MPPT Volts to Current Conversion

Lots of good input for him. Suggest he start reading and learning about solar as there tons of educational information available. The entire system ties together as in panels, solar controller, batteries, inverter, wiring etc.

The most basic question is "What are his energy requirements"? In 2014 my first solar purchase was a battery monitor which gave me volts, amps, kWH, SOC which allowed me get a very clear idea of my requirements. There are also manual methods and Excel calculators to help. A Kill A Watt is a good tool and often a watt rating on a device is a worst case vs a normal draw. A DC amp clampon meter is another good tool.

The watts to the battery get clipped about 10% to 15% to the battery voltage for the voltage conversion.

So in perfect conditions (that never exist) 1800 watt produced is maybe 1500 watts to the battery or over 100 amps at 14 volts. make sure the controller can handle the amps and volts (Voc) of the string of panels. The MPPT controller will also self limit to the output rating spec. A 30 amp MPPT will only put out 30 amps.

BTW at 30 volts and 8 amps the panels are closer to 240 watts. And voltage will drop some as the power begins to flow. Reality could be closer to 200w panels.

Where did you get the 30 volts open circuit and 8 amps numbers? They don't add up. The open circuit voltage of a panel is always higher than it's maximum power voltage. The maximum power voltage multiplied by the maximum power current should equal their rated wattage.
Most 300 watt panels will probably have a Vmp of around 30 and a Voc of around 37. They will usually have a Imp of around 10 amps.
Either way, Vmp x Imp = rated watts. Rated watts / battery charging voltage = potential charging amps. If you really have 6 300 watt panels for a total of 1,800 watts / 12 volts (if the batteries are very low) you could see 150 amps of charge current. I'm really curious what charge controller you have that can handle that much power.
To treat a battery well and make it have a long happy life a charge current of about 10% of capacity is usually recommended. Most people will go up to about 20% of capacity as a good compromise between battery life and quicker recharges. Going up to 25% won't really hurt anything except overall long term battery lifespan and even then it's not that much difference.
I don't think 100 Ah GC batteries exist. They are likely at least 200 Ah if not 225. You'll never be able to get 150 amps to flow into them even if the controller will output that much. The voltage will rise the set point really fast and controller will back off. That's the whole point of a charge controller.
So, I wouldn't worry about hurting the batteries but for the most part that big solar array is going to waste because the batteries (and likely the controller) can't handle it's output.

If the charge controller is properly set up it will not overcharge the battery bank.

jodeb720 wrote:
My buddy picked up 6 300 watt panels today - because he wants to be able to feed his ravenous Starlink - without running the generator.
The starlink uses 190 watts of power at 120v - it's a pig for power.

The panels have an open voltage of about 30 volts - and are rated at about 8 amps.

When the output is run through an MPPT controller - whats the conversion of higher voltage to output in amps?

One would assume that it would drop the voltage to 14.4 or 14.6 for bulk charging - but the amps would be higher than the panels rating of 8.

He's got two sets of 6 volt batteries (I think they are 100 amp Golf Cart batteries). if what i've read here is correct, he can only put 25% of the total battery capacity (lead acid) in per hour to recharge them.

I'm a bit concerned that the Controller could easily drive over that 25% amount and wipe out his batteries prematurely. Now before you tell me we can control the total amps on the controller (which we can), I'm just curious how many amps he should expect per panel assuming full sun - 30 volts and rated at 8 amps.

Thanks in advance!

josh

if they are 100amp gc batteries they are very small. most are around 200AH which means you can charge at 50 amps if you go by that. a good mppt controller will also have a profile for charging so it will only put out the max charging in bulk then it will switch to adsorption and the amps taper off as the battery becomes more charged. if you are going to be running that kind of load I would even consider adding two more batteries to the mix for more capacity.

if there rated at 8 amps and he has 6 panels then your looking at a max of what 48 amps? but 8 amps seams low for a 300watt panel. if it is a 24V panel then I could see 9 amps if it is at over a 30v output but I guess that all depends on the efficiency of the panel and equipment.

@CA Traveler - if it's a 220AH set of batteries - then what is the C (Charging?) rate for those batteries. I've read on rv.net from folks who are a LOT smarter than me that you can only put 25% per hour into Lead Acid Batteries during bulk charging. Is that not correct?

And Yes, it is WAY over sized for his battery banks - but that will change over time - as he'll be looking at moving to LiPo or some other technology for storage in the next year. As we all know we all have to start somewhere and evaluate our needs individually.

Personally, I've got 2 banks of GC batteries, and an Inverter. I alternate between each bank every other day - and do a daily recharge with my Honda - so my own needs are much less than his.

Though I'll be moving to Starlink at some point this year because some of the places we go in Colorado don't have internet (cell) based and for work, I need to have some internet access.

@2oldman - that's good to know. we're going to do a kill a watt test when we go next week to figure out real world usage.

By the way - he's had the worst luck with Starlink. When he originally bought is starlink system, he's in an area that's considered to be rural - and there's no High Speed internet available where he lives. He has a microwave dish on a tree pointing 12 miles south for his internet.

When he got his starlink system, he had great success - except, his original dishy was getting covered with snow (He's at 9300' in Conifer, CO) and the heater couldn't keep up with melting the snow and it was melting and freezing on the dishy. He purchased the new, bigger system - but was getting terrible throughput - only to find out even though his address is rural, he was considered to be in an over utilized zone and was getting terrible throughput.

After 4 months of going back and forth with Starlink - he had to go back to his old system and his performance is back to where it was. Seems that Starlink won't let you upgrade your system to a newer base unit and dish - and if you do, and you're in an oversubscribed area, you get "best effort". They don't have a way to "upgrade" and keep your current bandwidth.

It's a hot mess and he's out 1600 bucks!

For where we're going camping though, it's an under subscribed area so it's not an issue.

Thanks for the info by the way!!

Josh

According to google, Starlink uses about 50-75w. That's about what mine does.

1800W of solar is more than adequate to run 190W for starlink including battery discharge with clouds, night time, etc. However his battery system is totally inadequate for that much solar. The solar controller will not over charge the batteries as the batteries will only draw what they need for charging and the remaining solar power is loss unless he uses it with an inverter when the sun shines.

Best approach is to do a energy audit to determine his needs and size batteries and solar accordingly. Golf cart batteries are rated at 220 AH for 2 at 12V so he has 440 AH but best to discharge to only 50% so he has 220 AH available.

Ideally he could get 6*300=1800W/12v = 133ADC maximum from the controller minus losses.

Starlink uses 190/120 * 11 = 17A DC draw from the batteries by the inverter. But the batteries also have to power the RV lights etc. Basically a 10 to 1 draw of battery amps to convert to AC amps as in 120V/12V. I use 11 to account for losses.

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