Best Series/Parallel Cable connections

andyrud · September 2015

I have 8- 6 volt Golf Cart Batteries I want to connect to each other to make a 12 volt 800AH battery. Is this OK to use 8 batteries and if so what is the best way to hook them up? Is there a place that shows a schematic of what I want to do?
Thanks in advance,
Andy R

Iceni John · September 2015

That means you will have four parallel strings of batteries, and that amount of parallism will incur the wrath of some folk on this forum! Why only 12V for that much power? The consensus of opinion is that you should be using lower-voltage batteries in series to avoid the imbalance risks of having so many parallel strings.

John

vtmaps · September 2015

andyrud wrote: »

I have 8- 6 volt Golf Cart Batteries I want to connect to each other to make a 12 volt 800AH battery. Is this OK to use 8 batteries and if so what is the best way to hook them up? Is there a place that shows a schematic of what I want to do?
Thanks in advance,
Andy R

This smartgauge page shows you how to do it (method #3). I fault them for explaining how to do it without explaining why you shouldn't do it.

Four parallel batteries is very poor design for a frequently cycled system (parallel batteries are OK for backup/standby systems which rarely get cycled). Parallel batteries are in an unstable equilibrium (assuming you can get them into equilibrium at all). During charging the current does not divide equally among the parallel paths. This is caused by subtle differences in the internal resistance of the batteries, as well as the resistance of the wiring. The smartgauge site explains the part about the wiring, but doesn't explain how futile it is to get the wiring correct when the batteries themselves are all different.

Suppose you had a parallel bank of impossibly perfect batteries and impossibly perfect wiring... It is inevitable that one battery will be warmer than another (by just a tiny fraction of a degree). The warmer battery will get more of the charging current. The battery, getting more current than the others, gets warmer. As it gets warmer, it gets more of the charging current, which makes it warmer, which makes it get more charging current which makes it warmer.... This is called positive feedback and it is unstable. Another example of positive feedback is balancing a pencil on its point. You can get the pencil balanced, but in the real world it will deviate from equilibrium a tiny amount. As it leans to one direction, the gravitational forces pulling it in that direction increase, which pulls it further in that direction... positive feedback.

There are many other good (even better) reasons to avoid parallel batteries, but that's all the time I have right now...

--vtMaps

andyrud · September 2015

Iceni John wrote: »

That means you will have four parallel strings of batteries, and that amount of parallism will incur the wrath of some folk on this forum! Why only 12V for that much power? The consensus of opinion is that you should be using lower-voltage batteries in series to avoid the imbalance risks of having so many parallel strings.

John

I could get by using ONLY 6- 6 volt batteries in series/parallel 12 volt. Would this be acceptable? I have to keep the battery bank 12 volts because my 2800 watt pure sine wave inverter is 12 volts and I don't want to buy another one. My panels are set up 24 volts to a Midnight Solar controller that reduces it to 12 volts to the battery bank.

andyrud · September 2015

vtmaps wrote: »

This smartgauge page shows you how to do it (method #3). I fault them for explaining how to do it without explaining why you shouldn't do it.

Four parallel batteries is very poor design for a frequently cycled system (parallel batteries are OK for backup/standby systems which rarely get cycled). Parallel batteries are in an unstable equilibrium (assuming you can get them into equilibrium at all). During charging the current does not divide equally among the parallel paths. This is caused by subtle differences in the internal resistance of the batteries, as well as the resistance of the wiring. The smartgauge site explains the part about the wiring, but doesn't explain how futile it is to get the wiring correct when the batteries themselves are all different.

Suppose you had a parallel bank of impossibly perfect batteries and impossibly perfect wiring... It is inevitable that one battery will be warmer than another (by just a tiny fraction of a degree). The warmer battery will get more of the charging current. The battery, getting more current than the others, gets warmer. As it gets warmer, it gets more of the charging current, which makes it warmer, which makes it get more charging current which makes it warmer.... This is called positive feedback and it is unstable. Another example of positive feedback is balancing a pencil on its point. You can get the pencil balanced, but in the real world it will deviate from equilibrium a tiny amount. As it leans to one direction, the gravitational forces pulling it in that direction increase, which pulls it further in that direction... positive feedback.

There are many other good (even better) reasons to avoid parallel batteries, but that's all the time I have right now...

--vtMaps

Thanks for the reply Vtmaps. Right now I have 6 2volt 1100 AH Trojan batteries in series and after 5 years they are shot. I invested a lot of money in them, in the right amount of panels, in the best controller, have the necessary C10 to C12 current charging them, the correct absorb voltage, ETC according to the Manufacturer. And still, they are shot.
I went to Costco the other day and saw their 6 volt 208 AH batteries for $85.00. If I use 6 of these and they only last 2 years I could replace them 4 times and still be way ahead as far as money goes.

Andy R

vtmaps · September 2015

andyrud wrote: »

Right now I have 6 2volt 1100 AH Trojan batteries in series and after 5 years they are shot. I invested a lot of money in them, in the right amount of panels, in the best controller, have the necessary C10 to C12 current charging them, the correct absorb voltage, ETC according to the Manufacturer. And still, they are shot.

Yes, those Trojans are not a good deal if they only last 5 years, but they ought to do much better than that. You mentioned all the things you did correctly, but you did not mention the one thing that would convince me that you are not responsible for their untimely demise.... you didn't mention that you regularly checked the SG of each cell with a hydrometer or refractometer.

btw, Trojan makes a couple of large 2 volt batteries. In their industrial line they make 2 volt batteries where each battery is a single cell. They also make a 2 volt battery in a L-16 case where each battery is actually 3 cells in parallel. That's the one they peddle as an RE battery. I don't understand why people buy them... three times as many cells to water and check for SG.

--vtMaps

andyrud · September 2015

vtmaps wrote: »

Yes, those Trojans are not a good deal if they only last 5 years, but they ought to do much better than that. You mentioned all the things you did correctly, but you did not mention the one thing that would convince me that you are not responsible for their untimely demise.... you didn't mention that you regularly checked the SG of each cell with a hydrometer or refractometer.

btw, Trojan makes a couple of large 2 volt batteries. In their industrial line they make 2 volt batteries where each battery is a single cell. They also make a 2 volt battery in a L-16 case where each battery is actually 3 cells in parallel. That's the one they peddle as an RE battery. I don't understand why people buy them... three times as many cells to water and check for SG.

--vtMaps

I unfortunitely have the 3 cell designed batteries. I don't think they had the single cell ones at the time I purchased these. As far as " regularly checked SG" what do you consider regularly?
Andy R

vtmaps · September 2015

andyrud wrote: »

I unfortunitely have the 3 cell designed batteries. I don't think they had the single cell ones at the time I purchased these. As far as " regularly checked SG" what do you consider regularly?
Andy R

I check each cell every other month, just before I water and equalize them. --vtMaps

andyrud · September 2015

vtmaps wrote: »

I check each cell every other month, just before I water and equalize them. --vtMaps

I check water every two weeks, and every couple of months or so I checked SG. I equalized about 2 months or so even though Trojan said at the time to not equalize unless they needed it. When was that? So after you check your batteries and find a cell a little low what do you do?
Thanks, Andy R

BB. · September 2015

How hot do your batteries run (is the room cool/well ventilated)?

-Bill

andyrud · September 2015

BB. wrote: »

How hot do your batteries run (is the room cool/well ventilated)?

-Bill

Ventilated but not necessarilly cool. The batteries are in their own 3' x 4' x 8' enclosure behind the cabin and never in the sun. But sometimes in the summer the enclosure could get to 85 degrees in the afternoon.
Thanks, Andy R

vtmaps · September 2015

andyrud wrote: »

So after you check your batteries and find a cell a little low what do you do?

If the SG was low on a cell I would do a corrective equalization.

I also like to do a routine (non corrective) equalization every couple of months (monthly in winter). What I am trying to do is avoid slow, chronic deficit charging which is the most common cause of premature death of lead acid batteries.

Even though I use an hydrometer, I can't tell the difference between 99.9 and 100% SOC. If I repeatedly charge to 99.9% SOC, that 99.9 becomes my new 100% SOC. After a few years of 99.9% of 99.9% of 99.9% of 99.9% etc, you have lost real capacity to sulfation and raised the internal resistance of the battery. By doing a routine equalization I hope break through the 99.9% SOC and get to a real 100%. By the way, a very good way to know if you're getting to 100% on a regular basis is to monitor end amps.

Bill's point about temperature is very important... 10° C warmer than room temperature can cut a battery's life in half.

--vtMaps

andyrud · September 2015

vtmaps wrote: »

If the SG was low on a cell I would do a corrective equalization.

I also like to do a routine (non corrective) equalization every couple of months (monthly in winter). What I am trying to do is avoid slow, chronic deficit charging which is the most common cause of premature death of lead acid batteries.

Even though I use an hydrometer, I can't tell the difference between 99.9 and 100% SOC. If I repeatedly charge to 99.9% SOC, that 99.9 becomes my new 100% SOC. After a few years of 99.9% of 99.9% of 99.9% of 99.9% etc, you have lost real capacity to sulfation and raised the internal resistance of the battery. By doing a routine equalization I hope break through the 99.9% SOC and get to a real 100%. By the way, a very good way to know if you're getting to 100% on a regular basis is to monitor end amps.

Bill's point about temperature is very important... 10° C warmer than room temperature can cut a battery's life in half.

--vtMaps

I can't do much about the temp, but I have a thermometer in the battery shed and I have never seen it higher than 88 degrees. I pretty much did what you do with the Trojan batteries. I am a little frustrated with Trojan. Won't use them again.
Andy R

BB. · September 2015

5-8 years is probably "average" flooded cell battery life in a warm climate (guessing from reading here--not a battery expert).

What was the absorb voltage set point (and hours/conditions to terminate to float)?

How often did you need to add water (every two months to fill is probably average. More often than once a month--too much charging... Less than every 6 months, probably too little charging).

85F is not the worst (~6^oC over "room temperature"). Of course, battery temperature is what we need to know--Room temperature has an affect--But may not tell us all.

-Bill

andyrud · September 2015

BB. wrote: »

5-8 years is probably "average" flooded cell battery life in a warm climate (guessing from reading here--not a battery expert).

What was the absorb voltage set point (and hours/conditions to terminate to float)?

How often did you need to add water (every two months to fill is probably average. More often than once a month--too much charging... Less than every 6 months, probably too little charging).

85F is not the worst (~6^oC over "room temperature"). Of course, battery temperature is what we need to know--Room temperature has an affect--But may not tell us all.

-Bill

I have a battery fill system installed made by Fill Rite, so every two weeks when I went to the property to spray my grape vines I would just fill the batteries with the hand bulb, so I don't really know hoe much water they took each time. They always took some water. What do you mean buy "did you need to add water"? Every two week they took some water, but I don't think it was out of the ordinary.
Andy R

BB. · September 2015

If manual fill--You "fill" the area above the plates (after charging/equalization)--Then it takes ~2 months to get down to near the top of the plates. That is what I would suggest as "normal" water usage.

It does not sound like you are using too much/too little water.

-Bill

Bucho · September 2015

andyrud wrote: »

Is this OK to use 8 batteries and if so what is the best way to hook them up? Is there a place that shows a schematic of what I want to do?
Thanks in advance,
Andy R

Here's a tutorial on how to make 4 parallel connections: http://www.smartgauge.co.uk/batt_con.html

But my advice would be to go to 24 volts and cut your parallel connections in half. The size of your system's really to big for 12 volts.

cupcake · September 2015

For thy record, i got 6 autozone deep-cycles in PARALLEL (12 volts) -- doin just fine...

dunno where this anti-parallel sentiment originated from...

.

Vic · September 2015

cupcake wrote: »

For thy record, i got 6 autozone deep-cycles in PARALLEL (12 volts) -- doin just fine...
dunno where this anti-parallel sentiment originated from...

In general, this sentiment, or lack thereof comes from practical experience, and reality. Batteries are not like horses, pulling in common harness.

The issues with many, many parallel batteries has been discussed here many times. For cyclic applications, in general, parallel batteries are trouble IMO, two strings of batteries in parallel is not good. Three is bad, Four is very bad, five is very, very, very bad, six is exceedingly bad, bad, seven is profoundly ultra bad. eight is BAD beyond belief ... and so on.

If six strings is working well for you, then, it is probable that this configuration has not been in use for very long, or, perhaps,. you do not really need that much Capacity. In time, some of the batteries will become undercharged, and will not pull their share of the load ... in a few months or so, more batteries will not be charged well, and not pull their share ... then there is a good chance that one, or more batteries will start to pull the good one or two down. The progressive ruination will proceed, and then, if one is lucky, there will be one good battery, with five, or so others trying to pull that one down.

There are maintenance issues, with 6 X 6 cells to maintain, and interconnects to burnish, and grease and torque, and the Clamp-on DC ammeter readings to take, and batteries to separate and charge separately, and so on, and so on. Usually a real pain ...

If one needs Capacity, then it is much better to choose batteries, each one of which has enough Capacity that is needed, and connect these batteries in series to make string voltage that represents a system that will do the job needed.

In addition, a number of massively parallel battery, in low voltage systems ( like those wired for 12 V), have had batteries added, when the user discovers that there was a mistake made in going with a 12 V system, and more Capacity is needed. Paralleling batteries of differing ages is an additional misstep, IMO.

AND, it goes on, and on. Voltage drops for large currents at 12 V needed to provide significant power are a larger percentage loss than are for 24 V or 48 V systems, and so on. 12 V inverters are less efficient for the same amount of power output, and on, and on, ... and on. Many inexpensive 12 V inverters are of poor quality -- more so, than is the case for higher voltage inverters.

The 48 V systems here, have batteries with a total of 24 Cells, which can provide significant power ... fewer cells to check and water.

But, if one searches this Forum, all of this, and much more becomes apparent to those interested.

Just my take on it. FWIW, Vic

Bucho · September 2015

cupcake wrote: »

For thy record, i got 6 autozone deep-cycles in PARALLEL (12 volts) -- doin just fine...

For how long?

cupcake wrote: »

dunno where this anti-parallel sentiment originated from...

Because parallel connections put wear and tear on your batteries.

BB. · September 2015

Yes, larger number of parallel strings cian work OK... It is just, if possible to limit yourself to 2-3 maximum parallel strings--It does make life easier.

Similar to use 2x 6 volt batteries (especially if sealed/agm type where you cannot measure per cell specific gravity). Most batteries today is is near impossible to measure cell voltages--And with 12 volt batteries--You cannot use a volt meter to quickly look at the battery's health (all parallel 12 volt batteries on a 12 volt string have the "same" voltage).

These days, you can get a relatively cheap DC current clamp meter (like this one from Sears) and measure the charging/discharging/float current for each battery (assuming they are wired that way)--This does make it easier/safer/quicker to monitor the health of each battery (you don't want a shorted cell taking down the other batteries, or an open cell/wiring that reduces your bank capacity).

You get into other issues with total parallel current available (short circuit current) and people should (but probably most do not) put a fuse/breaker per string of batteries (especially with >2 parallel strings) to protect against short currents and wiring fires. Adding fuse/breaker per string just is more wiring/more complexity, and more costs.

-Bill

Best Series/Parallel Cable connections

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