Step Up Transformer 2

[zdas04]

I recently installed a 12V Solar Panel charger on a 24V system that didn't work too well. When I changed to a 24V charger (off the same panel) I got the batteries to 19V. Then I put a second 12V Solar Panel in series with the first and I'm getting about 24.9V. Working fine (and the deep discharge batteries don't seem to have been damaged by running for 5 days at around 6V).

This is about a 10-15W application and I really hate using two 40W panels to run it. I looked for a 24V solar panel, but the smallest I found was 100W. 20W panels are available here, but are really expensive (almost as much as the 40W panel, no one here knows why).

I found a 12VDC-24VDC step-up transformer, but it was limited to 1.5A and my control valves have a peak draw of 3.5A.

Does anyone have experience with 12VDC to 24VDC step-up transformers that could recommend a manufacturer that has a unit with 5A or greater peak current draw?

Thank you

David

 

[ScottyUK]

Hi David,

It's not a strictly a transformer, although it likely does contain at least one. The name we tend to use is 'DC-DC converter', which is accurate if not especially imaginative. Do you need 24V or do you really need about 28V to charge a pair of 12V batteries? The average 12 volt lead-acid bloc typically float charges at about 13.8V, and on fast charge might be as high as 14.4V.

You do quite often get an output trim control which allows you to bias the 24V nominal voltage by a few volts around nominal so it might be ok. I won't speculate on manufacturers until we know what the intention is.



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If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

David,

Before we dive into the DC-DC converter world, are the panels intended for battery charging duty? If they are then they should have some kind of charge controller built in to them which will allow the voltage to reach the float charge of 13.8V. If that's not behaving or not set up correctly then maybe there's a simpler solution than DC-DC conversion. Have you got a datasheet for these panels by any chance?


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Thank David for the clarification. Everything sounds great except for needing 2 panels.


Now we need to delve farther into the application...

You do NOT want to use a DC-DC as it will consume 99% of the solar energy. It will sit there sucking on the teat all the time, running at a very poor efficiency. This will also suck the cycle life out of your batteries.

You say a 15W load. However, when dealing with batteries that is only part of the required info.

Can you describe all the uses the battery power is being used for?

Like:
What wattage or current do your valves use when operating?
15W?

How long are they actually drawing this for in a day?

Is it just valves? How do the valves actually get commanded?

If it is just the valves how about just changing the coils? That would be the superior solution if it can be done. Just use 12VDC coils.




Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

I've got two motor operated valves that are controlled by floats in a vessel. One valve (2-inch) has operated once in 6 weeks, and that is about what I expected. That valve is 24VDC and takes 15 seconds to go from full open to full shut, and about the same to go the other direction. To start it moving with a low dP is around 1A, for a few miliseconds and then most of 15 seconds at around 0.6 A.

The other valve is a 1-inch, but the dP is higher. It rarely goes all the way open because it opens enough to allow the gas to leave the vessel and liquid to come in. When a couple of inches of liquid comes in, the float sents a signal to shut the valve. The valve usually moves towards open for 5-10 seconds at around 1A, then goes towards shut for 5-10 seconds. This happens every minute or so.

The batteries run the valves and the solar panel charges them. The electrician that installed the first one calculated the 15W requirement and speced the 40W panel.

David

 

[itsmoked]

Ahhh now we're getting somewhere. Does this operational recipe run 24/7 or just 8hrs?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

All day, every day, year round.

David

 

[ScottyUK]

How long is it dark for? Your batteries need to hold your load during the night, and during the day the solar panels need to charge the battery back to full capacity, meet the daytime power requirements, and have some margin for days when it's cloudy.

On a (very) quick run through the numbers I think your solar panels are way undersized, but there's another limitation in that the battery can only accept charge at a certain rate. You are fighting a problem on at least three fronts: finite amount of charging time to put energy back in to the battery; a limit on the amount of current you can throw at the battery; and a load which is depleting your battery as you try to charge it. That smaller valve is crudely equivalent to at a 300mA continous drain on the battery if it runs at 1A for 20 seconds of each minute (10s each way).

In my opinion you need a battery which can absorb a lot of charge fairly quickly, and a solar cell which can deliver enough power for that charge. I bet that 40W panel can't achieve that output for more than about 6 hours of the day. Gel cells hate high rates of charge too.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Scotty's hit the main problems.

I'll do the longer run thru the numbers.

Valve A: Its cycle rate places it fully in the ignore realm.

The other valve: 20s @ 1A, 20amp-sec every minute.

24hr x 60min 1440min in a day x 20As = 28800As/day

That's 28800/60 = 480Amin/day
or 20Ahr per day.

Batteries should not be discharged to less than 80% of their capacity or their lives will be counted in months instead of years.

This means you need a battery capable of 5 X 20Ahr = 100Ahr.

Now if you can have multiple days of overcast you need to up this battery size in an effort of not letting the battery go a lot below 80% regularly. Just two days would require 200Ahr of batteries.

For batteries I'd reach for these.
I'd use four of them in series as the minimum.

http://www.trojanbattery.com/Products/T-1056V.aspx

They are rated at 225Ahrs in your application.
They would only discharge down about 10% daily, and on a two day overcast case they'd get down to 80%, three day 70%, four day %60, five day - a mildly damaging %50. Six days, a damaging %40.

Now to recharge this drain in about 6hrs. Panels are rated in watts so we have to go there.
24V x 20Ahr = 480Whr a day.

If we can get this back over 6 hrs we need 480/6 = 80W of panels.

In reality you only get about %70 charge efficiency into LA batteries so you need to up this a bit.

You should probably use a 100W 24VDC panel or 2 ~50W 12VDC panels in series.

100W/24V = 4A. So a 5 or 10A charge controller is in order.

Bigger than you thought I bet... It always is.

Note these batteries will absolutely avoid Scotty's charge rate concerns.

If you need to run solar, you always want to do everything in your power to minimize energy consumption. Any other way to do this venting?




Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

We just finished monsoon season here, several days with less than 6 hours of sun. Since we put the second panel (in series), our sunrise voltage was never below 24.1 V. Two batteries seems to be adequate. Two 40W panels seems to do it.

With the nearly complete lack of electrical infrastructure in this area, we have learned how to size panels for cloudy days. We just don't seem to have learned how to deal with 24V systems since all of the automation systems are 12V. From everything you guys are saying it sounds like we stumbled on a pretty good solution with 2 40W panels and 2 deep-discharge batteries. I'll abandon the DC to DC converter idea.

I've looked at many ways to do the venting, and the dP is way too high for any solenoid on the market, the gas pressure is way too erratic for any pneumatic solution, and I can't afford to place an air compressor at these sites (and keeping an air compressor engine running would be too much fuel). The solar panel/battery/motor valve is the only technology I've found that meets the needs.

Thanks

David

 

[zdas04]

I forgot to mention that the 24V SunSaver I'm using is 10A. I also forgot to thank you for finally clearing up how to get from an Amp load to a Watt panel. That has confused me for years.

This configuration is probably working because most cycles are at the 5 second end of the 5-10 second travel I mentioned above, so the load is closer to 12 seconds/min instead of 20 seconds/min.

Thanks to both of you for the help.

David

 

[itsmoked]

Yeah I thought it was interesting that your two panels came out in the ball park.

Your busy valve process is really strange sounding to me. Can I ask what process your system is providing for?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

It is a gadget I invented. Traditional gas/liquid separators in my industry separate water out of gas. At low pressures that doesn't work very well for a lot of reasons. This separator pulls gas out of liquid. People say that the downhole pumps that they use don't allow any gas in. The first one of the gas knockouts I installed is averaging 250 MCF/d for the water stream from 4 wells--it paid for itself in 6 weeks and is still pulling 250 MCF/d from the water. The vessel internals strip gas out of the water stream, and the small motor valve vents the gas to a sales point. I made it to be self-sufficient so I could put it in remote parts of water gathering systems, no line power, no reliable source of pneumatics.

That is why I went with a 24 VDC motor valve that the motor drives in both directions. These particular valves get a reposition signal (say the float hits the "down" stop and tells the 1-inch valve to open) and the valve keeps moving in that direction until it hits a limit switch (full open) or it gets a close signal from the float being "up" or floating in liquid. Then it goes in that direction till it gets a different signal or hits the "closed" limit switch. With 3 wire float switches and these "dumb" valves (i.e., they do the last thing they were told till they hit the wall or get told something different), the system is really responsive.

David