quote:

Originally posted by Tom & Jan:
Stan, You may be adept at the method you advised, but most novice technicians may risk damaging their meter measuring what amounts to a short-circuit current across about 22 volts, which will produce a relatively high current, and possibly damage the panel too. I wonder what this information really reveals to us anyway considering we are only interested in the current flow through a battery when the panel is connected to it (real world).

Most meters are rated at 10 amps or more and if you follow my suggested method it is quite safe. Keep in mind that a 100 watt panel is rated at 6.5 amps or less depending on the brand and even the new 150s are just under 10 amps.

quote:

If you measure a solar panel that is smaller than your maximum amp rating you can use your VOM in Amp mode by connecting directly to the pos and neg terminals. Best to do it with the panel shaded and then remove the shade. That will give you your maximum amp rating from the panel. Don't leave it hooked up long, your meter will start to warm up fairly quickly at higher amp ratings.

There won't be 22 volts from the panel when the amp meter is connected across it, what voltage there is will be a result of the meter and the leads resistance.

What this test reveals is the maximum amperage that the panel can provide. That along with the voltage give you an idea of the peak values for that panel that you can compare to other panels in an attempt to figure out what you have. Sure it would be nice to have a full power curve showing voltage and current across the range but nobody I know carries the gear to do that in their rig.

As to leaving the meter connected for some time I'd be cautious about that with most meters at more than the low-range amperage. When using the internal shunt and the 10 amp plus range you can toast a meter if you leave it on long. I looked at a couple spec sheets and don't see any published limits but my old meter got pretty warm at anything above 5 amps for very long, haven't tried it with my new meter.

Arguments against MPPT controllers are often based on the fact that adding a panel with the extra money will make you more power. While this is true in many cases most RVers are space-limited and adding more panels that aren't shaded part of the day can be tough. Other arguments are that the MPPT controllers do little good in warm weather, again true but RVers tend to need the same amount of power or more in winter when the MPPT works well and with the lower solar input that 30% can be a big help.

Can't help you with cheap, I've never been rich enough to be able to afford cheap stuff and I've always saved my money until I could buy quality equipment. :-)

My Blue Sky MPPT 50 did everything I asked of it, quietly and with no fussing after the initial setup for several years and seems to still be working for the folks that bought our rig.

http://www.blueskyenergyinc.com/sb3048.htm

The 50 mounts near your batteries and uses a remote control panel giving less heavy wire to pull and better charging, the 2000 is not too shabby for smaller setups but you have to bring your wiring to it.

http://www.blueskyenergyinc.com/sb2000e.htm

quote:

Originally posted by Stanley P. Miller:
I've never been rich enough to be able to afford cheap stuff and I've always saved my money until I could buy quality equipment. :-)

Now that's an argument I like.

The MPPT controllers in combination with high voltage panels like AM Solar panels can do remarkable things. I recently installed an AM Solar system on a rig. Four panels and the Heliotrope controller that AM sells. The output in SW sun (UT) was truely impressive, and the panels were flat on the roof. Of course, the conditions were approaching ideal, but none-the-less, this is an impressive ssytem, and one I would recommend if you are starting from scratch. Price is actually less than going with 4 KC130's and I can tell you the output STOMPED a comparable set of 130's.

quote:

Originally posted by Stanley P. Miller:


Can't help you with cheap, I've never been rich enough to be able to afford cheap stuff and I've always saved my money until I could buy quality equipment. :-)

Snickers!! No, I don't buy cheap stuff, especially new gizmos.

What I really wanted to know was the WHY of Handy Bob's post that says some controllers don't keep up the proper V for a full charge. He mentioned the MPPT specifically (the low end models).

Now sure, Jack, if you say that the AM system rocks, how did you test it? amps to the controller, to the batts, or the Volts when the amps had gone way down indicating a really full charge?

I'm fixing to spend a few grand, and what I really want is to get my limited batt bank (2 6 volt GC) to the fullest charge possible in the shortest aomount of time. IF the MPPT system is that, then money well spent. IF, however, there is something else in this equation, then I want to know that, prior.

So, can anybody explain what the deal is here? Why the MPPT (some) controllers cut down the Volts, just when you reach the absorption phase? IOW, WHAT adjustment does the C40 have (or algorithm) that Bob thinks the low end MPPT's don't?

And, then dear friends, I can spend my cash with a smile

I can't help you with his thinking, about all I can suggest is to download the owner's manuals and read them. Having had both the C40 and the Blue Sky 50 I can tell you that both did a great job of getting me a decent charge, both were quite adjustable, both had a good temp sensor (as did my inverter/charger) and both have good service and support.

The C40 did have the radio noise issue for me but I don't see much mention of it from others.

The Blue Sky did give us a noticeably better amp rate in cool temperatures than the C40.

Adding another panel would have given us about the same power boost in winter and 20% more in the summer but there just wasn't a good space to add one on our roof. We had bad spaces for 5 more panels but all would have had shade issues.

Bob is referring to the lower end - 20 amp - wire-thru MPPT controllers. I never use or spec these devices.

Any good quality MPPT controller will provide you proper charge in a reasonable timeframe. Your bank is small so it is going to take on a pretty small charge.

For two 6 volts batteries, If I read correctly, I would use non-MPPT controller and a couple solar panels to save some money. MPPT system work better on larger panel system with large battery bank. Every time, extra feature like MPPT means additional component in the controller. That add heat and current lost in the process and a good MPPT would cost more.

Basically, MPPT converts extra voltage above 17V to electrical current which is what the battery looking for. But if your solar panel cannot generate more than 17V, MPPT is useless. In hot summer temperature, the panel voltage drop.

I have a large system (1200 watt solar @ 50VDC) on our rig and use MPPT controllers. In a large system, the extra expensive on MPPT and the lost thru these extra components are well worth it. The extra MPPT cost is small % compare to the whole system cost.

quote:

Bob is referring to the lower end - 20 amp - wire-thru MPPT controllers. I never use or spec these devices.

Jack, just wonder why? My recent poor experience with a Solar Boost 2000E (it went DOA) seems to mirror what I think you are saying?

Lew

quote:

Jack, just wonder why?

My reason is that I prefer a larger controller - one with some room to grow. I also require a controller that can be placed remotely - nearer the battery bank, and in a shorter line to the bank from the roof - NOT one that requires placement in your interior so you can monitor it's display. I may not spec the remote display, but I sure want the option. And I always run the wiring for the remotes when I do a system. I also want to be able to control the charge settings.

OK, Jack and thanks!

That got me to thinking a bit. I'm no dummy and am pretty tenacious, so I've started looking more deeply at the charging algorithms on the various models. There sees to have been not much study in these areas. Of course, this is not unique to the MPPT units, but they may need to accommodate some less-than-desirable charging set points to achieve their advertised claims. Not being an electrical engineer, I'll have to experiment a bit to see what general conclusions I can draw.


The general consensus is that an MPPT controller may help utilize a panels higher voltage, convert some into more charging amps using a DC to AC to DC transformer, and possibly get more charging under optimum conditions. OK, I'll go with that for a working theory.

What I'll try and determine is whether this gain has either a negative or positive real world application IF the charging algorithim needs to be less effective than say, a regular PWM charger. (which most MPPT units fall back on most of the time, If I understand this correctly.)

What I've found out is that the Xantrex C40, for example uses a set time (one hour) for it's absorbtion phase, and some others use a constant VDC and a set point based on the battery's ability to absorb amps. Each charging curve is different. This appears counter to what Bob was posting, in that my THEORY suggests that the most effective algorithm would be the opposite. He does say he likes the C40, however, so there may be adjustments that others may not have to make IT more effective under most conditions. I'll continue with that, and this:

What is the BEST algorithm for each type of battery and under what conditions are they most favorable? Everything has it's trade-offs, so I suspect this answer may be subjective enough for a college thesis. However, what I'm wanting to know (intellectual curiosity) is under MY specific conditions, what design will give me the most usable power for the least amount of money and complexity.

I'm researching the new Morningstar MPPT controller now. It promises to be the most efficient in the MPPT class. Does it matter that much?

I'll try and find out. Thanks to everyone for their input!!

A good, higher end MPPT in theory is the best. It converts the right amount of charge current from the voltage to track the condition of the battery and the sun light that hit the solar panels. It is a microprocessor based computer that constantly monitors these condition and provide the maximum current to the battery. The firmware are upgradeable on my MX-60 and I am not familiar with the other brand.

How efficient are the MPPT algorithms among different brands are hard to come by for practical purpose. I understand AM solar done experiments on various models and design the best system for RV use. But if you are in the detail, you have to find out what criteria they use to test different controllers.

Usually the decision in buying the best algorithms in MPPT or non-MPPT controller, are not the most important decision to make for a RV solar system. Since a better MPPT compares to a lesser MPPT algorithm doesn’t improve that much in amount of time to charge the battery. Due to the chemistry of battery, with three stages solar controller charger, the last 15% to get the battery full will take the longest time. So 1% or 2% better doesn’t make that much different plus sun light is free, other factors become more important like price, quality, dealer support, RMA or physical size.

Thanks, Jack.

Yup, I planned a maximum (2) 125W Panel system (constrained by roof space) so the 21A input Solar Boost 2000E was a good size. With one Panel I did see Boost but not $200 worth. For small systems I think I will stick to the simpler, cheaper, hopefully more rugged PWM controllers, which also "reach-and-hold" setpoint voltage for full battery charging. Morningstar PWM Controllers are rated for Parallel operation and twice the capacity costs roughly twice the money, so I will simply use one for each Panel. For larger systems like you guys deal with I think MPPT is the no-brainer choice as the cost of MPPT doesn't scale with larger capacity.

Lew

Right on, Pad.

FWIW, I've found out ALL MPPT controllers, including the Outback, use a timed absorption cycle. It goes from 1 hour on the C40 (this was actually designed by the current Outback engineers)> to 3 hours on the Morningstar. The Outback uses a more complicated scheme of averaging high voltage plus time, but occasional shading of the panels (a real world problem), factors in, so that it can't optimize the absorption cycle either. But it's currently the best algorithm out there. BTW, an engineer at Outback said that the Outback 60 can handle 80 amps with no tweaking.

A better solution would be to use amps accepted by the batts to time this cycle, but they all only measure the PV amps output. Which, as you know, will be wrong if a system is in use and some of that PV output is being used. A shunt measurement at the battery is possible, but nobody (except one made in Canada) does this.

My conclusion?

Well, my first instinct is to just get a Morningstar. Best bang for the buck. Most of them can be mounted outside, too. However, the set points for the absorption and bulk cycle are not adjustable. So, for me, this is a deal killer. I don't mind mounting inside, as I can always use bigger wire for the line loss. A temperature probe is a necessity. However, they only drop the DCV a specified amount (usually 1.4 or so) and aren't really that smart. Still, it is better than having to fool with the controller every season.

Now what about MPPT for me? Well, no. The algorithm necessary to get measurable results doesn't apply for my use. If I had a cabin in Alaska, then maybe. If I went to the high mountains in winter, then yes. But chasing average temps won't help me at all. Plus, I see where the "sniffing times" for setting the best Volt Amp output interferes with the charging cycle. It may also trick the PWM (most of the time it in this mode anyway) to complete a cycle that hasn't happened yet. This is the problem with some of the low-end MPPT controllers. It goes into float mode much too soon, and your battery's life span will shorten (not to mention the lack of available power for your use).

So then, what do I do. I have to have a float cycle. I've got to have temperature compensation. But the most important thing is to have manual set-point regulation. This is crucial as different brands (and naturally different types) of batterys have different requirement to get the most out of them. In addition, the algorithms of the standard (and MPPT) controllers aren't affected by battery age. They will charge all they can, if the charger is set up properly. They all require a minimum voltage at the battery to even work at all. So now I see WHY Bob recommended the:



Xantrex C40. It was designed by the Outback people. It is the smallest they have that has a heat sink. It costs around $100 without extras. It has headroom for me, and is a proven design. It's only downside is that it's not plug-and-play. You have to manually set it up and know what you're doing. But still, it's the best for me right now.

That's the end of my research on this one

I was told that one way to increase the usefulness of the Outback MX-60 was to wire the panels in series to increase the voltages since it can handle 72 volts. If you wire two panels in series you have 34 volts incoming, so some shading will still get you above the 14 to 15 you need. Not having infinite money, time and roof space I can't directly comment.