I have been building a PV system for a week now. Still in the purchasing and procurement mode.

A nice man accidentally purchased six 8v 170ah lead-acid deep-cycles for his golf cart instead of the required 6v.

Luckily for me, he purchaced them mail-order and the return postage would have killed him. He has sold them to me for a steal. So, these will be my solar/wind power bank.

I have been trying to figure the best way to wire for optimal use and charging.

My solar panels can be configured in any voltage arrangement that the 108 cells will allow. So, i figured on connecting two of the batterys in series for 16v groups, three of them.

I have been searching on good voltages for charging the batteries and have seen many different numbers. A lot of people have said I want 150% of capacity. Well the if I use 24v for the panels, that is 150% of the 16v groups. But I am wondering if anyone with experience with 8v batteries or the math equations to help me come to the most efficient setup. Thank you.

 

If you go to 24v, then you will be compatible with 2v, 4v, 6v, 8v, or 12v cells.

If you go to 16v, you will be stuck with just 2v, 4v, or 8v batteries.

 

Solar panels are still trivial to me as of now. I am more worried about the banking. I will adapt the array to the bank. I think it is more important to work to the bank. My major malfunction is the voltages of the groups. If I go 24v I will use 2 groups of three batteries each. Than parallel the 3 groups?

In my case, I think that is a catch-22. If I go with 3 groups of 2, or 16v I get a greater Ah rating. ~450....oh geez.

Overall, I am an idiot. How could I have gone this long without even consulting Ohms law. I was so involved in maximizing amp-hours, I forgot to convert to wattage for actual power. It will be the same no matter what configuration. So the best thing to do is make future expansion and voltage splitting less complicated.

So I will go with 24v.
Having read a post on another thread you told someone to use 28v for charging the 24v bank. So regarding the solar panels, would you recommend greater than 28v for charging?

 

I recommend that you consult the manufacturer's datasheet for their recommendations on charging.

 

Solar panels are still trivial to me as of now.

It might seem that way at the moment. However, it's doubtful that you will be getting another "steal deal" for 8v batteries. You might, but wouldn't you rather have the flexibility for if you happen across deals in a variety of other voltages?

Besides, have you investigated what inverters might be available for 12v, 16v, or 24v input?

The higher voltage your battery bank outputs, the less loss that you will have in power transmission. You would need much heavier cables to carry 2000W at 12v than you would at 24v.

I am more worried about the banking. I will adapt the array to the bank. I think it is more important to work to the bank. My major malfunction is the voltages of the groups. If I go 24v I will use 2 groups of three batteries each. Than parallel the 3 groups?

Three 8v batteries in series would output 24v.
If you then wired that in parallel with another series string of three 8v batteries, you would still have 24v, but your AH capacity would be double that of one cell. Instead of 170AH, you would have 340AH.

In my case, I think that is a catch-22. If I go with 3 groups of 2, or 16v I get a greater Ah rating. ~450....oh geez.

No, it would be 170AH x3 = 510AH. However, you would be stuck with an odd bank voltage.

Overall, I am an [inexperienced person]. How could I have gone this long without even consulting Ohms law. I was so involved in maximizing amp-hours, I forgot to convert to wattage for actual power. It will be the same no matter what configuration.

Not true. At lower voltages, you will need heavier cables to carry the power.

So the best thing to do is make future expansion and voltage splitting less complicated.

Yes.

So I will go with 24v.
Having read a post on another thread you told someone to use 28v for charging the 24v bank. So regarding the solar panels, would you recommend greater than 28v for charging?

It's necessary to occasionally deliberately overcharge the batteries for a short period of time. This helps to reduce plate sulphation, and "stirs up" the electrolyte.

You should find the manufacturer's datasheet.

 

If it were only that easy. That was my first stop. I tend to [try to] help myself before asking.

The Interstate U8VGC 's datasheet is super elusive. Interstates website leads to 404's for MSDS and others. That would have been the goal.

Thats why I wanted a guideline for design. Im only asking for advice. (Which you are very good at)

In your opinion, if you were designing an array for a 24v bank, would you build to 28v or above?

I am integrating a pwm(desulfinator) into the design for bank maintenance, as well as charging/waterer. I plan on using commercially available equipment for these tools.

 

Read here:
http://www.batteries-faq.com/activekb/questions.php?questionid=1

[eta]
General FAQ:
http://www.batteries-faq.com/activekb/search.php?q=interstate 8V U8VGC battery

 

[ed] I was reading while you were posting
These numbers will help others looking for this information.
[/ed]

I have found some generic data on batterys that start with 'U'.

24v system
C / 10 where C=20hr Capacity in AmpHours (for charging current)
28.4 Bulk charge voltage
31 Absorption Voltage
2 to 4 Absorption time (in hours)
26.4 Float Voltage
31.2 Equalization Voltage
2 Equalization time (in hours)

 

These are just for your manufacturer's battery model.

Note that just underneath, there are different numbers for a different model battery from the same manufacturer. That is why you must consult the manufacturer for the data for your particular battery.

They are not all the same.

[eta]
What I find interesting is that the bulk charge is given as C/10 (170/10 in your case, or 17A) until the battery voltage reaches 9.6v (or 28.4v for three in series) - and then the absorption charge voltage is 10.33 (or 31v) for 2 to 4 hours.

Normally, the absorption charge voltage is less than the bulk charge voltage; as absorption charge is usually given at a reduced current from the bulk charge phase. I would verify the numbers with Interstate tech support.

Note the equalization voltage. That's important.

 

So from the above data, a 24vdc battery system using 'U' Deep Cycle batteries, should have a source voltage for charging of 31.2vdc

Thats 63 solar cells (at .5v each) So for leeway with dirty power and voltage drops, a 72 cell panel should be produced. 36v.

 

Thank you sgt.wookie. Are you a psychiatrist by any chance? The way you guide people to answer their own questions is intriguing.

 

So from the above data, a 24vdc battery system using 'U' Deep Cycle batteries, should have a source voltage for charging of 31.2vdc

Thats 63 solar cells (at .5v each) So for leeway with dirty power and voltage drops, a 72 cell panel should be produced. 36v.

You need to investigate MPPTC; maximum power point tracking controllers.

Jumping in and building a bunch of stuff before you've planned things out adequately will result in many tear-downs and re-builds. Better to have a pretty solid plan up front, than spend a lot of time wringing your hands and turning the air purple later.

 

I agree. The project kind of started its self. The batteries were given to me. (I was asked to get them out of his face) So there was $1400 saved. I totally plan on having everything I can think of or can ask others to thing of layed out, and rendered before construction. I have looked into maximum power point tracking. I am putting that on the back burner. The current plans for the solar array will be built with modularity in mind. (Grouping with other panels, tracking system integration) I am doing this project in Baltimore city, Maryland.. The rooftop design will have to start as simple as possible. I want to keep things low-key until the permit for the VAWT is complete. Once I have that, I will incorporate further stages. A tilt table design with counter balances for E-W following is in the books. But not yet on the table.

The basement of my home is being converted into a DC only den. The goal is to see how far we can take our bank with out AC inversion. We are documenting the production and the hassles.

(The VAWT is being built for as a group project design with my co-workers in our shop)

I thank you for your knowledge and will continue to tap it as you allow.
Thank you.

 

See the attached, from an EDN contributor - several articles are in it, but the 1st one is what you're interested in; a relatively simple MPPT concept.

 

Hmm. looks like ill be building a mppt circuit tonight. I have a few motors and servos about. See if I can get it to follow a flashlight.

This is very good. But it makes me think. If the solar array is at the same lat. and lon. for its life, It would be possible to reference a solar chart to adjust the angle of the table for date and time. (This has to have been done already)

I cant leave well enough alone. I thank you for the inspiration. I will attempt to build and implement. Thank you again.

 

I don't think you're getting the concept of the article. It's not about tracking the sun with the array; it's about determining the most efficient way of using the available power from the array for charging. It's on a small scale, but it could be scaled up.

 

I just got done reading it. And you are correct. I jumped to conclusions after scanning the page.

The theory behind the modulation and phase comparason seems like a audio amp circuit I played with a few years ago. (I used to run a recording studio and started making my own effect processors) anyhoo, I have some studying ahead of me. Have you simulated the circuit? Im interested to see hoe the output of the modulated, bucked signal compares to auto EQing circuits.

 

No, I haven't simulated it. I have no immediate plans to build a solar array; however I found the article interesting when I found it a year ago (Jan 22, 2009, 1:24pm to be exact).

 

So, contrary to my original belief, the mppt-circuit adjusts the gain of the opamps and buck/boost regulator through feedback to keep the output in the maximum power point.? Using voltage comparators and temperatures as a reference against the modulated voltage line from the panel.

With this and a maximum solar exposure table, pretty good results should arise.

Let me know if my dissection was correct.

again, I thank you. Who knows where my mind would have went. Thanks.

 

Using the bulk charge of 28.4V, open circuit V of panel should be about 40V for peak power. A smart controller with V boost will allow a wider range of panel V.