looking for guidance on LiFePO4 protection

[bkfamily1]

I have been using a bare-bones solar system on an off-grid camper for a couple years but am upgrading to a (hopefully) better more reliable system with more capacity. I would like to share my thoughts and get feedback from the community. I recently purchased a Renogy 400W system with a 40A MPPT controller. It is installed with the (4) 100W panels in series. I want to be sure I am doing things in a way that I can go to 600W or 800W in a series-parallel arrangement in the future. I know this will start with a larger charge controller to handle the higher wattage. I am in a cold climate and depend on the system to keep the bare basics powered when I am away at times for weeks at a time. It is good if I never lose power on the camper, but it is not critical, especially if it is below freezing at the time. The system will see freezing temperatures. I am preparing to switch from lead acid to LiFePO4 batteries, and I want to be sure I properly protect my investment in the LiFePO4. I purchased an Eco-Worthy 280Ah LiFePO4.

From what I have read so far, I believe that I should maintain the charge between 12.5V (14%) and 13.3V (90%) to avoid shortening the battery life. I got that from reading various sources about “avoiding the shoulders,” and I verified the voltages that correspond to 14% and 90% in the battery manufacturer’s documentations. I also understand I should not charge below 32°F or discharge below 0°F.

The battery BMS has over- and under-voltage protection, but they are apparently at 14.8V and 8.0V. These seem too high and low and more for safety than protecting the battery life. Is this typical for the BMS, or is this because I bought a cheap battery?

I believe the charge controller can be programmed to avoid overcharging past 13.3V.

To avoid over-discharging below 12.5V, I think I can use a battery protection device like this https://www.victronenergy.com/battery_protect/battery-protect.

The BMS has high temp protection at 194°F, but again, it seems to be for safety, not for protecting the battery life. It does not have any low-temp protection on the charge or load side. Again, is this typical?

I am looking for a device that will disconnect the charging below 32°F and one that will disconnect the load below -4°F. Probably, that will be two of the same device, one on each side of the battery set at different temps, perhaps something like this, although I would need higher amperage https://www.ebay.com/itm/185814659317.

Any feedback would be appreciated, even if it is "let me better educate you" or “you shoulda bought a better battery.” Any advice on whether I have the right ideas to protect the battery or devices to do so would also be great.

Thank you for your time and expertise.

Bryan

 

[Rocketman]

You must protect the battery from charging below 32F - or you will destroy the lithium battery.

I use two levels of protection (actually three).

1. My Victron mppt gets the battery temp from the BMV712 (shunt based battery monitor w/temp sensor). Below ____F (user setable) it will not charge.

2. My bms doesn’t have low temp protection without shutting the entire battery down. But it does have a relay- that I use to run a heater when below about 45F. With the heater and insulation my battery has never been colder than 38F (when it was in the high 20’s).

3. My house has wheels - go south! (Baja is south).

Many of the FET based bms’s now have low temperature charging protection- I would not buy a battery without it.



Good Luck!

 

[Rocketman]

PS your lithium battery needs to spend some time fully charged so the bms can do cell balancing. Can you see the cell voltages on an app?

 

[mikefitz]

For low temperature control consider,

KSD301 Temperature Switch Control Sensor Thermal Thermostat 5°F-86°F/-15°C-30°C | eBay

KSD301 Thermostat Temperature Thermal Switch Normally Open/Closed NO/NC. Automatic reset. Application for household electric appliances, PC, microwave ovens, Irons, refrigerators, electronic oven, heating unit, coffeepot, water heater, and many more.

www.ebay.com

Needs no power and can interface to some chargers that have a remote disable.
As sugested the controler itself, Victron Smart MPPT, with additional sensor, can be programmed to disable charge at low temperature

believe that I should maintain the charge between 12.5V (14%) and 13.3V (90%

That's the resting voltage, for charging the voltage must be higher. Its holding the battery at a high voltage continuously that's considered to cause issues. Charging to a useful charge voltage, 14.0 or 14.2, will alow balancing and is not too stressful. If setting a float voltage 13.3 volts.

apparently at 14.8V and 8.0V. These seem too high and low and more for safety than protecting the battery life. Is this typical for the BMS

It's typical of badly engineered product. If not too late I would return the battery and select the a battery that has Bluetooth communication that will allow perameter modification if needed. Also low voltage protection or auto battery heating would be very desirable.

I can use a battery protection device

The Victron unit can disconnect loads, but is a 'one way' device so canot be installed in a path that has both discharge and charge flow.
It's also unsuitable for disconnection of inverter loads.

 

[aragonx]

Not sure which 40A charge controller came with your package. The Renogy Rover 40A charge controller that I have, has a battery temperature sensor that came with it. I believe, but can't be sure, that it would use that sensor and shut down if you were configured for lifepo4 batteries and it got too cold. I did just check the manual, and I don't see any reference to it stopping the battery charge when the battery is too cold.

Additionally, I was able to hook up 6 100 watt panels to my 40A controller but beware! Mine SAID it could go up to 100 volts. I had 4 panels in series once that totaled 95 volts. It triggered an over voltage error. Apparently, 95 volts is the real maximum. Found that out from their support. Which was pretty responsive.

 

[bkfamily1]

Thanks, Rocketman. I agree about getting a battery with the low temp built in for future battery purchases.

I like the heater idea.

I am also getting the idea that the Victron products are quite good. When I upgrade in the future, I will look more closely at them.

 

[Sat Manav]

PS your lithium battery needs to spend some time fully charged so the bms can do cell balancing. Can you see the cell voltages on an app?

When you say "full" do you mean that 90% mark or are you saying you should charge all the way to 100%? We just bought LifePO 4 batteries for the first time and are also trying to determine if we need to keep them below 90ish%.

 

[bkfamily1]

mikefitz, thanks for the temperature switch link - I was thinking something like that to control a small heater like rocketman controls from his battery relay

thanks also for the clarification on the charge vs resting voltages - I was wondering about that

 

[bkfamily1]

aragonx - I have the same controller - I spoke to Renogy - the temperature sensor does not shut down the charging at low temp - they said their batteries have low temp protection built (like rocketman suggested we should look for in a battery), so they do not provide for it in the charge controller

 

[bkfamily1]

So right now, my thoughts...

Go back and see if I can change direction on the battery and get one with low temp protection and high/low voltage protection and low temp battery heating

If I can't find that (or can't return what I bought or don't want to spend the $), then...

Use the temp switch to power a heater

Use a Victron Battery Protect for low voltage protection

Adjust the charge controller to avoid high voltage/overcharging

 

[aragonx]

So right now, my thoughts...

Go back and see if I can change direction on the battery and get one with low temp protection and high/low voltage protection and low temp battery heating

If I can't find that (or can't return what I bought or don't want to spend the $), then...

Use the temp switch to power a heater

Use a Victron Battery Protect for low voltage protection

Adjust the charge controller to avoid high voltage/overcharging

My Renogy charge controller did shut down on over voltage and also did something when the battery was over discharged. Honestly, I don't know what it did but it did notify me. And it was able to "kick start" the battery. I've read that they are hard to get started once they have shut down for over discharge. So, if you can't return what you have. It might be workable if you can insulate your battery or move it to a location where it won't freeze.

Since my last reply, I've upgraded to an all Victron system with a battery shunt. Moved my batteries to the basement when it will never freeze. I'm hoping that will be enough.