solar and Lithium batteries

[rbTN]

trying to understand the option of Lithium, but I keep thinking that with solar you can't just turn it off (unless you cover the panels) so how do you keep from the Lithium battery staying fully charged, which seems to be bad for them?

[JamesInTahoe]

Two easy options: 1) use a battery disconnect to disconnect the entire battery system when the rig is not in use (what I do) or 2) connect the solar to the batteries via a resettable circuit breaker that allows you to turn the solar off if you need to keep 12v power to other systems (like this: https://www.amazon.com/Bussmann-CB18...s%2C183&sr=8-8).

[DavidEM]

Originally Posted by JamesInTahoe

Two easy options: 1) use a battery disconnect to disconnect the entire battery system when the rig is not in use (what I do) or 2) connect the solar to the batteries via a resettable circuit breaker that allows you to turn the solar off if you need to keep 12v power to other systems (like this: https://www.amazon.com/Bussmann-CB18...s%2C183&sr=8-8).

Even though a solar panel doesn't need a fuse/breaker in between it and the controller (assuming the cable isn't woefully undersized), many install one just to be able to disconnect it from the batteries while the batteries still can continue to supply power to other devices.

David

[BCam]

From what I've read, this isn't an issue if you have a solar controller that with a "float voltage" of 13.6V or less. Many quality controllers have a selectable setting for LiFePO4 batteries that will take care of this. Check this write-up out under the "Charging" section:

https://www.solacity.com/how-to-keep...tteries-happy/

And there's more here:

https://battlebornbatteries.com/char...es-the-basics/

My take on all this is to rely on an appropriate float voltage from your charge controller in between uses during your RVing season and disconnect for long term storage. What you don't want to do is disconnect just your solar and leave the batteries connected to your RV where it will be subjected to parasitic loads (propane alarms, radio circuitry, etc.) that can deplete your charge over time.

If you go the complete disconnect route, it's best to do it with a proper combination of switches rather than repeatedly disconnect and connect cables which can cause issues by repeatedly flexing terminals or risking an inadvertent short.

[wolfgolden]

You can:

Customize the solar charge voltage settings.

Set solar charge controller for flooded lead acid and it should maintain about 75% charge.

Not worry about it and let the electronics do what they were set to do.

[vsheetz]

Float at 13.6v or less. Effectively little to no charging occurs. This is why many manufacturers spec this voltage.

I have a magnum shore/genny charger that will do CC/CV charge mode (no float) and a rebulk of 13.0v. Plus solar charging. So when on shore power the solar is providing all charging, unless the solar can't keep up. The batteries are cycled and not held fully charged.

Here's a pretty good article on the care and feeding of Lithium batteries.

https://www.solacity.com/how-to-keep...tteries-happy/

[astrocamper]

I have a battery disconnect switch between each of my solar panel groups and the MPPT controllers and a breaker/disconnect between the MPPT controllers and bus bars.

If I want to work on a de-energized system, I first turn of the solar disconnect switches and then turn off the main battery disconnect. I re-energize the system in the reverse order.

[Isaac-1]

My solution to this problem is storing my coach indoors, so the solar panels don't keep the LiFePo4 batteries fully topped off when it is not being used.

[darrellr]

Almost any off the shelf lithium aware controllers and the built-in BMS on most (all?) off the shelf lfp batteries manage this for you. No reason to overthink it.

[rbTN]

thanks all - have a better understanding now.

I didn't realize that 13.6 would not be charging them so that is where I was getting hung up I think

[hclarkx]

Originally Posted by rbTN

thanks all - have a better understanding now.

I didn't realize that 13.6 would not be charging them so that is where I was getting hung up I think

13.6 volts will take a LFP battery to over 99% state of charge. I first learned this from an engineer at a LFP manufacturer (Lifeblue) and not long ago performed an experiment to confirm this. I discharged a 200 Ah LFP to 90% and put a charger on it set to 13.5 volts. Seven hours later the current had dropped substantially and the Ah that went into the LFP was over 9%. As further confirmation, I then applied 14.4 volts (current limited) and it took about 1 Ah (0.5%) to get to 14.4V (effectively 100%). Any voltage above 13.4 V will take an LFP to over 99%; it just takes longer at the lower voltages.

There are two ways to get an LFP to around 50% SOC for storage. 1) Charge or discharge it to 50% and disconnect it. 2) Set a charger to charge to, and float at 13.1 or 13.2 volts. Most any solar controller with a "user" mode can be set to do this, though there are four or five settings that must be entered to achieve this.

Ideally one would have a solar controller with a suitable LFP mode AND a user mode and switch back and forth as needed. Some solar controllers reset the user mode to defaults if switched away from it. Also, some solar controllers with an LFP mode have rather bizarre parameters that I personally would not use.

The BMS is no help here. It only takes action if things get way out of hand (you are abusing the LFP). Even the user settable BMSs aren't suitable for anything more than preventing catastrophic damage.

[Isaac-1]

I would just say this about the previous post, floating at 13.1 - 13.2VDC for storage would roughly approximate storing at around 50%, by this I mean very roughly, you will likely be storing at somewhere between 40-70% state of charge, which is a lot better than storing at 95%+ state of charge. Don't get me wrong, it is probably as close to 50% that most of us can get without spending a lot of money, and is what I do with mine, I just want you to be aware this is not exactly 50%, or even likely all that close.

[hclarkx]

Originally Posted by Isaac-1

I would just say this about the previous post, floating at 13.1 - 13.2VDC for storage would roughly approximate storing at around 50%, by this I mean very roughly, you will likely be storing at somewhere between 40-70% state of charge, which is a lot better than storing at 95%+ state of charge. Don't get me wrong, it is probably as close to 50% that most of us can get without spending a lot of money, and is what I do with mine, I just want you to be aware this is not exactly 50%, or even likely all that close.

The thing I like about this approach is that I've got power in the RV without running a power cord that I would probably trip over. No problem if I leave lights on or forget to turn off the inverter when I leave. I have MaxxFans that are set in auto mode to help deal with the Sacramento (CA) Valley heat. As such there can be a bit of cycling of the batteries, but it's miniscule when it comes to life usage.

[cruizerEd]

You do not want to directly connect your panels to the battery. You need to use a charge controller which is programable and capable of charging your battery technology. This way you have no need to manually manage the battery charging.

These charge controllers can be programmed so keep your batteries fully charged without undue stress and optimize the power provided by your panels.

My batteries are LifeBlue in their "full charge" state have a voltage that is well short of any type of damage to the cells. Even so when I have no anticipated need to use the batteries I change the programming to keep them around 50% SOC. I do this to keep the system from going into the high voltages even briefly need to top off the batteries and to have the inverter/charger or solar provide most if not all of the 12v power.

The youtube channel DIY Solar has some good introductory videos and great product reviews.