Lithium battery and the cold temps

[Scalf77]

The Portland area (as much of the country) just went thru a cold spell, this was a pretty good chance to collect some battery temp data. I have a Lithionics 555 Ah battery with a built in heater. I have the heater going thru a switch, this is unneeded at it is regulated on it's own.

I have about 290 watts of solar on top of the van, this time of the year it is pretty useless. In my location we can flirt with freezing temperatures but it not what I would call normal. When not using the van, I am usually plugged in, and have my Truma Combi set to around 45 degrees. The Combi can be set use propane or electric or a combination of the two, I have it set to electric only.

I generally set the Victron Multiplus II 3000/120 to charge only and set the charger to 2 amps. This will pretty much cover most of my power overhead as it sits in the driveway, the battery stays pretty much at the same SOC level.

So anyhow that was the goal going into this freeze.



In looking at the above chart you can see we already below freezing, and starting to drop in the teens (blue line). The heater was pretty much doing its job keeping the inside of the van around 40 (green Line). The issue I saw here was that the battery internal temp was coming down. Granted the outside temp was getting lower, but this did catch me a little off guard. I quickly enabled the battery heater and you can see the battery temp start to go back up. (yellow line).

Shortly after that, I lost power at the house. The east winds were high and it did not look like power was coming back on soon. I quickly ran an extension cord to the van and turned on the inverter. I had the blower for the fireplace insert connected, a charging station for phones and lights. About a day later I included the fridge.

After that you can see the battery heater go thru multiple cycles. Eventually you will see two high current inputs where the battery temps goes even higher. This was where I was idling the van and charging the battery with my secondary alternator. Luckily my power came back on two days later.

After getting power back, I left the heater off. here is another shot of the battery heater in action.

[Spr]

As always this is good info. I have a couple questions relating to low temp charging and adding a heater. I debated on sending you this as a PM because I don't want to hijack this thread but the questions relate enough that I decided to post it here and get feedback from all.

Like you we are experiencing below freezing temps in the Midwest (Single digit last week). I have 2-200 amp hour Victron Lithium batteries with Victron components. I usually store the van outdoors plugged into house power to keep the batteries full. The charging current usually keeps the batteries over the suggested 41 degree relay set point (over 41 degrees the relay is "open"). We were out of town for the last 12 days. Knowing the cold was coming I decided to unplug the van while away just to have one less thing to think about. When we returned home yesterday the battery level was at 27% and the battery temps were each around 20 degrees with the relay "closed"

My thinking was that I could plug the van in to prevent the batteries from draining further knowing that the BMS would not start charging the battery until they were over that set point. After a couple hours of that van being plugged in what happened is confusing to me and what sparked the idea to post these questions.

- The BMS showed that state of charge rose from 27% to 39% even though the batteries still showed that charging was disabled under the SL-22 under temp alarm

- The battery temperature rose from 21 degrees to 34 degrees over the time plugged in indicating that there was some current raising the temps

-The battery voltage was 13.35V instead of the 14V+ that would show if the battery was in the bulk charging phase indicating that there was no charge actually taking place

This was weird to me. Do you have any idea why the BMS would show that it was charging when it doesn’t seem like the battery was even accepting a charge? Also, why the battery temperature would rise if no charge was taking place?

Separately the last couple years I have been thinking I should install a heating pad that I can plug into my 120v system on really cold days when the charging current isn’t enough to keep the battery above freezing. I found a great pad here https://www.amazon.com/Facon-Silicon.../dp/B08JGCNBLM that only uses 60 watts and also has a Thermostat but it says it isn’t advised to be used on Lithium. Katz also makes a silicone pad like this sold on etrailer but it is more money and doesn’t have the thermostat but makes no mention of battery type. It seems like heat is heat, do you have any idea why one would be bad for Lithium and one wouldn’t??

Here are a couple screen shots from last night when the van was plugged in. I immediately unplugged it thinking that the BMS was allowing a charge to freezing batteries, it was only while writing this that I realized that a battery voltage of 13.35V would indicate that there was no charge entering the battery which changed the question to why the BMS would be showing that it was. Any thoughts on this would be more than welcome

[Spr]

I think I was actually wrong about something on the above post. Earlier today I warmed up the batteries using a space heater. Once they were able to take a charge I plugged the van. I just came home and saw that the batteries are now at 48 degrees with a battery voltage of 13.38 and 13.32 and I realized that because of the cold the 13.32V I saw yesterday was probably indicating a bulk charge given how cold it is (I forgot batteries charge at a lower rate in colder temps).

Now realizing this it could mean that despite the cold and the under temp alarm it could mean that the rise in battery temperature yesterday was because the batteries were in fact charging.

Hmmmm, sounds like something isn't working right. Any thoughts?

[Scalf77]

Ok, a couple of things here.

Your voltage levels while in "Bulk" are just fine. the 14.X setting is actually the target voltage for the absorption phase. So at a low SOC i would expect a lower voltage as the battery is excepting charge. As the SOC increases the voltage does. Also Lithium will have very steep curve at the end. When it reaches 100% it will be your 14.x absorption level. Lithium absorption needs are not very long.

Lithium does not have temperature compensation for charge voltages like lead. You may find some batteries that support lower charge (current rates) at low voltages, but there are not many chargers out there to do that.

Conceptionally the terms of the relay being open and closed seem reversed to me. Give me a list of what Victron gear you have, Batteries , BMS, and such. Victron does have some ATC ports (Allow to charge) that can be used to stop specific charging devices. They also ATD or allow to discharge for their Smart Battery Protect devices.


You'll notice that while I am plugged in pretty much all the time in my driveway. I rarely am charging with shore. In the summer I have shore charger in bypass mode, and use solar to handle daily consumable, in the winter I will set the charger to a 2 amp charge load. This can pretty much keep the battery idle 90% of the time.

I was kind of surprised at how much my battery temp dropped, even while heating the inside of the van to 45 degrees. The floor is well insulated and the battery is mounted on some insulated foam. Certainly charging increases temperature the most, just remember that in the summer time.

As far as the heating pads, many people are using some sort of pads for their battery installs. Falcon tank pads are very popular. I think the problem is is where the heat is applied, does create hot spots and not provide adequate coverage. They are probably just in CYA mode because some people have not been happy with the results.

So give me more details on your system ( feel free to PM it if you like) and we can dive a little deeper. Oh, and none of your screen shots showed up.

[Scalf77]

Pondering things while trying to sleep, I believe the battery temperatures continuing to drop as the outside temperature dropped but the internal van temperature remained the same is caused by the ground connection. In my case I am running a large 4/0 ground wire to my secondary alternator as my DC ground to van frame.

With no real charging or discharging going on, battery is pretty much idle, all current produced by charger is to cover consumable items such as Cerbo GX , Insty-Connect and other items to monitor the van.

I guess I could run some testing, but fortunately are cold wave is starting to break up.

[vlagat]

Fascinating insights that I have not seen posted on other sites. Thanks for the detail. My company sells, services and supports LiPo batteries for aerospace applications so I have some familiarity with the details you posted.
I/we concur that the charge voltage of Li batteries when cold is way below their nominal voltage and certainly below the Absorption phase. Anything from 13.0 and above is acceptable in freezing or near freezing conditions. In fact on our aerospace batteries we have to maintain their ambient and case temperatures at 40 or above to have enough current for an engine start (small turbine) as below that the battery is as likely to fail and could catch fire with that sort of load.
Maintaining your SoC is critical in our experience as determined by the SoC shunt. Not below 25% and if possible not above 80% for long term (>a week) non draining storage. If you have current drag of any kind, you need to keep it within those parameters which most RV chargers will not permit. So for our 12V (low tech) aircraft, we put timers on the chargers and review the charge time frame every day irrespective of the SoC. Warmer ambients result in less time (~5 hours) and colder (< 40F), we will go to 12 or 14 hours and let the chargers limit the current by their hard wired maximum capacity.

[marret]

Thanks for all Greg, much appreciated.

[Kibo]

Originally Posted by vlagat

Fascinating insights that I have not seen posted on other sites. Thanks for the detail. My company sells, services and supports LiPo batteries for aerospace applications so I have some familiarity with the details you posted.

Just so there's no confusion, the lithium batteries being discussed here are LiFePO4 (or LFP) batteries, not LiPo. They are different technologies and the operating constraints are different as well.