# MPPT output

Options
Trying to understand the output of an MPPT charge controller.

If I am using a Sharp NT-175U1 to supply power to an MPPT charge controller, what would be the output to 12v batteries?

• Solar Expert Posts: 9,583 ✭✭✭✭✭
Options
Re: MPPT output

Since I have not memorized the lables on the back of every solar panel, what are the specs for your Sharp ?

If it produces enough voltage, the Charge controller will supply up to 15V to the batteries, depending on their state of charge. It won't boost a 13V panel up to 15V. It will work by operating the panels at their most efficient power point. That's the VmaxP spec on the panel, which is lower than the Vmax
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: MPPT output

Gunner,

The key module specs required to answer your question are Vmp and Imp. For the Sharp 175, those are 35.4 V and 4.95 A. The mathematical product of these two specs is 35.4 V x 4.95 A = 175 W STC. See: http://solar.sharpusa.com/files/sol_dow_175U1.pdf

Under ideal test conditions and using perfect equipment, the MPPT controller would take the module’s 35.4 V x 4.95 A (175 W) and convert it to, say, 14.6 V x 12 A (also 175 W) as the battery approaches the bulk-stage to absorption-stage transition voltage.

As a practical matter, PV modules rarely deliver rated specs, and the related system equipment isn’t perfect.

The STC spec for PV modules is based in part on a PV cell temperature of 25 C, or 77 F. Since fully illuminated PV cells typically operate at ~35 C above ambient, that means that the ambient temperature has to be ~ -10 C, or ~14 F. This is not a typical mid-day temperature. Accordingly, a good working number for Vmp is ~88% of STC spec, although this can range from ~75% for very hot modules to >100% for very cold modules.

There are also power losses in the wiring between the array and the controller (~98% efficient), and MPPT controller efficiency might be in the ~97% range, although these factors may also vary.

Combining all of these sample efficiency factors together (88% x 98% x 97% = 83.7%), the controller will deliver 14.6 V x (12 A x ~83.7%) = 14.6 V x ~10 A, or 146 W, from a “175 W” PV module.

HTH,
Jim / crewzer
Options
Re: MPPT output

Thanks for the response.

I am looking to install a small solar system at my cottage(primarily three season) which is off grid. Have estimated to need about 1000W/day for lighting, small TV, 12v water pump and radio. Have a small Honda gen (900W) that I am using now and will be used if needed as back up. Peak sun is 5.5 hrs/day.

I like the idea of using an MPPT controller, an inverter/charger and panels (12v or 24v) but have been racking my brain trying to figure out what combination to use.

Your thoughts on what I should use would be appreciated.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: MPPT output

Gunner,

We need to better understand your requirements and your cabin's location before we can recommend a system.

(1) First, there's no such thing a "1000 W/day". Perhaps you meant "1000 Whr/day"? That would be the equivalent of a 200 W load operating for 5 hrs/day, or 200 W x 5 hrs/day = 1,000 Whr/day.

(2) Which three seasons?

(3) "Peak Sun" varies by month. If possible, please select your cabin's location from this list. See: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/state.html You may have to check a neighboring state for a locality near your cabin.

(4) Finally, does the proposed site for your PV array have an unobstructed view of the eastern-to-southern-to-western sky, or will nearby trees shade the array during part(s) of the day?

Regards,
Jim / crewzer
Options
Re: MPPT output

Thanks Crewzer, here is the data you asked for.

1. I had used a solar calculator from Sandia Labs and I will give you what the form calls "Corrected Amp-Hour Load (AH/Day). 105 AH/Day

2. Months cabin is used, May - Sept.

3. Use International Fall MN. Tried to copy here but looks terrible, sorry.

4. Putting panels on roof, faces directly south, unobstructed view East to South to West.
• Solar Expert Posts: 5,183 ✭✭✭✭
Options
Re: MPPT output

Gunner, there are at least a couple of other postings in this BB that are very similar to yours as to what you want to run. Have a look at these for related info. Try a search for 'cabin'
here is one:

cheers
Eric

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: MPPT output

Gunner,

OK. A quick look at the Sandia form suggests the Ah/day required is a gross figure that contains all efficiency factors. I’ll assume this is for a “12 V” nominal system.

Average insolation from May through September on a south-facing array tilted up at ~33-1/2 degrees (latitude minus 15 degrees) for International Falls, MN, ranges from 4.3 hrs/day to 5.9 hrs/day.

Using the lower number suggests that you’ll need an array rated at 105 Ah/day / 4.3 hrs/day = 24.4 A. Assuming an STC Vmp spec of ~17.4 V, the array will need to be rated at 17.4 V x 24.4 A = 425 W STC.

Average insolation from May through August on a south-facing array tilted up at ~33-1/2 degrees (latitude minus 15 degrees) for International Falls, MN, ranges from 5.4 hrs/day to 5.9 hrs/day.

Using the lower number suggests that you’ll need an array rated at 105 Ah/day / 5.4 hrs/day = 19.4 A. Assuming an STC Vmp spec of ~17.4 V, the array will need to be rated at 17.4 V x 19.4 A = 338 W STC. This might be a more cost effective way to go, and then use the generator for backup in September.

One option would be an array of three ~125 W modules (~375 W STC total) wired in parallel through a Morningstar Tristar 45 PWM controller with a remote battery temp sensor. Candidate “12 V” modules of ~ that size include products from Mitsubishi, Sharp, and Kyocera. See: http://store.solar-electric.com/hiposopa.html and http://store.solar-electric.com/mochco.html

Another option would be a pair of larger, higher voltage modules of ~175 W STC each (~350 W to 380 W STC total) wired in parallel along with an OutBack MX60 MPPT controller and a remote BTS. Candidate modules include products from Sharp, Mitsubishi, Kyocera, and Evergreen. See: http://store.solar-electric.com/hiposopa.html and http://store.solar-electric.com/outpowmxmp.html or http://store.solar-electric.com/xaxwmp60amps.html

A cost effective 12 V battery bank would be three Interstate SRM-4D flooded-cell lead acid batteries wired in parallel for ~600 Ah. Assuming the batteries are located in the cabin, they’d need to be vented to the outside while being charged. See: http://www.solarseller.com/battery_box_power_vent_by_zephyr_industries.htm

An alternative that I’d recommend would be three side 4D VRLA (AGM or gel) batteries wired in parallel for 12 V X ~600 Ah. East Penn’s Deka and MK brands would be good candidates.

You’ll also need wire, conduit, an array combiner box, circuit breakers, etc. What about an inverter to provide 120 VAC?

HTH,
Jim / crewzer
Options
Re: MPPT output

I like the idea of using two larger panels with the MX60.

If I wanted more battery storage, would there be any issues if I used 4 Trojan L16H for 820 AH or 4 Surrette S-530 for 800 AH? Could I go to 6 batteries with the 2 larger panels?

I was looking at a small inverter/charger like the Magnum1212AE.

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Options
Re: MPPT output

For systems with arrays less than ~400 W to 500 W STC, the MX60 may not be the best choice, as its higher cost and its internal power losses take their toll. It’s worth running the numbers…

Also, if you find that you have to expand your system, it would be less costly to add another 125 W module to the system with the TS-60 controller than to add another large module to the MX controller. However, the latter would be able to deliver more power...

Choices, choices...

The systems I’ve described will deliver ~20 A peak battery charge current at mid-day. That’s about the miminum for a 600 Ah battery bank allowing for light use and battery aging. An 800 Ah bank would probably work initially, but, depending on day-time loads, may be come difficult to recharge as the bank ages.

Another option might be to go with the 800 Ah bank initially, and later add another PV module to the array.

Tip: try to postpone some daytime loads until after the controller has been in absorb stage for ~30 minutes or so. The controller operates in current limit mode when in the absorb- and float stages, so there’s unused current available from the array.

Other “small” true sine wave inverters to consider are the XP’s from Exeltech. If possible, try to turn off the inverter at night to reduce idling power drain. You’d need a separate charger.

See: http://store.solar-electric.com/exsiwain.html
And: http://www.exeltech.com/xpspecs.htm
And: http://store.solar-electric.com/bach2.html

HTH,
Jim / crewzer