diy solar

diy solar

On Keeping LFP Warm

Did you get a chance to look at/test the powermon? I'm looking to buy a few more Low Voltage Disconnects. I need one for my 12VDC dehumidfier, and another for my battery heater. I can hold off for a few more months before ordering.

Was it easy to program? Capable of very close voltages between cutout and cut in? Did it keep the settings after a power outage?

Thanks,
Carl
Yes, it's very easy to program. You just bring up the app on your phone, connect it via BLE (low energy Bluetooth), and configure it. There are several categories of disconnects that can be programmed. In this case, it's the low voltage disconnect. For my testing, I simply put in a disconnect at 26.0V, and a reconnect at 26.2V. You can also specify the delay time in milliseconds, where the disconnect will only happen if the voltage gets down to equal or below the disconnect voltage for the specified amount of time. To reconnect, the voltage has to be at or above the reconnect voltage for the same amount of time. Anyway, it works great.

I don't think you care, but it also has high voltage disconnect and an over current disconnect. You can also have up to 16 timers set to turn the relay on or off at certain times, certain days of the week, etc.

The non-timer disconnects are part of the device configuration, which is saved in non-volatile memory. So that info is saved if the power to the Pmon-5s is disconnected. The stuff that is lost when it loses power is the date and time (so the timer functions either won't work or won't work as intended) and any passwords you've set up for the app. That last part seems a bit crazy: They have password protection to keep people out, but if they just power-cycle the thing the password protection is removed.

The funny thing was that I wanted this device primarily for two things: Coulomb counting for an accurate SoC of the battery, and data logging. The thing can log voltage, current, SoC, and temperature for up to three years. It was after I got it that I discovered how handy the low-voltage disconnect would be the function you described. As it turns out I'm finding problems with the data logging and Coulomb counting. Oh well.
 
I should mention: I've mostly convinced myself to put the Pmon-5s on the battery side of the BMS, rather than on the outside. I'll put an inline fuse on it in case something goes crazy, but it isn't likely to be a problem. That way it is almost impossible for it to ever lose power, unless I break down the pack for some reason.

They (Thornwave labs) say they are working on the next version of the app, so hopefully they will be able to fix some of the little annoyances that I've discovered. I think it is worth the extra $$ over the Victron Smart Shunt, but only (at this point) because of the voltage-based relay control. If they can fix the other stuff, it is a clear winner.
 
I should mention: I've mostly convinced myself to put the Pmon-5s on the battery side of the BMS, rather than on the outside. I'll put an inline fuse on it in case something goes crazy, but it isn't likely to be a problem. That way it is almost impossible for it to ever lose power, unless I break down the pack for some reason.

They (Thornwave labs) say they are working on the next version of the app, so hopefully they will be able to fix some of the little annoyances that I've discovered. I think it is worth the extra $$ over the Victron Smart Shunt, but only (at this point) because of the voltage-based relay control. If they can fix the other stuff, it is a clear winn``
 
Thanks for the review. I'll look into the Pmon. I don't really need the password control. There's no one around that could/would want to mess with it.
 
Thanks for the review. I'll look into the Pmon. I don't really need the password control. There's no one around that could/would want to mess with it.
Yeah, I don't much care about power outage clearing the password stuff either, we're many miles away from anyone who would care. It just seems odd that that was one of the few things that they don't preserve in non-volatile memory.
 
That's a debate we've been having amongst the owners of the cabin (my brother, sister, and I). The floor of the basement is about 3 ft below grade. My sister and her husband think that the "solar room" where the batteries are doesn't get much below freezing, maybe to 20°F or so. We do have a little battery operated digital temp display in the cabin with an outside probe, and the display does keep track of the lowest and highest temp outside. My brother-in-law reset it before closing up the cabin, so I should at least have a guess at the lowest temp it got to outside over the winter.
Horsefly did you ever get the winter temp data (lowest from inside and out) from this past winter? I’m new to this website and to solar but have an old cabin I moved to a new foundation in a very cold part of the country (Island Park, ID). Though it’s only at 6,200ft elevation, it gets extremely cold. My only post on this website describes it. The basement has 9ft walls, but is only about 5.5ft below natural grade and unfortunately there is still ample opportunity for cold outside air infiltration and mixing. Having some idea of basement temp deltas with the outside, and some understanding of how buttoned up your basement was would be important info to me as I haven’t purchased my All-In-One charge controller inverter or decided on batteries yet and that stuff just don’t like the COLD….
 
Horsefly did you ever get the winter temp data (lowest from inside and out) from this past winter? I’m new to this website and to solar but have an old cabin I moved to a new foundation in a very cold part of the country (Island Park, ID). Though it’s only at 6,200ft elevation, it gets extremely cold. My only post on this website describes it. The basement has 9ft walls, but is only about 5.5ft below natural grade and unfortunately there is still ample opportunity for cold outside air infiltration and mixing. Having some idea of basement temp deltas with the outside, and some understanding of how buttoned up your basement was would be important info to me as I haven’t purchased my All-In-One charge controller inverter or decided on batteries yet and that stuff just don’t like the COLD….
Yes, I put in a temperature logger in the basement of our cabin, and it collected temps all winter. Our cabin is at just under 9,000 ft elevation, but at a bit lower latitude than you. Our cabin basement only has cinderblock walls, and is only about 3-4 ft below grade in the solar room.

I was very surprised the low never got below 27°F. However, there was one flaw in my test. I had the probe sitting on the shelf where the current AGM batteries sit. It may have been that the thermal mass of the batteries kept the probe a bit warmer than the surrounding room. Still, the real low was almost certainly not below 20°F. The box I'm building and the heat-control electronics that I'm putting in the box will certainly be able to handle that.

Here's the graph of the temps:

Solar_Room_Temps_Winter2020-2021.PNG
 
By the way, I'm progressing well on my box and the heating electronics. I'll probably post pics in a new thread of the "Show and Tell" sub-forum in a couple of weeks.
 
Horsefly THANK YOU for the speedy reply and your awesome data. Looks like your basement temps are typically 25F warmer than outside in the winter. Also looks like relatively mild temps last winter. I know we got down to -24F and much colder temps are typical. Is your basement sealed up Well? Just trying to gauge what effects the terrestrial heat and thermal mass of the basement slab vs outside cold infiltration have in a not yet sealed up basement. Batteries aside, I’m concerned my basement cold temps might have harmful effects on my future systems electronics.
 
Horsefly THANK YOU for the speedy reply and your awesome data. Looks like your basement temps are typically 25F warmer than outside in the winter. Also looks like relatively mild temps last winter. I know we got down to -24F and much colder temps are typical. Is your basement sealed up Well? Just trying to gauge what effects the terrestrial heat and thermal mass of the basement slab vs outside cold infiltration have in a not yet sealed up basement. Batteries aside, I’m concerned my basement cold temps might have harmful effects on my future systems electronics.
Unfortunately we didn't have a logger for outside over the winter, and no one goes up there from about mid-Nov until late-Apr. So, I don't really know what the weather was like. I also log the voltage of the battery through the winter, and you can generally see when the panels are covered with snow for more than a day as the batteries don't get back to float. For the four winters since we put the system in, there's never been more than 6 days TOTAL of the panels being covered. That was a real surprise. However, the sky is mostly clear and the air is very dry here in Colorado, so I think even if the temps remain below freezing the snow on the panels melts and maybe even sublimates.

Our basement is sealed up pretty well. Your thoughts about thermal mass of the slab and terrestrial heat from being partially underground are the same ones I've had since I started thinking about it. On the main floor of the cabin there are several windows facing south, so I would bet it gets into 50's on many days when it may be pretty cold outside. The attic isn't insulated as well as it could be, and we may add to that at some point.

I wouldn't worry too much about the cold temps and regular electronics, assuming you are talking about solar power components. We leave our inverter in standby and the SCC on as usual all winter.
 
By the way, I'm progressing well on my box and the heating electronics. I'll probably post pics in a new thread of the "Show and Tell" sub-forum in a couple of weeks.

Based on your data, if you have a well insulated box and it is sitting on the concrete slab of the basement, you will need very little heating to keep the batteries well above freezing. Resist the temptation to put large heat pads in.
1) It risks creating hot spots in the battery box
2) Switching large current is more prone to failure than low current.

I once calculated that with a small box insulated to R30 and reasonably well sealed, it only takes ~10Wh/day to maintain the temp above freezing if the outside of the box is -25deg. That means even a 10 W heat pad would do the trick. In your case, you do not need to maintain that large of a temp difference so a 10W heat pad would be more than enough.

You may want to consider this:

Alternatively, if you are not going to be there, just do an insulated box and make sure you have low temp cut-off on the chargers and BMS and call it good. It may never get below freezing and if it does, who cares if it is not charging if you are not even there?

BTW: If they are not in the sun, earth contact concrete slabs tend to have very stable and moderate temperatures. I am considering putting a large battery bank in my un-insulated garage. We almost never get to freezing so I will not have heat pads in the battery box (I will have low temp charge cut-off). However, we do get 100F+ days and the garrage gets hot so I was a bit worried about high temps. In June I put a recording thermostat on the floor of the garage where the batteries will go and so far, the hottest I saw was 78deg F and the coolest I saw was 69deg F. Most of the time the temp has been between 71 and 75 deg. In that time we have had multiple 100F+ days and in the open roof rafters of the garage it gets to above 95 almost every day (I know that because I have a vent fan that comes on at 95deg F).

The thermometer is just on the floor and not enclosed. My Next test will be to throw a folded towel over the thermometer to try to measure the actual slab temperature a little closer. My expectation is that it will rarely get much above 70 or 71F, if ever. My plan is to build an R10 insulated box with no insulation on the bottom. My expectation is that the batteries will rarely (if ever) get into the mid 70s and rarely (if ever) get below 60.
 
Based on your data, if you have a well insulated box and it is sitting on the concrete slab of the basement, you will need very little heating to keep the batteries well above freezing. Resist the temptation to put large heat pads in.
1) It risks creating hot spots in the battery box
2) Switching large current is more prone to failure than low current.
I mostly agree with you. I'll be using two 12V x 12W pads, in serial for my 8S pack. So that's only one amp. Some of my electronics is also to prevent the "hot spot" you are talking about. I've got two thermostats. One is the desired temp for the batteries (sensor on the top of the pack) and the other is to limit the heat (sensor on top of the 1/4" aluminum which is on top of the pads).
I once calculated that with a small box insulated to R30 and reasonably well sealed, it only takes ~10Wh/day to maintain the temp above freezing if the outside of the box is -25deg. That means even a 10 W heat pad would do the trick. In your case, you do not need to maintain that large of a temp difference so a 10W heat pad would be more than enough.
Well, R30 is not easy to attain. 2" XPS is only R10 (an R-value of 5 for every inch thick), so to get R30 I'd need six inches of XPS on all sides. My box is already bigger than I would originally imagined it. I think your sentence of "a small box insulated to R30" is an oxymoron. ;)

Early in this thread I tried to show the math of heat loss and specific heat of the batteries. I used pretty much what I had designed to that point, which is pretty close to what I've built, except that the box is quite a bit bigger. Based on 50°F inside and 32°F outside the box, I came up with a continuous loss of around 3.5W. So that's 3.5Wh per hour, or something over 80Wh/day. That's quite a bit higher than you are suggesting, and I didn't use as extreme a temperature outside the box (you said -25°F). I'd love for you to check my math in that first post.
 
Panasonic sells vacuum insulated panels that are hollow fiberglass with vacuum pulled and mylar pouch to seal.

Theres a 12 x 12 x 1 inch panel that is R20.

I am using these in my upcoming projects that need thermal insulation.

Pad them with something soft so they don’t pop. I put neoprene on mine before boxing them in with another hard material. Adhesive backed neoprene makes them much less vulnerable to puncture.

R40 in 2 inches sounds relatively small…? ?

1628884355465.png
 
Panasonic sells vacuum insulated panels that are hollow fiberglass with vacuum pulled and mylar pouch to seal.

Theres a 12 x 12 x 1 inch panel that is R20.

I am using these in my upcoming projects that need thermal insulation.

Pad them with something soft so they don’t pop. I put neoprene on mine before boxing them in with another hard material. Adhesive backed neoprene makes them much less vulnerable to puncture.

R40 in 2 inches sounds relatively small…? ?

View attachment 59995
Nice! How much is this stuff? Do you have a link?

"vacuum insulated" kinda scares me. I am virtually certain I would poke holes in them, probably while building the box, but certainly within the first year of it being deployed.
 
Panasonic sells vacuum insulated panels that are hollow fiberglass with vacuum pulled and mylar pouch to seal.

Theres a 12 x 12 x 1 inch panel that is R20.

I am using these in my upcoming projects that need thermal insulation.

Pad them with something soft so they don’t pop. I put neoprene on mine before boxing them in with another hard material. Adhesive backed neoprene makes them much less vulnerable to puncture.

R40 in 2 inches sounds relatively small…? ?

View attachment 59995
That's weird, I hit reply to ask for more data and the quote has an image that I don't see in your original post....

Do you have a link to where they can be purchased?
 
I found a link. They are cheaper than I feared...... but not cheap.


Edit: The forum SW seems to be acting up. I have no idea why the link I added above says 'invalid request' but if you click on it you will go to the product at Digikey.com
 
That's weird, I hit reply to ask for more data and the quote has an image that I don't see in your original post....
Edit: The forum SW seems to be acting up. I have no idea why the link I added above says 'invalid request' but if you click on it you will go to the product at Digikey.com

Hmmm. I was on firefox before. Now I am on MS Edge. I see the image in @curiouscarbon post, but it still says "invalid request" on my link.
 
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