MPPT Confusion

[Donager]

Hi folks. I'm new here and drinking through a fire hose.
I've been doing some searching and I don't really understand enough to know what is meaningful and it would help if someone could get me pointed in the right direction.

I have a Grand Design Transcend Xplor travel trailer. it has Furrion solar equipment, including a 165-watt panel and a 25A MPPT. it has a dealer-installed lead acid battery and I want to upgrade the battery to a 230ah LiFePO4.

What I don't understand is how the MPPT impacts the system. I know it has a setting to allow me to install the LiFePO4 by dropping it in and swapping out the cables. How does an MPPT impact the charging of my battery? Do I need to upgrade? Can someone point me to some specific literature or videos that will allow me to head down the correct rabbit hole?

I appreciate the help

 

[sunshine_eggo]

LFP is essentially drop-in replacements for lead acid in most cases.

Link the manual to your MPPT.

Any charger that can charge the LFP to 13.8-14.4V and float at 13.5V is acceptable, and there's some wiggle room.

 

[Donager]

Thank you for taking the time to reply.

Downloadable Contents for Furrion's Energy products

Download Spec Sheets and User Manuals for Furrion's Energy products.

furrion.com

It is the Wall Mount MPPT Solar Charge Controller. I can't find a direct link. It links to the spec sheets. Mine is the 25A, though the instructions are the same for 25A and 50A

 

[sunshine_eggo]

Thank you for taking the time to reply.

Downloadable Contents for Furrion's Energy products

Download Spec Sheets and User Manuals for Furrion's Energy products.

furrion.com

It is the Wall Mount MPPT Solar Charge Controller. I can't find a direct link. It links to the spec sheets. Mine is the 25A, though the instructions are the same for 25A and 50A

https://cdn.accentuate.io/6641527586999/5237057552473/IM-FEN00012_V5.0_Multi-v1617097026099.pdf

Not the best options:



Lithium is okay, but 13.6V is considered a little on the high side. 13.5V is preferred. Lithium mode also should disable temperature compensation. You should likely remove any installed temperature probe to ensure temp comp is always disabled.

AGM is also desirable for the lower float voltage.

One thing you'll need to do is bench charge the new battery. It's unlikely to tolerate charges to 14.6V without triggering BMS protection due to cell imbalance. You may find this results in a big voltage spike as the BMS cuts charging while the MPPT is still sending current. The MPPT can't clamp down on the voltage quickly enough to prevent a spike. Your inverter might object to seeing this voltage. Here's a recent thread discussing that issue:

Mysterious battery voltage spikes

Hey Guys, I was hoping that someone could give me an idea as to these battery voltage spikes. No inverter in play at the time. see image below. The 'Charging Status' reading "Not Charging" was there before I disconnected the PV. But, I could be wrong. Should have left it on before taking the...

diysolarforum.com

If you're going to charge/cycle daily, the sealed setting might be best, but the 13.2V float might consume more battery during the day as the MPPT is waiting for the voltage to drop before it starts charging.

 

[Donager]

If I am understanding this correctly, I should probably look at upgrading my MPPT if I want to move to a 230Ah LiFePO4. Am I correct in that?

 

[sunshine_eggo]

I can't give you a definitive yes or no. It's not the best solution for LFP, but it's not a bad one. It may work fine, or you may notice some odd behaviors. It's kinda a "try it and see" situation. Will it cause major problems or harm something? No.

Now the question becomes, what are you doing with AC-DC your converter? Have you succumbed to marketing and already plan to replace it? Does your current model support lithium?

 

[Donager]

I wasn't planning on replacing anything. I have an existing system from the factory that works just fine, but my battery doesn't last as long as I need it to if I get shade. My hope was I could simply swap out my existing lead acid battery for a 230 Ah LiFePO4 with no changes to the system.
That led me to wonder what exactly an MPPT did and how it is impacted by changing my battery out. The documentation said it was compatible with a LiFePO4, but I didn't know if a larger battery needed a new MPPT.

This entire thread is based on my solar ignorance, and I am struggling to wrap my head around it.

Now the question becomes, what are you doing with AC-DC your converter? Have you succumbed to marketing and already plan to replace it? Does your current model support lithium?

this is what my owner manual says about the converter

Converter
The Power Converter changes 120-volt AC power to usable
12-volt DC power when the shore power cord is connected to
an external power source. A thermal breaker is built-in to the
converter to protect it from overheating.
Overheating is usually caused by the converter operating above
its maximum power output for an extended time period, or with
too little air flow.
• To reduce converter heat, DO NOT run any unnecessary
12-volt lights/motors/appliances, and
• Keep the converter cooling fins and fan
clear of obstructions.
Inspection and maintenance
If the 12-volt power converter is NOT working, or
the auxiliary battery is not being charged:
1. Locate the converter fuse panel on one end
of the converter.
2. Check the reverse polarity fuse or fuses.
The manufacturer’s warranty will be void if the case has been
removed. There are no customer serviceable parts

 

[sunshine_eggo]

I wasn't planning on replacing anything. I have an existing system from the factory that works just fine, but my battery doesn't last as long as I need it to if I get shade. My hope was I could simply swap out my existing lead acid battery for a 230 Ah LiFePO4 with no changes to the system.
That led me to wonder what exactly an MPPT did and how it is impacted by changing my battery out. The documentation said it was compatible with a LiFePO4, but I didn't know if a larger battery needed a new MPPT.

It is important to match lead acid battery capacity with charge power. LFP (LiFePO4) is much more tolerant of both higher and lower charge rates than lead acid, so swapping lead out for a 230Ah LFP battery doesn't mandate you replace your MPPT.

Worth noting:
The amount of solar power and the solar conditions determines how much power you can use sustainably every day.
The amount of battery you have determines how long you can go between charges.

So, if you're running out of capacity on a daily basis, then the bigger battery is only going to prolong the inevitable. Using typical numbers, your panel/controller can only produce 165W * 4h = 660Wh/day. A 230Ah LFP battery has 230Ah * 12.8V = 2944Wh. That means it would take 2944Wh/660Wh/day = 4.5 DAYS to fully charge your battery from empty assuming you use ZERO energy during the recharge period.

Please note that this is in very good conditions - which aren't typical of flat panels mounted on an RV roof. Winter production can be as little as 30-40% of summer production.

Many converter manufacturers insist that converters be replaced if you install LFP. So, much like you're worried about replacing your MPPT, the converter is a charger too, so you should be concerned there as well. In many cases, it does not need to be replaced. It's just important to understand how it will behave relative to lead acid.

Do you have the make/model of your converter?

 

[Donager]

Thanks for the explanation. 230 Ah is overkill for me. I would be perfectly fine with a 100Ah battery. I can add another panel and improve the charging time. I don't need a lot. I ran into a problem where my 12v fridge wouldn't stay cold because of the power draw. I was in a shaded area and running a generator intermittently. I wanted a large enough battery to last when I couldn't run a generator.

The converter appears to be a WFCO brand and is one of the following:

Details for CONVERTER CHARGER W/DISTRIB CTR 30 AMP AC 55 AMP DC

legacy.granddesignrv.com

Details for CONVERTER CHARGER W/DIST CTR 30 AMP AC 55AMP DC AUTO DETECT (GE BREAKERS)

legacy.granddesignrv.com

I really appreciate your help.

 

[sunshine_eggo]

Thanks for the explanation. 230 Ah is overkill for me. I would be perfectly fine with a 100Ah battery. I can add another panel and improve the charging time. I don't need a lot. I ran into a problem where my 12v fridge wouldn't stay cold because of the power draw. I was in a shaded area and running a generator intermittently. I wanted a large enough battery to last when I couldn't run a generator.

Want to confirm that your 12V fridge isn't a 3 way that can use 12V, 120VAC and propane. If it's that type, you need to run it with propane. They use an insane amount of power.

The 25A MPPT should be good for 25A * 14.4V = 360W of solar.

The converter appears to be a WFCO brand and is one of the following:

Details for CONVERTER CHARGER W/DISTRIB CTR 30 AMP AC 55 AMP DC

legacy.granddesignrv.com

Details for CONVERTER CHARGER W/DIST CTR 30 AMP AC 55AMP DC AUTO DETECT (GE BREAKERS)

legacy.granddesignrv.com

I really appreciate your help.

WFCO have some downsides in that they rarely hit bulk voltage (around 14.4V) when charging LFP because LFP voltage is higher than lead acid - the WFCO thinks the battery is full since it's expecting lead-acid.

 

[Rednecktek]

including a 165-watt panel and a 25A MPPT.

That's going to be your biggest issue. The MPPT will work fine, and a larger battery will let you roll longer, but if it takes a week to charge the battery it's a net loss.

The 25A MPPT should be good for 25A * 14.4V = 360W of solar.

If it's flat on a camper I would recommend scrapping that 165w panel and getting a pair of 200w panels at least up there. That'll max out your potential charge and still leave you a bit of head room for flat panels and shade. Even a 100Ah battery is 1280wh / 165w = 1.7hrs of perfect clear directly straight on sun.

 

[Donager]

Want to confirm that your 12V fridge isn't a 3 way that can use 12V, 120VAC and propane. If it's that type, you need to run it with propane. They use an insane amount of power.

The 25A MPPT should be good for 25A * 14.4V = 360W of solar.



WFCO have some downsides in that they rarely hit bulk voltage (around 14.4V) when charging LFP because LFP voltage is higher than lead acid - the WFCO thinks the battery is full since it's expecting lead-acid.

The fridge is 12v only. It does not run on propane or 120 at all. The internet says it burns about 80Ah a day.

I have room for additional panels on the roof, so I will look into that. The 100Ah will get me through till I am on a genny and can charge the battery.

I may have to look into planning a more capable solar system than what I have.

 

[sunshine_eggo]

The fridge is 12v only. It does not run on propane or 120 at all. The internet says it burns about 80Ah a day.

Good to hear, but that sounds pretty inefficient. How big is it? We have a 10.1 cu-ft 120VAC fridge we used to replace our absorption fridge, and it uses about 0.6kWh or 50Ah @ 12V per day.

Assuming optimal summer performance, the 165W panel can only produce about 2/3 of the needed energy to power your fridge for 24 hours.

Whatever the case, a 230Ah LFP battery should help a lot as that should power the fridge for about 3 days from a full charge.

Obviously every ounce of juice you use for anything else will shorten that.

I have room for additional panels on the roof, so I will look into that. The 100Ah will get me through till I am on a genny and can charge the battery.

Just make sure that you don't exceed the 70V limit on that charge controller. That means no more than 2 12V (20-22Voc) panels in series. If you need more than about 500-600W, you'll want to replace the existing MPPT or add another.

I may have to look into planning a more capable solar system than what I have.

Beats the heck out of running a generator.

Something else to consider is to fabricate lightweight frames from pvc for lightweight flexible panels that you can put in parallel with your existing panel as a deployable array that you can tilt and position for optimal harvest.

 

[Donager]

Thanks!

I'm looking at the 400 watt setup on this site and contemplating that.

I'll do some more research and bring any questions back. I appreciate your patience with my ignorance. You've helped me understand what I need to start looking at and it will make the education process a bit easier.

 

[sunshine_eggo]

Thanks!

I'm looking at the 400 watt setup on this site and contemplating that.

I'll do some more research and bring any questions back. I appreciate your patience with my ignorance. You've helped me understand what I need to start looking at and it will make the education process a bit easier.

It's a constant learning experience. Until one has an Electrical Engineering degree AND are a certified electrician fluent in all aspects of the electric code, no one can say they know-it-all...

Please read the disclaimer in my signature...

 

[Rednecktek]

It's a constant learning experience. Until one has an Electrical Engineering degree AND are a certified electrician fluent in all aspects of the electric code, no one can say they know-it-all...

Naah, between @sunshine_eggo and I we know everything.

Grounding? That's a Sunshine question. ?

 

[sunshine_eggo]

Naah, between @sunshine_eggo and I we know everything.

Grounding? That's a Sunshine question. ?

Nope. @timselectric is the grounding master.

I know enough to be dangerous.

 

[glandpuck]

Batteries store power in the form of watts. This is also expressed as volts x amps. A battery can discharge fast or slowly. This is usually expressed as the C1, C2 or C20 rating. Standing for 1 hour, 2 hours and 20 hours. What an MPPT charge controller does is to take advantage of computer software built into the charger. When PV is present, the charger receives a raw voltage from the panel and an amperage. But the batteries may charge faster if the voltage is optimal for them and the amperage can be increased.

For example, suppose you had a 48 volt battery that was to be fully charged at 55 volts. Your panels may be bringing in 80 volts and 20 amps. But the 80 volts is wasted, it's too high for your batteries. The battery likes say a max voltage of 64 volts, so the MPPT program reduces the output voltage from 80 to 64 and increases the amperage from 20 to 25. As a result, your batteries charge faster and more efficiently than not having an MPPT controller.

In order for an MPPT controller to be useful, you need to have voltages usually about 25 percent or higher above the battery charging voltage so the program can work best. Charging a 12 volt battery with 16 volts is of no help with MPPT. Charging a 12 volt battery with 30 volts is very good. Understand?

 

[sunshine_eggo]

Batteries store power in the form of watts.

Nope. Watt-hours.

Watts is a power unit - a rate of energy consumption. Watt-hours is a unit of energy. Batteries store energy, not power. You can't store power.

In order for an MPPT controller to be useful, you need to have voltages usually about 25 percent or higher above the battery charging voltage so the program can work best. Charging a 12 volt battery with 16 volts is of no help with MPPT. Charging a 12 volt battery with 30 volts is very good. Understand?

Funny how you say it needs to be about 25% higher, then document a 33% higher voltage as an example, but say it's of no help.

It is absolutely, positively of help. When PV can work at their Vmp, you get the full benefit of the MPPT. Charging at higher voltage can actually reduce MPPT efficiency due to the greater DC-DC conversion step. This CAN be offset by reduced wiring losses, but there's no compelling reason to push Vmp higher unless the system design warrants it.