging voltage at the battery (causing it to possibly draw even more current).

At this point, swapping high current for low current battery positions is probably a good place to start. See if the current flow is position or battery dependent.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Odd Battery Questions

I'd like to test a set of paralleled batteries with a data logging setup where you could get some definitive information on 4-6 different perimeters. I'v tried to do it manually, but lose interest and end up moving on to something else.

The question I have is it consistent over time or it always changing. Seems like someone would have already nailed it down and produced a solution. Like I said the EV guys are always trying different things. They have grid power, so it's different for them, some use 5 or more separate chargers and switches. They even charge batteries in series with Distributive Charging. If you used blade switches, you could use different charge controllers, but it all a manual operation and a pain. The other factor is ours is in use when charging.
• Solar Expert Posts: 336 ✭✭✭
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Re: Odd Battery Questions

I'd noticed a few days ago that the battery voltage in the 2nd series were odd. One had 13.1V and the other 11.1V. Bill suggested that " the 11.1 volt battery may have a shorted cell or the 13.1 volt battery may have a high resistance cell ".

Would this explain why only half (2A) the expected current is passing through the 2nd series???..... but then there is still the problem on how to explain the 8A in the fourth series.

cheers
Larry
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Re: Odd Battery Questions

A shorted cell can increase current flow in that parallel string.

An open cell can reduce current flow in a parallel string.

• 13.1+11.1=24.2 Volts
If that is resting voltage, the bank is, roughly 50% state of charge (assuming around 77F).

If that is the bank voltage with significant charging current, it is probably way under 50% state of charge.

If that is under load--bank state of charge may be ok.

In any case, with lots of parallel strings, a shorted cell can take down the entire bank. An open cell may not be detected and talk out the rest of the batteries in that paralleled string.

Monitoring both voltage and current flow (during loading and charging)--And looking for oddball readings--Is the place to start.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Odd Battery Questions
BB. wrote: »
A shorted cell can increase current flow in that parallel string.

An open cell can reduce current flow in a parallel string.

• 13.1+11.1=24.2 Volts
If that is resting voltage, the bank is, roughly 50% state of charge (assuming around 77F).

If that is the bank voltage with significant charging current, it is probably way under 50% state of charge.

If that is under load--bank state of charge may be ok.

In any case, with lots of parallel strings, a shorted cell can take down the entire bank. An open cell may not be detected and talk out the rest of the batteries in that paralleled string.

Monitoring both voltage and current flow (during loading and charging)--And looking for oddball readings--Is the place to start.

-Bill

If a cell is really open, no current at all can flow through the battery. The cells are in series.