Can't charge lithium batteries when cold?

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Comments

  • NANOcontrol
    NANOcontrol Registered Users Posts: 260 ✭✭✭
    I've heard that 12V RV heater mats to keep a water heater from freezing work well for batteries and have built in thermostat.
  • JRHill
    JRHill Registered Users Posts: 277 ✭✭✭
    Off grid, plugging a load in to heat batteries is a non-starter. Yup, the stuff is out there. But no one has done a cost justification of Kw out of the battery vs benefit of keeping it above 34f and how big of a battery it takes to do that so you don't run low in the house to keep the battery warm. Duh.. Oh, and for those of us that have to charge because there is no sun and run a generator in the off season?! This is insanity. Run the generator to fill the batteries and the resistance blanket under the batteries? This is the stuff of weekenders.
    Off Grid. Two systems: 1) 2925w panels, OB VFXR3648, FM80, FNDC, Victron BMV-712, Mate3s, 240 xformer, four SimpliPHI 3.8; 2) 780w, Morningstar 30a, Grundfos switch, controller and AC/DC pump, 8 T105. Honda EU7000is w/AGS. Champion 3100. HF 4550, Miller Bobcat.
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    edited September 2022 #34
    Coyyote said:
    Has anybody had any experience using thermostatically controlled mats used for germinating seedlings?  Relatively inexpensive and use 20-40 watts. Thought about strapping them onto the side of the battery. 
    I've done it. The details are in another forum. Not sure if it is OK to post a link here. Bill ?

    Summarizing: I built an insulated battery box with electronic thermostats, and a couple of small 12W 12V heating pads (my battery is 24V). I tested it in some pretty extreme temps over last winter, and it was able to keep the LiFePO4 cells between 50°F and 60°F with less than 5Ah (@ 24V) per 24 hr period. I had a couple of fail-safes, in that the heater won't run if the battery is below about 40% SoC (but can run direction off the SCC in that case), and the BMS has a low-temp cutoff that will prevent charging if the cells get down to 35°F.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Horsefly (and everyone else),

    Links to other forums are fine. We are here to help spread good and useful information.

    -Bill "moderator" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    Ok, thanks Bill.

    Here's a thread where I posted my battery upgrade from lead acid to LiFePO4 (the first post was the original system in 2017 that was already posted here, everything after is about the upgrade):

    https://diysolarforum.com/threads/horseflys-cabin-solar-lifepo4-upgrade.27472/

    The details are in that thread, but I'll post some of the main stuff here.  Some main takeaways:
    1. The physics tells us that how heat escapes the box you have the cells in is the main issue. If there is no heat escaping, you would just have to heat it once over the summer and leave it alone. But that isn't the case. Insulation is pretty important if you want to minimize how much energy you use to heat.
    2. Keeping LiFePO4 warm is a bit like smoking a good pork shoulder: Low and slow wins the day. I only used 24W of heating pads for my roughly 2'x2'x2' box. With the temperature outside the box in the low teens or single digits, it took a bit over 3 hours to warm the cells from 50°F (where the thermostat turned ON) to 60°F (where the thermostat turned OFF). But that is only 3Ah pulled from my 230Ah 24V battery.
    3. The thermal mass of the cells is high enough that it takes quite a while for them to cool off. After warming the cells to 60°F, it took between 15 and 21 hours for them to cool back down to 50°F.
    Here's a pic of the not-quite-completed battery box, with 2" XPS on all sides and on the bottom (the lid is not shown, but it also has 2" XPS). The aluminum plate on the bottom has the two 12W 12V heating pads under it.


    Here's a graph of the multi-day test when the temps were in the single digits and low teens outside the box.  


    Here the green line is showing when the power was applied to the heating pads. The blue line shows the temperature on the aluminum plate under the cells, and the red line shows the temperature on the top surface of the cells. I had originally worried that the pads might heat up too fast, which would cook the bottom of the cells before the top got warm. But by using just 24W of pads the bottom didn't heat up all that much.

    Bottom line: Keeping LiFePO4 cells warm enough to operate through sub-freezing temps is not really difficult, and doesn't consume all that much energy. Even if my panels get covered with snow, I think my setup can keep the cells warm and toasty for a least a couple of weeks.


    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    A couple of other things worth mentioning.

    There are many folks who have done similar things to keep their LiFePO4 cells warm: Insulated box, heating pads, thermostat.  As was mentioned earlier, some of the assembled LiFePO4 batteries also include a heater (at a much higher price than DIY).

    Also, if you are not going to be attempting to charge LiFePO4, it is perfectly fine to allow discharge or storage of cells at well below freezing.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,728 ✭✭✭✭✭✭
    And also mention, as little as one charging session below 32F can ruin an LFP that you paid alot of money for.

    You do not want to know the stories, so build in the failsafe's as Horsefly has. Or,  verify the failsafe's are there and have a reputable manufacturer that will stand by you.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

  • Graham Parkinson
    Graham Parkinson Registered Users Posts: 160 ✭✭✭
    Dave is so right - I can't believe that some otherwise high end LiFePO4 batteries such as the Discover AES don't yet implement self heating. 

    Many do have BMS's that inhibit charging in cold conditions, but so much better to have integrating heating since when you have energy to charge you should have energy to heat as well.

    Offgrid in cloudy PNW

    MacGyver'ed museum collection of panels, castoff batteries and generators - ready for state of art system install .... parade of surviving and dead generators: H650, Ryobi 900, Briggs and Scrap Iron 2000, H2200, H3000, Kubota 3500, Kubota 4500, Onan 7500

  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,728 ✭✭✭✭✭✭
    edited September 2022 #40
    Thanks but I would not want heating built-in for several reasons. The AES battery does shut down the positive terminal when cold and logs everything for warranty BTW.   People in cold places need to keep a power system at a decent temperature if they care about reliable AC power. The electronics do not like moisture forming on components also.

     Candles are also an option😉
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

  • Graham Parkinson
    Graham Parkinson Registered Users Posts: 160 ✭✭✭
    Discover told me that their latest AES / Link II firmware will have logic built in that could be used to trigger external heating via a relay port.

    Offgrid in cloudy PNW

    MacGyver'ed museum collection of panels, castoff batteries and generators - ready for state of art system install .... parade of surviving and dead generators: H650, Ryobi 900, Briggs and Scrap Iron 2000, H2200, H3000, Kubota 3500, Kubota 4500, Onan 7500

  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,728 ✭✭✭✭✭✭
    That is a good failsafe! 

     Still would rather have the battery room well above refrigerator temps and not cycling temperature for offgrid full-timers.
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net