Solar conversion

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Solar conversion

Postby timujin » Thu Mar 14, 2013 3:54 pm

I have a 170W solar panel that puts out 36v. I need to convert that down to 12v to keep my deep cell batteries charged. I tried a convertor built for a golf cart, but it fried after about a day. Suggestions? I'd be willing to build something if I just know what to use...

I'd also like to put something between the batteries and the rest of my gear to keep things from getting fried.
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Re: Solar conversion

Postby BeachBum » Fri Mar 15, 2013 10:58 am

Sounds like you got one of the BP Solar panels, like i did from a gentleman off Craigslist last year for like $1/Watt in LB.

Good panel, very solid. A few tricks on how to use it:

- For using with a battery system, rather than a grid-connected system, it's for a 24V battery system. (The 36V is kinda a spec for bigger solar systems)
- I got a 24V Morningstar charge controller, and a "24Vdc -> 110Vac" inverter to use with this panel. This requires TWO batteries to be connected IN SERIES to get the 24Vdc.
- DON'T get a "MPPT" controller to try to jury-rig it to use the full 36V converted to 12Vdc. Those are far too expensive and quirky to justify using them on a 12Vdc, one battery system. Most Chinese "MPPT" controllers are a joke anyways, there's no fed law defining "MPPT", so anything can get sold using this designation. And, if you look deep into the specs of any charge controllers that imply charging a 12V battery from a 36V panel, you'll find that they'll probably overheat and burn up immediately, as you experienced, so it's not worth trying to figure out if you may, perhaps, be able to find one that might possibly work.

An easy way to use this good panel with a 12Vdc (one battery) system:
- NO charge controller. Put a solar BLOCKING diode in-line from the solar panel "+" output to the battery. The MBR1645 is a good 16 Amp solar blocking diode. The blocking diode is NECESSARY to prevent the battery from draining through the solar panel at night and when it is shaded. Google "MBR1645" and solar BLOCKING (not bypass) diode for info. You can mount the blocking diode to the solar panel module metal using a plastic screw/nut and non-electrically-conductive, isolating "TO-220 insulator" and thermal grease. It's easy, just verbally ask someone who understands what i just said to give you a hand.
- Monitor the battery voltage, and put a small tarp over the solar panel when the battery voltage reaches near it's limits, or point the solar panel away from the sun. It takes a full day of good sunshine for this panel (5 Amps) to recharge a depleted deep-cell, so it's easy to do. You can get an automotive cigarette-lighter-battery-voltage-meter for like $12 to watch the voltage.
- The panel is able to recharge two 12V batteries IN SERIES at the same time, with the same current as when recharging only one battery, thus recharging two batteries in the same amount of time that it takes to recharge one battery. Make up a "battery-positive 12V terminal to battery-negative 12V terminal" cable for doing this without having to swap around cables and perhaps blowing things up, and leave this cable connected to the normally-used 12V battery's positive terminal.
- An article with info that you need to do this: http://www.wholesalesolar.com/Informati ... ticle.html (Note to mod: this article is just info needed by the OP, not a rec for a company)

Best wishes!
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Re: Solar conversion

Postby BeachBum » Fri Mar 15, 2013 2:14 pm

Actually, timujin, here's one real-MPPT charge controller that will probably do what you want. Your solar panel pushes to very near the amperage limits of this charge controller, but this company has very solid specs (i.e., they're not Chinese, and they sell to industrial plants/portable roadside signs/oil industry, which require the reliable specs). Your solar panel will only put out like 13 or 14 Amps max to the battery with this charge controller, so it's barely within the 15 Amp device spec.

Downside is that it's $200 to $300, probably significantly more than you paid for your panel. It would probably be cheaper to do one of the two methods i mentioned in the previous post. Or, to just get another panel if you need the power ;-)

If you do get one of these charge controllers, please check out the specs yourself beforehand, and i would strongly recommend the current version of this charge controller. Earlier versions probably have different specs, which may not work.

http://www.morningstarcorp.com/en/sunsavermppt If anyone knows of a different, non-Chinese, real-MPPT, charge controller that's reliable and definitely meets the 36Vdc-to-12Vdc and open-circuit-voltage and current specs, for a lower price, please feel free to post.
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Re: Solar conversion

Postby timujin » Fri Mar 15, 2013 6:06 pm

It is, indeed, one of the BP panels. Best price I've ever seen per watt and it's made in the U.S.

The last time I tested it in full sun, though, it was putting out 36v. I'm running two deep cell and was had them connected in parallel in order to double my amp hours. The idea was to get 12v in from the converter to the batteries and then from them out to everything that ran on 12v... Which was great right up until the converter went tits up.

Thanks for the info on the diode, last year I just put a switch between the panel and the batteries and "turned it off" at night, but that seems like a better solution.

Another concern is that there are things I'll need to run during the day, which means I'll have to be charging and draining at the same time, and I want to make sure I'm not blowing stuff out. It's not like the batteries are going to just dump it down to a clean 12v, so I'll need something between them and the rest of my gear. What can I do on that end to clean up the line?

In case it matters, I'm only running a large 12v cooler, a small water pump and a radiator fan, plus the occasional device on a standard car 12v-to-5v USB plug.
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Re: Solar conversion

Postby BeachBum » Fri Mar 15, 2013 11:33 pm

Yea, recognized the specs for the BP panel. Good company, which BP bought for it's last iteration. Too bad BP had this solar subsidiary go belly up, forcing it to dump onto the market all it's inventory, which resulted in the low prices. But, now many of the 36V solar panel complete modules are now being advertized for like 84 cents/Watt, with the Chinese production lines making them for like 60 cents/Watt, soon to go to 42 cents/Watt. Insane.

A battery is like a big capacitor, or "Leyden" jar as Ben Franklin said, it cleans up the DC voltage extremely well. The real issue is that you don't want to accidentally put the 36Vdc solar panel voltage directly onto your equipment, even for a second. A trick to prevent this is to connect the solar panel to the big "+" and "-" battery terminals, and connect the your electrical loads to the smaller battery posts beside the big battery terminals. The battery is such a HUGE capacitor, that it's nearly impossible to get it above normal battery voltage. If you do, it'll blow up, it's that rare. Use crimp-on ring terminals, which are available in any electrical store or auto parts supply store, for wiring your equipment to the smaller battery posts.

A Youtube on why cheap Chinese solar panel charge controllers go tits up: http://www.youtube.com/watch?annotation ... UbYAZ4eCHw
Also, the solar panel charge controller's max input voltage is often less than the solar panel Voc, which is not good, to put it mildly. And, very few controller brands, other than the controller brand mentioned above, are conformal coated, so they need protected from playa dust.

And, if you need more power than like 60 Watts (12 Vdc x 5 Amps) from your solar panel, info on how to get more power is above.

Best wishes.
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Re: Solar conversion

Postby timujin » Sat Mar 16, 2013 9:16 am

Awesome, thanks for all that.
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Re: Solar conversion

Postby Dork » Sat Mar 16, 2013 10:08 am

If you get a good mppt controller such as a Morningstar it will convert the voltage and you won't have to worry about anything else. Just plug it in and go. The expense is a bit much if you're only using it for one week, but if you have some other year round use it's worth it. Or, put it on ebay and get most of your money back in the fall. I've been using one for the last year in my camper van with a 24v panel and it's been trouble free.
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Re: Solar conversion

Postby timujin » Wed Mar 20, 2013 3:02 pm

Alrighty... I finally got around to watching the video and if I understand correctly this one will handle up to 50v and about 200w (even though it says 250) which is more than I'm going to push:

https://www.google.com/search?q=mppt250 ... 22&bih=844

Most of the morningstar ones I saw that weren't crazy expensive were rated at 12/24... but I don't know if that's input only or output. Can the one posted above handle the 36v input? If I'm understanding this correctly, my 170w panel should be putting out about 4.7 amps.
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Re: Solar conversion

Postby BeachBum » Wed Mar 20, 2013 4:32 pm

Hi timujin,

Congrats on doing this!

As far as the BZ MPPT 250 goes, based on their specs, i don't have a clue about whether it is good or not. Their specs say it can handle the 36 Vdc (41 Voc) panel, so it won't blow up at least by over-voltage. But the specs have so much singing and dancing in them, i don't know anything beyond that. They say it will work with 12V or 24V solar panels, and give pseudo-specs for that, but don' t give any specs at all for how it works with a 36Vdc solar panel. They quote a high efficiency rate, that the good Morningstar doesn't even quote, and they don't say what voltage the efficiency is at. Only, in their marketing blurb, they say that the BZ MPPT 250 can provide up to a 30% boost.

The Morningstar solar panel controller will provide about a 250% boost to current from a 36 V Pmax panel into a 12V battery, according to the very detailed power curves in their spec. You can get one of their controllers, mentioned above, for just a hair over $200. The Morningstar controller mentioned above is perfectly sized for the BP 170 panel.

BZ MPPT 250 spec: http://www.bzproducts.net/sitebuilderco ... PPT250.pdf , the Morningstar spec is in a post above.

Bottom line, i wouldn't trust the MPPT 250 to provide significant benefit beyond probably not blowing up. I think this solar controller is sourced with other company nameplates on it also, probably built generically in China, but i don't have any time to look at this further. If you can find the power curves of voltage & current in with the curves going past 36Vdc, versus current going out into a 12Vdc battery, that would be information necessary to consider if this device provides much benefit at all. But, I don't think you'll find it for this device. (BTW, i think i remember one Youtube from a guy trying to calculate power from this device where he made some basic algebra errors)

I think i previously talked myself into getting one of the Morningstar controllers, linked above, for my BP 170 panel. Also, i forgot to mention before, a fuse in the output from the battery to the electronic devices is always a good idea.

Best wishes!
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Re: Solar conversion

Postby BeachBum » Wed Mar 20, 2013 4:51 pm

Whoops, one mis-spoke in my previous reply, i'm not able to edit it anymore. It should read "The Morningstar solar panel controller will provide about a 150% boost to current from a 36 V Pmax panel into a 12V battery, resulting in 250% of the solar panel's 36V current, according to the very detailed power curves in their spec."

Best wishes!
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