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Old 09-21-2021, 11:41 AM   #1
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Why is my LiFePO4 BMS going into protect mode?

I installed a SOK 206 Ah LiFePO4 (LFP) a while back and it worked great. I had used cheapo cutoff switches and circuit breakers, so I swapped those out for the good stuff. Specifically, I replaced the generic battery cutoff switch and 100A circuit breaker coming from the battery with a Blue Sea 3 pole 250A circuit breaker that I also use as a switch.

The issue since the upgrade is that my LFP goes into protect mode when I flip on the breaker (nothing is on but the propane detector). The power comes on for a moment, and then cuts off and the battery is in protect mode. The only way to get it out of protect mode is to unhook the negative terminal from the battery and hook it back up. That's a pain. Interestingly, once I do that, everything works fine. I can flip the master breaker as many times as I want and the power cycles fine without the BMS going into protect mode.

If I leave the system off for a while and turn it back on with the breaker, the same thing happens again.

If I leave the system off for a while but disconnect the converter before I turn the system on (via its circuit breaker), the BMS does not go into protect mode. All works fine. I can turn it off and on without issue. The converter is a Progressive Dynamics 9160ALV made specifically for LFP duty. If I turn the converter on after I turn the main 250A breaker on, all works fine. Hmmmmm....

I cannot figure out what is causing the BMS to go into protect mode if the converter is connected.

Any ideas?

Here's my wiring diagram:
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Old 09-21-2021, 12:16 PM   #2
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What is the converter output voltage when the breaker is off and you only have the propane detector load?

Do you have both batteries in the circuit?
If so then both shut down?

Is the solar controller on at the time?
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Old 09-21-2021, 12:28 PM   #3
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Have you tried this with the Inverters disconnected from the positive bus, not just turned off.

With converter on and batteries off the inverter's capacitors will remain charged. When batteries are turned on the capacitors can join the converter and deliver a huge jolt of charging current to the batteries causing the BMS to think a "fault" has occurred and it (they because you have two batteries, each with a BMS) shuts down.

With battery C/B off and converter off the cap's will bleed down due to other parasitic draws so this may explain how things go back to normal after converter is shut down for a while.

If you can then turn the batteries on with converter running then reconnect the Inverters one at a time, repeating test after each is connected. This will show if you have an Inverter problem.

Relying only on the Inverters "switch" can still leave the input caps online. My inverter (Renogy) specifically warns about this.

You may have to add a warning tag to your Circuit breaker "Turn Off Converter Before Connecting Batteries".

I don't have this problem with my Battleborn batteries but without more knowledge of the actual BMS used by SOK it's hard really know what's going on. What's their fault sensing threshold?
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Old 09-21-2021, 12:36 PM   #4
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What happens if you put a larger load on the 12V and repeat the experiment?
Ideally an incandescent bulb or something purely resistive.
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Old 09-21-2021, 12:37 PM   #5
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Everything worked fine until I replaced the separate main breaker and cutoff with the single breaker.

I am going to install the old cutoff after the 250A main breaker and see if it works. I will power up with the breaker on and the cutoff switch off. Then turn on the cutoff switch and see what happens. Might be that fancy 250A breaker is causing the problem.
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Old 09-21-2021, 12:48 PM   #6
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Have you tried this with the Inverters disconnected from the positive bus, not just turned off.

With converter on and batteries off the inverter's capacitors will remain charged. When batteries are turned on the capacitors can join the converter and deliver a huge jolt of charging current to the batteries causing the BMS to think a "fault" has occurred and it (they because you have two batteries, each with a BMS) shuts down.

With battery C/B off and converter off the cap's will bleed down due to other parasitic draws so this may explain how things go back to normal after converter is shut down for a while.

If you can then turn the batteries on with converter running then reconnect the Inverters one at a time, repeating test after each is connected. This will show if you have an Inverter problem.

Relying only on the Inverters "switch" can still leave the input caps online. My inverter (Renogy) specifically warns about this.

You may have to add a warning tag to your Circuit breaker "Turn Off Converter Before Connecting Batteries".

I don't have this problem with my Battleborn batteries but without more knowledge of the actual BMS used by SOK it's hard really know what's going on. What's their fault sensing threshold?
I'll give this a whirl. I think 3000W of inverter caps is too much for the 100A BMS. Once I add the second battery (on the way) and hook up the solar (in work), the caps won't be an issue. I hope.
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Old 09-21-2021, 12:56 PM   #7
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I'll give this a whirl. I think 3000W of inverter caps is too much for the 100A BMS. Once I add the second battery (on the way) and hook up the solar (in work), the caps won't be an issue. I hope.

You may also want to consider adding C/B's to the input of each Inverter. C/B's like these are around $25 and are commonly used for Inverter inputs.



I have several installed in my electrical system for various loads and they have been working great. Work as switches too for isolating equipment for service/repair.
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Old 09-21-2021, 04:00 PM   #8
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With converter on and batteries off the inverter's capacitors will remain charged. When batteries are turned on the capacitors can join the converter and deliver a huge jolt of charging current to the batteries causing the BMS to think a "fault" has occurred and it (they because you have two batteries, each with a BMS) shuts down.
I can't find it now, but I swear Battle Born recommends some kind of battery protector to stop in-rush loads like this.

This isn't what I remember seeing but sounds like it could be helpful if that's really the issue:
https://battlebornbatteries.com/prod...surge-limiter/
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Old 09-21-2021, 04:28 PM   #9
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I can't find it now, but I swear Battle Born recommends some kind of battery protector to stop in-rush loads like this.

This isn't what I remember seeing but sounds like it could be helpful if that's really the issue:
https://battlebornbatteries.com/prod...surge-limiter/
I took the advice from DIYsolar and added a precharge circuit for the inverters. It's a 6 ohm 50W resistor wired to a momentary horn button between the + battery terminal and the + bus that feeds the inverters. Hooked it up. Pressed the horn button for 1 sec and flipped on the main breaker. Worked like a champ.

I'm sure there is a more elegant solution, but I need to go camping on Thurs. Looks silly having that 14ga wire running around all that 4/0 cable, doesn't it? I'll see what else is out there, but this works.
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Old 09-21-2021, 06:58 PM   #10
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I took the advice from DIYsolar and added a precharge circuit for the inverters. It's a 6 ohm 50W resistor wired to a momentary horn button between the + battery terminal and the + bus that feeds the inverters. Hooked it up. Pressed the horn button for 1 sec and flipped on the main breaker. Worked like a champ.

I'm sure there is a more elegant solution, but I need to go camping on Thurs. Looks silly having that 14ga wire running around all that 4/0 cable, doesn't it? I'll see what else is out there, but this works.
Perhaps there is a more elegant solution but that often translates to "more expensive" ($280 for the BB surge limiter).


For the difference in cost you could buy yourself a real fancy horn button.

I like this solution. No more difficult than when you own a diesel engine and have to run the glow-plugs for a given time before hitting starter.

Adding: Rather than calling this a "pre-charge circuit" I'd rather call it an "equalizer circuit". Since your issue occurs when the converter is on when batteries are switched online. The Inverter caps are charged from the Converter. When the batteries are switched online the current is flowing out of the Inverter caps and into the batteries. The BMS will shut down due to a surge in either direction.

Semantics for sure but what the heck, it's been a slow afternoon.
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Old 09-21-2021, 11:50 PM   #11
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I took the advice from DIYsolar and added a precharge circuit for the inverters. It's a 6 ohm 50W resistor wired to a momentary horn button between the + battery terminal and the + bus that feeds the inverters. Hooked it up. Pressed the horn button for 1 sec and flipped on the main breaker. Worked like a champ.
Interesting experience! If I'm following this, the converter sets the inverter capacitor voltages higher than the battery voltage and the "inrush" that triggers BMS protection is from the caps INTO the battery. It's just a few volts difference, but the near-zero LiFePO4 internal resistance and the miniscule BMS voltage drop (about 11 mv with Overkill BMSs) results in considerable current. The BMS is probably very sensitive to current in this direction because ANY current above max charge current can and should be interrupted immediately. I.e., the battery only has to ride through starting currents and such of loads in the RV in the load direction (Overkill BMSs have three levels of protection in the "load" direction.

Removing the converter allows parasitic loads to drag the inverter cap voltage down to zero. You then bring the caps back up with the 6 ohm resistor, so they match the battery voltage, and the BMS sees little or no current when the battery breaker is closed (though I think most BMS will ride through this high current in that it's flowing out of the battery). Though it's a really good idea to bring the caps up slowly with an amp or two (6 ohms) than to allow hundreds of amps to surge into them.

Normally you will leave the battery breakers closed and have all other breakers open when the RV is in storage (the 50% SOC storage rule). So you won't run into this inverter voltage problem in practice. I.e., you won't if you don't introduce the converter until the rest of the system is up and running .. meaning the inverter caps are charged and the inverter breakers are closed along with other loads when you are heading out.

Just had a thought. Since break-a-way trailer brakes require the trailer battery to be connected to at least the trailer brakes, that load must be energized before leaving home. Just wondering if I always do this!
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Old 09-22-2021, 06:44 AM   #12
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Interesting experience! If I'm following this, the converter sets the inverter capacitor voltages higher than the battery voltage and the "inrush" that triggers BMS protection is from the caps INTO the battery. It's just a few volts difference, but the near-zero LiFePO4 internal resistance and the miniscule BMS voltage drop (about 11 mv with Overkill BMSs) results in considerable current. The BMS is probably very sensitive to current in this direction because ANY current above max charge current can and should be interrupted immediately. I.e., the battery only has to ride through starting currents and such of loads in the RV in the load direction (Overkill BMSs have three levels of protection in the "load" direction.

Removing the converter allows parasitic loads to drag the inverter cap voltage down to zero. You then bring the caps back up with the 6 ohm resistor, so they match the battery voltage, and the BMS sees little or no current when the battery breaker is closed (though I think most BMS will ride through this high current in that it's flowing out of the battery). Though it's a really good idea to bring the caps up slowly with an amp or two (6 ohms) than to allow hundreds of amps to surge into them.

Normally you will leave the battery breakers closed and have all other breakers open when the RV is in storage (the 50% SOC storage rule). So you won't run into this inverter voltage problem in practice. I.e., you won't if you don't introduce the converter until the rest of the system is up and running .. meaning the inverter caps are charged and the inverter breakers are closed along with other loads when you are heading out.

Just had a thought. Since break-a-way trailer brakes require the trailer battery to be connected to at least the trailer brakes, that load must be energized before leaving home. Just wondering if I always do this!
If you used a storage scope with current probe you could determine how much inrush current there was. Might be possible to resolve with a torroid choke in the battery line.
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Old 09-22-2021, 12:42 PM   #13
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If you used a storage scope with current probe you could determine how much inrush current there was. Might be possible to resolve with a torroid choke in the battery line.
You ARE an engineer. Good suggestion. And maybe not bother with the storage scope; a bit of trial and error would do it. Or a very rough calculation of the circuitry impedance and an estimate of the capacitor values. Might get an idea of the cap values on-line somewhere. The torroid needs to carry a couple of hundred amps so it's not going to be small.
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Old 09-22-2021, 01:28 PM   #14
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You ARE an engineer. Good suggestion. And maybe not bother with the storage scope; a bit of trial and error would do it. Or a very rough calculation of the circuitry impedance and an estimate of the capacitor values. Might get an idea of the cap values on-line somewhere. The torroid needs to carry a couple of hundred amps so it's not going to be small.
I'm following this as it's very similar to what I will likely put together myself.
Since I have the tools in my shop I'd measure and then calculate to see if a torroid would work. I have some large ones I use in amateur radio work. Amidon is a source for cores. Yes. Would be large core to wind a few turns of that heavy wire. You could use a torroid on each invertor input wire. I believe Corn18 has two inverters.

My preference is for a solution that doesn't require manual intervention.
Another option is to use some large mosfets as a switch across the resistor. The transistors would have a time delay for turn on. For this, the resistor/switch would be in line with the invertors so would also slow transients when the convertor comes on as well. In my case I'd use multiple mosfets and design a PCB. I've had PCBs made in China for my own projects and can be done for $25 or so. This isn't a solution for people who don't have electronics skills.

BTY, currently working on a battery heater to extend battery charge temp range. For this project I started out thinking I'd do a custom design but changed direction to use commonly available STEM components. It will require some loading of Arduino code. Which I would make public for use and modification for those so inclined.
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Old 09-22-2021, 01:46 PM   #15
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You ARE an engineer. Good suggestion. And maybe not bother with the storage scope; a bit of trial and error would do it. Or a very rough calculation of the circuitry impedance and an estimate of the capacitor values. Might get an idea of the cap values on-line somewhere. The torroid needs to carry a couple of hundred amps so it's not going to be small.
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I'm following this as it's very similar to what I will likely put together myself.
Since I have the tools in my shop I'd measure and then calculate to see if a torroid would work. I have some large ones I use in amateur radio work. Amidon is a source for cores. Yes. Would be large core to wind a few turns of that heavy wire. You could use a torroid on each invertor input wire. I believe Corn18 has two inverters.

My preference is for a solution that doesn't require manual intervention.
Another option is to use some large mosfets as a switch across the resistor. The transistors would have a time delay for turn on. For this, the resistor/switch would be in line with the invertors so would also slow transients when the convertor comes on as well. In my case I'd use multiple mosfets and design a PCB. I've had PCBs made in China for my own projects and can be done for $25 or so. This isn't a solution for people who don't have electronics skills.

BTY, currently working on a battery heater to extend battery charge temp range. For this project I started out thinking I'd do a custom design but changed direction to use commonly available STEM components. It will require some loading of Arduino code. Which I would make public for use and modification for those so inclined.

You guys definitely ARE engineers. Rather than a simple push button solution, and following the suggestion in Occam's Razor, a more "sophisticated" solution is required

Whatever works

On the battery heater project, why not just use tank heater pads like those made by FACON and sold on Amazon.

Low density heating pads that operate on 12 volts, internally temperature regulated to turn on~ 40 F and off ~60 F.

Since this is only an issue when charging, a simple external temp sensor that isolates the batteries (Solid State Relay) if battery temp is lower than 40 F. but leaves heater pads still energized. OR just let BMS do the work.
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Old 09-22-2021, 02:53 PM   #16
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I'm following this as it's very similar to what I will likely put together myself.
Since I have the tools in my shop I'd measure and then calculate to see if a torroid would work. I have some large ones I use in amateur radio work. Amidon is a source for cores. Yes. Would be large core to wind a few turns of that heavy wire. You could use a torroid on each invertor input wire. I believe Corn18 has two inverters.

My preference is for a solution that doesn't require manual intervention.
Another option is to use some large mosfets as a switch across the resistor. The transistors would have a time delay for turn on. For this, the resistor/switch would be in line with the invertors so would also slow transients when the convertor comes on as well. In my case I'd use multiple mosfets and design a PCB. I've had PCBs made in China for my own projects and can be done for $25 or so. This isn't a solution for people who don't have electronics skills.

BTY, currently working on a battery heater to extend battery charge temp range. For this project I started out thinking I'd do a custom design but changed direction to use commonly available STEM components. It will require some loading of Arduino code. Which I would make public for use and modification for those so inclined.
I haven't needed a battery heater yet, but would probably add a tank heater matt to the underside of the battery and add a switch near my tank heater switches. But a thermostat on the battery would be easy and probably more reliable. The mats on my tanks are about 12x18" and 1/4" thick.

BTW, ex-KA2R here.
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Old 09-22-2021, 02:56 PM   #17
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For the 12V fridge aficionado only ............

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Bottom line, plan on at least 80 ah a day, maybe 100ah for hot ones and higher use. Better have 300 ah of lithium on board to get through a couple of crappy days unless you want to be a slave to your generator.
I too agree. But, in true engineer-with-nothing-better-to-do fashion, I have a heavily modified Nova Kool 9.1 cu ft unit with bottom freezer and the compressor/condenser below the freezer (uses indoor air to cool the compressor and condenser coils).

I didn't read the FAQs on this fridge before buying and only later learned that the freezer is designed to run only 20F cooler than the fridge. That's a theoretical 18F in the freezer with the fridge running at 38F.

I didn't see 18F on a good day. In fact I rarely saw 20F inside the box formed by the freezer evaporator plate (to the left) and was seeing 27-28F opposite that box (to the right).

Part of this result was my own fault. The fridge comes with about 2" of insulation, presumably about R13 or a bit more. I added 2" of Rmax R13.1 on the sides and back and 6" on top with the idea of reducing energy usage. It certainly did that, but the insulation I added benefitted the fridge more than the freezer in both coverage and relative heat flow (higher for the freezer) so probably compounded the freezer temp problem. Ideally I would have added R26 to the freezer but there was no space to do that anyway.

The refrigerant flows through the evaporator plate in the freezer then up to the evaporator plate in the fridge and the single thermostat is in the fridge. I.e., the freezer is a slave to the fridge and gets cooling only to the extent the fridge calls for cooling.

So, I had a fridge that was freezing lettuce at the bottom when I set the fridge temperature lower to help the freezer and still a not cold enough freezer. The frozen raw meat catfood we use needs to be colder.

So the experiments started.

1) Ran the fridge as low as I could without freezing lettuce at the bottom (about 36F).

2) Added an 80mm box fan running at half speed in the freezer to circulate air thus extracting more "cold" from the freezer evap plate and making the freezer temp more uniform. This helped a lot in that it dropped the evap box by 2F and the right side by 6-7F.

3) Added some insulation around the fridge evap plate so the fridge was getting less cooling and requiring the compressor to run longer. This helped a little but made it difficult to find the right setting for the fridge thermostat. And seemed to require a different setting for a different ambient temperature. BTW, the thermostat controls fridge evap plate temperature, not fridge air temperature.

4) Added 2" of Rmax to the bottom of the fridge. This slowed heat flow from the fridge to the freezer below. The big benefit was much more uniform fridge temperature (about 1F difference top to bottom instead of 2 or 3F and freezing lettuce). But it also let me set the fridge at 34F without freezing lettuce at the bottom. The lower fridge temp and lower heat flow into the freezer picked up a couple of degrees in the freezer.

5) Added 2" of Rmax to the bottom of the freezer (inside). This reduces heat flow into the bottom of the freezer from the compessor and condenser coils that are under it. There was no room for insulation under the freezer (between it and the coils). This picked up a couple of degrees in the freezer.

6) Swapped in a quieter and more capable compressor/condenser fan. Quieter but no help temperature-wise and maybe hurt since there's more convection going on under the freezer floor.

7) Added 2" of Rmax to the inside of the freezer door. This was wasted space in any event and the insulation probably halved the heat flow through the freezer door. This picked up a couple of degrees.

8) Added 2" of Rmax in the side walls of the cupboards to the right and left of the freezer. This took those areas from 4" to 6" of insulation. probably helped a little, but the effect was not obvious.

9) Added some fixed air vanes in the compressor/condenser compartment to reduce the bit of re-circulation of warmed exhaust air that was occurring. Probably helped efficiency a little.

10) This one has not yet been implemented. I'm going to reverse the air flow of the compressor/condenser fan so that air flowing over the hot compressor does not flow inward and across the condenser coils before being exhausted by the fan. I'll also add fixed air vanes so most of the air will flow across the compressor as it leaves the compressor/condenser cavity and be deflected away from the air inlet (the condenser is near the front). This will reduce the convection heating of the freezer floor and improve efficiency a bit.

11) With these improvements, I do not need to run the Danfoss/Secor variable speed compressor at full speed, even at 100F ambient (the max design temperature). The compressor is up to 15% more efficient at its minimum speed. I hoped more running time at the lower speed would help in spite of the refrigerant being less cold. No freezer temperature benefit that I could discern. And the compressor is not audibly quieter at the lower speed, so no benefit there. I now usually leave the compressor at 3500 rpm.

Long story short ..... the freezer now stays under 15F to the left and under 16-17 to the right most of the time. After extra run time from door opening at meal time the freezer air may get down to 10F briefly. The freezer is probably cold enough for the cat food if it's kept only a month or less (we put it in the box formed by the evaporator plate).

But there's still one problem!!!!!!!!!!! When the ambient is low, say in the 40's at night, the fridge runs very little and the freezer warms up. The freezer contents can be in the low 20's by morning. With all the added insulation, this is probably better than it would otherwise be, but not great.

Any suggestions would be appreciated. I'd like the freezer to be down around 10F.

Nova Kool makes a dual compressor model. I didn't buy it because of cost and Nova Kool rates it at a higher current. I think their current rating is pessimistic, at least if the fridge and freezer temperatures are set the same as for the single compressor model. However, I'm betting that most users, including me, would set the freezer to a lower temperature and thus use more energy. I could live with that, solar isn't that costly (much of the fridge energy use occurs during the hot hours when there is plenty of sun). I'm currently trying to get my son to take the single compressor unit so I can order a dual compressor model.

As for the battery and solar requirements. I'd bet that Half Ton Heavy's estimates are right on for the typical installation. The manufacturer states that duty cycle is 40-50% at 70F ambient. I'd guess 50% is most realistic. With the BD35F compressor running at full speed (5.2 amps) that would be about 63 Ah over 24 hours. Assuming the compressor duty cycle hits 100% at 100F in a typical installation, the day time half of that Ah use could double to 63 Ah. Ergo about 100 Ah on a day that is 70F at night and 100F during the day.

With all of my added insulation, I'm well below those numbers, not half but well less than 2/3. But, I don't recommend the route I went. More solar and battery would be a lot easier.

And a dual compressor model is essential if you are fussy about freezer temperature. At least this is the case with the Nova Kool RFU9000.
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Old 09-22-2021, 06:02 PM   #18
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Originally Posted by Hclarkx View Post
I haven't needed a battery heater yet, but would probably add a tank heater matt to the underside of the battery and add a switch near my tank heater switches. But a thermostat on the battery would be easy and probably more reliable. The mats on my tanks are about 12x18" and 1/4" thick.

BTW, ex-KA2R here.
I moved my batteries into a small storage compartment that is open to the heated interior air (via under bed/couch space). No need for heated batteries. When batteries are in use and being charged during cold weather the furnace is running to keep both the interior water pipes and ME from freezing. When winterized the batteries are just shut down after receiving a top-off charge in non-freezing temps.

For anyone reading these posts regarding battery heating it's important to note that you DO NOT want to use Battery Heating Pads like are common in the Arctic. Tank Heaters use low density heat with thermostats incorporated in their construction. BATTERY HEATERS get far too hot and since the inside of a LiFePo4 isn't liquid filled like Lead Acid batteries, damage can occur from the high density heat source.

Just because they have the name "Tank Heater", they can still be used to keep LiFePo4 batteries above the critical temp for charging.
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2018 Flagstaff Micro Lite 25BDS
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Old 09-22-2021, 07:21 PM   #19
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My batteries are in my basement which is heated and cooled. How do you get the basement warmed up before charging the batteries? Especially with solar.
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800W solar > 412 Ah LiFePO4 > 3,200W Inverters
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Old 09-22-2021, 07:30 PM   #20
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Originally Posted by corn18 View Post
My batteries are in my basement which is heated and cooled. How do you get the basement warmed up before charging the batteries? Especially with solar.
Pull the switch on Solar Panels and Converter then run the furnace. You did say the basement was heated.

Remember, the batteries can still be charged down to 32 degrees and since keeping LiFePo4 batteries fully charged when winterized is not recommended I'd have the solar shut off so it doesn't try to charge below freezing.
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