MPPT Upgrade?

[rickst29]

And all "12V" panels are actually providing a lot of that, when operating at maximum power. With a PWM controller running in PWM mode, it gets thrown away by disconnecting and reconnecting the Array (at high freqeuncies). In "Bulk Mode", the Solar circuit is connected "almost directly" to the battery circuit - running the array at something between 13.2 and 14.4 Volts (far below the panel Mpp Voltage). Current is slightly higher than I(mpp), but total power is only a little bit higher than running PWM mode at "the same voltage".

Originally Posted by tentcamper

I get it with the foot print reduction and that there are many days you don't get enough sun to full charge the batteries, so lets get as much amps as we can with the MPPT. But I'm not sure you get the numbers on an RV?
I have been reading a lot on the MPPT. I'm getting the impression you get the large gain in efficiency when your are running much higher voltage differences between the panes and battery banks, then a typical RV setup. If I understand it correctly, I may get 10% more with my low voltage system that I have??

It depends on wiring losses and MPPT efficiency (MPPT typically consume about 4-6% of the incoming power to run the inductor and electronics.) If you have two panels, as I do, you can cut your wiring losses in half by re-wiring in Series (twice the volts, half the Amps. The "savings" on a pair of 100W panels is about equal to the loss inside the MPPT - but you're starting with the Panel operating at V(mpp), instead of degrading it to match Battery Voltage (by "direct connection", or by PWM.)

Big installations with lots of panels and a long "home run" from the last panel to the Solar Controller (e.g., a house) do benefit greatly by wiring the Panels in Series: Instead of the having 110 A at 17V (typical for about 20 panels), they can have 22A at 85V, saving 3/4 of the power loss which occurs in that last wire.

But let's ignore the wiring losses completely, and look at the factor which makes a bigger difference in RV installations: An MPPT converts "unusable" power at high Voltage (higher Voltage than your batteries can handle), into more Current at the desired Voltage. In PWM mode, PWM controllers simply leave that power unused, by disconnecting from the Solar array (thousands of times per second) to keep the average Voltage stable and lower. In Bulk mode, they degrade the panel Voltage to a battery-compatible Voltage, gaining only a tiny bit of additional current. The advantage of "Bulk Mode" over "PWM Mode" is merely the ratio of current increase at battery voltage versus current at V(mpp); and the differnence is usually not much).

With a controller operating in "PWM" Mode, the power into the batteries is only I(mpp) * PWM charging voltage- 14.4 VDC or less. Power which the panels could have provided, while creating "excessive" voltage, is simply not accepted into the Solar Controller during PWM. With my Controller doing "PWM" charge at, for example, 13.7V, the "wasted Solar power" is roughly 19.1V - 13.7V = 5.4V, times 5.2A each = 28 Watts being "wasted" by each of thee two panels.

With the MPPT, nearly all 30W is sent to the batteries as higher current. The Panels offer 200W as either 10.4A @ 19.1V (wired in Parallel), or 5.2A @ 38.2V (re-wired into Series). But the output to the batteries will be about 14.1A @ 13.7V if I choose to keep the same the "Bulk Charge" voltage. I just gained about 50W of charging power at the battery terminals, roughly 1/2 of an entire new Panel (which would cost $50 less to buy, but weigh too much on the roof.)

- - - warning: complex discussion of MPPT internal operations follows - - -
BTW, an electric stove in the kitchen works exactly like a PWM controller in PWM mode: When cooking at "low" heat, The Voltage coming into the Stove is still 240V (120V * 2). But you hear clicks every few seconds, as the element is turned off and on to consume less "average" energy. PWM Solar controllers work exactly the same way, but turn on and off many thousands of times per second. In contrast, an MPPT controller functions as a DC-DC "Buck" Converter (power in = power out, but delivered at higher current/lower voltage). They can't do the reverse (convert low PV array voltage into higher battery voltage), because that would require nearly double the logic circuitry, a very expensive upgrade with nearly zero return. Mine also contains an enormous Inductor, to store and release energy smoothly- that's one of the parts which led me to say that it seems 'too expensive to build' for it's price.

But it also contains lots of "monitor" circuitry to keep maximum output to the battery bank in balance with input accepted form the PV array. When the batteries aren't accepting ALL of the maximum output current which the MMPT can deliver at the specified charge Voltage, the MPPT switches to "partial" PWM mode, reducing the input power accepted from the PV array. But it still sends somewhat higher current to the batteries, unless the batteries are accepting the same or less current than the PV array is putting into the Controller: in that case, it behaves exactly as a PWM.)

MPPT is not 100% efficient, but the secondary advantages from converting Panels wired in Parallel to Series (reduced wiring loss, and providing a few Watts during slightly longer charging times at the start and end of the day). The gain with my 2 * 100W panels, is slightly larger than 4-5% net power consumption of good MPPT controllers all by themselves.

[tentcamper]

That make sense to run them in series.

[rickst29]

Originally Posted by scrubjaysnest

My understanding is mppt breaks even with 400 watts of panel.

It breaks even (financially) at 300-400W of PV, depending on the MMPT you buy (yours and mine are both higher-end, pushing the "break even" point into the 400W range... you're exactly right. But on TM, there's also the weight and roof-space restrictions - you might not be able to add another $150 panel with comfort, so accomplishing even half as much with an MPPT might be worthwhile (if the batteries are NOT reaching 95-100% SOC, and COULD recover into that range if you add just the relatively small amount of power which MPPT provides). That seems to be my situation.

Originally Posted by scrubjaysnest

That said, with the TS-45 morningstar pwm and 160 watts in winter full sun conditions we are only seeing panel ratings which means a longer charge time to reach 100% soc. What this means is 4.5 amps per battery when they really would like to 5+ amps. Summer we can do this but not in winter with sun angles. We went with the ts-45 expecting this and will add one or maybe two more panels in the 100 to 150 watt range. At this point Monday morning quarter backing says we should have gone with about 300 watts of solar, 24 volt panels and a morningstar pro 30.

I strongly agree with your assessment. 160W of PV would definitely be a bit short for my needs as well, I need at least the 200W I've got on the roof, and the 140W maximum which I now get from the PWM doesn't do the job.

Originally Posted by scrubjaysnest

With a pro 30 pwm and two 150 watt 24 volt panels the system would still be portable but you could drop from 8 awg wire to 10 awg for a little savings. A mppt controller might give you C/20, battery capacity in aH divided by 20 gives the minimum boost charging current, under winter conditions full sun. That is the first 80 % of charge and after that when the CC goes to absorb, holding battery voltage at 14.8 or higher, there seems to be no problem with the mppt. Hope fully we will get some better data once Rickst29 gets his up and running.

If I had a chance to do it over from the start, I would configure almost exactly as you describe: roughly 360W as 2 panels, each running at "24V" (more like 37V Peak), but wired in Series (for a maximum PV Voltage of about 75V), wired up with with MPPT from the beginning. The PV side of my MPPT can handle 100V short-circuit max, but the battery output side is current-limited to only 21A - so 310W is the maximum power which my MPPT can push into a 12V battery bank. The "extra" PV capacity would provide for my continously less-than-ideal sun angles, chances of shade, and similar factors which limit Panel output.

But my future upgrade path (if needed) is like your add-on: Just pile on another 18/22V panel (flex for weight). For you, run in Parallel (more Amps). For me, run all 3 in Series (more Volts). It appears to me that "Crazy People" are thinking alike on this one.

[scrubjaysnest]

Ordered a used 120 watt Kyoreca 12 volt panel today for $111.62 and free shipping. What I actually get out of it will depend on how long it was in the sun which results in some reduced output. This matches my existing panels very close for Vmp. I'll mount it on the rear shell using Riv nuts or nutserts as some call them in the panel. This will allow removal if desired for semi portable use. Brackets will be mounted to the shell using VHB tape and pop rivets. Additional water proofing will be 3m 4200. Wire will be run like Wayne, HarveyRV, did his in the cord mate II. In portable mode I'll have a length of 10 AWG wire rolled up in the bumper with mc4 connectors on one end. The other will be wired through a breaker to the TS-45. Currently there is a 8 AWG rolled up in the bumper for the existing two 80 watt panels. Total wattage will be 280 with the three panels. This should be a pretty flexible system. One could with all three panels in portable mode have the 80 watt getting the morning sun and the 120 watt getting the afternoon when not at the camp site.
We still have the ability to pull one battery and charge it in the back of the truck while exploring if we should run into days without sun.

[rickst29]

Although I finished too late in the evening to do any useful "testing". Because it's attractive to thieves, I installed it inside - on two vertical pieces of wooden molding, each with multiple screws into the inside layer of front "box" aluminum - and slightly into the underlying insulation (not all the way through). The placement is front center, where the table "leaf" used to be stored.

It needed those wooden mounts to handle the weight - the relatively thin TM aluminum sandwich surface would let the screws go loose in direct mounting, probably on the very first big "road bump". This is a HEAVY device.

It looks very nice under there, although the table "leaf" can no longer be inserted in the usual storage spot. We'll just leave the table opened all the time, even on the road. (Full size, the "leaf" inserted). I rewired the panels in Series: Vmp a bit less than 38V for the two of them, with Short Circuit Voltage at right around 45 VDC. That leaves lots of hearoom for a 3rd panel, someday.

The Rogue keeps lots of measurements, and they can be downloaded into a Windoze computer. I might give that a try tomorrow, although an 80-degree day won't be much of a test for running the Fridge.....