Advice please on a SCC

[RKDowner]

I'm installing solar on my travel trailer and I'm on a low budget. I'm not trying to run A/C or anything stupid, I know my batteries won't do anything on solar they can't do on a generator, I'm just trying to make less noise and leave the generator at home. LiFePO4 batteries are expensive and I'd need to replace the factory power converter/distribution panel with one that can charge Lithium, thus I'm sticking with the existing lead-acid batteries. To save money I bought some used panels and they're rated at 16.1 volts with open circuit at 20.1 volts. As near as I can tell (PLEASE correct me if I'm wrong) 16.1 volts is not enough for MPPT, so I would put them in series for 32.2 volts. But 32.2 volts on PWM would be wasteful, so for that I'd connect them in parallel.

So those are my options: 32.2v MPPT or 16.1v PWM on 12 volt lead-acid batteries. The https://www.renewablewise.com/pwm-charge-controller-calculator/ tells me I need a 60 amp PWM controller, and the https://www.renewablewise.com/mppt-calculator/ tells me I'd need an 80 amp MPPT controller. So I'm looking at about $60 vs $360, depending on who you believe (I've seen "MPPT" controllers for as little as $15). I've already spent more on this project than my better half would like, so is MPPT worth the extra money? If you were in my shoes, what would you do? I'd appreciate any advice and opinions, thanks.

 

[sunshine_eggo]

I'm installing solar on my travel trailer and I'm on a low budget. I'm not trying to run A/C or anything stupid, I know my batteries won't do anything on solar they can't do on a generator, I'm just trying to make less noise and leave the generator at home. LiFePO4 batteries are expensive and I'd need to replace the factory power converter/distribution panel with one that can charge Lithium, thus I'm sticking with the existing lead-acid batteries.

This depends on the installed converter. The idea that one must replace their converter for lithium comes from the manufacturers looking to sell new converters.

To save money I bought some used panels and they're rated at 16.1 volts with open circuit at 20.1 volts. As near as I can tell (PLEASE correct me if I'm wrong) 16.1 volts is not enough for MPPT,

You are wrong. Many MPPT require a certain difference to start. Victron publishes PV +5V over battery to start working, i.e., the Voc value, not the Vmp value.

so I would put them in series for 32.2 volts.

You could.

But 32.2 volts on PWM would be wasteful, so for that I'd connect them in parallel.

Correct.

So those are my options: 32.2v MPPT or 16.1v PWM on 12 volt lead-acid batteries. The https://www.renewablewise.com/pwm-charge-controller-calculator/ tells me I need a 60 amp PWM controller, and the https://www.renewablewise.com/pwm-charge-controller-calculator/ tells me I'd need an 80 amp MPPT controller. So I'm looking at about $60 vs $360,

There are MANY 60A MPPT for < $200 on Amazon. I personally have an older version of this one:

https://www.amazon.com/Controller-12V24V48V-Battery-Regulator-Batteries/dp/B09S9PL6DS

depending on who you believe (I've seen "MPPT" controllers for as little as $15).

These are complete and total fakes. PWM controllers with MPPT in the company name or redefining MPPT in the description to mean Max current rather than max power. Total BS.

I've already spent more on this project than my better half would like, so is MPPT worth the extra money? If you were in my shoes, what would you do? I'd appreciate any advice and opinions, thanks.

I don't see that you disclosed the total wattage of the panels. The 60 and 80A controllers can handle the following amount of power:

60A * 14.4V = 846W
80A * 14.4V = 1,152W

If you buy PWM, the controller MUST be rated for MORE than the panels can deliver. PWM can't regulate current, only voltage. If you go MPPT, you can safely over-panel.

PWM will suffer a ~25% permanent penalty by forcing panels to work at battery voltage. This penalty is less severe in high temperatures because panel voltage decreases in high temperatures on either type of controller and the voltage decrease results in less power than rated.

Even with an MPPT, your panels will RARELY put out rated power, and when they do, it will only be for a couple hours per day. Most of the time, they will output < 90% rated just due to conditions, so if your panels might deliver a little more than the controller can use, you're likely losing very little juice overall.

You need to tailor your PV charging to battery capacity. Lead-acid can only be charged in a 0.1-0.2 range (depends on battery, check your battery datasheet), i.e., 10-20% of their rated capacity. If you actually had 60A of charging power, your batteries would need to be 300-600Ah.

Lastly, it's so worth investing in a quality battery monitor that actually counts current in and out and compares that to the programmed capacity. Many "% meters" just go based on voltage, and it's a complete lie.

 

[RKDowner]

I'm installing solar on my travel trailer and I'm on a low budget. I'm not trying to run A/C or anything stupid, I know my batteries won't do anything on solar they can't do on a generator, I'm just trying to make less noise and leave the generator at home. LiFePO4 batteries are expensive and I'd need to replace the factory power converter/distribution panel with one that can charge Lithium, thus I'm sticking with the existing lead-acid batteries.

This depends on the installed converter. The idea that one must replace their converter for lithium comes from the manufacturers looking to sell new converters.

Mine is 9 years old and the design is older than that, it's made for lead-acid. Not sure I'd trust it to lithium.

There are MANY 60A MPPT for < $200 on Amazon.

Yes, but few 80A. The recommendation for 80A was because of cold operation, but what you're saying is I could use 60 and in the cold the MPPT would just not use the "extra" juice. I can live with that.

depending on who you believe (I've seen "MPPT" controllers for as little as $15).

These are complete and total fakes. PWM controllers with MPPT in the company name or redefining MPPT in the description to mean Max current rather than max power. Total BS.

Oh, I totally understand that. They put MPPT in the model number but just looking at them you can tell they're PWM. I wouldn't touch those even as a PWM, probably burn my trailer down.

I don't see that you disclosed the total wattage of the panels. The 60 and 80A controllers can handle the following amount of power:

60A * 14.4V = 846W
80A * 14.4V = 1,152W

If you buy PWM, the controller MUST be rated for MORE than the panels can deliver. PWM can't regulate current, only voltage. If you go MPPT, you can safely over-panel.

PWM will suffer a ~25% permanent penalty by forcing panels to work at battery voltage. This penalty is less severe in high temperatures because panel voltage decreases in high temperatures on either type of controller and the voltage decrease results in less power than rated.

I bought six 100 watt panels. They're used, so I'm guessing 90 watts now.

So you're basically saying (as I stated above) that 60A is adequate and in cold weather the MPPT will just not use all the panels can give (for the rare case of a cloudless day with the sun directly overhead, which never happens in winter). Thanks! 60A it is, you saved me a bunch of $$$.

Lastly, it's so worth investing in a quality battery monitor that actually counts current in and out and compares that to the programmed capacity. Many "% meters" just go based on voltage, and it's a complete lie.

Thanks for that, too. I'll look into this as well.

 

[sunshine_eggo]

Mine is 9 years old and the design is older than that, it's made for lead-acid. Not sure I'd trust it to lithium.

Age is irrelevant, and 9 years old is a spring chicken. My youngest one is 16 years old. Charger profiles and voltages matter. Lithium will protect itself. The vast majority of installed converters will work with Lithium; however, there may be concessions.

Link the manual to your converter.

Yes, but few 80A. The recommendation for 80A was because of cold operation, but what you're saying is I could use 60 and in the cold the MPPT would just not use the "extra" juice. I can live with that.

There is no way you need anything near 80A. Extreme cold operation might get you 10% above rated. The most I've ever seen in +20% on my optimally tilted/oriented array NOT flat on a vehicle roof.

If more PV is available, MPPT will just not use it.

I bought six 100 watt panels. They're used, so I'm guessing 90 watts now.

So you're basically saying (as I stated above) that 60A is adequate and in cold weather the MPPT will just not use all the panels can give (for the rare case of a cloudless day with the sun directly overhead, which never happens in winter). Thanks! 60A it is, you saved me a bunch of $$$.

600W/14.4V = 42A
600/12V = 50A

Most of your charging will be around 13.6V:

600/13.6V = 44A

Again, it only does this in perfect conditions and for maybe ±30-60 minutes around high noon.

Factor in any age related degradation , and 40A is likely more appropriate.

Back to the battery bank size, this would suggest you need a 200-400Ah lead-acid bank.

 

[RKDowner]

Just a follow-up: After reading several posts here and elsewhere, and the various comments on Amazon, I decided to get the MakeSkyBlue 60 amp V119. I found it on Amazon for $130, free shipping. I found it on eBay for $108, free shipping from China, get here late June early July. And I found it on makeskyblue.com for $79 plus $30 shipping from USA. I don't know yet when it will arrive, but I'm sure it will be before the end of June. Just putting it out there that Amazon isn't always the best price, and I'd rather buy direct from the manufacturer anyway.

 

[RKDowner]

Age is irrelevant, and 9 years old is a spring chicken. My youngest one is 16 years old. Charger profiles and voltages matter. Lithium will protect itself. The vast majority of installed converters will work with Lithium; however, there may be concessions.

I only mention the age because it pre-dates lithium and wasn't designed for it

There is no way you need anything near 80A.

As I said above, https://www.renewablewise.com/mppt-calculator/ said I needed more than 60 amps, which put me in the 80 amp zone. But you've talked me down to 60, thank you.

 

[sunshine_eggo]

I only mention the age because it pre-dates lithium and wasn't designed for it

LFP has been around for more than 9 years. It doesn't matter that it wasn't designed for it. LFP is essentially a drop-in replacement for lead acid and requires similar charge voltages. Oddly enough, lead-acid converters can be more appropriate for Lithium than they are for lead acid - because there is variation between lead acid manufacturers. Trojan likes to charge to 14.82V, and you won't find a converter that does that.

Link the manual to yours.

As I said above, https://www.renewablewise.com/mppt-calculator/ said I needed more than 60 amps, which put me in the 80 amp zone. But you've talked me down to 60, thank you.

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As I showed

Just a follow-up: After reading several posts here and elsewhere, and the various comments on Amazon, I decided to get the MakeSkyBlue 60 amp V119. I found it on Amazon for $130, free shipping. I found it on eBay for $108, free shipping from China, get here late June early July. And I found it on makeskyblue.com for $79 plus $30 shipping from USA. I don't know yet when it will arrive, but I'm sure it will be before the end of June. Just putting it out there that Amazon isn't always the best price, and I'd rather buy direct from the manufacturer anyway.

above, that calculator is wrong if it's telling you you need more than 60A. The math is simple, and I demonstrated it above. How did I not talk you down to 40-50A?

Just a follow-up: After reading several posts here and elsewhere, and the various comments on Amazon, I decided to get the MakeSkyBlue 60 amp V119. I found it on Amazon for $130, free shipping. I found it on eBay for $108, free shipping from China, get here late June early July. And I found it on makeskyblue.com for $79 plus $30 shipping from USA. I don't know yet when it will arrive, but I'm sure it will be before the end of June. Just putting it out there that Amazon isn't always the best price, and I'd rather buy direct from the manufacturer anyway.

You don't have to convince me about amazon, but the above will bite you in the ass more often than not with cheap Chinese components. Sometimes it's worth a little extra to be able to return it to Amazon rather than get the run around from an ebay purchase from the manufacturer, repeatedly blaming the customer for the problem, asking for the same information over and over, unable to be convinced that something is actually wrong with the unit and eventually having to send it back for $50 shipping. The nightmares are prolific.