Solar controller for LiFePO4 question

[reilym]

I am stepping through upgrades on my 2016 Pleasureway Lexor TS class B and have come to the solar charging system.
The RV came with two 100 Ah LiFePO4 batteries, three 95 watt roof panels and a GoPower 30 Amp PWM SCC without a LFP charging profile. The manufacturer says to use the AGM profile, which sets bulk/absorption at 14.4V and float at 13.7V (no equalization). That SCC also has a GEL setting that sets bulk/absorption at 14.1V and float at 13.7V. Eventually, I want to upgrade to an MPPT controller, but for various reasons that might take a while. In the meantime, should I continuing using the original SCC with the recommended settings (14.4V/13.7V) or purchase an inexpensive PWM SCC that has a Lithium profile (thinking 14.2V with no float)? I realize that float is not necessary, but is there a negative impact of using a float voltage of 13.7 and is 14.4V too high for bulk/absorption?

 

[LakeHouse]

Given those options for the current PWM, I would change to the GEL profile. The 14.1 absorption is a pretty good balance of SOC usage and easy on the cells. The 14.4 is on the high side, but not entirely unreasonable.
I don’t think I’d bother ‘upgrading’ to a different PWM. Save your money and stick with what seems serviceable now until you upgrade to a MPPT, which will give you more bang for your buck.

 

[time2roll]

14.4 is the top of fine. 13.7 is a bit high but is OK with solar that is about off 12 hours a day.
GEL is fine too if there should be any battery shut down issues.

Not that a "lithium" controller is needed but most any that have the option to adjust the setting manually.

I set mine at 14.2 and 13.2

 

[MisterSandals]

Not that a "lithium" controller is needed but most any that have the option to adjust the setting manually.

Agreed. Is there a user defined battery type option in your SCC? You can probably do better than a specific lithium profile with a user defined battery profile.

Otherwise, if your batteries accept a charge to 14.4V without any BMS cutouts from cell over voltage, then you can proceed as you are setup now. If its working, not sure i'd try to fix it...

 

[reilym]

Your replies put my mind at ease, thank you. Unfortunately, there is no option to define charging profiles on the existing SCC.

From what I have read here and elsewhere, I understand why LiFePO4 should be charged with a fixed bulk/absorption voltage 14.2 +/-, but the opinion seems split on whether to float in the low 13's or have no float. What are the pro's and con's of using a float voltage and why low 13s? I suppose that when using a float on a full battery, loads can pull from the charger and not the battery?

If the BMS prevents overcharging, why is a profile necessary at all - i.e. why not just apply 14.2 and forget about it? Seems like so much effort has been put on applying lead-acid battery theory to lithium, but it really is an entirely different beast.

 

[time2roll]

No one recommends holding LFP indefinitely at 100% charge. This is the direct opposite of lead-acid. This why the float down in the working zone is normal. No float has the same effect. This allows the LFP to cycle some and reduce the time holding at 100%.

The lower float will allow the battery to cycle and at some point the float will help support the load. Otherwise the low float is essentially off if the LFP never depletes to that level.

If the LFP is primarily maintained by solar and is marginal to get through the night 13.4 will hold very close to full without stressing the battery. With a very adequate battery a lower float just starts the discharge cycle as soon as the charge cycle is complete.

This not the same as lead-acid that has a working range much lower than LFP. (11.8 - 12.7). This lead needs to stay 100% for as long as possible and will be held at 13.2 to 13.8 depending on conditions.

 

[reilym]

This why the float down in the working zone is normal. No float has the same effect. This allows the LFP to cycle some and reduce the time holding at 100%.
The lower float will allow the battery to cycle and at some point the float will help support the load. Otherwise the low float is essentially off if the LFP never depletes to that level.
If the LFP is primarily maintained by solar and is marginal to get through the night 13.4 will hold very close to full without stressing the battery. With a very adequate battery a lower float just starts the discharge cycle as soon as the charge cycle is complete.

That's helpful. In my case, the battery is more than capable of getting us through the night and solar is only supplemental, so no float would be OK. The only option on my older GP SCC is a 13.7 float which is 0.4-0.5 volts than the no-load voltage of my fully charged LFPs . Interestingly, the newer Go Power PWM controllers have an LFP option but it's fixed at 14.4 with a 14.0 float - not good based on this discussion. If I understand correctly, floating at 13.7 volts will not achieve the battery cycling that you referred to or is it harmless? For $50 the Renogy Adventurer would allow me to select whatever bulk voltage I want with no float and would fit in the same spot as my existing SCC. Open to other suggestions on SCCs.

 

[reilym]

Otherwise, if your batteries accept a charge to 14.4V without any BMS cutouts from cell over voltage, then you can proceed as you are setup now. If its working, not sure i'd try to fix it...

Agree with you that "if it ain't broke, don't fix it", which is why this has been down on my list of things to do. Part of this is trying to better understand how all these systems in the RV work. My model year was the first that Pleasureway went from AGM to LFP. In 2016, LiFePO4 in RVs was fairly new and there are lots of carry-over systems from the lead acid platform that I am re-engineering.
What exactly does a BMS cutout look like? Does the battery shut down or might it be more subtle? I can't query my battery's BMS.

 

[MisterSandals]

Open to other suggestions on SCCs.

I am wondering what your use case is for your batteries and solar system. Are you full time RVers or weekend warriors or ?

The reason I ask i because if you are full time RVers, the concept of float is rather moot because you probably have some continual loads. With continual loads, you would not be floating much at all so its rather moot.

If you have an RV sitting in the driveway for many days or months, then floating, especially at a higher voltage is a real concern. My RV sits for weeks and sometimes months. I simply turn off the array at 13.2V and check on it every week or two. I turn on solar for a day or two when it gets around 13.0V or lower. I go in the RV to putz around fairly often so its not a concern that i forget about it.

So, what's your use case?

 

[reilym]

Given those options for the current PWM, I would change to the GEL profile. The 14.1 absorption is a pretty good balance of SOC usage and easy on the cells. The 14.4 is on the high side, but not entirely unreasonable.
I don’t think I’d bother ‘upgrading’ to a different PWM. Save your money and stick with what seems serviceable now until you upgrade to a MPPT, which will give you more bang for your buck.

I would hate to replace one PWM with another unless it's to make it easier on my batteries. One of my hold-ups in going to MPPT is that the 3 parallel panels on my roof are also older and relatively low voltage (Vmp 17.5). Since an MPPT requires that the panels be at least several volts higher than the battery bank, I am not sure that I can put one on without re-wiring the panels in series. Given that I am under trees a lot, I am trying to figure out if the partial shade losses with series panels would negate the benefits of the MPPT. Thoughts on that are welcome too.

 

[reilym]

The reason I ask i because if you are full time RVers, the concept of float is rather moot because you probably have some continual loads. With continual loads, you would not be floating much at all so its rather moot.

If you have an RV sitting in the driveway for many days or months, then floating, especially at a higher voltage is a real concern. My RV sits for weeks and sometimes months. I simply turn off the array at 13.2V and check on it every week or two. I turn on solar for a day or two when it gets around 13.0V or lower. I go in the RV to putz around fairly often so its not a concern that i forget about it.

So, what's your use case?

It's an excellent point. The RV lives in a barn when we're not using it. When we are using it, we're charging with the alternator (though a DC-DC charger - one of my first improvements) and solar or plugged into shore power. Occasionally we boondock but only for a few days at a time (my wife likes her hair dryer). Sometimes, we hang in a hotel for a week with the RV sitting in the parking lot with the roof fan and a couple of other light loads on. My buddy and I have been planning an off-grid trip that would make the solar much more relevant.

 

[time2roll]

My experience is with Morningstar controllers where nothing is a low cost leader. About $100 minimum.
Renogy seems fine for what is needed. Although verify the functions are available. IIRC there was a renogy with user settings in the manual but they were for a future model not available yet.
Leave the panels in parallel. Nothing wrong with PWM for anything under 500 watts.

 

[MisterSandals]

The RV lives in a barn when we're not using it.

Well all this talk about float voltage is pretty much for nothing then. I am back to "if it ain't broke don't fix it".

My buddy and I have been planning an off-grid trip that would make the solar much more relevant.

I'd put it thru its paces a little before the trip to verify your existing setup does what you need it to do.
If you need more solar, i'd consider a 4th panel, configure in 2S2P and get an MPPT SCC.

 

[reilym]

My experience is with Morningstar controllers where nothing is a low cost leader. About $100 minimum.
Renogy seems fine for what is needed. Although verify the functions are available. IIRC there was a renogy with user settings in the manual but they were for a future model not available yet.
Leave the panels in parallel. Nothing wrong with PWM for anything under 500 watts.

Morningstar looks to be top notch, but pricey. I am not a big fan of Renogy's documentation or customer support, but the Adventurer is a flush mount (like my existing) and the current model does appear to allow selecting what bulk/absorption voltage to use. The BlueSky SC30 ($100) appears to be fully programmable and is flush mount.

 

[reilym]

After reading the Blue Sky SC30 manual, I liked it so much that I ordered one. It should fit right into the existing cabinet cutout. One nice thing is that they don't have fixed profiles for different battery types, but rather allow for the user to select values for every parameter. Not a big fan of the red LED display, but it autodims at night and can be turned off entirely. This is good since it's right at the foot of my bed.

 

[reilym]

I finally have the BlueSky SC30 installed. I had to slightly enlarge the hole in the panel where the Go Power SCC was. While I had the saw out, I added a DC breaker so that I could disconnect the PV + input when necessary. The unit is very easy to program and all parameters are available. One bonus of this effort was to discover a significant voltage drop across an inline blade fuse that was installed by Pleasure Way. The total drop was 0.8V at 13.5 amps. Swapping this out for a MIDI fuse reduced this to 0.4V at 15 amps. Customer service at BlueSky is exceptional and Ryan Gurin at BlueSky was extremely helpful with setup and troubleshooting the voltage drop.